Naval Base Ventura County,
Port Hueneme, California
EPA Characterization Test Cell
Report on Electromagnetic Surveys in the Test
Cell Area
U.S. EPA Research Brief
EPA/600/S-04/073
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U.S. EPA Characterization Test Cell Geophysical Survey Report EPA/600/S-04/073
Naval Base Ventura County, Port Hueneme, California
EPA Characterization Test Cell
Report on Electromagnetic Surveys in the Test Cell Area
D. Dale Werkema
U.S. EPA, ORD, NERL, ESD-LV, CMB
944 E. Harmon Ave.
Las Vegas, NV. 89119
702-798-2263
werkema. d@epa. gov
1. Executive Summary
The objective of the geophysical surveys at the EPA Characterization Test Cell (CTC)
area (Site) at Naval Base Ventura County, Port Hueneme, California is to locate
geophysical anomalies indicative of metallic objects within the area of the cell. The goal
was to provide background metallic object content at the Site for future construction and
research activities. To achieve the objective, detailed reconnaissance geophysical mapping
using Electromagnetic Induction (EMI) was conducted throughout the Site. The EMI
survey was performed using the Geonics EM-31 and the Geonics EM-61. The following
series of geophysical property maps were produced from the survey results: an EM-31
Quadrature response (bulk ground conductivity), EM-31 In-Phase response, EM-61 Bottom
Channel, EM-61 Top Channel, and the EM-61 Normalized Differential Channel. The EM-
31 revealed bulk ground conductivities, while the EM-61 revealed responses due to ferrous
and/or non-ferrous metallic objects. The EM-31 maps show no anomalous responses
within the area surveyed. An increasing bulk ground conductivity gradient was observed
along the eastern and southeastern portions of the Site interpreted as due to a nearby chain-
link fence as well as an underground utility along the southeastern boundary. The EM-61
maps reveal several discrete anomalies indicative of small metallic objects throughout the
site and one large discrete anomaly of no known cause. In addition a few north-south
trending linear anomalies may represent elongate pipe-like metallic objects or surface
material contrasts. Overall, the EMI surveys suggest the locations of several subsurface
metallic objects, which may be encountered during excavation and construction and if so,
should be removed prior to construction of the CTC.
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U.S. EPA Characterization Test Cell Geophysical Survey Report EPA/600/S-04/073
This report is organized according to the following table of contents.
1. Executive Summary 2
2. Objective 3
3. Methodology 3
Theory 3
Geonics EM-31, Electromagnetic Induction Survey Instrument 3
Geonics EM-61, Electromagnetic Induction Survey Instrument 4
Field Acquisition 5
EM-31, Electromagnetic Induction Survey Instrument 5
EM-61, Electromagnetic Induction Survey Instrument 5
4. Results and Interpretation 5
EM-31 Quadrature Response 6
EM-31 In-Phase 6
EM-61 Bottom Channel 6
EM-61 Top Channel 6
EM-61 Normalized Differential Channel 7
5. Conclusions 7
6. Disclaimer 8
7. Figures 9
Figure 1: EM31 Quadrature Component and Ground Elevation (ft.) 9
Figure 2: EM31 In-Phase Component and Ground Elevation (ft.) 10
Figure 3: EM61 Bottom Channel and Ground Elevation (ft.) 11
Figure 4: EM61 Top Channel and Ground Elevation (ft.) 12
Figure 5: EM61 Normalized Differential Channel and Ground Elevation (ft.) 13
8. Appendix 14
Quality Assurance / Quality Control Plot EM-31 14
Quality Assurance / Quality Control Plot EM-61 15
2. Objective
The objective of the geophysical surveys at the EPA Characterization Test Cell at the
Naval Base Ventura County (Site) is to locate geophysical anomalies indicative of
subsurface ferrous and/or non-ferrous metallic objects.
3. Methodology
To achieve the above objective, detailed reconnaissance geophysical mapping using
Electromagnetic Induction (EMI) was conducted throughout the Site. The EMI survey
used the Geonics EM-31 and the Geonics EM-61 instruments.
Theory
Geonics EM-31, Electromagnetic Induction Survey Instrument
This instrument operates on the principle of electromagnetic induction. The EM-31
generates an electromagnetic field by sending a low frequency (9.8 kHz) alternating
current (AC) along a wire coil. The AC generates a magnetic dipole perpendicular to the
coil and induces an electromagnetic (EM) wave emanating orthogonally to the coil.
Based on the orientation of the coil to the ground (or simply the instrument orientation),
the EM-wave propagates through the ground. As the wave moves through the ground, a
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U.S. EPA Characterization Test Cell Geophysical Survey Report EPA/600/S-04/073
secondary wave is generated based on the ground properties (e.g., electrical
conductivity). A second coil on the instrument receives the two EM waves (primary and
secondary) and then generates two results. The first result is the Quadrature component
(out-of-phase with the primary field) which is directly calibrated to the bulk electrical
conductivity of the ground and is measured in milliSiemens/meter (mS/m). The second
result is the In-phase component (in-phase with the primary field), which is the ratio of
the secondary wave to the primary wave amplitude and is measured in parts-per-
thousand (ppt). Generally speaking, the In-phase component reveals the presence of
very good electrical conductors (e.g., metals). Due to the geometry and separation of the
coils, this instrument will only detect objects that have at least one dimension that is
large relative to the coil spacing of 3.66 meters. The anomaly maps produced indicate
general ground conductivity as well as anomalously large or small bulk ground
conductivity (Quadrature component) and the presence of large metallic conductors (In-
phase component).
Geonics EM-61, Electromagnetic Induction Survey Instrument
The EM-61 is a time-domain metal detector, which detects both ferrous and non-
ferrous metals. A powerful transmitter generates a pulsed primary magnetic field into the
ground, which induces eddy currents in nearby metallic objects. The eddy current decay
produces a secondary magnetic field measured by the horizontal receiver coils. By
taking the measurement at a relatively long time after the start of the decay, the current
induced in the ground has fully dissipated and only the current in the subsurface metal is
still producing a secondary field. According to the manufacturer, the EM-61 can detect
a single 200-litre (55-gallon) drum at a depth of over 3 meters beneath the instrument,
yet is relatively insensitive to nearby cultural interference, such as fences, buildings and
power lines. The EM-61 can detect much smaller objects closer to the surface. The
amplitude of the response depends on the distance between the coil assembly and target.
Observing the output from two coils and processing them in differential mode can
reduce the effect of any near surface material. That is, four values are recorded as the
results from a survey: the response from the top coil, the bottom coil response, the
differential, and a normalized differential. The differential channel is calculated by the
equipment as the signal at the bottom coil subtracted from the signal at the top coil. The
bottom coil receives information from all targets within the reach of the EM-61 system.
The top coil receives information primarily from near surface targets, and potentially
large deeper targets. The differential channel shows mostly deeper targets with the
removed or largely suppressed response from near surface material. As a result,
negative values on the differential channel are often associated with metallic objects
located at or above the surface. Finally, the normalized differential channel is a
calculated channel with the purpose of removing noise in the data. The reduction of
noise is based on the fact that each of the two receiver coils is receiving noise from the
same source. An order of magnitude reduction of noise can be achieved by selecting the
gain from each coil and subtracting this from the channel outputs. The normalized
differential channel will have target responses very similar to the target response of the
bottom coil and, in many cases, at a drastically reduced noise level.
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U.S. EPA Characterization Test Cell Geophysical Survey Report EPA/600/S-04/073
Field Acquisition
The geophysical surveys were conducted on a reference grid determined in the field
by measuring distance (in meters) from a nearby fence. The survey grid was established
with coordinates increasing distance to the east and the north from the point of origin, as
determined from field measuring tapes. The corners of the grid, as well as several
additional positions, were marked with survey paint on the asphalt surface. The weather
was overcast with no precipitation, and the data were collected on October 28, 2003.
After completion of the survey, the U.S. Navy relocated the grid coordinates and
surveyed the site according to a local Base Coordinate System. This system includes
distances north and east in feet and elevations in feet above mean sea level. The EMI
data are presented referenced to this coordinate system as presented relative to the Base
Survey Coordinates by the U.S. Navy.
EM-31, Electromagnetic Induction Survey Instrument
After initial instrument calibration this survey was conducted along the north to
south survey lines incrementing in an eastward direction every 2 meters. The data were
collected with a 0.4-second cycle time in unidirectional survey mode where the data
were collect only when surveying from south to north. Line 0 m E (4068 ft. east) was
repeated upon the completion of the survey for the EM-31 quality assurance/quality
control (QA/QC - see appendix).
EM-61, Electromagnetic Induction Survey Instrument
After initial instrument calibration this survey was performed along the same south
to north survey lines as the EM-31 and incremented in an eastward direction every 2
meters. Data were collected with 3-readings per second cycle time in bi-directional
survey mode (i.e. surveying south to north then north to south for the next line). For
quality assurance/quality control (QA/QC) Line 26 m E (4153 ft. east) was repeated after
the survey was completed; however, the repeated survey was completed in the opposite
direction than the first.
4. Results and Interpretation
The geophysical results were compiled into maps, transformed to the Navy Base
Coordinate System, and gridded onto a 0.65 meter mesh using Kriging.
In the Appendix are the QA/QC plots for the EM-31 and the EM-61. These plots
show that the equipment functioned within specifications and data quality was good.
Slight variations in these plots are due to not exactly re-locating the same spatial position
during the repeated survey. Furthermore, horizontal displacement of the profile can
occur if the repeated survey was completed in the opposite direction than the first, as in
the EM-61 survey.
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U.S. EPA Characterization Test Cell Geophysical Survey Report EPA/600/S-04/073
EM-31 Quadrature Response
The EM-31 Quadrature map (Figure 1) reveals no significant areas of anomalous
bulk ground conductivity. The bulk ground conductivity response ranges from 43 mS/m
to 65 mS/m. This is a small range and does not represent significant conductivity
variations within the surveyed area. Increases observed along the east and southeast
portions of the map are interpreted as due to the chain-link fence along the east boundary
and the underground utility (marked as telephone on the asphalt) along the southeast
boundary of the surveyed area. One anomalous small monopolar high is observed on the
northern edge of the survey at approximately 4160 ft east. Its cause is unknown.
EM-31 In-Phase
The EM-31 In-phase (Figure 2) shows a range of response from 0.2 to 6 ppt.
Relative highs are observed along the northern boundary of the survey grid between
4133 ft E. and 4182 ft E and the western edge of the Site at 13810 ft. N. The northern
anomalies appear to corroborate with the anomaly in this area in the Quadrature data.
The cause of the small feature on the western edge cannot be determined as the survey
only detected the eastern half of the anomaly. A small linear feature is observed along
line 4160 ft E; although its cause is unknown. The increasing gradient along the eastern
boundary and the southeastern edges are interpreted as the effects of the fence and the
utility.
EM-61 Bottom Channel
The response from the EM-61 Bottom Channel (Figure 3) reveals anomalies due to
ferrous and non-ferrous metallic objects within the depth range of the instrument. The
range of response is from -200mV to over 100 mV indicating a good likelihood of
metallic objects present within the survey. The results show several monopolar
anomalies scattered around the northern, eastern and southeastern portions of the Site.
The anomaly at 4075 ft. E, 13860 ft N is a monitoring well. Additionally a linear or N-
S elongate anomalous feature is observed along line 4165 ft. E. This and the other
anomalies have no known cause with the exception of the southeastern area as
attributed to the fence and underground utility.
EM-61 Top Channel
The results from the EM-61 Top Channel (Figure 4) reveal responses due to ferrous
or non-ferrous metals closer to the surface than the bottom channel. These data reveal a
response range from -100 to 100 mV and corroborate with the bottom channel for
several of the discrete monopolar anomalies along the eastern half of the survey as well
as further reveal the linear or elongate features occurring about line 4165 ft. E. These
linear features are more predominant in this channel and may also be due to an
instrument acquisition effect caused by surveying in bi-direction mode or an instrument
problem. However, since this linear effect was only observed at this location and the
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U.S. EPA Characterization Test Cell Geophysical Survey Report EPA/600/S-04/073
QA/QC data does not suggest instrument problems, it is interpreted as either due to the
bidirectional survey or linear features in the subsurface. Furthermore, the EM-31 In-
phase response also observed a small linear feature in this area.
EM-61 Normalized Differential Channel
The EM-61 Normalized Differential Channel (Figure 5) can be used to interpret the
location of objects with respect to the ground surface. Typically, objects that give a
positive response for both the Top and Bottom Channel but a negative response for the
Normalized Differential Channel are located very near to, at, or above the surface.
These data again show the monitoring well at 4075 ft. E, 13860 ft. N, and various
discrete monopolar anomalies along the eastern and northern portions of the survey.
The linear features along line 4165 ft. E show values up to 15 mV which suggests these
are deeper linear sources or perhaps an exacerbated effect of the instrument survey
method as discussed above. All these anomalies are interpreted as ferrous or non-
ferrous metals of various sizes and orientations. Linear anomalies suggest sources
which are pipe-like and are oriented horizontally. Monopolar anomalies suggest
vertical orientation of objects similar to monitoring well covers or drums. Dipolar
anomalies may indicate sources which are inclined from the vertical and their location
is approximated to the inflection point between the dipole. One such anomaly occurs at
approximately 4170 ft. E and 13780 ft. N.
5. Conclusions
In general, the EM-31 data do not reveal any significant anomalies within the
surveyed area around the CTC. These data do provide background bulk ground
conductivity values and confirm the presence of the utility along the southeast edge of
the survey. The EM-61 data reveal several discrete monopolar anomalies suggestive of
vertically oriented ferrous or non-ferrous metallic objects. One of these is the
monitoring well along the western edge of the survey. Additionally, the larger linear or
N-S elongate features are observed which may represent horizontal pipe-like sources or
perhaps a linear material property contrast in the near subsurface. All the EM-61 data
also confirm the utility along the southeastern edge.
This investigation has identified several geophysical anomalies which are indicative
of ferrous or non-ferrous metallic objects. These results provide a background for
future research as well as suggest the location where metallic materials may be present
and should be removed during excavation and construction activities.
Finally, it is important to note as with any geophysical survey the instruments detect
material physical property contrasts on the surface and to the depth limitation of the
instrument. If a significant physical property contrast exists on the surface, this can and
will mask responses from any materials at greater depths. Also, while the instruments
used in this survey were designed for these types of investigations, various
combinations of physical property contrasts can potentially exist to yield results similar
to those observed in this survey. Therefore, while the anomalies should represent
metallic objects, the anomalies truly represent significant material property contrast
from the nearby material to produce an anomaly.
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U.S. EPA Characterization Test Cell Geophysical Survey Report EPA/600/S-04/073
6. Disclaimer
The U.S. Environmental Protection Agency, through its Office of Research and
Development, funded and collaborated in the research described here. It has been
subjected to an external peer review, the Agency's peer and administrative review, and
has been approved for publication as an EPA Research Brief. Mention of trade names
or commercial products does not constitute an endorsement or recommendation for use.
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U.S. EPA Characterization Test Cell Geophysical Survey Report
EPA/600/S-04/073
7. Figures
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ting (feet)
Figure 1: EM31 Quadrature Component and Ground Elevation (ft.)
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Figure 2: EM31 In-Phase Component and Ground Elevation (ft.)
10
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OJ
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Figure 3: EM61 Bottom Channel and Ground Elevation (ft.)
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.00 4100.00 4150.00 4200.00 4250
Figure 4: EM61 Top Channel and Ground Elevation (ft.)
12
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Figure 5: EM61 Normalized Differential Channel and Ground Elevation (ft.)
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8. Appendix
Quality Assurance / Quality Control Plot EM-31
EM-31 QA/QC Line 4068 feet East
55
45
14
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U.S. EPA Characterization Test Cell Geophysical Survey Report
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Quality Assurance / Quality Control Plot EM-61
EM-61 QA/QC Line 4153 feet East
Bottom Channel Start
Top Channel Start
Bottom Channel End
Top Channel End
10 20
Northing (m)
15
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