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NOTICE
The information in this document has been funded by the United
States Environmental Protection Agency under interagency
agreement DW-14932583 with the U. S. Geological survey. It has
been subjected to the Agency's peer and administrative review,
and it has been approved for publication as an EPA document. Use
of trade names and commercial product brand names does not
constitute endorsement by the USGS or EPA or recommendation for
use.
ii
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CONTENTS
Forward iv
Abstract vi
Figures viii
Abbreviations and Symbols ix
Acknowledgment x
1. Introduction ... 1
Background
Purpose and Scope
Geologic and Hydrologic Summary
Geophysical Surveys
2. Conclusions ,
General
Aeromagnetic Data
Passive Electromagnetic Data
Active Electromagnetic Data
3. Recommendations 9
Magnetic Data
Passive Electromagnetic Data
Active Electromagnetic Data
Integrated Interpretation
4. Airborne Geophysical Instrumentation 11
General
Auxiliary Equipment
Total Field Magnetic Measurement System
Passive Electromagnetic Measurement System
Active Electromagnetic Measurement System
5. Total Field Magnetic Survey. 16
6. Electromagnetic Survey 19
Passive Source VLF Survey
Active Source EM survey
References 27
Appendices
A. Technical Specifications for RFP 28
B. Subcontractor's Response to RFP 37
C. Subcontractor's Report 77
iii
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FORWARD
The Branch of Geophysics of the U. S. Geological Survey
(USGS) has conducted airborne geophysical studies near
Brookhaven, Mississippi, as part of a general program to
evaluate various geophysical methods to detect and delineate
near-surface brine pollution associated with oil fields. This
research project is specifically designed to evaluate
suggestions from previous ground electromagnetic (EM) studies
that airborne electromagnetic (AEM) methods might be effectively
applied to mapping vertical and lateral distribution of brine
(Fitterman, 1986; Fitterman, Raab, and Frischknecht, 1986). A
particular problem in the use of either ground or airborne EM
methods in mapping brine associated with oil fields is
interference in electrical measurements caused by cultural
sources. These sources of electrical noise include power lines,
pipe lines, radio frequency transmissions, rail lines, and
fences. The major objective of this study is to test whether AEM
methods can be effectively used in an area such as Brookhaven
with heavy cultural noise. Secondary objectives of the study are
to evaluate different types of AEM methods which could be
applied to the Brookhaven area and to suggest specific further
processing and interpretation methods that are applicable to
detection of shallow brine.
The study of brine pollution at the Brookhaven oil field was
carried out in the following steps:
> EPA contracts for ground electrical geophysical
surveys and preliminary evaluation of various types of
airborne electromagnetic systems.
> EPA and USGS formulate a general project work plan
funded through interagency agreement DW-14932583.
--> Existing borehole and surface electrical
measurements for the area were evaluated to derive an
interpreted generalized geoelectrical section.
> On the basis of previous electrical studies,
technical specifications were drawn up for a request
for proposals (RFP) for an airborne electromagnetic
survey of the Brookhaven area.
> Proposals submitted to the USGS were evaluated
according to contracting procedures with a resulting
award to DIGHEM Surveys and Data Processing Inc.
(referred to as DIGHEM) of Ontario, Canada.
> Prior to the beginning of the airborne survey, a
quality assurance document for the project was prepared
and approved by USGS and EPA officials. The document
follows the form of "Interim Guidelines and
Specifications for Preparing Quality Assurance Plans
(EPA-600/4-83-004)".
> In May of 1988 the helicopter airborne geophysical
survey was started by DIGHEM and completed within ten
days. On site quality control as given in the final
contract and the quality control document was carried
iv
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out by USGS and EPA officials.
> Preliminary data from the subcontractor was
accepted by the USGS contract officer's representative
in September 1988. By mid-November, final calibration
corrections were completed for the active EM system. A
multiplicative error in one of the data sets was
discovered by the contractor and corrected in early
December.
The following report describes details of the above steps in the
investigation which are directly related to the airborne
geophysical study. A USGS open-file report is currently being
prepared to make the digital geophysical data available to the
public.
While the major objective of the study has been
accomplished, there is much more that can be done with the
airborne data. An integrated interpretation of both hydrologic
and geophysical data will be undertaken by the USGS, Branch of
Geophysics, in cooperation with the EPA.
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ABSTRACT
The Brookhaven oil field, one of the oldest in Mississippi,
has produced a significant amount of brine associated with oil
production. As a result of various brine disposal methods, there
has been brine contamination of near-surface aquifers and some
streams. Brine contamination is known to decrease the electrical
resistivity (increase conductivity) of fresh water aquifers. In
studies of other areas, various types of electrical geophysical
ground surveys have been applied to mapping the distribution of
shallow brine. However, a major problem in applying these
methods in oil field environments is extensive cultural features
such as cased wells, pipelines, electrical pumps, oil tanks,
power lines, and fences. The major objective of this study is to
evaluate whether airborne electromagnetic (EM) surveys could be
used to map subsurface electrical features in the presence of
high cultural noise present at the Brookhaven area.
A helicopter geophysical system was chosen for the survey
based on better spacial resolution than faster flying fixed wing
systems. The helicopter system, flown at a line spacing of 1/8
mile (200 meters) made total field magnetic and active and
passive EM measurements of the area of brine contamination. The
magnetic field data, corrected for regional variations, show
numerous small semicircular anomalies that are due to steel oil
tanks, pipe lines, and well casings. There are not any magnetic
features that can be directly related to geologic features
associated with subsurface brine distribution.
Passive EM systems are those which measure electromagnetic
signals from sources other than those flown with the helicopter
system. Data from the passive EM systems (60 Hz monitor and very
low frequency EM) systems are dominated by noise from cultural
features, mainly power lines. The data, as presented here, are
not of very much potential use in mapping near-surface brine.
However, these data are critical in determining possible
cultural effects in the active EM data. The 60 Hz and VLF data
may also be of use in interpreting cultural features such as
abandoned wells that are indirectly associated with the
subsurface brine distribution.
The active EM system has both transmitter and receiver
instrumentation which are carried by the helicopter. In this
survey, the transmitters and receivers are horizontal coils. The
primary objective of this study is to evaluate the performance
of active airborne EM systems in mapping subsurface resistivity
variations. The DIGHEM system as used here measures EM signals
at three frequencies, 56,000, 7,200, and 900 Hz which yield data
reflecting an increasing depth of penetration into the earth.
These measurements also show different responses to cultural
noise. Measured EM signals were corrected for system
calibrations and reduced to apparent resistivities by the
contractor.
Cultural noise for the active EM systems varies between maps
of apparent resistivity for each frequency. This type of noise
vi
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is characterized by narrow (short-wavelength) anomalous apparent
resistivity responses that may or may not cross several flight
lines. The amount of cultural noise is greatest for the highest
frequency and least for the lowest frequency. The levels of
apparent resistivity over power lines for the 7,200 Hz maps are
almost the same as the background level. Consequently additional
information, such as the passive EM data, needs to be used to
determine areas of possible signal corruption by cultural
features.
In spite of the high level of cultural noise, apparent
resistivity data appears to map shallow vertical and horizontal
resistivity variations. The highest frequency (56 kHz) has the
largest apparent resistivities (median value of 150 ohm-meters)
since it mostly maps the surface resistive loess deposits. The
lowest median apparent resistivity (about 10 ohm-meters) is
associated with the lowest frequency. Areas of very low
resistivity could indicate areas of subsurface brine or low
resistivity"clay zones within the near-surface formations.
This study demonstrates that airborne electromagnetic
methods can be used to map subsurface resistivity variations in
areas of extreme cultural noise. Further interpretation is
needed to more specifically relate low resistivity areas to
possible brine contamination.
Vll
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FIGURES
Number Page
1 Location of the Brookhaven oil field (after 2
Kalkhoff, 1986). Geologic cross section C-C'
is given in Figure 2.
2 Geologic cross section of the Brookhaven oil 5
field given by Kalkhoff (1986). Letters are
keyed to Figure 1. See text for description
of lithologies for various formations.
3 Boundaries of the airborne geophysical survey ....12
(heavy lines) as specified in USGS contract
number 7-4350 (Appendix A). Major cities are
shown with major roads given by thinner lines.
4 Contour map of the reduced total field 18
magnetic data for the survey area shown
in Figure 3. Contour interval is 2 nanotesslas.
Areas identified by letters and data reduction
methods are discussed in the text.
5 Contour map of the filtered airborne VLF 20
total field (light lines). Heavy solid
lines are some of the possible power lines
identified by the 60 Hz monitor and cultural
features on topographic maps.
6 Grey scale apparent resistivity map computed 22
from the 56,000 Hz EM data. Light areas
are less resistive (more conductive) than
the darker areas. Areas labeled A, B, and C
are interpreted cultural responses discussed
in the text.
7 Grey scale apparent resistivity map derived 23
from the 7,200 Hz EM data. Light areas
are less resistive (more conductive) than
the darker areas (see text for explanation).
8 Grey scale apparent resistivity map derived 26
from the 900 Hz EM data. Light areas
are less resistive (more conductive) than
the darker areas (see text for explanation).
viii
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LIST OF ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS MEANING
AEM Airborne electromagnetic, refers to various
geophysical methods
EM Electromagnetic
EPA Environmental Protection Agency
Hz Hertz, the fundamental frequency of a periodic
signal measured in cycles per second
kHz 1000's of Hz
nT nanotessla (unit of magnetic field intensity)
RFP request for proposal
UHF ultra high frequency
USGS U. S. Geological Survey
VLF very low frequency
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ACKNOWLEDGMENT
Frank C. Frischknecht undertook the initial stages of this
study including conception of the work and formulation of
technical specifications for the geophysical contract.
Unfortunately, due to his untimely accidental death, he did not
see the fruits of his efforts. David V. Fitterman served as
project chief during the remaining part of the study. Mr. Fred
Hille, Mississippi Department of Natural Resources, visited the
study area while the airborne geophysical survey was being done
and helped to obtain current information for this study.
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SECTION 1
INTRODUCTION
BACKGROUND
The Brookhaven oil field, located in northwestern Lincoln
County, west of the city of Brookhaven, (Figure 1) is one of the
oldest fields in Mississippi with completion of the first well
in March of 1943 (Kalkhoff, 1986). Oil production from over 75
wells peaked in 1949. Since then production gradually decreased
with production being limited to 20 wells by 1984. Water high in
dissolved solids (brine) is produced along with the oil from the
Cretaceous Tuscaloosa Formation. Kalkhoff (1986) gives the
chemical composition of the brine as characterized by the
average dissolved solids.
Very little brine in proportion to oil is produced in the
early development stages of this type of oil field. As the oil
resource is depleted, more brine is pumped to the surface. Since
1943, approximately 54.2 million barrels of brine have been
pumped to the surface with a peak in the brine to oil production
ratio of about 5.6. The three following brine disposal methods
have been used at the oil field:
1) The earliest method of brine disposal was to
pump the brine onto the ground or into a
nearby stream.
2) A later method was to pump the brine into
evaporation pits.
3) Since the above disposal practices were
prohibited by 1978, brine has been reinjected
by Class II wells into the deep (greater than
4000 feet below ground surface) oil producing
formations.
All three of these disposal methods pose a threat to the quality
of near surface water supplies. Impact of the first two disposal
methods is obvious. The third method of disposal could
contaminate near surface ground water by several possible
mechanisms including defective or inadequate casing in the
injection well. In addition it is possible that deep saline
waters could seep to the surface through older improperly
plugged abandoned wells or subsurface vertical fractures. The
near-surface brine contamination probably is due to a
combination of the above sources. Geophysical methods may help
to delineate brine contamination and possibly identify the
effects of these different sources.
PURPOSE AND SCOPE
The primary objective of this study is to evaluate the
application of airborne electromagnetic (AEM) methods to mapping
of near-surface brine bearing waters. The survey area is typical
of many other oil fields which have possible brine contamination
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EXPLANATION
I I Study area
Brookhaven oil field
90*39'
00*25'
T.7N.
T Surface-water control
site and number
Ground-water control
site and number
A-A' Geologic cross section
R.6E. R.7E.
Bait map from U.S. OMtogleal 8un*y
NalctMZ, l:J50.000
T.6N.
R.8E.
ON tttkJ location liom Mliilnlppl
SKI* Oil and Qaa Bond (1»Wti)
Figure 1 Location of the Brookhaven oil field (after Kalkhoff, 1986). Geologic cross
section C-C* Is given in Figure 2.
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problems in that there are many sources of cultural electrical
noise. These sources of electrical noise may make it impossible
to carry out meaningful ground and airborne electromagnetic
surveys. Consequently the most important consideration in the
evaluation of AEM methods is whether any useful measurements can
be made in areas such as Brookhaven that have many sources of
cultural electrical noise. Two secondary objectives of this
study are to continue evaluation of airborne magnetic methods to
locate cased wells and to evaluate methods to further process
the geophysical data to address specific environmental problems.
GEOLOGIC AND HYDROLOGIC SUMMARY (from Kalkhoff, 1986)
The shallow geologic units important to the geophysical
study are unconsolidated sedimentary deposits of Tertiary and
Quaternary age shown in Figure 2. Pleistocene loess and alluvium
irregularly covers the area. In some areas, stream erosion has
removed the loess and parts of the upper Pliocene Citronelle
Formation. The upper Citronelle contains mostly sandy clays
grading into thicker gravel in the basal part of the Formation.
The Miocene Hattiesburg Formation has a clay and silt dominated
lithology with three sand units (designated A, B, and C) which
range in thickness from 10 feet to more than 90 feet (3 to 30
meters)
Gravels of the Citronelle Formation and sand layers in the
Hattiesburg Formation are the main sources of waters for local
use and serve as the main aquifers in the study area. All of the
aquifers in the oil field have been contaminated by brine to a
depth of at least 300 feet and contaminated outside of the oil
field to an unknown degree. Brine can move from its sources in
the Citronelle aquifer to discharge into nearby streams and can
move vertically into underlying Hattiesburg aquifers.
GEOPHYSICAL SURVEYS
Two previous geophysical investigations have been contracted
by the EPA for the Brookhaven area. The first investigation was
an exploratory study of the near surface electrical conductivity
variations (Nacht and Barrows, 1985). However, data gathered
from these ground surveys was not interpreted in detail. In the
second geophysical investigation, Becker and Morrison (1987)
interpreted the shallow resistivity sounding data in order to
input parameters for theoretical modelling of different airborne
EM methods. They conclude from analysis of resistivity soundings
that brine contaminated lithologies have a factor of 10 higher
conductivity (or factor of 10 lower resistivity) than areas
without brine. Both of these contracted studies were used to
formulate the work plan for the present study.
The report by Becker and Morrison (1987) evaluates
helicopter-borne EM (HEM) and fixed wing airborne EM (AEM)
systems for mapping the distribution of near surface (within 300
feet) brine. Their conclusion is that HEM systems are best
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suited to mapping water contaminated by brine in the shallowest
part of the Citronelle aquifer. The deeper penetrating fixed
wing AEM system is most suitable to mapping deeper brine
distribution within the Hattiesburg aquifer. However,
conclusions from this analysis do not consider hypothetical
effects of geological or cultural noise in measurements made
with either system. Such sources of noise are difficult to
estimate particularly in areas like Brookhaven because very few
if any airborne EM measurements have been made in oil field
envi ronments.
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UA LEVEL
MA LEVEL
HORIZONTAL SCALE
0 1 Mill
VERTICAL SCALE OMEATLY EXAMEMATEO
1 KILOMETER
Figure 2 Geologic cross section of the Brookhaven oil field given by Kalkhoff
(1986). Letters are keyed to Figure 1. See text for description of
lithologies for various formations.
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SECTION 2
CONCLUSIONS
GENERAL
The helicopter borne electromagnetic survey of the
Brookhaven oil field, carried out by DIGHEM accomplished the
main objective to test and evaluate airborne EM measurements as
described in the work plan for this project. One unanticipated
problem in doing this survey was that the flight plans had to be
somewhat modified to avoid impacting populated areas and
residences in the area. Flight lines which were planned to be
flown straight east-west, deviated from a straight line to avoid
houses and small communities. Using a helicopter versus a
fixed wing system for the geophysical instrumentation greatly
facilitated adjustment of flight line locations and also helped
improve the spacial resolution of geophysical anomalies. Other
conclusions are given in outline form below.
AEROMAGNETIC SURVEY
1. The most obvious features of the total field magnetic map
(Figure 4) are circular and semicircular positive anomalies of
10 to several hundred nanotesslas (nT) which have a width less
than 300 feet (96 meters). From previous USGS research on
magnetic field measurements over oil fields (Frischknecht and
others, 1985), these anomalies can be associated with well
casings and other magnetic metallic features such as oil tanks,
pipe lines, abandoned cars, and metal sheds. Identification of
anomalies due to casings may be important in locating abandoned
wells that could serve as flow paths allowing deep brine to seep
into surface aquifers.
2. The 1/8 mile (660 feet, 200 meters) flight line spacing
is not sufficient to define more subtle magnetic anomalies from
well casings located between flight lines (Frischknecht and
others, 1985).
3. The UHF radio positioning system which has a sensitivity
of 3 feet (1 meter), produced an accuracy on the order of 30
feet (10 meters) for this survey. A radio positioning system is
absolutely necessary for this type of survey to accurately
locate well casing anomalies in ground follow-up studies.
4. The high resolution of the Cesium magnetometer (0.01 nT)
was not really needed to resolve the magnetic anomalies from
well casings. Motion of the magnetic field sensor towed beneath
the helicopter caused a sinusoidal signal to be generated in the
recorded data which was filtered out of final data.
5. A critical factor in defining the shape and location of
magnetic anomalies from well casings is the data sampling rate
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from the magnetic sensor. The measurement sampling rate of 10
per second was adequate to define magnetic anomalies along
flight lines.
PASSIVE ELECTROMAGNETIC SYSTEM DATA
1) The VLF map of the survey area as given by the
contractor, does not show any major features directly related to
subsurface variations in the shallow electrical section.
Cultural noise from power lines and other electrically
conductive features dominates the map (Figure 5).
2) Standard commercial VLF instrumentation used in this
survey does not measure a sufficient number of parameters of the
EM field to be useful in this environment of high conductivities
and cultural noise. However, ground VLF surveys which have
higher resolution of EM signals and measure more parameters
should be considered for ground follow-up surveys.
3) The 60 Hz monitoring system normally used to identify the
location of power lines, appears to show anomalous responses
away from the power lines. These responses could be due to other
cultural features such as metallic fences or geologic features.
Further data processing will be required to evaluate the
usefulness of these data in mapping subsurface resistivity
variations.
ACTIVE ELECTROMAGNETIC SYSTEM DATA
1) Apparent resistivity maps at different frequencies (900,
7,000, and 56,000 Hz; Figures 6, 7, and 8) computed from the
survey data show a general increase in resistivity with
increasing frequency. The highest frequency is most sensitive to
the shallow resistive loess. Lower frequencies sense the deeper
lower resistive formations.
2) The 56 kHz apparent resistivity map shows the most narrow
or short wavelength features due to the variable thickness and
character of the surficial material. In addition this part of
the measurement system is most susceptible to corruption of
signals by cultural noise which also causes some of the short
wavelength features.
3) The 900 and 7,200 Hz data generally show the same
apparent resistivity variations. The general area of the oil
field has a lower subsurface resistivity (higher conductivity).
However spatial variations are not easily interpreted from the
black and white contour maps of resistivity.
4) The contractor's choice to use an algorithm to compute
apparent resistivity which relies on the phase of the EM signals
appears to minimize the influence of noise from power lines.
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5) The most important conclusion from this part of the study
is that it is possible to collect airborne EM data which is not
completely corrupted by cultural noise in an area like the
Brookhaven oil field.
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SECTION 3
RECOMMENDATIONS
Helicopter geophysical surveys using electromagnetic
resistivity mapping and magnetic methods should be considered as
one of the tools to efficiently map shallow oil field brine
bearing water in geologic settings similar to Brookhaven. The
technical specifications given in appendix A can provide general
guidelines for such contracted surveys. The technical design of
airborne surveys (such as selection of particular frequencies
for EM measurements) is greatly aided by selected ground surveys
and analysis of any available drill hole geophysical logs.
It is strongly recommended that an integrated interpretation
be carried out of all available hydrologic and geophysical data.
Application of geographic information systems (CIS) would
probably greatly help to interpret the wide variety of digital
data involved in the project. Recommendations for specific
components of the project are given in outline form below.
MAGNETIC DATA
1) A current location map of operating and abandoned wells
should be obtained for the area and compared with the location
of semi-circular magnetic anomalies.
2) Particular anomalies could be modeled to estimate the
magnetic characteristics of well casings.
3) If warranted, selected areas could be further evaluated
with ground checks and ground magnetic profiling.
PASSIVE ELECTROMAGNETIC DATA
1) Limited further processing of VLF data may be warranted.
However from preliminary analysis of the other airborne EM data,
it is doubtful that the VLF measurements will be very useful to
map brine distribution in this particular geologic and
hydrologic setting. However, ground VLF surveys which measure
more parameters and have greater resolution than airborne
measurements should be considered for further ground geophysical
surveys.
2) A theoretical evaluation should be made of the possible
application of the more advanced USGS VLF system which measures
more components of the EM fields than currently available
commercial systems.
3) Data from the 60 Hz monitoring system should be processed
to evaluate possible relationships between brine distribution
and any anomalous EM responses away from power lines.
4) A theoretical evaluation should be made of the possible
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application of the more advanced USGS airborne 60 Hz system.
This evaluation should incorporate detailed analysis of signal
to noise in the contractor's EM data.
ACTIVE AIRBORNE ELECTROMAGNETIC DATA
1) The contractor's resistivity maps should be enhanced to
facilitate interpretation of spatial trends and variations.
Several different enhancement methods should be tried including
preparation of color maps and perhaps shaded relief maps.
2) Several different methods can be applied to the airborne
data to estimate earth resistivity parameters. This
interpretation can then be compared with the parameters
interpreted from the ground electrical soundings.
3) A more comprehensive analysis of possible application of
fixed wing airborne EM methods can be made based on the
helicopter EM data.
INTEGRATED INTERPRETATION
1) The integrated interpretation of data pertaining to brine
distribution at the Brookhaven oil field should be a cooperative
effort of the USGS, EPA, and local offices of Water Resources
Division and the Mississippi Department of Natural Resources.
2) A key factor in effectively carrying out an integrated
interpretation is putting all appropriate data into a digital
format. An example of data which is currently not in digital
format includes known location of oil wells, power lines and
other cultural features. Once a uniform formatted digital data
base is assembled, a geographic information system (GIS)
computer program can be used to analyze the data sets.
3) A complete integrated interpretation may require that
additional supplemental data be acquired such as more current
water quality measurements, ground checks of well locations, and
additional ground geophysical surveys.
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SECTION 4
AIRBORNE GEOPHYSICAL INSTRUMENTATION
GENERAL
Data from previous ground geophysical studies of mapping
subsurface brine bearing water by the USGS (Fitterman, 1986;
Fitterman, Raab, and Frischknecht, 1986) suggest that airborne
geophysical methods can be applied to various environmental
problems such as locating cased wells and mapping subsurface
water bearing brine. On the basis of these and other studies,
technical specifications were drawn up for a request for
proposals (RFP) for an airborne magnetic and electromagnetic
(EM) survey of the Brookhaven (MS) oil field. The technical
portion of the RFP (USGS #7-4350), given in Appendix A, allowed
proposals to be submitted from contractors using both fixed wing
and helicopter based instrumentation.
Proposals from contractors were evaluated on the basis of
technical merits by a panel of four scientists with the
following results. Proposals using fixed wing geophysical
instrumentation were generally judged to have lower spatial
resolution of EM anomalies and are more susceptible to noise
from cultural features such as power lines. Of the proposals
using helicopter geophysical instrumentation, the proposal by
DIGHEM SURVEYS AND DATA PROCESSING (subsequently referred to as
DIGHEM) of Canada was awarded the airborne contract. The
technical part of the DIGHEM proposal given in Appendix B
describes details of the geophysical hardware.
The AEM survey is limited to - the immediate area of the
Brookhaven oil field (Figure 3). The flight line spacing for the
survey is 1/8 mile (200 meters). Flight lines are in an east-
west direction since the oil field is slightly elliptical in a
north-south direction (Figure 1). Two lines were flown north-
south through the survey area in order to check and adjust base
level changes in geophysical data between east-west flight
lines. Though the planned flight line location is optimal for
the geological setting, it was found that scattered dwellings
and small population centers which had to be flown around
produced less than straight flight lines.
The geophysical survey equipment used in the helicopter
survey can be divided into the following four groups: 1)
auxiliary equipment, 2) magnetic sensor, 3) passive EM sensors,
and 4) active EM system. All of these systems are described in
detail by the contractor's report given in Appendix C. The
following subsections give a brief nontechnical description for
each of the above systems.
AUXILIARY EQUIPMENT
This type of equipment consists of navigational and data
recording instrumentation. In this survey, a UHF (ultra high
11
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LOCATION MAP
JI'ST'SO"
90»SO'
Scale 1:250,000
THE SURVEY AREA
Figure 3 Boundaries of the airborne geophysical survey (heavy lines) as specified
in USGS contract number 7-4350 (Appendix A). Major cities are shown
with major roads given by thinner lines.
12
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frequency) radio system was used as the primary navigation
system. The system uses two or three beacons or transponders
that are located outside of the survey area. Real-time
navigation information determined from the transponder signals
is displayed for the helicopter pilot and digitally recorded
with the other survey data. The UHF navigation system yields a
much higher accuracy in retrieval of the flight path location
than the standard method of using photographs as described
below. The UHF navigation system is important in this particular
survey because the flight lines had to be changed from straight
east-west in order to avoid flying too close to houses and small
populated areas.
Another component of the navigation system is the tracking
camera on the helicopter which takes pictures of an area
directly below the aircraft. A video camera was used in this
survey which recorded on video tape images of the ground below
the'helicopter. The system also provides a real time display for
the helicopter navigator.
Vertical position of the helicopter is digitally recorded
from a radar altimeter. The altimeter senses the elevation above
the nearest radar reflector. Ideally the measured elevation is
from the ground surface. However in practice the nearest radar
reflector below the helicopter can be the tops of trees
(approximately 15-20 meters high) or buildings. The estimated
altitude is used in reduction of the geophysical measurements'^
Both analog and digital data records are made of the
ancillary and geophysical measurement systems. The analog
records provide real time display of data for the
operator/navigator on board the helicopter. They are also used
in evaluation of data quality during the course of the airborne
geophysical survey.
TOTAL FIELD MAGNETOMETER SYSTEM
The magnetic field measurement system consists of a
magnetometer towed beneath the helicopter and a base station
recording magnetometer. The magnetometer used in this survey is
a high sensitivity (0.01 nT) Cesium sensor towed about 50 ft.
below the helicopter. The base station magnetometer is a
standard proton precession system with a sensitivity of 0.50 nT.
This magnetometer, located near the Brookhaven airport, provided
an analog record of changes in the total magnetic field every
five seconds during the geophysical survey. The timing for the
base station magnetometer recording is recorded for correlation
with the clock used in the airborne data acquisition system.
The records from the base station magnetometer serve two
purposes. The first purpose is to provide an indication during
the survey as to whether time changes in the magnetic field are
too fast to provide reliable measurements from the helicopter
surveying system. Large and rapid changes in the magnetic field
occur during magnetic storms. Specifications for the rates of
change that are permissible during the geophysical survey are
13
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given in Appendix A (subsection 2.7). The second use of the base
station records is to correct the helicopter data for small
amplitude time changes in the main magnetic field that take
place during the survey.
PASSIVE ELECTROMAGNETIC SYSTEMS
These systems passively sense electromagnetic signals
generated from sources external to the helicopter
instrumentation. The two passive systems used in this survey
sense EM signals from VLF (very low frequency) Navy transmitting
stations and EM signals generated by power lines. The VLF
measurements were made from three transmitting stations located
at Cutler (Maine), Seattle (Washington), and Annapolis
(Maryland). These stations transmit signals at frequencies
ranging from 21 kHz to 25 kHz. Specifications for the VLF
instrumentation is given in Appendix C.
The other passive EM system is termed a 60 Hz monitor and is
used in most airborne EM surveys to sense the location of power
lines which produce signals that usually corrupt the
measurements of other EM systems. In this survey, the amplitude
of the horizontal and vertical components for the 60 Hz magnetic
field was measured and digitally recorded.
Recent research by the USGS (Frischknecht and others, 1986)
on applying developing technology to new airborne EM mapping
methods has resulted in a prototype system which uses signals
from power lines. Data processing and interpretation methods are
being developed to map variations in subsurface resistivity.
Based on experience from this research, an informal arrangement
was made with the contractor to modify their normal measurement
procedure for the 60 Hz system. Normally the gain or
amplification of the signals is minimal so that when the EM
system is flown over a power line a characteristic signal is
recorded. For this survey, gains for these channels were
increased so that variations in the 60 Hz magnetic fields could
be measured further away from power lines. Preliminary
inspection of the data in the field and on the digital profiles
shows that there are significant variations in amplitude of the
magnetic fields which may be associated with either subsurface
or surficial (cultural) conductive features. Further processing
and interpretation needs to be done in order to determine how
useful the 60 Hz monitor data might be for mapping subsurface
brine. The contractors report (Appendix C) gives a good
discussion of different anomalies in the 60 Hz data produced by
various cultural features.
ACTIVE SOURCE ELECTROMAGNETIC SYSTEM
The term active source used in connection with airborne EM
surveys indicates that both the EM transmitter and receiver are
part of the geophysical system. The DIGHEM V EM system employed
in the geophysical survey, described in Appendix B, is the
14
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primary EM system that was tested for possible mapping of
subsurface brine bearing water. In this system the transmitter
and receiver are horizontal coil pairs operated at frequencies
of 900, 7,200 and 56,000 Hz. The horizontal coil configuration
is ideally suited to mapping variations in subsurface
resistivities in the Brookhaven area since these variations are
confined to horizontal layers. EM measurements were also made
with a vertical coaxial transmitter-receiver coil system which
was not specified in the final contract (Appendices A and B).
This coil system is typically used in mineral exploration to
define near narrow vertical areas of low resistivity. Further
data processing of these data will be needed to determine
possible applications to mapping lateral boundaries between
fresh and brine bearing waters.
The broad range of frequencies in the EM system yield
information about resistivity variations from near surface (10's
of feet) to depths on the order of 200 to 300 feet. The depth
of penetration or mapping generally is deepest for the lower
frequencies (900 Hz) and shallowest for the higher frequencies
56,000 Hz. More quantitative estimates of the mapping depth
require computer modeling of the EM data.
15
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SECTION 5
TOTAL FIELD MAGNETIC SURVEY
The digital data and maps supplied by the contractor have
been corrected for magnetic field drift recorded by the base
station magnetometer as specified in the contract with DIGHEM
(Appendix A). Further processing was carried out by the USGS to
reduce the data supplied by the contractor. The first step in
the processing was to remove the IGRF (international geomagnetic
reference field) which caused a large south to north gradient in
the contractor's magnetic contour map. This large gradient
tended to obscure subtle magnetic anomalies.
After removal of the IGRF, a strong east to west magnetic
gradient remained in the reduced data due to a large regional
magnetic low to the west of the survey area. This gradient was
removed by fitting a planar surface to the IGRF corrected
magnetic field data. The map shown in Figure 4 is the residual
magnetic field after removing this planer surface. Removal of
the IGRF and a planer regional magnetic field allows small
magnetic anomalies to be more easily seen in maps of airborne
magnetic data.
The most obvious features of the reduced magnetic field data
shown in Figure 4 are isolated circular and semicircular
magnetic anomalies. These magnetic anomalies have positive
amplitudes ranging from 10's to 100's of nanotesslas (nT). The
short wave-length of these anomalies requires that the magnetic
sources be at or near the ground surface. Detailed
interpretation of these data is not a primary objective of this
report. A description of interpretation of aeromagnetic data in
oil field environments to locate well casings has been given by
Frischknecht and others (1985). They conclude that semicircular
magnetic anomalies such as those in Figure 4, can be caused by
steel well casings in addition to a number of other sources.
These sources include many different types of cultural features
made of steel such as pipelines, buildings, storage tanks, and
large machinery. In addition some oil fields are associated with
detrital or authigenic magnetic minerals which can cause short
wavelength magnetic anomalies (Frischknecht and others, 1985).
Short wavelength magnetic anomalies can be indirectly
associated with the distribution of magnetic features related to
oil production in the Brookhaven oil field. Several magnetic
anomalies have been identified in Figure 4 which are typical of
different magnetic features. The magnetic anomaly labeled A
(Figure 4) is typical of large amplitude semicircular anomalies.
This particular anomaly has an amplitude of 160 nT and is
located over an oil storage tank shown on the topographic base
map. The linear magnetic anomaly labeled B (Figure 4) which
trends east-west is associated with a pipeline between oil tanks
also shown on the topographic base map. In contrast to the high
amplitude magnetic anomalies, there are many other lower
amplitude semicircular magnetic anomalies such as C (Figure 4).
16
-------
This positive 6 nT anomaly is not associated with any cultural
feature shown on the topographic base maps. Consequently this
particular positive magnetic anomaly could be caused by an steel
casing in an abandoned oil well. Further interpretation should
concentrate on obtaining information on locations of known
locations of oil wells and other cultural features that could
cause magnetic anomalies.
If the assumption is made that many of the short wavelength
magnetic anomalies are at least indirectly associated with oil
production, then the extent of drilling and other development
activity is more extensive than the approximate boundaries shown
in Figure 1. In particular, short wavelength magnetic anomalies
that possibly indicate cased wells extend to the northeast and
southeast of the oil field boundaries (Figure 1).
Additional data processing may help to enhance the magnetic
signatures of steel well casings. In particular, computer
modelling programs as described by Frischknecht and others
(1985) may help to discriminate magnetic anomalies caused by
shallow and deep well casings.
17
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90e34'30'
90«33' 0"
90»3I'30'
90030' 0'
90«28'30'
3I°39' 0'
31"37'30'
3I»36' 0'
31034.30-
.3I»33' 0'
Figure 4 Contour map of the reduced total field magnetic data for the survey area
shown In Figure 3. Areas Identified by letters and data reduction methods
are discussed in the text.
18
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SECTION 6
ELECTROMAGNETIC SURVEYS
PASSIVE ELECTROMAGNETIC MEASUREMENTS
Neither of the two passive EM systems, described in Section
4 of this report, were considered to be the primary airborne
systems for mapping the subsurface distribution of brine. The
inexpensive VLF (very low frequency) method is usually used in
electrically resistive geologic settings, such as crystalline
rocks, to locate narrow low resistivity zones. The other passive
EM system, the 60 Hz power line monitor, (also described in
Section 5) is used in commercial airborne EM surveys to estimate
the location of power lines and other sources of cultural noise.
A description of the VLF method and data reduction is given
in the subcontractor's report (Appendix C). The total field
digital data has been filtered to remove long wavelength
anomalies and enhance short wavelength features. A contour map
of the processed VLF total field data is shown in Figure 5.
The 60 Hz monitor data and topographic maps have been
examined to estimate the location of power lines in relation to
the linear features shown in the VLF contour map. Heavy dark
lines in Figure 5 show possible locations of power lines.
Feature A (Figure 5) follows a road and feature B is a large
power line both of which are shown on the topographic map.
The VLF anomalies (Figure 5) are narrow linear features
which cross several flight lines or are small circular features
that seldom cross more than one flight line. The longer linear
features are associated with north-south trending power lines
such as anomaly A in Figure 5. The east-west power lines are not
as prominent in the VLF data because they are nearly parallel to
the direction of the flight lines. For example, the power line
identified as B (Figure 5) only has a few associated small
circular VLF anomalies because it trends almost parallel to the
flight lines.
A vast majority of the VLF anomalies are probably due to
cultural features such as power lines and metallic structures
(for example, oil tanks and fences). However, old pits which
were used to store brine (Kalkhoff, 1986) which produce small
circular VLF responses that resemble cultural effects.
Consequently a more comprehensive interpretation of the VLF data
uis warranted. The association of VLF and positive magnetic
anomalies should also be evaluated for possible additional
information about location and characterization of geophysical
anomalies from oil well casings.
Preliminary analysis of VLF data as presented by the
contractor has limited indirect application to the general
problem of mapping subsurface brine. However, this observation
should not be taken to indicate the usefulness of ground VLF
measurements. Ground surveys are made with closer station
19
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90*34'30"
90*31 '30-
90*30' 0'
90*28'30*
31*39' 0'
3I»37'30'
31 "36' 0'
31*34'30'
3|e33. 0.
Figure 5 Contour map of the filtered airborne VLF 'total field (light lines). Heavy
solid lines are some of the possible power lines identified by the 60 Hz
monitor and cultural features on topographic maps.
20
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spacing and measure more parameters than is possible in airborne
systems. In particular, ground surveys can measure apparent
resistivity by directly contacting the ground. Ground VLF
surveys should be considered in a more comprehensive geophysical
investigation of the Brookhaven area.
ACTIVE ELECTROMAGNETIC MEASUREMENTS
The primary objective of this study is evaluation of active
EM systems to estimate the quality of data that can be acquired
in an area with heavy cultural contamination. There are three
major steps in data reduction , described below, that must be
carried out to produce geophysical maps that can be used to
interpret and map subsurface resistivity variations. The first
important step in data reduction is removal of system
calibrations done both on the ground and during the airborne
survey. The low resistivity of subsurface units in this area
requires that special care be taken with calibration procedures
and system response removal in final contour maps. Preliminary
maps delivered by the contractor showed some subtle variations
in the reduced EM data that were due to problems in correction
of system calibration which were subsequently corrected in the
final delivered products. The second major step in processing
is to convert the reduced EM data to apparent resistivity as
discussed in Appendix C. Fraser (1978) also gives technical
details of these computations. The third step in data
processing, not required in the contract specification (Appendix
A), was done by the contractor to remove small level changes in
the apparent resistivity between a few flight lines. These level
changes, mostly at the highest frequency, are caused by
instrumentation drift that could not be corrected by the normal
calibration procedures. The procedure used to remove level
changes in geophysical data between flight lines is commonly
termed decorrugation (Urquhart, 1988) and is a filtering
process. All of the apparent resistivity maps given in this
report have been filtered by DIGHEM using this method.
A grey scale presentation of the apparent resistivity map
from the 56,000 Hz EM data is given in Figure 6. These and other
apparent resistivity maps discussed below have been plotted at a
scale suitable for page size presentation as required for this
report. Original map plots of the data are at a scale of
1:24,000 which is much more suitable for evaluation and
interpretation. One of the visual effects produced by decreasing
the size of the maps is a loss in the resolution of small (short
wavelength) features. Consequently many features of the map due
to cultural noise are not prominent on maps given here.
There are two other considerations in the grey scale used
for Figure 6 and other maps of apparent resistivity. The first
is that the grey scale intervals are not linearly spaced. An
approximate logarithmic interval was used because the apparent
resistivity values span almost three decades in magnitude. A
second consideration in the grey scale is the negative lower
21
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BROOKHAVEN RESISTIVITY 56KHZ
31:39: 0
31:37:30
31:36: o
31:34:30
31:33: 0
u>
a
:
»
u
w
8M.«
IM.i
n.
15.I
OHM-M
Figure 6 Grey scale apparent resistivity map computed from the 56,000 Hz EM
data. Light areas are less resistive (more conductive) than the darker
areas. Areas labeled A, B, and C are interpreted cultural responses
discussed in the text.
22
-------
BROOKHAVEN RESISTIVITY 7200HZ
31:39: 0 ,
31:37:30
3t:36: o
31:3-4:30
3i:33: o
u>
u
u
jo
w
tt.t
29.9-
13.
OHM-M
Figure 7 Grey scale apparent resistivity map derived from the 7,200 Hz EM data.
Light areas are less resistive (more conductive) than the darker areas
(see text for explanation).
23
-------
bound of values (Figure 6). This was required by the USGS
gridding and plotting programs and does not reflect the actual
values computed from the measurements. Negative apparent
resistivities are not possible for most electrical geophysical
measurements.
The grey scale contour map (Figure 6) shows some features
that can be directly related to the location and trend of power
lines. The arcuate trend of resistivity highs in the northwest
part of the survey area anomaly in (labeled A in Figure 6)
follows the trend of a power line (Figure 5). A similar
northeast trending the south central part of the survey (B in
Figure 6) is associated with a power line. Though these two
cultural features produce narrow high apparent resistivity
trends, other power lines are associated with linear low
resistivity trends. For example, the north-south power line
located at area C (Figure 6) is associated with low and high
resistivities. The difference in expression of power lines in
apparent resistivity maps depends on a complicated interaction
of the active EM system with the power line and also the
background subsurface resistivity. In the Brookhaven survey, the
variable signature of cultural noise is particularly hard to
assess for the 56,000 Hz apparent resistivity maps because
background shallow resistivity is highly variable.
Though there are many local areas where cultural noise have
distorted the apparent resistivity signature of the subsurface,
there are broad areas of both high and low resistivities shown
in Figure 6. The areas of high resistivity, generally greater
than 100 ohm-meters, probably reflect the distribution of loess
and more resistive alluvium. The darkest areas in Figure 6 have
resistivities greater than 200 ohm-meters with local areas
approaching 1000 ohm-meters. These latter areas are most likely
produced by corruption of the EM signals by cultural features
since such high resistivities are not expected in this
geological setting. Areas of low resistivity (Figure 6) are
shallow (less than 10 meters) conductive features that could be
cultural or geologic. Though the 56,000 Hz apparent resistivity
map reflects the distribution of surficial features such as
alluvium and loess, it provides a control for interpretation of
the lower frequency data discussed below.
The grey scale used for the apparent resistivity map from
the 7,200 Hz data (Figure 7) has a much more restricted range of
values than the 56,000 Hz map (Figure 6). Computed values of
apparent resistivity from the 7,200 Hz data are on the average a
factor of seven lower than those computed from the higher
frequency. Theoretically the lower frequency has approximately a
factor of three greater depth of penetration than the higher
frequency (21 versus 59 meters as described in Appendix B).
Consequently the apparent resistivities shown in Figure 7 are
not as sensitive to the thin surface layer of resistive loess.
The signatures of cultural features are not as obvious in Figure
7 as they are in Figure 6. For example the arcuate narrow high
resistivity anomalies labeled "A" in Figure 6 are more subtle in
24
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Figure 7. Difficulty in recognition of responses caused by
cultural sources is due to two factors. The first factor is the
scale of the maps required for this report. On the larger maps
at a scale of 1:24,000, subtle correlations can be seen in the
trends of apparent resistivity and cultural features such as
power lines.
The second factor is that in this particular geoelectric
setting, the apparent resistivities from areas of cultural noise
are nearly the same as produced from the subsurface lithology.
Thus apparent resistivity maps must be examined very carefully
in order to avoid interpreting surface cultural variations as
subsurface features.
In general the Brookhaven oil field (Figure 1) is associated
with a broad resistivity low of less than 15 ohm-meters which is
most likely due to a decrease in the subsurface resistivity.
This decrease is expected from near surface brine contamination
(Becker and Morrison, 1987). Areas of low resistivity are
probably not all due to near surface brine unless contamination
is much more extensive than described by Kalkhoff (1986). One
possible geologic source of the low resistivity zones are clay
rich sand units within the Citronelle or the upper part of the
Hattiesburg Formation.
Another major trend in the 7,200 Hz apparent resistivity map
(Figure 7) is the general increase in apparent resistivity in
the southeastern part of the survey area. There is approximately
a factor of ten increase in the apparent resistivity between
light (northwest) and dark (southeast) parts of Figure 7. The
northeast trending irregular contact between these two areas is
probably due to a number of different geologic and hydrologic
sources. However without further geologic information, it is not
possible to specifically interpret this feature. Grey scale
intervals used for the apparent resistivity map derived from the
900 Hz EM measurements (Figure 8) are the same as used for the
7,200 Hz map (Figure 7). Generally the 900 Hz map has less dark
areas indicating resistivities greater than 15 ohm-meters than
the apparent resistivity maps for higher frequencies (Figures 6
and 7). Since the 900 Hz EM measurements have the greatest depth
of penetration the general conclusion is that the subsurface
lithologies sensed by these signals have lower resistivities
than the shallower features mapped by the higher frequencies.
There are some obvious correlation of trends in low apparent
resistivities and power lines indicated by the letters shown in
Figure 8. However these and other cultural effects do not
obscure the broader apparent resistivity variations probably
related to changes electrical characteristics of the subsurface.
The Brookhaven oil field (Figure 1) is associated with a broad
resistivity low of less than 10 ohm-meters. Low resistivities
occur throughout the northern 2/3 of the survey area (Figure 8).
Consequently further interpretation will be needed to evaluate
the possible signature of shallow subsurface brine.
25
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BROOKHAVEN RESISTIVITY 900HZ
31:39: 0
31:37:30
3i:36: 0
31:34:30
31:33: o
tM.«
u.*-rffl
8
OHM-M
a
K>
Figure 8 Grey scale apparent resistivity map derived from the 900 Hz EM data.
Light areas are less resistive (more conductive) than the darker areas
(see text for explanation).
26
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References
Becker, A. and H.F. Morrison. 1987. Detection of brine
contamination in the Brookhaven oil field with airborne
electromagnetics, EPA Internal Report, Environmental
Monitoring Systems Laboratory, Las Vegas, Nevada, 29
pp.
Fitterman, D.V. 1986. Transient Electromagnetic Sounding in the
Michigan Basin for ground water evaluation. Proceedings
National Water Well Association Surface and Borehole
Geophysical Methods and Ground Water Instrumentation
Conference, National Water Well Association, Dublin,
OH., pp. 334-353.
Fitterman, D.V., P.V. Raab, and F.C. Frischknecht. 1985.
Detection of brine contamination from injection wells
using transient electromagnetic soundings. EPA Report,
Environmental Monitoring Systems Laboratory, Office of
Research and Development, U.S. Environmental Protection
Agency, Las Vegas, Nevada, 62 pp.
Fraser, D.C. 1978. Resistivity mapping with an airborne
multicoil electromagnetic system. Geophysics, v. 43,
No. 1., pp. 144-172.
Frischknecht, F.C., R. Grette, P.V. Raab, and J. Meredith. 1985.
Location of abandoned wells by magnetic surveys:
acquisition and interpretation of aeromagnetic data for
five test areas. U.S. Geological Survey Open-file
Report 85-614A, 64 pp.
Frischknecht, F.C., B.D. Smith, D.B. Hoover, and C.L. Long.
1986. New Application of geoelectrical methods in
mineral resource assessment. U.S. Geological Survey
circular 980, pp. 221-247.
Kalkhoff, S. 1986. Brine contamination of shallow ground water
and streams in the Brookhaven oil field, Lincoln
County, Mississippi. USGS Water-resources
Investigations Report, 86-4087, 57 pp.
Nacht, S.J. and L.J. Barrows. 1985. Electrical Reconnaissance
Survey of the Brookhaven Oil Field, Lincoln County,
Mississippi. EPA Administrative Report, Environmental
Monitoring Systems Laboratory, Las Vegas, Nevada, 43
pp.
Urquardt, T. 1988. Decorrugation of enhanced magnetic field
maps. Expanded Technical Program Abstracts, Society of
Exploration Geophysicists, Fifty-eighth Annual Meeting,
pp. 371-372.
27
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APPENDIX A
TECHNICAL SPECIFICATIONS USED IN RFP
Material given in this appendix gives the technical
specifications section of the request for proposals (RFP 17-
4350) as issued by the USGS Branch of Procurement and Contracts.
The following text is presented in the format required by the
USGS and presented as it originally appeared in the RFP. The
contents of these specifications are presented as part of this
report for the following reasons:
1) These specifications and the
response of the contractor
(Appendix B) serve as the basis for
quality control documentation.
2) The technical section of the RFP
may serve as a guideline for future
geophysical contracts for airborne
surveys in similar geological
settings, and
3) Distribution of the specifications
can result in improvements for
subsequent applications.
The complete RFP is not included since many details of contracts
vary between government agencies.
28
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PART I STATEMENT OF WORK
1.0 Objectives; In many oil fields large quantities of brine are produced
along with oil. Generally, the brine is separated from the oil and
reinjected into the producing horizon or another horizon that already
contains poor quality water. However, leaks in the injection wells or
conduits between aquifers, such as leaky abandoned wells, sometimes allow
brine to enter aquifers containing fresh water. As part of a research
program on identification and mitigation of brine contamination of fresh
water the U.S. Geological Survey, in cooperation with the U.S.
Environmental Protection Agency plans to test the use of airborne
electromagnetic methods as a means of mapping brine pollution in near-
surface aquifers.
The Brookhaven Oil Field and parts of the surrounding area in Mississippi
have been selected as the test area (see attached map). In parts of the
Brookhaven field the Citronelle formation, which is a near surface
unconfined aquifer, is contaminated. Locally, sand layers which serve as
aquifers in the underlying Hattlesburg formation also are contaminated.
The objectives of the airborne survey(s) are to map significant
variations in resistivity that occur within 100-120 m of the surface. A
possible secondary objective is to make high resolution magnetic
measurement to locate abandoned wells.
2.0 Scope of work: The area to be flown encompasses 45 square miles (see
attached map) the area is generally flat or gently rolling although
locally the elevation changes as much as 70 feet in 1/4 mile or less. It
may be difficult to adequately map resistivity variations throughout the
depth range of interest using a single system. Therefore, the
possibility of using both a helicopter system that can operate at several
high frequencies and a fixed wing transient system will be considered.
Proposals for either or both types of systems are solicited.
3.0 General requirements for survey
3.1 The preferred flight direction is east-west but the proposal should
include costs for both east-west and north-south flight lines.
3.2 The desired line spacing is ISO to 400 meters. The proposal should
include costs per line mile for EM surveying only and for combined EM and
magnetic surveying for spacings of 150, 200, 250, 300 and 400 meters.
3.3 Tie lines shall be flown at intervals of approximately 4 miles.
3.4 The flight height shall be specified in the proposal along with a
brief discussion of the instrumental and operational factors that were
considered In selecting the height.
3.5 Before beginning routine surveying approximately 20 line miles will
be flown and evaluated by the Contractor and the Contracting Officers
Technical Representative (COR). The COR's evaluation will be done
overnight in the field. To the extent possible system parameters will
then be optimized before routine surveying is started.
29
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PART II TECHNICAL SPECIFICATIONS
1.0 Electromagnetic System
To sense changes in the resistivity of both aquifers, measurements must
be made at several frequencies or times. In particular, the response
from the lower layer will exhibit a maximum at a frequency that depends
on the parameters of all the layers.
Part of the area is covered by loess and alluvium which may be as much as
20-30 feet thick. Immediately beneath the loess, or outcropping where
loess and alluvium are absent is the Citronelle Formation which is
composed of discontinuous sand and gravel units separated by sandy clay
lenses. Beneath the Citronelle Formation is the Hattiesburg Formation
which consists mainly of silty clays; however, a number of thick
discontinuous sand layers occur within the Hattiesburg Formation.
Shallow resistivity soundings indicate that the resistivity of the
Citronelle Formation Is relatively high except where it contains
brines. Generally the resistivity of the upper unsaturated part of the
Citronelle Formation is highest. The most extreme gradients in the water
table are only 20 to 40 feet per mile so by following the surface
topography the height of an aircraft above the water table will vary.
Electric logs indicate that the resistivity of the confining layers in
the Hattiesburg is relatively low and that the resistivity of the sand
layer is much higher. Typical estimated parameters for the various
layers are:
Thickness Resistivity
Loess - 0-30 ft 50-200
Citronelle (without brine) 20-120 400-2000
Hattiesburg (confining layer)
Hattiesburg (sands)
To the extent possible the system should be sensitive to variations in
resistivity of the sand layers in the Hattiesburg Formation as well as to
variations in the Citronelle Formation.
The proposal shall contain a general description of the electromagnetic
system plus specific Information as follows:
1.1 Coil configuration The orientations and spacings of all coil
configurations that will be used should be specified.
1.2 The frequencies and/or gate times that will be available for use
shall be specified.
1.3 Static and in-flight system noise levels shall be specified assuming
that sferic levels are typical for morning hours excluding the summer
period May 1 - Sept. 15. The specified noise levels will become a
requirement. Measurements of the static noise level and in-flight noise
level at altitude shall be made before the survey.
30
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1.4 Maximum rates of system drift due to both mechanical and electronic
changes shall be specified. The specified drifts will become a
requirement: The procedures that will be used for monitoring and
correction of drift shall be specified.
1.5 The techniques that will be used to calibrate the system shall be
described briefly. The system must be calibrated as often as necessary
to ensure that it operates within stated specifications.
1.6 The level of 60 Hz power line noise shall be monitored; a brief
description of the monitor shall be provided.
2.0 Magnetometer System
Acquisition of magnetic data is an option which may be exercised by the
Government). A general description of the magnetometer shall be given in
addition to the following specific information:
>
2.1 The static resolution of the system shall be given. It must be 0.1
nanoteslas (nT) or better.
2.2 The maximum sample rate shall be specified for each sensitivity.
Readings shall be taken and recorded at a rate of no less than two per
second.
2.3 The inflight noise envelope shall be specified. "Quiet" air
conditions may be assumed but the specified noise envelope will be a
requirement.
2.4 The figure of merit or other measure of performance for pitch, roll
and yaw maneuvers shall be specified and shall become a requirement. It
shall be verified before the survey begins.
2.5 The errors due to changes in heading shall be specified and shall
become a requirement. Heading error shall be verified before the survey
begins by flying over the same point on magnetic north, east, south, and
west directions at least twice. Verification shall be repeated if
mechanical parts of the aircraft are changed or if the magnetometer is
repaired or modified.
2.6 An analog record of variations in the Earth's magnetic field shall
be made during periods when airborne data is being collected. The
monitor station may be placed near the contractors base in a magnetically
quiet location. The ground monitor station shall have a noise envelope
less than 0.4 nT. The chart speed shall be no less than 1 inch/per 5
minutes and the vertical scale shall be no less than 25 nT per inch. The
analog records shall contain time marks that permit recovery of actual
time to within 15 sees and the record shall be annotated to indicate
date, absolute value of the magnetic field, time and vertical and
horizontal scales.
2.7 Airborne surveys shall not be conducted when variations of the
earth's magnetic field exceeds 2 nT from a chord two minutes long, as
determined from the analog record on the ground monitor.
31
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2.8 A digital record of the earths magnetic field shall be made during
the periods when airborne data is being collected. The station shall be
placed in a magnetically quiet area within 10 miles of the survey area.
The resolution of the magnetometer shall be 0.1 nT or better and the
noise envelope shall be 0.2 NT or better. The field shall be measured
and recorded at least once per 2.0 seconds. The airborne and digital
base station magnetometer shall be synchronized with an accuracy of 1.0
second or better. Synchronization shall be checked at the end of each
days flights.
3.0 Altimeter Specifications
Continuously recording radar and barometric altimeters shall be employed.
3.1 The resolution of both altimeters shall be specified, and shall
become a requirement.
3.2 The absolute accuracy of the radar altimeter over flat terrain shall
be specified and shall become a requirement.
3.3 The procedures that will be used to correct the barometric altimeter
for changes in air pressure shall be described.
3.4 The methods used to calibrate the altimeters shall be specified.
The altimeters shall be calibrated at the beginning of the survey and as
often is required to ensure that the altimeters are operating within
specifications.
4.0 Navigation and Flight Path Recovery Systems
4.1 The system used for aircraft navigation shall be specified (visual,
electronic indicator). If the spacing between survey lines exceeds 1.5
of the stated flight line spacing for more than 0.5 mi, a fill-in line
shall be flown at the contractors expense unless the deviation is caused
by safety requirements or FAA regulations.
4.2 The flight path shall be recovered within ± 50 feet of the true
position in the along track and cross track directions. This accuracy
shall be verified by use of a tracking camera or other specified means.
5.0 Analog Records
The system used for analog recording shall be specified.
5.1 The analog records shall be of sufficient resolution to enable
visual checks to be made of the system performance (e.g. noise levels).
5.2 At least four channels of EM data shall be recorded by the
contractor. The information on these channels will be mutually agreed
upon by the COR and the contractor.
5.3 The remaining analog data shall consist of: 60 Hz noise level,
magnetometer readings, radio altimeter data and fiducial data.
32
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5.4 Analog records shall be adequately labeled with at least: date,
line number, flight direction, and project name.
5.5 Records shall be made available to the COR upon request during the
survey.
6.0 Digital data recording
6.1 Digital data shall represent the analog traces including (EM), and
altimeters, within (0.32) or better of full scale value of analog trace.
6.2 Final data tapes shall include x, y position of the flight lines and
fiducial points along with the other digital data. These position points
shall be in a coordinate system (e.g. lat., long., etc.) agreed upon by
the COR and the contractor.
6.3 The contractor shall provide the government with sufficient
information to establish the integrity of the digital data.
6.4 Tape specifications: (a) 9 tract, NRZI compatible, odd parity with
800 bpi recording density. Recording mode to be EBCDIC (formatted).
Character set limited to IBM 028 Keypunch code set. (b) All floating
point data to be in F format. Fortran E format is not acceptable, (c)
Blocksize shall not exceed 8,000 characters in length.(d) Data may be
blocked (more than one logical record per physical record or block). If
blocking is performed the logical record size shall remain fixed in any
one field and, except for the last block shall be equal in size. If
blocking is not performed, physical records may be of varying length.
Computer system recording methods employing "link words", key or other
system dependent recording modes are not acceptable, (e) Computer system
labels shall not be recorded (i.e., "unlabeled" tapes). Optionally, user
readable identification records may be supplied, (f) More than one data
file may be present on one data tape (multifile reels), (g) Each tape
supplied shall have a unique visual label attached to each reel, and
correlating with supplied descriptive material or its contents.
7.0 General Specifications for Data Processing
7.1 The analog records and logs must be examined daily in the field to
make certain that the equipment is operating properly and that altitude
deviations are not excessive. Proper functioning of the flight path
recovery system shall be verified in the field.
7.2 The flight path shall be recovered and plotted at a scale of
1:12,000 over a topographic base derived by enlargement of USGS 1:24,000
topographic maps.
7.3 Contour maps at a scale of 1:12,000 shall be prepared for the area
flown showing the reduced total field magnetic data overlain on a
topographic base. Diurnal variations are to be removed by use of the
base station magnetometer data and levelling shall be verified by use of
tieline data. The contour interval shall be decided by mutual agreement
between the contractor and COR.
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7.4 The electromagnetic data shall be levelled and contour maps at a
scale of 1:1.2,000 overlain on a topographic base shall be prepared for
each component at each frequency or each time gate. The contour interval
shall be decided by mutual agreement between the contractor and COR.
7.5 The corrected magnetic, four channels of corrected electromagnetic,
60 Hz monitor and the altimeter data shall be plotted as stacked profiles
at a scale of 1:12,000. The contractor and COR shall agree on which four
electromagnetic channels shall be plotted.
8.0 Interpretation of Electromagnetic Data
Interpretation of the reduced EM data is an option which may be exercised
by the government.
8.1 All EM anomalies that are thought to be caused by cultural features
shall be identified and indicated on a map. The region around the
cultural feature within which the EM response is substantially affected
shall be estimated and outlined on a map at a scale of 1:12,000.
8.2 Ideally, the data would be inverted for the entire area, excluding
regions of cultural anomalies, using four or five layer one-dimensional
models.However, proposals to produce other products based on simpler
models, including a homogeneous earth model, will be considered.
t
PART III QUALITY CONTROL
1.0 Data.acquisition, processing, and interpretation must be carried out
under the requirements of a Quality Assurance Plan written by the U.S.
Geological Survey and approved by the Environmental Protection Agency.
The contractor will be responsible for adjustment and calibration of
equipment, operation of equipment to meet specifications, and processing
and interpretation of data to meet specifications.
1.1 All geophysical and navigation equipment shall be checked, adjusted
and calibrated according to manufacturers recommendations immediately
before commencing data acquisition or within the time specified by the
manufacturer. The contractor shall provide copies of manufacturers
adjustment and calibration procedures for each piece of equipment and a
log giving the dates and procedures that were followed.
1.2 Before commencing surveys the readings of the airborne and ground
magnetometers shall be compared by making successive measurements with
the sensors in the same location. To carry out this procedure the
aircraft must be moved after readings with the airborne magnetometer are
taken on the ground.
1.3 The analog portion of the data acquisition system shall be
calibrated by injecting known signals from a dc standard and recording
and recovering the output.
1.4 All steps in data processing in which data is corrected, transformed
or changed in any way shall be described and included as part of the
final report. 34
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1.5 The models used and all steps in which data are altered or
transformed in interpretation shall be described and included in the
final report.
1.6 The contractor shall allow the COR to inspect equipment and data in
the field and data and results in the Contractors Office to verify
adherence to specifications.
PART IV DELIVERABLE ITEMS
1.0 Original data and Quality Control items.
1.1 All Quality Control items specified under Part III.
1.2 Flight logs indicating production times, lines flown, operational
problems and other relevant data.
1.3 All analog records for airborne systems and ground monitor systems.
1.4 Records documenting magnetic heading effect as described in Part II.
1.5 Records documenting magnetometer manuever noise as specified in Part
II.
1.6 Records documenting accuracy and of navigation system as specified
in Part II.
1.7 Records of static and in-flight noise level for the EM system as
specified in Part II.
1.8 Records of altimeter calibrations and control for pressure
variations for barometric altimeter.
1.9 Original digital data tapes with a complete description of the
format.
2.0 Processed Data Single copies.
2.1 Flight line maps on stable base as specified in Part II.
2.2 Magnetic contour maps on stable base as specified in Part II.
2.3 EM contour maps on stable base as specified in Part II.
2.4 Stacked profiles as specified in Part II.
2.5 Magnetic tape containing digital information written according to
specifications in Part II, containing the information given in the
stacked profiles as specified in Part II.
2.6 Magnetic tape containing the flight path, written according to
specifications in Part II.
35
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2.7 Magnetic tapes, written according to specifications in Part II
containing gridded data used in preparation of magnetic and
electromagnetic contour maps.
2.8 Map or maps showing Interpretation of electromagnetic data.
2.9 Magnetic tape, written according to specifications in Part II,
containing gridded data used in preparation of interpretative map of
electromagnetic data.
3.0 Multiple copies.
3.1 Four copies of the final report including all required information
on data acquisition and processing.
3.2 Four copies of the final report on interpretation of the data.
PART V INSPECTION AND ACCEPTANCE
1.0 The Government reserves the right to visit the contractor in the field or
at the contractor's place of business to ascertain that proper procedures
are being employed in the acquisition and compilation of the data. Any
data processing or field techniques that are deemed proprietary by the
Contractor will be maintained proprietary by the USGS inspectors).
1.1 The Government will conduct a review of preliminary paper copies of
contour maps and profiles as described in Part IV 2.1, 2.1, 2.3, and 2.4
entitled "Deliverable Items", within twelve (12) calendar days after
receipt of the above-described rough data and return the rough drafts of
the completed data to the contractor by the fourteenth day.
1.2 The Government will conduct a review of preliminary paper copies of
interpreted data as described in Part IV, 2.8 within twenty (20) calendar
days after receipt and return the rough drafts by the twenty-second
day.
1.3 If the Government review exceeds the periods referenced above, the
contract delivery data shall be automatically extended one (1) day for
each day of delay caused by the Government review.
1.4 With the submittal of all final deliverables as listed the
contractor shall have met all criteria as specified herein and shall have
made all corrections required resulting from the Government. The
Government reserves the right to review again the deliverables for
compliance prior to acceptance by the Contracting Officer. Until final
acceptance, deliverables may be returned to the contractor for compliance
with corrections listed during the Government review.
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APPENDIX B
SUBCONTRACTOR'S RESPONSE TO RFP
Responses to the RFP (#7-4350) were evaluated on the basis
of technical merits and cost. This evaluation resulted in award
of a contract for helicopter airborne geophysical survey of the
Brookhaven area to DIGHEM Signal Processing and Surveying Inc.
(Toronto, Ontario, Canada). Material in this appendix is the
contractor's proposal. This material is presented in the form
submitted by.the contractor with page numbers added to conform
to EPA specifications for this report. The vitae of the
personnel have been deleted since this information is not
particularly relevant to the technical performance of the
geophysical equipment.
The contractor's proposal is presented without any editorial
'.cAang.es. Acceptance of the proposal by means ofaward of the
contract means that exceptions to the technical specifications
given in the proposal replace those given in appendix A. The
conclusions presented in the proposal are the sole
responsibility of the contractor and are not directly or
uindirectly endorsed by the EPA or the USGS.
37
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U. S. G. S.
SOLICITATION NO.
7-4350
DIGHEM SURVEYS
& PROCESSING INC.
VOLUME II
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TABLE OF CONTENTS
1. Introduction 1
2. Technical Approach 3
2.1 Modeling 3
2.1.1 Depth of Penetration Considerations 5
2.2 Data Acquisition 6
2.2.1 EH System 6
2.2.2 Magnetometer System 8
2.2.3 Ancillary Equipment 9
2.2.4 Recording of Data 10
2.2.5 Technical Approach to Data Processing 11
2.2.5.1 Resistivity Analysis 13
2.2.5.2 Electromagnetic Map 13
2.3 Project Management 14
3. Statement Of Compliance 15
4. Capabilities of Organization, Personnel and Equipment 16
4.1 Organizational Experience 17
4.1.1 Subcontractors 17
4.2 Personnel Qualifications 17
4.3 Equipment and Procedures 17
4.3.1 Survey Equipment 17
4.3.2 Data Processing Facilities and Procedures 18
4.3.2.1 Facilities 18
4.3.2.2 Procedures 19
5. Quality Control Procedures 24
6. Contingency Plan 25
6.1 Aircraft 25
6.2 Geophysical Equipment 25
6.3 Personnel 26
7. Summary 27
Figures 1-9
Curricula Vitae
39
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1. INTRODUCTION
Dighem proposes to map areas of brine saturation in the
Citronelle and Hattiesburg formations using its DIGHEMIV
electromagnetic system. The system is also capable of
mapping abandoned well casings using a Cesium magnetometer
flown in conjunction with the DIGHEMIV sensor.
We anticipate that the apparent resistivity of the ground
within the survey area is complex, with significant vertical
and lateral variations of resistivity within each horizon. A
small-scale helicopter-borne EM (HEM) system will be best
able to define these variations of resistivity. A HEM system
with maximum coil separation will give maximum penetration.
Dighem's 7.98 m coil separation is not surpassed by any
commercially available system. The depth penetration of the
DIGHEMIV system should be adequate for the anticipated
depths to brine saturated earth.
The discontinuous zones to be mapped may be assumed to be
roughly horizontal. Because of EM response characteristics,
horizontal transmitter-receiver coil-pairs are more suitable
for data acquisition than vertical coil-pairs. The
DIGHEMIV system has 3 horizontal coil-pairs, more than any
other HEM system. A HEM system with a broad frequency range
will have the best chance at separately identifying various
layers. Dighem1s 62-fold frequency spread (900 Hz to 56000
Hz) is the largest of any commercially available system.
4Q
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Since 1967, Dighem has flown numerous surveys requiring
high signal-to-noise electromagnetic and magnetic data for
mapping applications. Dighem has been furnishing its clients
with airborne resistivity maps since 1975. The DIGHEMIV
system, first flown in 1984, meets the above mentioned
criteria for optimum data acquisition on this project.
Dighem1s 12 years of experience with identifying cultural
effects and delivering accurate resistivity maps from a wide
variety of surveys should contribute to the usefulness of the
resistivity maps from this survey.
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2. TECHNICAL DISCUSSION OP APPROACHES
2.1 Modeling
To quantify system selection parameters, the mapping problem
has been simplified into three workable models. The first
model (depicted in Figures 1 through 3) shows a 3, 6 and 9
meter thick loess layer (100 ohm-m) on top of a half space of
dry Citronelle formation (1000 ohm-m). These figures
illustrate the response of this model to the DIGHEMIV
frequencies (900, 7200 and 56000 Hz). In all the figures,
CPI is the horizontal coil inphase response, CPQ is the
horizontal coil quadrature response, DP is the apparent depth
to conductive earth and RES is the apparent resistivity of
the earth.
The 900 Hz resistivity (Figure 1) is relatively indifferent
to loess thicknesses up to 9 m. At 7200 Hz (Figure 2), a
little more resistivity variation is seen. The 56000 Hz
resistivity (Figure 3) gives the most accurate measure of
loess resistivity, although it significantly overestimates
the resistivity where the loess is thin. For the most likely
thickness and resistivity of loess, the lower frequencies
yield resistivity estimates of about 600 ohm-m for a combined
loess and dry Citronelle sequence.
Model 2 (Figures 4-6) and model 3 (Figures 7-9) illustrate
the response of the DIGHEMIV system to the Citronelle
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formation saturated with fresh water and with brine. The
most probable combined sequence of loess and dry Citronelle
formation is represented by a 25 m thick layer of 600 ohm-m
material. The Hattiesburg formation is represented by a 20
ohm-m half space. Fresh water saturated Citronelle
formation is represented in model 2 (Figures 4-6) as a 120
ohm-m layer of variable thickness. The 900 Hz frequency
(Figure 4) maps the Hattiesburg formation. The 7200 Hz
frequency (Figure 5) is increasingly influenced by the
Citronelle formation as it thickens. Where the loess and
Citronelle formation are thin, the 56000 Hz frequency
(Figure 6) yields a half space resistivity matching the
Hattiesburg formation. However, the resistivity climbs
rapidly with increasing thickness of dry or fresh water
saturated Citronelle formation.
Model 3 (Figures 7-9) represents the same layered situation
as model 2 (Figures 4-6), but brine replaces fresh water as
the pore fluid of the Citronelle formation. The 900 Hz
frequency (Figure 7) maps the Hattiesburg formation well in
the absence of brine. The 7200 Hz frequency (Figure 8)
maps the brine layer very well. The 56000 Hz frequency
(Figure 9) also responds to the brine layer but much of its
energy is absorbed by the 25 m of 600 ohm-m cover.
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- 5 -
2.1.1 Depth of Penetration Considerations
The maximum depth of exploration is 250 m for a lower half
space of 1 ohm-m. This yields a coplanar signal of 4 ppm
(vs noise of 1 to 2 ppm). These signal levels assume a
sensor flying height of 30 m. A 1 ohm-m resistivity would
equate to sea water salinity in rock having a porosity of
25%. These figures also assume that the overlying material
is infinitely resistive. Since the overlying material is
not infinitely resistive, skin depths at our three
frequencies will provide limitations. For example, if the
resistivity of the cover is 100 ohm-m, the skin depths at
our frequencies are as follows:
Frequency
Hz
900
7200
56000
Skin Depth
m
168
59
21
These figures illustrate that the low frequency of 900 Hz
is more than adequate for the program. They also indicate
that our broad range of frequencies are necessary to
provide various sounding depths. For example, the 56000 Hz
frequency samples the near surface, while the 900 Hz
frequency yields deep sounding.
44
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2.2 DATA ACQUISITION
2.2.1 EH System
A DIGHEMIV system would be utilized for EM data
acquisition. A concise description of the system follows:
1. Coil Configuration: 1 vertical coaxial coil-pair
3 horizontal coplanar coil-pairs
2. Coil Spacing: 7.98 m for the vertical coaxial
and two of the three horizontal
coplanar coil-pairs. 6.32 m for
the third horizontal coplanar
coil-pair.
3. Frequencies: 900 Hz for the vertical coaxial and the
first horizontal coplanar coil-pairs.
7200 Hz for the second horizontal
coplanar coil-pair.
56000 Hz for the third horizontal
coplanar coil-pair.
4. Noise Levels: Less than 1 ppm static and less than 3
ppm in straight and level flight for the
horizontal coplanar coil-pairs.
5. System Drift: System drift is less than 1 ppm per
minute and is linear over a time period
of ten minutes. Drift compensation is
accomplished by taking the sensor out of
ground effect in order to establish zero
levels. This procedure is performed at
the end of every survey line.
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6. Calibration:
7. Sferics and
60 Hz
Monitoring:
System calibration consists of phasing
and gain adjustment. Phasing control is
accomplished by introducing a ferite rod
into the primary field. This produces a
negative inphase response and no
quadrature response.
System gain is adjusted by introducing
calibrated coils into the primary
field. These coils are calibrated to
provide a 100 ppm response.
Sferics and 60 Hz noise are monitored
utilizing the coaxial coil-pair. A
separate sferics channel indicates the
presence of sferics and 60 Hz
interference.
The horizontal coil-pair response is twice the vertical
coil-pair response from a layered earth. The horizontal
coil-pair is null coupled to sferic fields, and this
coil-pair is less susceptible to aerodynamic noise. For
these signal and noise reasons, horizontal coils are most
suitable for this survey.
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- 9 -
Static Resolution:
Sample Rate:
Inflight Noise Envelope;
Figure of Merit:
Heading Error:
Diurnal Recording:
Standard:
Optional:
Diurnal Specifications:
0.01 nT
10 Hz
0.2 nT
not applicable
not applicable
towed bird
towed bird
Digitally recording Geometries
proton magnetometer
Resolution 0.5 nT
Sample Rate - 0.2 Hz
Digitally recording Scintrex
Cesium magnetometer
Resolution 0.01 nT
Sample Rate - 2 Hz
2 nT from a chord two minutes
long
2.2.3 Ancillary Equipment
1. Radar Altimeter: Sperry AA220
Sensitivity: ± 1 foot
Accuracy: ± 5 feet
Altitude will be recorded in both analog and digital
formats.
Calibration is by voltage input.
2. Flight Camera: Geocam 35 mm camera.
47
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Primary field variation due to aerodynamic vibration is the
principal source of noise in a HEM system. The primary
field strength at the receiver coil decreases with the cube
of coil separation. While a 7 m coil separation is common
in the industry, the separation on Dighem's low and medium
frequency coils is 7.98 m. This wider spacing yields a 50%
increase in signal-to-noise ratio. The shorter coil
spacing of 6.32 m at 56000 Hz is not a problem due to the
high signal levels generated by this frequency.
The modeling of Figures 1 to 9 shows that distinctively
different responses are received at the three frequencies
of 900, 7200 and 56000 Hz. Our survey experience shows the
breadth of this frequency range is entirely satisfactory
for the variation of resistivities expected in the survey
area.
2.2.2 Magnetometer System
Should the government require magnetic data acquisition, a
Scintrex/Varian Cesium magnetometer will be used. It is
flown in a bird approximately 45 metres above the ground.
The following magnetometer specifications apply:
48
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2.2.4 Recording of Data
1. Analog
2. Digital
RMS GR33 32 channel graphics recorder
Scintrex GDI 6 digital acquisition system
with a Digidata 9-track magnetic tape
transport.
RECORDING SENSITIVITIES
Channel
Number
01
02
03
04
05
06
07
08
09
10
11
12
Parameter
coaxial inphase ( 900 Hz)
coaxial quad ( 900 Hz)
coplanar inphase ( 900 Hz)
coplanar quad ( 900 Hz)
coplanar inphase ( 7200 Hz)
coplanar quad ( 7200 Hz)
coplanar inphase (56000 Hz)
coplanar quad (56000 Hz)
altimeter
magnetics, coarse
magnetics, fine
coaxial sferics/60 Hz
range 1 Navigation
range 2 Navigation
Analog
Sensitivity
per mm
2.5 ppm
2.5 ppm
2.5 ppm
- 2.5 ppm
5.0 ppm
5.0 ppm
7.5 ppm
7.5 ppm
10 ft
10 nT
2 nT
2.5 ppm
Sensitivity on
digital tape
0.20 ppm
0.20 ppm
0.20 ppm
0.20 ppm
0.40 ppm
0.40 ppm
0.80 ppm
0.80 ppm
1 ft
0.01 nT
0.20 ppm
1 ft
1 ft
Real time verification of the header block information is an integral
part of the data system. All digital data will be verified post flight,
in the field.
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2.2.5 Technical Approach to Data Processing
Dighem proposes to produce the following deliverables at a
scale of 1:12000:
1. Apparent resistivity contour maps for each of the three
coplanar frequencies of 900, 7200 and 56000 Hz.
2. .Apparent depth contour map. for the 900 Hz frequency if
signal levels are strong, otherwise for the 7200 Hz
frequency.
3. Inphase and Quadrature contour maps for each coplanar
frequency.
4. Stacked magnetic, eight channels of electromagnetics,
60 Hz monitor, altimeter, three resistivity and three
depth profiles.
If the material overlying the fresh water or
brine-saturated layer is reasonably resistive, Dighem's
apparent depth map will accurately show depth to the
layer. Refer to the depth profiles of Figures 4 and 7.
50
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If the overlying material is fairly conductive, the
apparent depth will underestimate the true depth. In this
case, Dighem will supply the following optional contour
maps if requested:
1. Resistivity of the upper layer (above the water table.
This assumes that the major conductivity contrast
occurs at the top of the water table).
2. Depth to the water table.
3. Resistivity of the lower layer (below the water table).
These optional maps will be produced using Dighem1s
pseudo-layer two-layer inversion program.
Figure 10 (in map pocket) supports the above comments. It
illustrates DIGHEM17 data over the Night Hawk test range
in Ontario. The conductor is a wide graphite body buried
beneath 90 m of sand. True resistivities of graphite and
cover are respectively 1 and 250 ohm-m. The apparent
resistivities illustrate that, (a) the 56,000 Hz frequency
does not penetrate to the graphite, while (b) the 900 Hz
frequency senses the target well. This separation of
response helps ensure a reliable two-layer inversion.
51
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2.2.5.1 Resistivity Analysis
Apparent resistivity maps are normally prepared from DIGHEM
survey data. We use the "pseudo-layer half space model"
described in the attached publication entitled "Resistivity
Mapping with an Airborne Multicoil Electromagnetic System"
(map pocket). This half space model avoids errors in the
apparent resistivity calculations resulting from altimeter
errors. Altimeter errors can result from trees or
topographic variations. If such errors do not exist, the
pseudo-layer half space algorithm will also give a reliable
thickness of the upper layer providing it is transparent to
the frequency.
As an option, we can employ our two-layer analytic'
technique, termed the "pseudo-layer two-layer model". This
method yields separate resistivity maps of the upper and
lower layers, and a thickness map of the upper layer.
2.2.5.2 Blectroaagnetic Map
The desired geological responses are best displayed as
resistivity and depth contour maps. An EM map would
comprise two classes of anomalies:
1. Anomalies due to culture, eg., power lines and fences.
2. Anomalies due to broad conductivity increases and to
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- 14 -
local decreases in flying height.
The second (geological) class of anomalies is of limited
use assuming resistivity maps are generated. The first
(cultural) class may be of interest. We can produce a map
of either or both classes as an optional product.
Regardless of whether such a map is requested, we shall
indicate areas thought to be affected by cultural features.
2.3 Project Management Plan
Project Schedule:
As per F901 (page 15 of 44) of the U.S.G.S. solicitation.
Base of Operations: Brookhaven, Mississippi
Helicopter Contractor: Midstate Helicopter
(1st. choice)
Pilot: Mike Ward, Chief Pilot
Experience: 13500 hours
Alternate Pilot: Richard Santmyer, Director of Operations
7500 hours
The DIGHEMIV survey equipment will be mobilized to
Brookhaven. It is anticipated that installation will take
two days. Testing requirements (See sections 1.2.5, 2.1.3,
2.2.3, 2.2.4 and 3.1.2 of the RFP) will take place
immediately following installation. At this point, any
optimization of system parameters as requested by the
U.S.G.S. will be undertaken where practical. Actual survey
flyinq will commence immediately upon acceptance of test
data by the U.S.G.S. It is anticipated that 7 days will be
required to complete the survey.
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3. STATEMENT OF COMPLIANCE
Dighem agrees to comply with the requests set forth in the
Statement of Work, with the following exceptions:
1. Barometric altimeter is quoted as an optional item under
this proposal. This is because lack of knowledge of
barometric pressure, as height above sea level, does not
affect the integrity of EM or magnetic data.
2. A 2 Hz, 0.01 nT, digitally recording ground magnetometer is
quoted as an optional item. This is because our experience
has indicated that the standard 0.2 Hz, 0.5 nT, digitally
recording magnetometer provides sufficient control for the
removal of diurnal variations to the 1 nT level. Further,
the removal of long wave length diurnal variations does not
contribute to the detection of well hole casings as their
signature is primarily in the high frequency components.
3. A price reduction is provided if inphase and quadrature ppm
contour maps are deleted from the requirements. Such maps
will primarily highlight flying height variations rather
than conductivity variations. Conductivity variations are
shown, without distortion caused by flying height
variations, on the resistivity contour maps.
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4. CAPABILITIES OF ORGANIZATION, PERSONNEL & EQUIPMENT
4.1 Organizational Experience
Dighem has continously flown HEM surveys since 1969.
Almost all of these have been combined EM/magnetometer
surveys. Clients have included most of the major
exploration companies in the world, as well as the
governments and agencies of the U.S.A., Canada, Japan,
Germany, Italy, France and Austria. Dighem has flown 2
surveys for the U.S.G.S.
In recent years, electronic navigation has been employed on
the majority of our surveys.
The following is a summary of Dighem1 s work for the U.S.
Government:
LINE-KM CONTRACT NO.
AGENCY COST (US?)
$/KM
FHA 82 193,000
USGS 70 144,700
NOO 139 44,500
USGS 75
18,714
2366 DOT-FH-11-9144
2080 14-08-0001-18881
320 N62306-84-C-0013
251 5-4400-1310075
CONTRACT
OFFICER
Mr. J.F. Koca
Dr. D.B. Hoover
Mr. D.G. Burkell
Ms. L.M. Davidson
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Note: Dighem acted as principal subcontractor in the Federal
Highways Administration (FHA) and Naval Oceanographic
Office (NOO) contracts. The dollars shown indicate
Dighem1s billings. Other surveys have been flown for
U.S. State governments.
4.1.1 Subcontractors
See section 2.3 of this proposal.
4.2 Personnel Qualifications
Curricula Vitae follow, after figures 1 to 9, for the
Dighem personnel to be involved in the execution of this
project.
4.3 Equipment and Procedures
4.3.1 Survey Equipment
The equipment to be used will be the DIGHEMIV helicopter
EM system which was developed in 1984. This system was
developed under a grant from the Canadian Federal
Government. The first commercial surveys were undertaken
in 1985. In addition to numerous mineral exploration
surveys, projects flown with this system have included
salinity mapping for the U.S.G.S., structural mapping in
various environments/ mapping of shear zones and
identification of poorly conductive mineralization.
This unique system operating at frequencies up to 56000 Hz
allows the mapping of resistivities from 0.1 to 30,000 ohm
meters.
56
.1
-------
- 18 -
4.3.2 Data Processing - Facilities & Procedures
4.3.2.1 Facilities
Dighem computer facilities for processing geophysical
survey data include:
(i) Computers
* VAX 11/780 virtual memory computer with 650 Mbytes of
on-line data storage, a high performance magnetic tape
drive, multiple CRT terminals and printers.
* MicroVAX-II virtual memory computer with 1.2 Gbytes of
on-line data storage, magnetic tape drive and multiple
CRT terminals.
* Multiple IBM PC/AT compatible imaging workstations with
image processors and high resolution monitors.
All of the above computers are linked via a high
performance Local Area Network using Ethernet links and
OECnet software.
(ii) Plotters
* Versatec Model 8224, 200 dot per inch, 24 inch width
electrostatic plotter for preliminary display and
working copy plots.
57
-------
- 19 -
* Calcomp Model 970, 52 x 80 inch wet-ink plotter for
final cartographic quality plots.
Both plotters are interfaced to the VAX computers. Many of
our products are displayed in color using Applicon and Iris
ink-jet plotters.
(iii) Digitizer
* Hi-State Model 4260, high precision 42 x 60 inch table
digitizer with VAX computer on-line interface.
4.3.2.2 Procedures
(i) Data Compilation
The object of the data compilation is to ensure an
error-free database before actual data processing
commences. This is achieved by:
* Input/ checking and editing of the flight log.
* Input and editing of the raw field tape data including
checking for invalid characters, misaligned data
records, noise, sensor equipment errors and fiducial
problems. Errors are corrected by reporting or
displaying the affected lines and correcting the
problems.
* Input and editing of positional data. Digital
positional data from the electronic positioning system
is plotted and fitted to the 1:12,000 topographic maps.
58
-------
- 20 -
(ii) Electromagnetic Processing
Dighem's electromagnetic data processing procedures are
well known to a number of U.S.G.S. personnel from surveys
flown earlier. The EM signals are used to compute the
ground resistivity, to yield resistivity profiles and
contour maps. For this survey, the resistivity will be
computed for 900, 7200 and 56000 Hz, and contour
resistivity maps will be prepared for all frequencies.
(iii) Magnetic Processing
The Cesium magnetometer will typically yield a ±0.1 nT
noise envelope. In the presence of the horizontal coplanar
EM fields, this may at times rise to ±0.2 nT. Our data
processing yields maps contoured at 1, 2, 5 or 10 nT,
depending on the client's instructions. The IGRF field
will be removed.
59
-------
- 21 -
Tie lines and ground base station data are used in the
magnetic data processing to eliminate diurnal responses.
Optional processing is available to extract magnetic
signatures caused by culture, e.g., drill hole casings.
(iv) Gridding
The line gridding algorithm yields uniform grids from
survey data collected along nominally parallel survey
traverses. Control of the gridding operation is given by
Akima's function which is a modified local slopes method.
Line gridding features and benefits include:
* Setting a minimum separation for adjacent survey
traverses to prevent the creation of sharp steps in the
final grid;
* Setting a limit to the excursion of the Akima function
to prevent the creation of spurious "high" and "low"
enclosures between adjacent survey traverses;
60
-------
- 22 -
* Accepting bi-directional survey traverse data.
Initially, each of the two sets of survey data are
gridded separately. A final grid is calculated as the
average of the two initial grid values, each of which is
weighted by its inverse square of distance to the survey
traverse from which it was derived;
* Gridding of irregular shaped survey areas and areas with
"holes"; and
* Optional, automatic detrending of narrow linear features
at acute angles to the nominal survey line direction.
The benefits of detrending are a reduction of oval
contour enclosures, more aesthetically pleasing contour
maps, and grids more suitable for derivative work such
as vertical gradient, reduction-to-pole,
depth-to-basement, etc.
(v) Contouring
The contouring algorithm utilizes an advanced contour
threading routine that is based on logic rather than
mathematics^ This greatly improves the contour mapping
speed and quality. The contouring package is especially
suited to geophysical survey data because of its capability
to depict fine detail over large amplitude variations.
61
-------
- 23 -
The principal features of the contouring program include:
* Gradient dependent supression of annotation and
contour lines;
* Multiple contour levels given in incremental or discrete
mode;
* LEROY template characters for contour annotation;
~*~~ Control of the frequency of contour annotation,
orientation, content, etc. Annotation is always
up-gradient;
* Different line widths and styles, e.g., solid, dashed,
dotted, for different contour levels. Enclosed lows are
marked by triangular teeth;
* Grid windowing for map sheet presentation.
(vi) Additional Processing Information
Magnetic archive tape will be provided as per paragraph
2.6.4 of the U.S.G.S. solicitation (page 8 of 44).
62
-------
- 24 -
5. QUALITY COHTROL PROCEDURES
Beading Compensation:
Inflight Data Acquisition:
Flight Path Location:
Not applicable to a towed
bird system.
Data quality is monitored
utilizing a analog display
(RMS GR33) which is visible
to the operator at all
times. Selective portions
of the digital data are
dumped after the completion
of the survey flight.
Flight path will be guided
by an electronic positioning
system.
Ground Magnetometer:
Monitored
utilizing
recorder.
at all times
an analog
Data is also
recorded in memory and then
transferred to a floppy
disk.
Data acquisition, processing, and interpretation will be
carried out under the requirements of a Quality Assurance
Plan written by the U.S.G.S. and approved by the E.P.A.
These specifications are defined in sections 3.1.1 to 3.1.6
of the U.S.G.S. solicitation.
63
-------
- 25 -
6. CONTINGENCY PLAN
Three major areas cover the resources that will be required
for this project. They are aircraft, geophysical equipment
and personnel.
6.1 Aircraft
Should the helicopter subcontractor default on any critical
commitment to the project, the following nearby companies
have suitable survey helicopters and external cargo
licenses:
Air Logistics, Lafayette, LA
Commercial Helicopters Inc., Lafayette, LA
Industrial Helicopters, Lafayette, LA
Each will be contacted after contract award to determine
rates and availability in case of need.
6.2 Geophysical Equipment
The complete DighemIV system, including magnetometer and
all ancillary equipment, has been used in Dighem's ongoing
survey operations since July, 1985. During this time, it
has had an average production rate of 90 line km per d_ay_
on-site. Dighem stocks sufficient spares to maintain four
systems in the field.
-------
- 26 -
6.3 Personnel
Curricula vitae follow, after Figures 1 to 9, for Dighem
personnel who will substitute for other personnel if
required.
65
-------
- 27 -
7. SUMMARY
The U.S.G.S. requires rapid, cost-effective and detailed
mapping of brine saturated portions of near surface
aquifers. Since this survey is experimental in nature, a
state-of-the-art system designed for the mapping of
discontinuous horizontal layers should be selected to maximize
the likelihood of success.
To- satisfy these requirements, Dighem proposes to acquire
and process data from a DIGHEMIV system, outfitted with an
optional Cesium magnetometer. We believe this is the most
appropriate system for the survey because:
1. Helicopter-borne: Better mapping detail than
can be provided by fixed-wing
systems.
2. 7.98 m coil spacing: Greater depth penetration than for
the common 7 m HEM birds, due to
50% larger signals.
3. 3 horizontal coil-pairs: Greater signal from horizontal
layers; strike independent.
4. 900, 7200, 56000 Hz Broad spectrum of response for
frequencies: maximum differentiation of
resistivity modeling.
66
-------
- 28 -
5. Cesium magnetometer: Maximum sensitivity and sample
rate for detecting small, weak
magnetic anomalies.
With three years of commercial experience with the DIGHEMIV
system and 12 years of experience with resistivity mapping,
Dighem has refined its systems to minimize inherent noise and has
developed procedures to keep noise and drift insignificant
relative to geological variability.
Dighem's processing techniques for producing reliable resistivity
and magnetic maps, and recognizing cultural effects, have been
developed over numerous surveys in populated areas of North
America and Europe.
i
In summary, the U.S.G.S. can be confident that its objectives are
most likely to be met if it selects Dighem's system, people and
experience for this project.
6.7
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73
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FIGURE 7
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FIGURE 9
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APPENDIX C
SUBCONTRACTOR'S FINAL REPORT
DIGHEM was required as part of the contracted work (see
appendix A) to write a final project report. This report is not
meant to be an in-depth scientific analysis of the geophysical
data. It describes the instrumentation, data processing methods,
and makes general recommendations about further data
processing. The report included in this appendix has not been
edited to conform to any USGS or EPA report standards.
Conclusions and recommendations are the sole 'responsibility of
the contractor.
Also part of the deliverable items for the contract are
digital data on 9-track computer compatible tape. These data
will be made public through USGS open-file procedures.
77
-------
Report f537
DIGHKM IV 5DRVEY
FOR
U.S. GEOLOGICAL SURVEY
PROJECT 8-9380-4038
MISSISSIPPI
DIGHBM SURVEYS t PROCESSING INC. Douglas L. McConnell
MISSISSAUGA, OHTARIO Geophysicist
September 6 1988
A0537SEP.89R
78
-------
A DIGHEMIV survey was flown for the United States
Geological Survey, over a survey block near Brookhaven,
Mississippi.
The purpose of the survey was to detect resistivity
contrasts in order to map the contamination of fresh water
acguifers with brine. A secondary objective was to make high
resolution magnetic measurements to locate abandoned oil
wells.
The 900, 7,200 and 56,000 Hz data were used to produce
resistivity maps. The different levels of penetration of the
three frequencies through conductive earth, results in
resistivity maps that show the conductive properties at
different depths. The total field magnetic contours show
numerous bull's-eye anomalies due to cultural sources. The
VLF contours have also been influenced by cultural sources.
A comparison of the three resistivity parameters, and
additionally the calculated depth channels, should be useful
in determining the depths and extent of conductive sources
below surface.
79
-------
LOCATION MAP
tO'30'
Scale 1:250,000
SlMT'SO"
FIGURE 1
THE SURVEY AREA
80
-------
Section
IBXRODOCTION 1
SUKVBX BQUIPMEHT 2
PRODUCTS AMD PROCESSIMG TKCHHIQDES 3
SORVKT RESULTS 4
BACKC31OOND IHFORMATIOH 5
COHCLUSIOHS AND RECOMMEMDATIOliS 6
81
-------
- 1-1 -
INTRODUCTION
A DIGHEMIV electromagnetic/res 1st ivity/magnetic/VLF
survey was flown for the U.S. Geological Survey, from May 12
to May 16, 1988, over a survey block near Brookhaven,
Mississippi. This block is located on the Zetus and
Brookhaven, Mississippi, U.S.G.S. map sheets (Figure 1).
Survey coverage consisted of approximately 422
line-miles. Flight lines were flown with a line separation
of approximately 1/8 of a mile (200 metres) in an azimuthal
direction of 090*/270*. Tie lines were flown perpendicular
to the flight line direction.
IV
The survey employed the DIGHEM electromagnetic
system. Ancillary equipment consisted of a magnetometer,
radio altimeter, video camera, analog and digital recorders,
a VLF receiver and an electronic navigation system.
This report is divided into six sections. Section 2
provides details on the equipment used in the survey and
lists the recorded data and computed parameters. Section 3
reviews the data processing procedures, with further
information on the various parameters provided in Section 5.
Section 4 describes the geophysical results.
82
-------
- 1-2 -
The survey results are shown on 1 separate map sheet for
each parameter. Table 1-1 lists the products which can be
obtained from the survey. Those which are part of the
contract are indicated in this table by showing the
presentation scale. These total 6 maps.
Recommendations for additional products are included in
Table 1-1. These recommendations are based on the
information content of products that would contribute to
meeting the objectives of the survey.
83
-------
- 1-3 -
Table 1-1 Plata Available from tha Survey
NO. OF
MAP SHEETS
Flight Lines 1
EiectroDtagneuc Anrmni IBB
Probable Bedrock Conductors
Resistivity ( 900 Hz) 1
Resistivity ( 7,200 Hz) 1
Resistivity (56,000 HZ) 1
EM Magnetite
Total Field Magnetics 1
Enhanced Magnetics
Vortical Gradient Magnetics
2nd Vertical Derivative Magnetics -
Magnetic Susceptibility ' -
VU- 1
EUecucmacpiBLic raoLLLes( yuu HZ)
Electromagnetic FroLLLes(72UU HZ) -
(-km i l.IUj|ji.i fit«i<«u»uui<»
cvuxuunjkHi iiiicjaises
Digital Prof ilflB
ANOMALY
MAP
N/A
-
-
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
PRCFII£S
CN MAP
-
-
N/A
-
-
-
-
-
-
-
-
-
-
-
-
-
CONTOURS
INK COLOR
24,000
N/A
N/A
24,000
24,000
24,000
-
24,000
-
-
-
-
24,000
-
-
-
-
N/A
N/A
**
**
**
-
-
-
-
-
-
-
-
-
-
Worksheet profiles
Tntjfti HtMljfyi OPOf ileS
SHADOW
MAP
-
N/A
N/A
-
-
-
-
-
-
-
-
-
-
-
-
-
-
12,000
N/A Not available
*** Highly reconnended due to its overall Information content
** RaunuBftJad
* Qualified reccnmendation, as it may be useful In local areas
24,000 Scale of delivered nap, i.e, Ii24,000
84
-------
- 2-1 -
SUKVKI
BOUIPMENT
This section provides a brief description of the
geophysical instruments used to acquire the survey datat
Electromagnetic System
Modeli DIGHEMIV
Type: Towed bird, symmetric dipole configuration,
operated at a nominal survey altitude of 100
feet. Coil separation is 26.2 feet.
Coil orientations/frequencies: coaxial / 900 Hz
coplanar/ 900 Hz
coplanar/ 7,200 Hz
coplanar/56,000 Hz
Sensitivity: 0.2 ppm at 900 Hz
0.4 ppm at 7,200 Hz
1.0 ppm at 56,000 Hz
Sample rate: 10 per second
The electromagnetic system utilizes a multi-coil
coaxial/coplanar technique to energize conductors in
different directions. The coaxial transmitter coil is
vertical with its axis in the flight direction. The coplanar
coils are horizontal. The secondary fields are sensed
simultaneously by means of receiver coils which are maximum
coupled to their respective transmitter coils. The system
yields an inphase and a quadrature channel from each
85
-------
- 2-2 -
transmitter-receiver coil-pair. The system is also equipped
to provide two environment noise monitor channels.
Magnetometer
Model: Picodas Cesium
Sensitivity: 0.01 nT
Sample rate: 10 per second
The magnetometer sensor is towed in a bird 50 ft. below
the helicopter.
Magnetic Base Station
Model: Geometries G-826A
Sensitivity: 0.50 nT
Sample rate: once per 5 seconds
*
An Epson recorder is operated in conjunction with the
base station magnetometer to record the diurnal variations
of the earth's magnetic field. The clock of the base station
is synchronised with that of the airborne system to permit
subsequent removal of diurnal drift.
VLF System
Manufacturer: Herz Industries Ltd.
86
-------
- 2-3 -
Type s To tern- 2 A
Sensitivity s 0.1%
The VLF receiver measures the total field and vertical
quadrature components of the secondary VLF field. Signals
from two separate transmitters can be measured
simultaneously. The VLF sensor is towed in a bird 33 feet
below the helicopter.
Radar
Manufacturers Honeywell/Sperry
Types AA 220
Sensitivitys 1 ft
The radar altimeter measures the vertical distance
between the helicopter and the ground. This information is
used in the processing algorithm which determines conductor
depth.
Analog Recorder
Manufacturers RMS Instruments
Types 6R33 dot-matrix graphics recorder
Resolutions 4x4 dots/mm
Speeds 1.5 mm/sec
The analog profiles were recorded on chart paper in the
87
-------
. 2-4 -
aircraft during the survey. Table 2-1 lists the geophysical
data channels and the vertical scale of each profile.
Digital Data Acquisition System
Manufacturer! Scintrex
Typei CDI-6
Tape Deck: RMS TCR-12, 6400 bpi, tape cartridge recorder
The digital data were used to generate several computed
parameters.
Tracking Camera
Type: Panasonic Video
Model: AG 2400/WVCD132
Fiducial numbers are recorded continuously and are
displayed on the margin of each image. This procedure
ensures accurate correlation of analog and digital data with
respect to visible features on the ground.
Navigation System
Models Del Horte 547
Types OHF electronic positioning system
Sensitivity! 3 feet
Sample rate! 0.5 per second
88
-------
- 2-5 -
The navigation system uses ground based transponder
stations which transmit distance information back to the
helicopter. The ground stations are set up veil avay from
the survey area and are positioned such that the signals
cross the survey block at an angle between 30* and 150*.
After site selection, a baseline is flown at right angles to
a line drawn through the transmitter sites to establish an
arbitrary coordinate system for the survey area. The onboard
Central Processing Unit takes any two transponder distances
and determines the helicopter position relative to these two
ground stations in cartesian coordinates.
Aircraft
The instrumentation was installed in an Aerospatiale
Lama 315B turbine helicopter. The helicopter flew at an
average airspeed of 70 nph with an EM bird height of
approximately 100 feet.
89
-------
- 2-6 -
2-1.
Channel
Name
GUI
CX1Q
CP2I
CP2Q
CP3I
CP3Q
CP4I
CP4Q
CXSP
CPSP
ALT
YL1T
YL1Q
YL2T
VL2Q
MAGC
MAGF
Parameter
coaxial inpha^e ( 900 Hz)
{yviyijil quad ( 900 Hz)
ooplanar inphase ( 900 Hz)
ooplanar quad ( 900 Hz)
ooplanar inphase (7200 Hz)
coplanar quad (7200 Hz)
coplanar inphase ( 56 kHz)
ooplanar quad ( 56 kHz)
coflxiffll nferics
ooplanar sferics
altimeter
VU- total: primary station
VLF-quad: primary station
Vlf -total: secondary stn.
VLP-quad: secondary stn.
magnetics, coarse
magnetics, fine
Sensitivity
per mm
2.5 ppn
2.5 ppm
2.5 ppm
2.5 ppm
5.0 ppm
5.0 ppm
10.0 ppm
10.0 ppm
2.5 ppm
2.5 ppm
3m
5%
5%
5%
5%
20 nT
2.0 nT
Designation on
digital profile
CXI ( 900 Hz)
CXQ ( 900 Hz)
CPI ( 900 Hz)
CPQ ( 900 Hz)
CPI (7200 Hz)
CPQ (7200 Hz)
CPI ( 56 kHz)
CPQ ( 56 kHz)
AUT
MAG
HAG
2-2. Ite
profilee
Channel
Name- Freal
MAG
ADT
CXI
CXQ
CPI
CPQ
CPI
CPQ
CXPL
CDT
RES
RES
RES
DP
DP
DP
900 Hz)
900 Hz)
900 Hz)
900 Hz)
(7200 Hz)
( 56 kHz)
( 900
(7200 Hz)
( 56 kHz)
( 900 Hz)
(7200 Hz)
( 56 kHz)
Sf°
magnetics
bird height
vertical
coil-pair inphase
horizontAl
horizontal
coil-p&ir
horizontal nnplnnnr coil-pair
quadrature
tarizontal
-------
- 3-1 -
The following products are available from the survey
data. Those which are not part of the survey contract may be
acquired later. Refer to Table 1-1 for a summary of the
maps which accompany this report and those which are
recommended as additional products. Most parameters can be
displayed as contours, profiles, or in colour.
Base Maps
Base maps of the survey area were prepared from
Is24,000 topographic maps.
Flight Path
The cartesian coordinates produced by the electronic
navigation system were transformed into UTM grid locations
during data processing. These were tied to the UTM grid on
the base nap. In the case of a photomosaic base map, the UTM
grid mist be transferred from a topographic map to the
photomosaic.
Prominent topographical features were correlated with
the navigational data points, to ensure that the data is
accurately registered on the base map.
91
-------
- 3-2 -
Electromagnetic Anomalies
Anomalous electromagnetic responses are selected and
analysed by computer to provide a preliminary electromagnetic
anomaly map. This preliminary EH map is used, by the
geophysicist, in conjunction with the computer generated
digital profiles, to produce the final interpreted EM anomaly
map. This map includes bedrock, surficial and cultural
conductors. A map containing only bedrock conductors can be
generated, if desired.
Resistivity
The apparent resistivity in ohm-m may be generated from
the inphase and quadrature EM components for any of the
frequencies, using a pseudo-layer halfspace model. A
resistivity map portrays all the EM information for that
frequency over the entire survey area. This contrasts with
the electromagnetic anomaly map which provides information
only over interpreted conductors. The large dynamic range
makes the resistivity parameter an excellent mapping tool.
EM Magnetite
The apparent percent magnetite by weight is computed
wherever magnetite produces a negative inphase EM response.
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The results are usually displayed on a contour map.
Total Field Magnetics
The aeromagnetic data are corrected for diurnal
variation using the magnetic base station data. The regional
IGRF gradient is removed from the data, if required under the
terms of the contract.
Enhanced Magnetics
The total field magnetic data are subjected to a
processing algorithm. This algorithm enhances the response
of magnetic bodies in the upper 1,500 feet and attenuates the
response of deeper bodies. The resulting enhanced magnetic
map provides better definition and resolution of near-
surface magnetic units. It also identifies weak magnetic
features which may not be evident on the total field
magnetic map. However, regional magnetic variations, and
magnetic lows caused by remanence, are better defined on the
total field magnetic map. The technique is described in more
detail in Section 5.
Magnetic Derivatives
The total field magnetic data may be subjected to a
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variety of filtering techniques to yield naps of the
following:
vertical gradient
second vertical derivative
magnetic susceptibility with reduction to the pole
upward/downward continuations
...All., of these filtering techniques improve the
recognition of near-surface magnetic bodies, vith the
exception of upward continuation. Any of these parameters
can be produced on request. Dighem's proprietary enhanced
magnetic technique is designed to provide a general
all-purpose" map, combining the more useful features of the
above parameters.
VLF
The VLF data can be digitally filtered to remove long
wavelengths such as those caused by variations in the
transmitted field strength. The results are usually
presented as contours of the filtered total field.
Digital Profiles
Distance-based profiles of the digitally recorded
geophysical data are generated and plotted by computer.
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These profiles also contain the calculated parameters which
are used in the interpretation process. These are produced
as worksheets prior to interpretation, and can also be
presented in the final corrected form after interpretation.
The profiles display electromagnetic anomalies with their
respective interpretive symbols. The differences between the
worksheets and the final corrected form occur only with
respect to the EM anomaly identifier.
Contour» Colour and Shadow Map Displays
The geophysical data are interpolated onto a regular
grid using a cubic spline technique. The resulting grid is
suitable for generating contour maps of excellent quality.
Colour maps are produced by interpolating the grid down
to the pixel size. The distribution of the colour ranges is
normalized for the magnetic parameter colour maps, and
matched to specific contour intervals for the resistivity and
VLF colour maps.
Monochromatic shadow maps are generated by employing an
artificial sun to cast shadows on a surface defined by the
geophysical grid. There are many variations in the shadowing
technique, as shown in Figure 3-1. The various shadow
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techniques may be applied to total field or enhanced magnetic
data, magnetic derivatives, VLF, resistivity, etc. Of the
various magnetic products, the shadow of the enhanced
magnetic parameter is particularly suited for defining
geological structures with crisper images and improved
resolution.
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Dlghan software provides several shadowing techniques. Both monochromatic (ccnmonly
green) or polychromatic (full color) maps, may be produced. Monochronatic shadow
naps are often preferred over polychromatic maps for reasons of clarity.
Spot Sun
The spot sun technique tends to mimic nature. Hie sun occupies a spot in the sky at
a defined azimuth and inclination. Tha surface of the data grid casts shadows. This
is the standard technique used by industry to produce nonochronatic shadow maps.
A characteristic of the spot sun technique is that shadows are cast in proportion to
how well the sunlight intersects the feature. Features which are almost parallel to
the sun's azimuth may cast no shadow at all. Tb avoid this problem, Dighem's
hemispheric sun technique may be employed.
The hemispheric sun technique was developed by Dighem. The method involves lighting
up a hemisphere. If, for example, a north hemispheric sun is selected, features of
all strikes will have their north side in sun and their south side in shadow. The
hemispheric sun lights up all features, without a bias caused by strike. The method
yields sharply defined monochromatic shadows.
The hemispheric sun technique always improves shadow casting, particularly where
folding and cross-cutting structures occur. Nevertheless, it is important to center
the hemisphere perpendicular to the regional strike. Features which strike parallel
to the center of the hemisphere result in ambiguity. This is because the two sides
of the feature may yield alternating patterns of sun and shadow. If this proves to
be a problem in your survey area, Dighem's ami sun technique may be employed.
The ami sun technique was also developed by Dighem. The survey area is centered
within a ring of sunlight. This lights up all features without any strike bias.
The result is brightly defined lonochranatic features with diffuse shadows.
Two or three spot suns, with different azimuths, may be combined in a single
presentation. The shadows are displayed on one map by the use of different colors,
«.g. , by using a green sun and a red sun. Some users find the interplay of colors
reduces the clarity of the shadowed product.
Any of the above monochromatic shadow maps can be combined with the standard
contour-type solid color nap. The result is a polychromatic shadow map. Such maps
are esthetically pleasing, and are preferred by sane users. A disadvantage is that
ambiguity exists between changes in amplitude and changes in shadow.
Pig. 3-1 Shadow Mapping
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SUKVKY RRSTTT.TS
Resistivity
Apparent resistivity naps were prepared from the 900,
7,200 and 56,000 Hz coplanar EM data. These maps show the
conductive properties of the survey area.
The 56,000 Hz data has the greatest dynamic range but is
biased towards surficial conductivity. In general, the
56,000 Hz map shows higher resistivities than either the 900
or 7,200 Hz maps. This is indicative of a relatively
resistive upper layer overlying a conductive layer or layers.
This resistive cover may be thinner where the 56,000 Hz
resistivity agrees closely with the lower frequencies. For
example, a low resistivity trend of 10 to 15 ohm-m extends
from fiducial 2200 on line 10270, to fiducial 3640 on line
10410 on the 56,000 Hz nap.
The 7,200 Hz data is likely penetrating through the
surficial layer to a greater extent than the 56,000 Hz
resistivity. It generally agrees with the 900 Hz
resistivity, except in a few isolated areas. In areas where
it yields higher resistivities the relatively resistive
urficial layer may be thicker.
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The resistivity contours do not appear to have been
affected to a high degree by cultural sources. EM anomalies
due to cultural sources, such as power lines, are primarily
the result of current channelling. This yields a high
amplitude response but little phase shift and therefore no
appreciable change in the inphase to quadrature ratio. As
the resistivity calculation is based on the phase, which is
changed as a result _of inductive coupling, the cultural
sources do not usually distort the resistivity map to a high
degree. In some areas excessive noise in the form of spiking
or hash on the EM channels resulted from culture. In such
areas the data was left out of the resistivity calculation to
ensure that the resistivity contours were not distorted.
Magnetics
The total field magnetic data have been presented as
contours on the base map using a contour interval of 1 nT
where gradients permit. The IGRF gradient across the survey
block has not been removed. The maps show the magnetic
properties of the rock units underlying the survey area. The
isolated bull's-eye anomalies are likely due to cultural
sources. Some of these may be attributed to oil wells. The
narrow response yielded by such a source may easily be missed
at a 1/8 mile line spacing, therefore many of these sources
ay not have been detected.
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VLF
VLF results were obtained from three transmitting
stations, Cutler, Maine (NAA - 24.0 kHz), Seattle, Washington
(NLK - 24.8 kHz) and Annapolis, Maryland (NSS - 21.4 kHz).
The use of three different stations was necessitated by
signal interruptions at the source of transmission. Results
from the transmitter at Annapolis, Maryland were presented as
contours of the filtered total field. The contour patterns
are greatly influenced by cultural sources.
The VLF method is quite sensitive to the angle of
coupling between the conductor and the proposed EM field.
Consequently, conductors which strike towards the VLF
station will usually yield a stronger response than
conductors which are nearly orthogonal to it.
Closely-spaced conductors, conductors of short strike
length or conductors which are poorly coupled to the VLF
field, nay escape detection with this method. Erratic
signals from the VLF transmitters can also give rise to
strong, isolated anomalies which should be viewed with
caution.
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Electromagnetic Anomalies
The electromagnetic anomalies are displayed on the
digital profiles. Corresponding to each anomaly identifier
is either an "L" or "H" interpretive symbol. The "L" symbol
reflects an anomaly that is due to a line source or culture.
The "H* interpretive symbol is used to denote a response from
a conductor which fits a half space model, such as a buried,
flat lying layer. The coplanar EM channels will be maximum
coupled to these flat lying conductors, and therefore these
sources are best represented on the resistivity parameters.
Refer to the sections on "Recognition of Culture" and
Resistivity Mapping" in section 5 for more information.
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PACKGROUND IMFORMATIOH
This section provides background information on
parameters which are available from the survey data. Those
which have not been supplied as survey products may be
generated later from raw data on the digital archive tape.
Resistivity mapping
Areas of widespread conductivity are commonly
encountered during surveys. In such areas, anomalies can be
generated by decreases of only 5 m in survey altitude as well
as by increases in conductivity. The typical flight record
in conductive areas is characterized by inphase and
quadrature channels which are continuously active. Local EM
peaks reflect either increases in conductivity of the earth
or decreases in survey altitude. For such conductive areas,
apparent resistivity profiles and contour maps are necessary
for the correct interpretation of the airborne data. The
advantage of the resistivity parameter is that anomalies
caused by altitude changes are virtually eliminated, so the
resistivity data reflect only those anomalies caused by
conductivity changes. The resistivity analysis also helps
the interpreter to differentiate between conductive trends in
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the bedrock and those patterns typical of conductive
overburden. For example, discrete conductors will generally
appear as narrow lows on the contour map and broad conductors
(e.g., overburden) will appear as wide lows.
The resistivity profiles and the resistivity contour
maps present the apparent resistivity using the so-called
pseudo-layer (or buried) half space model defined by Fraser
(1978)1. This model consists of a resistive layer overlying
a conductive half space. The depth channels give the
apparent depth below surface of the conductive material. The
apparent depth is simply the apparent thickness of the
overlying resistive layer. The apparent depth (or thickness)
parameter will be positive when the upper layer is more
resistive than the underlying material, in which case the
apparent depth may be quite close to the true depth.
The apparent depth will be negative when the upper layer
is more conductive than the underlying material, and will be
sero when a homogeneous half space exists. The apparent
depth parameter must be interpreted cautiously because it
will contain any errors which may exist in the measured
altitude of the EM bird (e.g., as caused by a dense tree
1 Resistivity mapping with an airborne multicoil
electromagnetic systems Geophysics, v. 43, p.144-172
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cover). The inputs to the resistivity algorithm are the
inphase and qaudrature components of the coplanar coil-pair.
The outputs are the apparent resistivity of the conductive
half space (the source) and the sensor-source distance are
independent of the flying height. The apparent depth,
discussed above, is simply the sensor-source distance minus
the measured altitude or flying height. Consequently, errors
in the measured altitude will affect the apparent depth
parameter but not the apparent resistivity parameter.
The apparent depth parameter is a useful indicator of
simple layering in areas lacking a heavy tree cover. The
DIGHEM system has been flown for purposes of permafrost
napping, where positive apparent depths were used as a
measure of permafrost thickness. However, little
quantitative use has been made of negative apparent depths
because the absolute value of the negative depth is not a
measure of the thickness of the conductive upper layer and,
therefore, is not meaningful physically. Qualitatively, a
negative apparent depth estimate usually shows that the EM
anomaly is caused by conductive overburden. Consequently,
the apparent depth channel can be of significant help in
distinguishing between overburden and bedrock conductors.
The resistivity map often yields more useful information
on conductivity distributions than the EM map. In comparing
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the EM and resistivity naps, keep in mind the followingt
(a) The resistivity map portrays the absolute value
of the earth's resistivity, where resistivity =
I/conductivity.
(b) The EM map portrays anomalies in the earth's
resistivity. An anomaly by definition is a
change from the norm and so the EM map displays
anomalies, (i) over narrow, conductive bodies
and (ii) over the boundary zone between two wide
formations of differing conductivity.
The resistivity nap might be likened to a total field
nap and the EM map to a horizontal gradient in the direction
of flight2. Because gradient maps are usually more sensitive
than total field naps, the EM map therefore is to be
preferred in resistive areas. However, in conductive areas,
the absolute character of the resistivity map usually causes
it to be more useful than the EM nap.
2 The gradient analogy is only valid with regard to the
identification of anomalous locations.
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Recognition of culture
Cultural responses include all EM anomalies caused by
man-made metallic objects. Such anomalies may be caused by
inductive coupling or current gathering. The concern of the
interpreter is to recognize when an EM response is due to
.-culture. Points of consideration used by the..interpreter,
when coaxial and coplanar coil-pairs are operated at a common
frequency, are as follows:
1. Channel CXPL monitors 60 Hz radiation. An anomaly on
this channel shows that the conductor is radiating
power. Such an indication is normally a guarantee that
the conductor is cultural. However, care must be taken
to ensure that the conductor is not a geologic body
which strikes across a power line, carrying leakage
currents.
2. A flight which crosses a "line* (e.g., fence, telephone
line, etc.) yields a center-peaked coaxial anomaly and
an m-shaped coplanar anomaly. When the flight crosses
the cultural line at a high angle of intersection, the
amplitude ratio of coaxial/coplanar response is 4. Such
an EM anomaly can only be caused by a line. The
geologic body which yields anomalies most closely
resembling a line is the vertically dipping thin dike.
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Such a body, however, yields an amplitude ratio of 2
rather than 4. Consequently, an in-shaped coplanar
anomaly with a CXI/CPI amplitude ratio of 4 is virtually
a guarantee that the source is a cultural line.
3. A flight which crosses a sphere or horizontal disk
yields center-peaked coaxial and coplanar anomalies with
»
a CXI/CPI amplitude ratio (i.e., coaxial/coplanar) of
1/4. In the absence of geologic bodies of this
geometry, the most likely conductor is a metal roof or
email fenced yard. ^ Anomalies of this type are
virtually certain to be cultural if they occur in an
area of culture.
4. A flight which crosses a horizontal rectangular body or
wide ribbon yields an in-shaped coaxial anomaly and a
center-peaked coplanar anomaly. In the absence of
geologic bodies of this geometry, the most likely
conductor is a large fenced area.5 Anomalies of this
type are virtually certain to be cultural if they occur
in an area of culture.
3 It is a characteristic of EM that geometrically
similar anomalies are obtained from* (1) a planar
conductor, and (2) a wire which forms a loop having
dimensions identical to the perimeter of the
equivalent planar conductor.
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5. EM anomalies which coincide with culture, as seen on the
camera film or video display, are usually caused by
culture. However, care is taken with such coincidences
because a geologic conductor could occur beneath a
fence, for example. In this example, the fence would be
expected to yield an m-shaped coplanar anomaly as in
case 12 above. If, instead, a center-peaked coplanar
anomaly occurred, there would be concern that a thick
geologic conductor coincided with the cultural line.
6. The above description of anomaly shapes is valid when
the culture is not conductively coupled to the
environment. In this case, the anomalies arise from
inductive coupling to the EM transmitter. However, when
the environment is quite conductive (e.g., less than 100
ohm-m at 900 Hz), the cultural conductor may be
conductively coupled to the environment. In this latter
case, the anomaly shapes tend to be governed by current
gathering. Current gathering can completely distort the
anomaly shapes, thereby complicating the identification
of cultural anomalies. In such circumstances, the
interpreter can only rely on the radiation channel CPS
and on the camera film or video records.
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The magnetometer data are digitally recorded in the
aircraft to an accuracy of 0.01 nT for cesium magnetometers.
The digital tape is processed by computer to yield a total
field magnetic contour map. When warranted, the magnetic
data may also be treated mathematically to enhance the
magnetic response of the near-surface geology, and an
enhanced magnetic contour map is then produced. The response
of the enhancement operator in the frequency domain is
illustrated in Figure 5-1. This figure shows that the
passband components of the airborne data are amplified 20
times by the enhancement operator. This means, for example,
that a 100 nT anomaly on the enhanced map reflects a 5 nT
anomaly for the passband components of the airborne data.
The enhanced map, which bears a resemblance to a
downward continuation map, is produced by the digital
bandpass filtering of the total field data. The enhancement
is equivalent to continuing the field downward to a level
(above the source) which is 1/2 Oth of the actual sensor-
source distance.
Because the enhanced magnetic map bears a resemblance to
a ground magnetic map, it simplifies the recognition of
trends in the rock strata and the interpretation of
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ui
o
10
to-1
CYCLES/METRE
Fig. 5-2 Frtqutney response of mognttlc
nhancemtnt operator for a tamplt
Inttrval of 60 m.
no
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geological structure. It defines the near-surface local
geology while de-emphasizing deep-seated regional features.
It primarily has application when the magnetic rock units are
steeply dipping and the earth's field dips in excess of 60
degrees .
Any of a number of filter operators may be applied to
the magnetic data, to yield vertical derivatives,
continuations, magnetic susceptibility, etc. These may be
displayed in contour, colour or shadow.
VLF transmitters produce high frequency uniform
electromagnetic fields. However, VLF anomalies are not EM
anomalies in the conventional sense. Elf anomalies primarily
reflect eddy currents flowing in conductors which have been
energized inductively by the primary field. In contrast, VLF
anomalies primarily reflect current gathering, which is a
non-inductive phenomenon. The primary field sets up currents
which flow weakly in rock and overburden, and these tend to
collect in low resistivity zones. Such zones may be due to
massive sulfides, shears, river valleys and even
unconformities .
The VLF field is horizontal. Because of this, the
ill
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UJ
o
_J
0.
CYCLES /METRE
Fig. 5-3 Frequency response of VLF operator.
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method is quite sensitive to the angle of coupling between
the conductor and the transmitted VLF field. Conductors
which strike towards the VLF station will usually yield a
stronger response than conductors which are nearly orthogonal
to it.
The Herz Industries Ltd. Totem VLF-electromagnetometer
measures the total field and vertical quadrature components.
Both of these components are digitally recorded in the
aircraft with a sensitivity of 0.1 percent. The total field
yields peaks over VLF current concentrations whereas the
quadrature component tends to yield crossovers. Both appear
as traces on the profile records. The total field data are
filtered digitally and displayed as contours to facilitate
the recognition of trends in the rock strata and the
interpretation of geologic structure.
The response of the VLF total field filter operator in
the frequency domain (Figure 5-2) is basically similar to
that used to produce the enhanced magnetic map (Figure 5-1).
The two filters are identical along the abscissa but
different along the ordinant. The VLF filter removes long
wavelengths such as those which reflect regional and wave
transmission variations. The filter sharpens short
wavelength responses such as those which reflect local
geological variations.
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rniinr.nsTnwfi AMD
This report provides a brief description of the survey
results and describes the equipment, procedures and
logistics of the survey.
The various maps included with this report display the
magnetic and conductive properties of the survey area. The
survey results should be reviewed in detail, in conjunction
with all available geological, geophysical and geochemical
information.
It is also recommended that additional processing of
existing geophysical data be considered, in order to extract
the maximum amount of information from the survey results.
Resistivity colour plots may aid in identification of
resistivity contrasts.
Respectfully submitted,
DIGHEM SUKVKIS i PROCESSING INC.
*~~ ^
Douglas L. McConnell
Geophysicist
DLM/mg
A0537SEP.89R
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