December 1987
EPA 440/6-8 *<01,Sr
Surface Geophysical
Techniques for Aquifer
and Wellhead Protection
Area Delineation
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SURFACE GEOPHYSICAL TECHNIQUES FOR AQUIFERS
AND WELLHEAD PROTECTION AREA DELINEATION
Prepared by:
Paul Violette
Office of Ground-Water Protection
Office of Ground-Water Protection
Office of Water
US Environmental Protection Agency
401 M Street, S.W.
Washington, D.C. 20460
December 1987
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FORWARD
This document, "Surface Geophysical Techniques for
Aquifer and Wellhead Protection Area Delineation," is one
in a continuing series of technical reports prepared by
the U. S. Environmental Protection Agency's Office of
Ground-Water Protection. These publications report on
miscellaneous scientific topics which may be of interest
to State ground-water program managers. The methodologies
described in these reports do not represent EPA policy
but are intended to assist State decision-makers as well
as contribute to the scientific literature. This latest
report is a companion document to OGWP's "Guidelines for
the Delineation of Wellhead Protection Areas."
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EXECUTIVE SUMMARY
Surface geophysical techniques developed by the min-
erals and petroleum prospecting industries are applicable
to ground-water investigations. Through measurements
taken at the earth's surface, these techniques detect
subsurface physical property changes which are related to
hydrogeologic conditions.
The Safe Drinking Water Act Amendments (SDWAA) of
1986 establish that the States must define Wellhead
Protection Areas (WHPA) for public water supply wells.
The Environmental Protection Agency's (EPA) Office of
Ground-Water Protection (OGWP) has developed the "Guide-
lines for the Delineation of Wellhead Protection Areas"
which presents a variety of criteria and methodologies for
delineating WHPAs.
The WHPA delineation methods are used to transfer
the delineation criteria to the ground, and the spatial
limits of these mapped criteria represent the WHPA. Many
of the WHPA delineation methods such as the analytical
flow method and the numerical modeling method require
subsurface hydrogeologic data as input; however, surface
geophysical techniques can also provide information which
estimate subsurface hydrogeologic conditions. In many
ground-water investigations, surface geophysical data are
used to supplement well data, thereby reducing the need for
extensive water well drilling programs. Common approaches
are to use surface geophysical data to correlate between
boreholes or to extrapolate borehole information into
nearby areas. In these applications, surface geophysics
functions as a rapid, inexpensive, supplement to test
drilling.
Hydrogeologic mapping is the WHPA delineation method
outlined in the Guidelines that can be used to map the
flow boundary WHPA criterion. Surface geophysics is one
technique within this method that can support the flow
boundary criterion in unconfined aquifer systems.
The nature of the hydrogeologic setting determines
the applicability of a particular geophysical method. In
many ground-water studies, several different geophysical
methods are applied to the same survey area. Although
each method in these integrated surveys responds to
different property changes, the results of each data set
often support a single interpretation. In general, the
selection of a geophysical technique depends on the phy-
sical nature of the survey area, the desired depth of
penetration, the data resolution requirements, and the
available resources.
m
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TECHNIQUE
Seismic
Refraction
Seismic
Reflection
Electrical
Resistivity
Electromagnetic
Induction
(EMI)
Very
Low
Frequency
Resistivity
(VLF)
Ground
Penetrating
Radar
(GPR)
Gravimetry
Magne tome try
PARAMETER CHANGE
acoustic velocity
acoustic velocity
electrical resis-
tivity
electrical conduc-
tivity
electrical resis-
tivity
dielectric con-
stant
density
magnetic suscept-
ibility
MEASUREMENTS
stations with
ground contact
stations with
ground contact
stat Ions mea-
surements with
ground contact
continuous and
station measure-
ments ; no ground
contact
continuous
continuous with
ground contact
not necessary
stations
continuous air-
bourne or land-
based ; land-
based station
measurements
RESOLUTION
good vertical
description of
3 or 4 layers
excellent
vertical res-
olution
good vertical
resolution of
3 to 4 layers
excellent la-
teral resolu-
tion; good
vertical re-
solution ot 2
layers
same as EMI
Excellent
vertical and
lateral re-
solution
poor
poor
PENETRATION DEPTH
depth is limited
by space on the
surface; equip-
ment dependent
depth is equip-
ment dependent;
limited accuracy
above 100 tt;
probes to 1000 ft
depth is equip-
ment dependent
depth is deter-
mined by the coil
spacing; common
depths are from
1 to 100 feet
depth is deter-
mined by the re-
sistlvity ot the
terrain ; performs
like EMI
depth limited by
conductivity of
terrain; probes
from 1 to 100 tt
not relevant
not relevant
APPLICATIONS
depth to water table in uncon-
thickness of aquifer; depth to
bedrock in alluvial valleys;
stratlgraphic mapping
napping of bedrock in valley-
fill aquifers; detailed strat-
igraphic mapping of sediment-
ary units
depth to water table and salt-
fresh water interface; deline-
ation of clay layers or fine
and coarse sediments; mapping
uncon fined aquifer boundaries
same as electrical resistivity
same as electrical resistivity
map water table and shallow
stratigraphy in unconflned
aquifer systems
map bedrock and thickness of
alluvial sediments
map sedimentary units which
contain magnetic materials;
map fractures and fault zones
depth of alluvium when bedrock
has a measureable magnetic
susceptibility
ADVANTAGES
quantitative results;
refined acquisition and
data acquisition faster
tlon
velocity not required to
increase with depth; mod-
erate geophone spreads ;
high resolution in areas
provides some litho logic
tion is quantitative; ac-
quisition and processing
procedures are refined
rapid and simple data
acquisition—no ground
contact; good resolution
rapid and simple data
collection; good resolu-
tion
rapid data acquisition;
excellent data resolution
economical and unaffected
by cultural noise
economical
LIMITATIONS
can require explosives or
long geophone spreads ;
velocity required to
increase with depth
complex field and inter-
pretation procedures; may
require explosives; data
is recorded digitally;
data impaired by noise;
poor resolution tor shal-
low reflect ions- -espec i-
ally shallow water table
limited resolution; non-
requires large surface
area tor deep soundings;
acquisition is slow and
data is impaired by noise
data impaired by noise ;
depth limitations; Inter-
pretation is qualitative;
can define only 2 to 3
layers; limited to simple
stratigraphy
depth limitation; qualit-
ative interpretation; VLF
signal Is intermittent;
1 in i ted to simple strati-
graphy
depth of penetration
severely limited in con-
ductive terrains; complex
poor resolution ; care
required in performing
measurements ; Interpret-
atlonal ambiguities ;
elevations required for
each station
poor resolution; limited
to crystalline bedrock ;
interpretational ambigu-
ities
Table 1: Summary of geophysical techniques considered in this document.
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Table 1 summarizes the technical characteristics,
applications, advantages and limitations of the geophy-
sical techniques which have been reviewed in this docu-
ment. This summary is expected to give the reader an
idea of the capabilities of each technique. Seismic
and electrical methods are most suited to aquifer mapping
studies with the gravity and magnetic methods having only
secondary applications. Recent technology advances have
resulted in the development of new techniques which have
ground-water applications.
The seismic refraction technique is used to estimate
the water table depth, the saturated thickness and the
areal extent of unconfined alluvial aquifers. With
increasing target depths, data acquisition procedures
become more sophisticated while the uncertainty of data
accuracy remains at ten percent.
An advantage of this technique is that quantitative
data interpretation procedures have been developed.
Refraction data can resolve three or four layers. Dis-
advantages of this technique are restrictive data acqui-
sition requirements, depth limitations, and blind zone or
velocity inversion problems. Since data acquisition and
processing are labor-intensive, seismic refraction can be
relatively expensive.
The petroleum exploration industry has developed the
seismic reflection technique for use in detailed strati-
graphic mapping of sedimentary basins. Due to recent
technological advancements, this technique has been used
to map the hydrogeologic boundaries in shallow unconfined
aquifers. As compared to seismic refraction, this tech-
nique provides greater resolution within the saturated
zone, has slow but routine field operations, and does
not require the acoustic velocity to increase with depth.
Unfortunately, data resolution is limited to depths
shallower than 100 feet, and data acquisition and pro-
cessing techniques are still being refined. Thus, the
refraction technique is the preferred seismic method for
targets at depths less than 100 feet; but, due to tech-
nology advances, seismic reflection is gaining increased
popularity in ground-water applications.
Although the seismic refraction and the electrical
resistivity techniques measure different physical property
changes, both techniques have similar ground-water applica-
tions. As with seismic refraction, the electrical resis-
tivity technique can identify the water table and the
alluvium/bedrock interface under certain hydrogeologic
conditions. The effectiveness of each technique in these
mapping studies depends on the relative strengths of
subsurface earth property contrasts. In many cases, both
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electrical and acoustic velocity contrasts are sufficient
to allow detection by both methods. In these settings,
both techniques are sometimes applied to verify or com-
plement the results of each other.
There are differences in the applicability of
the electrical resistivity and the seismic refraction
techniques. For example, the refraction technique is
generally more accurate in delineating the water table
or the alluvium/bedrock interface. The resistivity
technique can identify lithologic variation within al-
luvial sediments. Typical resistivity surveys map the
relative positions of coarse and fine sediments or clay
layers. Electrical methods can also map conductivity
variations within ground water. When conducting an
electrical survey, it is the responsibility of the in-
vestigator to separate-electrical response of lithologic
and water quality variations.
Data acquisition, data processing, and data resolu-
tion characteristics are similar for the refraction and
resistivity techniques. Both refraction and resistivity
measurements require ground contact and are therefore
labor-intensive. Similarly, both techniques utilize
quantitative processing routines which can resolve three
or four subsurface horizons. Since data acquisition and
processing requirements are similar, survey costs are
similar. Both the similarity in cost and in applications
contribute to the frequency of integrated refraction/resis-
tivity surveys.
In addition to electrical resistivity, there are many
electrical techniques which have been applied to ground-
water studies. This document reviews three techniques
which have gained recent popularity due to technology
advances. These are the electromagnetic induction (EMI),
very-low-frequency resistivity (VLF), and ground penetrating
radar (GPR) techniques.
The EMI technique detects subsurface changes in elec-
trical conductivity (inverse of electrical resistivity).
Although EMI measures the same quantity as the electrical
resistivity technique, there are many data acquisition
and resolution characterisitics which distinguish the two
techniques.
EMI instruments are portable units which provide a
direct measurement of subsurface conductivity. Moving
the instrument along a survey traverse results in a con-
tinuous profile of subsurface conductivity. Using one or
several instruments which measure subsurface conductivity
variations at different depths, the investigator can
generate a cross-section. In general, EMI processing and
interpretational procedures can only accurately resolve
VI
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two layers. These vertical resolution restrictions com-
bined with the simplicity of data acquisition and inter-
pretation make the EMI technique most effective as a
shallow reconnaissance profiling tool.
In general, EMI measurements are more sensitive to
lateral variations than electrical resistivity measure-
ments. Likewise, the resistivity technique is more
accurate at resolving vertical variations. In spite of
these resolution differences, EMI and resistivity data
have similar ground-water applications and are often
collected over the same area.
The VLF technique is very similar to the EMI tech-
nique but instead uses low frequency radio waves as a
source. The VLF instrument is a portable unit which
measures subsurface electrical resistivity. The penetra-
tion depth of the instrument is dependent on the resis-
tivity of the subsurface like the EMI technique, there-
fore the VLF technique is well-suited for shallow recon-
naissance profiling surveys.
GPR is a reflection technique which used high-fre-
quency radio waves to continuously map shallow hydro-
geologic boundaries. GPR is a reflection technique which
measures electromagnetic contrasts and can be rapidly and
continuously acquired. Unlike seismic reflection, GPR is
limited to shallow penetration depths. Input signal
attenuation is caused by high-conductivity media such as
clay-rich lithologies. The GPR technique can map shallow
water table surfaces, but the success of this application
is highly site-specific.
The primary advantage of the GPR technique relates
to its data acquisition capabilities. The GPR instrument
is mounted on wheels and pulled across the survey area.
The resulting data profile can be viewed directly in the
field. Thus, GPR combines continuous spatial sampling
with qualitative in-field interpretation and is most
effective as a reconnaissance tool.
In the gravity method, the spatial variation of
surface gravity measurements are interpreted as subsurface
density contrasts. Since different lithologies are
characterized by different densities, the gravity method
can detect lithologic changes. Unfortunately, a single
gravity data set can be related to an infinite number of
geologic interpretations. As a result, gravity inter-
pretations must be constrained by other geologic or geo-
physical information.
The density contrast between unconsolidated alluvial
sediments and bedrock is detectable by the gravity method.
Since gravimeters are portable units, gravity data are
vii
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often used in reconnaissance surveys of valley-fill
aquifers. Gravity data can define the spatial extent of
aquifer boundaries as well as locate topographic variations
in the bedrock. Although these reconnaissance data can
be rapidly acquired, interpretation can be tedious and
inconclusive. In any case, gravity data often provide
inexpensive reconnaissance information which can supple-
ment other geophysical data in ground-water studies.
The gravity and magnetic methods measure potential
field quantities at the earth's surface and relate these
data to theoretical models. These methods have similar
data acquisition, processing, and interpretation charac-
teristics. Since water-bearing units are typically non-
magnetic, the magnetic method has only limited applications.
There are special circumstances however where the magnetic
method is applicable to ground-water studies. In most of
these cases, the magnetic method provides qualitative
data which has poor resolution and is difficult to
interpret.
In conclusion, the applicability of a particular
geophysical technique depends on the subsurface geology
and the available resources. Selecting a particular
technique often depends on what data are already available.
In most cases, geophysical data are used to supplement
surface geological and well information. Thus, when
delineating aquifers for WHP efforts, investigators may
utilize surface geophysics as a rapid, inexpensive,
companion technique to test drilling.
vm
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CONTENTS
FORWARD i
EXECUTIVE SUMMARY ii
CONTENTS . viii
1 INTRODUCTION 1
1 .1 WELLHEAD PROTECTION 1
1.1.1 Legislative Authority 1
1.1.2 Delineation Criteria & Methodologies 1
1 .2 SURFACE GEOPHYSCIAL METHODS 3
1 .3 DOCUMENT OBJECTIVES 4
2 SEISMIC METHODS , 6
2.1 SEISMIC REFRACTION 6
2.1.1 Introduction.. 6
2.1.2 Theory 6
2.1.3 Methodology. 9
2.1.4 Data Processing & Interpretation 11
2.1.5 Ground-Water Applications 11
2.1.6 Limitations 13
2.1.7 Summary 14
2.2 SEISMIC REFLECTION 14
3 ELECTRICAL METHODS 17
3.1 ELECTRICAL RESISTIVITY 17
3.1.1 Introduction.... 17
3.1.2 Theory 17
3.1.3 Methodology 18
3.1.4 Data Processing & Interpretation...... 22
3.1.5 Ground-Water Applications 23
3.1.6 Limitations 24
3.1.7 Summary 24
IX
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CONTENTS (cont'd)
3.2 ELECTROMAGNETIC INDUCTION 26
3.2.1 Introduction , 26
3.2.2 Theory 27
3.2.3 Methodology 27
3.2.4 Ground-Water Applications 29
3.2.5 Summary 29
3.3 VERY-LOW-FREQUENCY RESISTIVITY 30
3.4 GROUND PENETRATING RADAR 30
3.4.1 Introduction 30
3.4.2 Theory 31
3.4.3 Methodology 31
3.4.4 Data Processing & Interpretation 33
3.4.5 Ground-Water Applications.... 33
3.4.6 Limitations 34
3.4.7 Summary 35
4 POTENTIAL FIELD METHODS 36
4.1 GRAVITY 36
4.2 MAGNETIC 37
5 METHOD COSTS 39
5.1 INTRODUCTION 39
5.2 SEISMIC & ELECTRICAL RESISTIVITY 40
5.3 GRAVITY & MAGNETICS 41
5.4 EMI, VLF, & GPR 41
5.5 CONCLUSIONS 41
6 WHPA & AQUIFER DELINEATION 42
6.1 INTRODUCTION 42
6.2 AQUIFER ASSESSMENT 42
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CONTENTS (cont'd)
6.2.1 Reconnaissance. 40
6.2.2 Detailed Site Investigation 41
6.3 WHPA DELINEATION 42
REFERENCES 44
xi
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CHAPTER 1
INTRODUCTION
1.1 WELLHEAD PROTECTION
1.1.1 Legislative Authority
The Safe Drinking Water Act Amendments of 1986 require
the States to delineate Wellhead Protection Areas (WHPA)
for all public water wells. A grant program is included
to assist the States in this effort. A WHPA is defined
by the Amendments as "the surface and subsurface area
surrounding a water well or wellfield, supplying a public
water system, through which contaminants are reasonably
likely to move toward and reach such water well or well-
field." Although the statute gives this careful legal
definition of a WHPA, it does not specify WHPA delinea-
tion approaches. States are allowed maximum flexibility
in designing WHPA programs. The U. S. Environmental
Protection Agency's (EPA) Office of Ground-Water Protection
(OGWP) has issued the technical guidance, "Guidelines for
Delineation of Wellhead Protection Areas." The Guidelines
outline a variety of criteria and methodologies believed
most effective in establishing WHPA's.
1.1.2 Delineation Criteria and Methodologies
In general, a WHPA consists of all or part of the
zone of influence (ZOI) or the zone of contribution (ZOC)
around a pumping well. Figure 1 presents the ZOI and the
ZOC for an unconfined aquifer system. The ZOI is the
area on the earth's surface overlying the pumping well's
cone of depression. The ZOC consists of the entire flow
system which contributes water to the well. The hydro-
geologic boundaries of the ZOC are defined by ground-water
divides, surface water features, and permeability contrasts,
A WHPA is established within or including a ZOI or a
ZOC according to specific conceptual standards or criteria.
These criteria are determined by the protection goals of
the WHP program. For example, States may conceive of a
WHPA as: 1) remedial action zones which protect wells
from unexpected contaminant releases, 2) attenuation zones
which provide for the proper assimilation of the conta-
minant before it reaches the wellhead, or 3) wellfield
management zones which apply land use restrictions to
recharge areas.
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^. GROUNDWATER
f DIVIDE
PREPUMPING
WATER LEVEL
CONE OF
DEPRESSION
[A] VERTICAL PROFILf
DRAWDOWN
CONTOURS
[B] PLAN VIEW
LEGEND:
V Water table
• Ground-water Flow Direction
* Pumping Well
ZOI Zone of Influence
ZOC Zone of Contribution
Figure 1: Terminology for WHPA delineation (after Guidelines
for Delineation of Wellhead Protection Areas, U.S.
EPA, 1987)
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All but one of the WHPA criteria outlined in the
Guidelines require subsurface data for their determin-
ation. This criterion is the distance criterion which
is a simple distance measurement from the well. The
hydrologically-based criteria are: drawdown, time of
travel, flow boundaries, and assimilative capacity.
The delineation methods outlined in the Guidelines
range in complexity from the very simple arbitrary fixed
radius method which consists of drawing a circle around
a well to the numerical flow model method which uses
high-speed computers to predict ground-water flow. The
methods of intermediate complexity are the: calculated
fixed radius, simplified variable shapes, analytical flow
model and hydrogeologic mapping methods. The reader
should consult the Guidelines to familiarize himself with
the details of the WHPA criteria and methods.
Hydrogeologic mapping is the WHPA delineation method
which uses geologic, hydrologic, geophysical, and tracing
observations to map the flow boundary criterion of an
^aquifer system. This document, "Surface Geophysical
Techniques for Aquifer and Wellhead Protection Area
Delineation," discusses surface geophysics as one mapping
technique within the hydrogeologic mapping method. This
document is designed as an auxiliary paper to EPA's
technical Guidelines.
In the example of Figure 1, the flow boundary
created by the ground-water divide could be selected as
the upgradient limit of the WHPA. This flow boundary
criterion could then be mapped to the ground by the
application of one or more WHPA delineation methods.
In this case, the WHPA would correspond to the ZOC
outlined in Figure 1.
1.2 SURFACE GEOPHYSICAL METHODS
Surface geophysics has been developed by the mining
and petroleum industries to measure subsurface physical
property contrasts. More recently, geophysical techniques
have been used by the environmental community to detect
buried waste and waste migration (Benson et al., 1982;
Olhoeft, 1986; Walther et al., 1986). These geophysical
methods map the earth's response to either natural or
artificially generated energy fields. The parameter
changes that these methods measure are caused by subsur-
face elastic, density, electrical, or magnetic constrasts.
Since the earth is a complicated system, geophysical
methods are interpreted using models that treat the
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subsurface according to simplifying assumptions. Survey ,
data are acquired and interpreted in accordance with these
assumptions, and the accuracy of these interpretations
depend on how well the actual geology agrees with the assumed
model. Subsurface interpretations are generally improved
when information from test borings or observation wells is
available to constrain the data sets.
Surface geophysical technologies are commonly applied
in reconnaissance ground-water surveys that identify the
major flow boundaries in unconfined aquifers. These
surveys may integrate data from several field methods.
For example, seismic refraction and electrical resistivity
data are often collected over the same area to verify and
complement the results of each other.
In addition to reconnaissance information, geophysics
can also be used to obtain detailed subsurface information;
typical surveys may result in a map of the hydrologic,
lithologic, or salinity changes in alluvial aquifer systems.
In some cases, surface geophysical studies are tailored
to the geological complexities of the survey area. For
example, although the magnetic method cannot generally
detect changes in alluvial aquifers, it has been used to
map the subsurface extent of some basaltic aquifers. In
general, the applicability of a particular technique
depends on the relative magnitudes of subsurface physical
property changes.
1.3 DOCUMENT OBJECTIVES
In this document, the principles and methodologies
are presented for some of the surface geophysical tech-
niques which have been applied in ground-water investi-
gations. The text is divided by section according to
technique, and the level of detail in each section cor-
responds to the relative utility of that technique with
regards to aquifer assessment as reflected in the liter-
ature.
The principal techniques used in ground-water studies
have been seismic refraction and electrical resistivity.
As a result, these data acquisition and processing technol-
ogies have been refined, and there is a substantial liter-
ature describing these procedures. Both seismic refraction
and electrical resistivity are presented at the beginning
of this document so that fundamental geophysical concepts
can be introduced. It is expected that the reader will
acquire a basic understanding of geophysical principles
by reading these sections. These principles are relevant
to the other techniques, although they are not discussed
at the same level of detail. This document focuses on
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geophysical techniques that have found increased ground-
water applications based on new technology advances
(Collett and Haeni, 1987; Walther et al.,~ 1986; Dobecki
and Romig, 1985).
It is assumed that the reader of this document has a
technical background in geology or engineering hydrology,
and some knowledge of geophysics. This document is not
a textbook on geophysical principles or a cookbook on
how to perform a geophysical survey. Instead it is
intended to serve as a resource document that introduces
the reader to some of the ground-water applications of
geophysical techniques. The reader should use this as
one starting point when investigating the various aquifer
delineation methods that can assist in defining Wellhead
Protection Areas.
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CHAPTER 2
SEISMIC METHODS
2.1 SEISMIC REFRACTION
2.1.1 Introduction
The seismic method was developed for use in petroleum
and mineral exploration as well as in engineering studies.
As pointed out by Haeni (1986a), recent technological
developments have made the seismic refraction technique
highly effective and economical for obtaining data for
ground-water investigations. Seismic refraction is most
often used to delineate the hydrogeological boundaries
characterized by high-velocity surfaces in unconfined
aquifer systems. In many of these aquifer settings,
refraction data can determine: the depth to the water
table, the saturated thickness of the aquifer, the allu-
vium/bedrock contact, and the spatial limits of the
aquifer. These results often provide the information
necessary for the development and implementation of a
test drilling program.
2.1.2 Theory
Seismic refraction is a surface geophysical technique
which attempts to obtain information about shallow sub-
surface geologic formations. In the seismic techniques
considered in this document, geologic formations are
modeled as acoustic layers. The seismic refraction
technique consists of introducing seismic energy into the
ground and recording the arrival of direct or refracted
sound waves at various distances, along the earth's surface.
This seismic energy travels in each layer of the earth with
a characteristic velocity as a pressure or compressional
wave (see Figure 2). The speed of these compressional
waves depends on the density and the compressibility of
the geologic formation. Thus, by measuring compressional
wave velocities and the travel times of the sound waves,
scientists can infer the nature and depths of the sub-
surface geologic units. Usually this subsurface inter-
pretation is dependent on other geological or geophysical
information. The success of this technique in defining
lithologic and hydrologic boundaries depends on how
accurately the layered acoustic model simulates the
actual geology.
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TIME
T1
DIRECT
ARRIVALS,
I
•SLOPE=1/V1
SHOTPOINT
SLOPE
I
FIRST REFRACTED ARRIVAL
RECEIVERS
DISTANCE
GROUND
SURFACE
DIRECT
ARRIVALS
LAYER 2
REFRACTING INTERFACE
[Vj < V2]
V2
Figure 2: Schematic representation of a single interface
seismic refraction experiment and the corres-
ponding time-distance plot (after Costello, 1981)
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The impulsive source which initiates the seismic
energy is located at or near the earth's surface. A
linear array of seismic sensors or geophones located some
distance away on the earth's surface detects the arrival
of these sound waves. If the compressional wave velocity
of each layer increases with depth, then seismic energy is
refracted along each subsurface interface. The seismic
waves refracted along these interfaces continue to release
energy upward into the lower velocity layers. These
refracted waves are detected by the geophones on the
surface.
When interpreting the recorded data, only the direct
wave travelling in the surface layer and the waves refrac-
ted from deeper interfaces are considered. The reflected
energy and other elastic wave components are not relevant
in a refraction survey. Figure 2 represents a cross-section
of a layered acoustic earth with ray paths representing
the direct and refracted waves travelling from source to
receiver. This linear array of receivers is connected to
a multichannel field recording system. Plots of the
recordings of the first arrival from each channel constitute
a time-distance profile. Therefore, refraction data
consists of: 1) the travel times of the direct and refracted
waves from source to receivers, and 2) the associated
distances between the source and the receivers. From
these times and distances, lithologic thicknesses and
velocities are calculated.
The relative times of arrival of the refracted and
the direct waves depend on the subsurface velocity distri-
bution and the source-receiver spacing. For a close
source-receiver spacing, the first wave to arrive at the
receiver is the "direct wave" travelling in the near-surface
layer. The next recorded pulse is the wave which travels
from the source through the first layer, refracts along
the higher velocity interface, and travels up through the
near surface layer to the receiver. As the source-receiver
spacing is increased, the refracted wave eventually
reaches the receiver at the same time as the direct wave.
The distance which corresponds to the simultaneous arrival
of the direct and refracted waves is known as the "cross-
over distance." For all source-receiver spacings beyond
this distance, xc, the refracted wave reaches the receiver
before the direct wave. At increasingly greater receiver
spacings, the refracted waves from the deeper interfaces
may become the successive first arrivals.
From the plot of first arrivals against their corres-
ponding source-receiver distances, the subsurface velocity
configuration can be inferred. The time-distance plot for
the single interface horizontal geometry is shown in
Figure 2. In this plot, the two intersecting lines
8
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represent arrivals from each of the two layers. The line
passing through the origin corresponds to the direct wave
which travels through the surface layer while the other
line represents the arrivals which have refracted along
the top of the higher velocity medium. This plot provides
graphical information which allows an inference on the
subsurface velocity structure. The reciprocals of the
slopes of these lines are the media velocities, and their
graphical point of intersection on the distance axis is
the crossover distance. The crossover distance and the
layer velocities lead to a calculation of the interface
depth for a horizontally layered earth:
D = Xc/2 • [(V2 - V1)/(V2 + V})} 1/2
Thus, these refraction data can characterize the subsur-
face velocity configuration of this simple model. For
multi-interface and dipping models, similar but more
complex depth "inversion" equations can be derived.
Graphical and analytical solutions for some of these
more complicated models are presented by Haeni (1986b)
and Dobrin (1976).
2.1.3 Methodology
In a seismic refraction survey, the acoustic energy
generated by the seismic source is converted to an elec-
trical signal by the seismic sensors. This signal is then
amplified, filtered, and recorded. The specific energy
source, geophone array, and recording system are chosen
according to the objectives of the survey.
The type of energy source utilized in a refraction
survey depends on the targeted depth of penetration. For
target depths less than 100 feet, a hand-held impact ham-
mer is commonly used. For intermediate depths between
100 and 300 feet, various explosive and non-explosive
technologies are available. For depths greater than 300
feet, explosives are required.
The selection of a sound source requires both econo-
mic and scientific considerations. For instance, a
hammer source can provide inexpensive shallow data in a
minimum of time (see Underwood et al., 1984). In contrast,
buried charges produce data at a greater cost and time
commitment (Tucci and Pool, 1987). The advantages and
disadvantages of the various seismic refraction sources
are discussed by Haeni (1986b).
-------
The geophones which react to the impinging seismic
energy are connected by cables to the recorder. These
spread cables are multiconducting with geophone "takeouts"
located incrementally along their length. The geophones
are arranged linearly over the target area, and their
spatial arrangement with respect to the shot determines
the subsurface coverage of the recorded data. This can
be understood by tracing the ray paths from the source to
the receivers according to Snell's law.
In a typical refraction survey, the distance between
the shot and the geophones is such that the refracted
wave is the primary recorded wave component. The geophone
spread length is three to five times the maximum depth of
interest. Usually the geophone spread is advanced along
a line to obtain a profile of continuous coverage. Many
times however, shots are fired in the middle of the
linear array to acquire near-surface information. Deep
information is obtained by "shooting" off either end of
the spread. Shooting data from both directions produces
forward and reverse traveltime plots. This procedure is
known as reversed profiling. Reversed profiling data
can be inserted in the appropriate inverse equations to
search for the dip of the geologic formations. In order
to achieve more accurate assessments of lateral velocity
changes, reversed profiling is an absolute requirement.
The spacing between adjacent geophones determines
the resolution of the data, with closer geophone spacings
providing higher resolution. Common geophone spacings
range between 15 and 50 feet. When a refraction survey
is initiated in a new area, the geophones are usually
spaced close to each other to allow construction of
a highly detailed time-distance curve. This information
helps to define the spread geometry necessary to map the
lithologies of interest.
The recording system used in a refraction survey
can contain between 1 and 24 channels. The signal trans-
mitted by each geophone is amplified and filtered before
it is recorded. The filtering of seismic data is designed
to selectively eliminate the temporal frequencies associ-
ated with seismic noise. Noise is defined as any form of
seismic energy which is generated by natural or cultural
sources and interferes with the signal. The final output
of these amplified and filtered signals is typically dis-
played on an oscilloscope, or recorded on heat sensitive
paper or magnetic tape for later processing.
A minimum of three people are necessary to efficiently
run a seismic refraction survey. Such a crew could com-
plete three or four reversed refraction profiles in a
typical work day. Usually four-wheel drive vehicles are
used to carry equipment and personnel. In some surveys
10
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with deep target depths, an explosive truck and a drill
rig are necessary. In general, the more complicated the
field system or the deeper the target, the more experienced
and numerous the field crew. The details of seismic
refraction survey field procedures are outlined by Haeni
(1986b).
2.1.4 Data Processing and Interpretation
Raw refraction data must be processed before a sub-
surface interpretation can be made. One essential pro-
cessing step is the systematic elimination of travel time
errors introduced by topographic variations. This pro-
cedure is known as data reduction. Once the data have
been reduced, then the arrival times are plotted against
source-receiver distances. Finally, the time-distance
data are inverted to form a subsurface velocity structure.
Well logs or core information from the survey area can
insure more accurate interpretations and are highly
desirable.
This entire process from data reduction to inter-
pretation may be automated by digital seismographs and
microcomputers, but an experienced geophysicist is
required to insure the accuracy of these procedures.
Many of these processing systems can accomplish these
tasks while still in the field. The degree of automation
depends on the scale of the survey. If many refraction
lines are anticipated, then it may be more cost efficient
to program a computer to reduce the data, pick the travel
times, plot the points, fit the lines to the plotted
points, and offer an interpretation. For smaller scale
surveys, a programmable hand calculator is often adequate.
2.1.5 Ground-Water Applications
Seismic refraction surveys are most useful in mapping
high velocity contrasts such as the water table, bedrock
surfaces, and some lithologic changes. In general, any
shallow geologic feature which has a velocity increase of
greater than 20 percent can be detected by seismic refrac-
tion. This condition is usually satisfied in sedimentary
environments or along the contact between sedimentary
lithologies and basement. Haeni (1986b) discussed the
effectiveness of the refraction method for many hydrogeo-
logic settings.
Unconsolidated surficial materials commonly occur as
valley-fill deposits in channeled bedrock. Such geologic
features are known as buried valleys or buried channels.
The contact between these alluvial materials and bedrock
11
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forms an important flow boundary; and in temperate regions,
these deposits provide an extensive and valuable source
of ground water.
Seismic refraction techniques are useful in defining
the areal extent of channel deposits due to the consider-
able velocity contrast which sometimes characterizes the
bedrock/alluvium interface. One approach for delineating
valley-fill aquifer deposits is to complete several pro-
filing surveys in parallel sequence. Mapping the contact
for each profile and extrapolating between profiles
produces an areal map which reveals the channel. Survey
maps are presented as contour maps or profiles.
The literature is replete with examples concerning
bedrock surface mapping as a means to delineate valley-
fill aquifers. In two survey papers, Zohdy et al (1974),
and Sendlein and Yazicigil (1981) describe some typical
bedrock surface mapping surveys which have appeared in
the literature. These mapping procedures apply identical
principles although they differ in scale, equipment,
available data, and specific objectives. Other inter-
esting investigations not included in these two papers
are presented by Tucci and Pool (1987), Sverdrup (1986),
Underwood et al. (1984), Frohlich (1979), and Sander
(1978). Also, Haeni (1986b) provides an annotated
bibliography for refraction applications to ground-water
investigations. In general, the literature provides
information on the practicality of this technique as well
as a glimpse at the available technology.
The velocity increase occurring at the interface
between the saturated and unsaturated zones in an uncon-
fined aquifer is usually detectable by refraction pro-
filing techniques. In fact, water table refractor
information is often implicit in valley-fill aquifer
mapping data. Some surveys however focus primarily on
obtaining water table information. The ability of
refraction techniques in detecting the water table depends
on the size of the velocity increase at the saturated zone.
In some cases, the acoustic properties of the saturated
zone are not sufficiently different from the surrounding
media to allow water table detection. A more serious
obstacle results when a lithologic refractor lies directly
beneath the zone of saturation. When this occurs, the
water table refractor does not occur as a separate branch
on the time-distance curve, and the zone of saturation
cannot be located. This situation commonly arises when
the water table in an unconfined alluvial aquifer lies
just above bedrock. The significance of the vertical
separation between the two refractors becomes less
critical when the lithologic acoustic contrast is less
12
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severe. Despite these obstacles, seismic refraction
is an rapid and cost-effective technique for both the
delineation and the areal mapping of the water table in
unconfined aquifers.
Seismic refraction can also be used to define strat-
igraphic boundaries which often correlate with permeability
or flow boundaries. These stratigraphic surveys are more
ambitious than bedrock or water table mapping surveys;
they generally require high resolution data and some
subsurface information. Examples of these surveys are
discussed by Sendlein and Yazicigil (1981).
2.1.6 Limitations
In the simplest use of the seismic refraction tech-
nique, the subsurface geologic formations are modeled as
homogeneous acoustic layers whose compressional wave
velocities increase with depth. These requirements are
satisfied in most sedimentary environments, but deviations
must be accounted for in subsequent interpretations. For
example, glacial or alluvial deposits usually include
sand and clay lenses or channel deposits. These geologic
features must be accounted for when both collecting and
interpreting the data.
The alluvium/bedrock interface as well as the
unsaturated/saturated zone in unconsolidated deposits
are characterized by a velocity increase. Occasionally,
the geologic configuration is such that a low-velocity
layer occurs beneath a high-velocity layer. This
situation is known as "velocity inversion." The energy
refracted from this type of interface travels downward
into the subsurface and cannot be detected by the geophones,
As a result, the time-distance plot leads to an incorrect
determination for that layer velocity and layer depth.
This situation commonly occurs when glacial till overlies
finer-grained, unconsolidated glacial sediments.
The seismic refraction method also requires that the
stratigraphic layers be of sufficient thickness, and that
successive layers form a sufficient velocity contrast to
allow their detection. Many times however, a refracting
layer is too thin to be detected by the seismic method,
but thick enough to cause a significant change in travel
time. This undetected layer is known as a "blind zone."
Blind zones are common in geologic formations which
contain high-velocity limestone sequences. In many
surveys areas, well logs or core .information may be used
to estimate velocity structure so that blind zones may be
recognized and the time data corrected. Sander (1978)
has provided a detailed discussion on the implications of
blind zones on the interpretation of refraction data.
13
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Haeni (1986b) and Zohdy et al. (1974) examine typical
geologic settings which depart from the simplified strat-
ified model. Among other things, they consider blind
zones, velocity inversions, multi-layered media, and
dipping layers. They present the time-distance plots
which correspond with these geologic features. These
documents are important resources which can be consulted
when interpreting refraction data. In general, the
refraction technique most accurately models three or four
flat layers which are characterized by substantial velocity
increases.
2.1.7 Summary
Seismic refraction is an effective means for mapping
the subsurface hydrologic conditions of some unconfined
aquifers. Refraction data can provide estimates to
within ten percent on: the depth to the water table,
the areal extent of the aquifer, the saturated thickness
and the hydraulic gradient. Refraction data are often
used to correlate between wells or to extrapolate well
information into new terrain. Thus, the refraction
technique provides an economical supplement to extensive
test drilling programs and is well-suited for mapping the
flow boundaries in WHPA programs.
2.2 SEISMIC REFLECTION
The seismic reflection technique utilizes the same
principles as seismic refraction but measures the reflected
instead of the refracted component of the acoustic waves
(see Figure 3). Since the receivers are close to the sound
source, sound waves reflected from shallow horizons and the
surface waves arrive at the receivers at the same time.
As a result, near-surface lithologic changes are not
clearly defined in the recorded response. In general, the
seismic reflection technique is most effective in mapping
reflecting horizons below 100 feet. Shallower targets are
usually better handled by refraction.
The primary application of seismic reflection has been
in petroleum exploration where exploration targets are
thousands of feet deep. The petroleum industry has developed
sophisticated data acquisition and processing techniques
which enhance reflection data at the expense of the near-
surface response. The seismic reflection technique has not
been routinely used in shallow aquifer studies, but has
had some application in ground-water studies where probing
depths exceed 100 feet.
14
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SHOTPOINT
RECEIVERS
GROUND
SURFACE
LAYER 1
LAYER 2
REFLECTING INTERFACE
J
Figure 3: Schematic representation of a single interface
seismic reflection experiment.
15
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The processing and acquisition of seismic reflection
data is complicated and will not be discussed here. The
rudiments of reflection data acquisition and processing are
discussed by Telford et al. (1976). In general, reflection
data are more expensive to collect than refraction data.
Spread lengths are shorter, sources are highly sophisti-
cated, and the acquisition procedure is routine. Also,
the reflection method does not require the velocity
structure to increase with depth which reduces thin bed
and blind zone problems.
Recent research efforts have focused on increasing
the applicability of seismic reflection to shallow aquifer
mapping studies. These surveys minimize the deleterious
surface wave component of the seismic wave by using an
optimal travel time window. Such parameters are site
specific and must be determined in the field.
Shallow aquifer reflection mapping studies offer
innovations in both acquisition and processing procedures.
For example, Hunter et al. (1982, 1984) have used a multi-
channel engineering seismograph and a hammer source to
acquire reflection data which they process with algorithms
developed on microcomputers. Work is also being done to
improve the useful frequency range of shallow reflection
data (Knapp, 1986a, b). In a another related study,
Haeni (1986c) has used continuous seismic reflection
profiling techniques to locate hydrogeologic boundaries
in formations beneath water covered areas. After review-
ing these new advances in the light of conventional
technologies, Dobecki and Romig (1985) predict that the
reflection technique will have more widespread application.
16
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CHAPTER 3
ELECTRICAL METHODS
3.1 ELECTRICAL RESISTIVITY
3*1.1 Introduction
The electrical resistivity technique measures varia-
tions in subsurface electrical resistivity and has been
employed in both mineral exploration and ground-water
investigations. In an electrical resistivity survey,
separate electrode pairs are used to inject a current in
the earth and measure the resulting potential difference.
Varying the positions and configurations of these electrodes
provides information about subsurface resistivity variations.
As in seismic methods, the electrical resistivity inter-
pretation is based on a stratified earth model. Resistivity
data can be used to infer the lithologic and the hydrologic
characteristics of the survey area. There are many publi-
cations discussing the rudiments of electrical resistivity.
Three papers which relate resistivity techniques to ground
water are presented by Senglein and Yazicigil (1981), Zohdy
et al. (1974), and Bisdorf (1985).
3.1.2 Theory
Electrical resistivity is the measure of a material's
resistance to the flow of an electrical current. Since most
geologic materials behave as electrical insulators, surface
measurements of earth resistivity are governed by the
electrolytic properties of interstitial water. The subsurface
distribution of water is controlled by the porosity of the
formations. Thus, resistivity values generally are governed
by the amount of porosity and the degree of water saturation
in a geologic formation. Additionally, clay minerals are
capable of conducting electricity; and as a result,
geologic materials which contain significant amounts of
clay minerals have relatively lower resistivities. For the
most part though, it is the amount and conductivity of pore
water and not the rock matrix of a formation which controls
resistivity values.
The electrical resistivity technique consists of intro-
ducing an electrical current (I) into the ground and measuring
a voltage response (V) along the earth's surface. The
subsurface resistivity can be calculated from the injected
current and the measured voltage. Conventional resistivity
methods utilize four electrodes. A direct or low-frequency
current is injected into the ground by a pair of current
17
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electrodes C1 and C2, and the resulting potential difference
AV is measured by a potentiometer at a pair of potential
electrodes P1 and P2 (see Figure 4). From these data, the
resistivity (r) is calculated according to:
KAV/I
where K is a geometric factor based on the relative posi-
tions of the electrodes. The calculated resistivity is
the true resistivity if the geologic medium is electrically
homogeneous over a volume which is large compared to the
electrode spacing. This is usually not the case, and the
measured resistivity is known as the apparent resistivity.
3.1.3 Methodology
r Resistivity surveys are designed to independently
resolve the horizontal and vertical components of sub-
surface resistivity variations. The horizontal profiling
procedure measures lateral resistivity variations while
depth soundings measure vertical resistivity variations.
There are many different electrode configurations which
have been developed according to different survey objec-
tives. Three common configurations are the Wenner,
Schlumberger, and dipole-dipole arrays. The specific ,
geometric configurations and K formulas for these arrays
are given in Figure 5. The Wenner and Schlumberger
arrays are commonly used for vertical sounding investi-
gations. The dipole-dipole array is often used for
lateral investigations and is sometimes used for deep
sounding investigations.
Vertical sounding are made by symmetrically expanding
the current electrodes for the Wenner or Schlumberger
arrays along a line about the array center. This procedure
is based on the fact that greater and greater electrode
spacings probe deeper and deeper beds. In general, the
maximum electrode spacing should be three to four times
the depth of investigation to allow proper penetration of
the current.
Horizontal profiling is generally performed by moving
a fixed-spacing array incrementally along the earth's
surface and recording the apparent resistivity values.
This fixed spacing is at least one to two times the
depth of interest. A simple plot of these values against
their spatial coordinate reveals lateral resistivity
variations for a given depth. If several of these profiles
are performed in a parallel sequence, then the data may
be contoured to give an areal map of resistivity variations.
Sounding measurements are commonly made in conjunction
18
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CURRENT
SOURCE
CURRENT METER
EARTH'S
SURFACE
CURRENT FLOW
THROUGH EARTH
______ Current
—_— Voltage
Figure 4: Typical four electrode electrical resistivity
arrangement (after Benson et al., 1982).
19
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K = 2 IT A
I
WENNER ELECTRODE ARRANGEMENT
c
1
1
1
1
p 1 p
c
M
N
** ..... L * L *"
SCHLUMBERGER ELECTRODE ARRANGEMENT
c c
.*>-« a
K = IT IV
, tr
DIPOLE DIPOLE ELECTRODE ARRANGEMENT
Figure 5: Electrode configurations for the various
electrode arrays. P represents the potential
electrodes and C the current electrodes. K
is the geometric factor used to convert the
measured quantities to apparent resistivity
values (after Benson et al., 1982).
20
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with profiling data in order to determine the appropriate
array spacing or target depth.
A second profiling method utilizes the dipole-
dipole array. In this procedure, measurements are made
with one of the electrode pairs held fixed while the
other is moved incrementally along a line. The fixed
electrode pair is then advanced down the line, and the
procedure is repeated. This process yields both lateral
and depth information which can be presented as a cross-
section.
Zohdy et al. (1974) have discussed these arrays.
After comparing the Schlumberger array to the Wenner
array, they conclude that the Schlumberger array requires
less field effort, offers somewhat greater resolution, is
less sensitive to noise and lateral homogeneities, and is
suited to more sophisticated interpretation techniques.
The Schlumberger method however requires more current
to achieve the same potential difference (Fretwell and
Stewart, 1981). In a more recent work, Carrington and
Watson (1981) have conducted both laboratory and field
experiments comparing nine different electrode configur-
ations. Dobecki and Romig (1985) have suggested that
this wide variety of electrode configurations may have
hindered geophysicists by confusing the issue of the
appropriate geometry for a specific field problem.
The equipment for a resistivity survey consists of
electrodes, cables, a current source, and a potentiometer.
The specific equipment selected for a survey depends on
survey objectives and field conditions. There are many
source and receiving technologies commercially available.
In general, the degree of sophistication increases as the
target depth increases. To probe to depths of 150 to 300
feet, portable battery-powered units are sufficient;
deeper surveys require generator-powered transmitters.
Most current sources provide a direct reading of the
transmitted current, and some receivers have signal-
enhancing circuitry.
In electrical resistivity surveys, noise is defined
as any unwanted electrical current or voltage which
interferes with the signal. Equipment-related sources,
cultural sources, and natural sources are responsible for
this noise. Equipment-related noise is generated by
insufficient ground-electrode coupling, or by any of the
other electrical contacts associated with the cables.
Cultural noise is caused by stray currents related to
such things as power lines, and metallic objects. Natural
sources are ca\ised by spontaneous potential and other
earth currents. Noise can be reduced by careful field
procedures and filter mechanisms. For instance, some
potentiometers contain special circuitry which filter
power line noise.
21
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There are many types of electrodes used in electri-
cal resistivity surveys. For shallow surveys, stainless
steel or copper rods are often used as current electrodes
For deep surveys, buried copper screens or culverts are
often used. Stainless steel rods or non-polarity elec-
trodes (porous pots) are used to measure the potential
difference. For sounding surveys, the current and poten-
tial electrodes are made of the same material. For
profiling or dipole-dipole surveys, non-polarized elec-
trodes are used. Once in place, the soil around the
electrodes is sometimes soaked with water to improve
electrical contact. The cables which are connected to
the source electrodes carry the full input current and
are well insulated. The cables connected to the poten-
tiometer do not carry high currents and are of a lighter
grade. A minimum of three field personnel are usually
required to complete a resistivity survey.
3.1.4 Data Processing and Interpretation
As an initial processing step, field measurements
must be converted to apparent resistivity values. This
is accomplished by forming the ratio of observed potential
difference to input current and weighting the result by
the appropriate geometric factor. For profiling data,
these values are plotted as a profile or contoured on a
map. For sounding data, these resistivity values are
plotted against electrode spacing in a logarithmic
format. These procedures are collectively known as data
reduction.
Converting geophysical sounding data to a subsurface
earth model is known as inversion. In this process, the
apparent resistivity plots are compared to theoretical
curves which are calculated from stratified earth models
(Koefoed, 1979). This comparison procedure is commonly
done automatically by a computer (Zohdy, 1973). This is
often an iterative process whereby the differences between
the observed and the theoretical curves are progressively
minimized according to established criteria such as least
squares error (Anderson, 1979). The theoretical model
data which correspond to the observed curve are inferred
to be the subsurface resistivities and thicknesses of the
discrete layers.
Before the widespread use of computers, sounding
data were inverted visually by matching the actual data
to a set of "master curves" which were calculated for a
wide range of layer thicknesses and resistivity contrasts
(see Orellana and Mooney, 1966). Curving-matching, how-
ever, requires a high level of expertise, and sounding
data are generally inverted and by automated techniques.
22
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3.1.5 Ground-Water Applications
Since the electrical resistivity values are largely
controlled by the spatial and salinity distributions of
interstitial water, this method Is well-suited to ground-
water studies. As stated above, the spatial distribution
of water in these systems is controlled by lithologic or
porosity changes. Superimposed on these lithologic
changes are salinity variations within the water. Thus,
some surveys measure lithologic or porosity variations
(Poole, 1986; Heigold et al., 1984; Page, 1968) while
others map the location and migration of buried waste,
or the salt water/freshwater interface (Stewart et
al., 1983; Sweeney, 1984; Rudy and Caoile, 1984; Gorhan,
1976). Usually, both lithology and water quality vary
simultaneously which results in interpretational problems.
It is the responsibility of the interpreter to assess
the validity of survey assumptions.
Although the electrical resistivity and seismic re-
fraction techniques measure different physical properties,
their results may be complementary for ground-water
studies. Refraction and resistivity data are often applied
to the same survey area to resolve the ambiguities inherent
in each data set (Tucci and Pool, 1987, Denne el al., 1984;
Underwood et al., 1984; Frohlich, 1979; Wachs et al.,
1979). Each technique has its own strengths or weaknesses
depending on the geology of the survey area. For example,
the resistivity technique is particularly useful in de-
tecting clay layers in alluvial sequences whereas the
refraction technique is often more accurate in determining
the depth to the water table. In general, the applicability
of each method depends on the geology of the survey area,
the availability of well data, and the objectives of the
survey.
Resistivity values are always site-specific, and
so the reliability of subsurface interpretations depends
on subsurface information. If well logs are available,
then resistivity sounding data are correlated with these
data. This approach can lead to accurate inversions
which often incorporate lithologic information. Many
resistivity surveys result in geoelectric correlations
which distinguish clay from sand, coarse from fine,
and saturated from unsaturated sediments. Some researchers
have even concluded that resistivity measurements can
be used to estimate the hydraulic properties of aquifers
(Kelly and Reiter, 1984; Kosinski and Kelly, 1981).
23
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3.1*6 Limitations
The inversion of resistivity sounding data does not
produce a unique model. In general, a resistivity data
set can be related to an unlimited number of stratigraphic
models. For example, a thin resistive layer may create
the same response as a thicker, less resistive layer.
This is the "equivalence" problem. In many cases, res-
trictions can be placed on layer thicknesses and resis-
tivity contrasts, based on geologic or other geophysical
information such as resistivity logs. These controls can
be entered as constraints in some computerized inversion
routines and can lead to a more accurate inversion.
In addition to interpretative limitations imposed
by the nonuniqueness of solutions, the accuracy of the
resistivity method is also limited by other factors.
The resistivity method is most successful when resis-
tivity contrasts are at least twenty percent. However,
extremely conductive or resistive layers can dominate
the measured response and often preclude further elec-
trical penetration. Common examples of these barriers
are coal seams and the salt water/fresh water interface.
In general, this method's ability to resolve resistivity
contrasts decreases with increasing depth.
3.1.7 Summary
As with seismic refraction, the electrical resisti-
vity technique is used to map hydrologic and lithologic
boundaries in unconfined aquifer systems. In settings
where stratigraphic units are composed of distinctly
different resistivities, this technique can identify
lithology. Resistivity data have successfully defined:
the depth to the water table, the depth and thickness of
clay layers, the relative positions of coarse and fine
sediments, and the depth to the salt-water/fresh-water
interface. Thus, electrical resistivity is another
surface geophysical technique which can provide data for
WHPA delineation programs.
3.2 ELECTROMAGNETIC INDUCTION
3.2.1 Introduction
The electromagnetic induction (EMI) method was devel-
oped for use in mineral exploration and has been applied
to ground-water investigations. These applications
include salt water intrusion studies (Stewart, 1982) and
aquifer mapping studies (Haeni, 1986d). With the
24
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enforcement of new environmental standards, EMI surveys
nave also been applied to contaminate mapping investi-
gations (Sweeney, 1984; Rudy and Caoile, 1984; Grady and
naeni, 1984). The recent popularity of the EMI method
relates also to the development of new technology which
provide for rapid data acquisition and interpretational
procedures.
3.2.2 Theory
The electromagnetic induction method uses coils
instead of electrodes to induce current into the earth
without ground contact. A time-varying magnetic field is
generated by the transmitting coil, and this primary
field induces currents and a secondary EM field in the
subsurface. This secondary EM field is measured by the
receiving coil as a voltage (see Figure 6). These
voltage measurements are related to the subsurface elec-
trical conductivity (inverse electrical resistivity).
EMI conductivity measurements reflect the cumulative
response of the materials stretching from the earth's
surface to an approximate depth known as the "penetration
depth." Surface materials contribute more to these bulk
measurements than do the deep materials. Thus, the
electromagnetic technique respond to the same subsurface
variations as the electrical resistivity technique, even
though each method weights the contributions from each
subsurface region differently.
3.2.3 Methodology
Terrain conductivity meters or low induction number
instruments provide direct readings of conductivity
in the field. For these instruments, the penetration
depth of the EM field is a function of the coil spacing,
the transmitter frequency and the coil orientation. For
profiling measurements, all three variables are held
fixed and the coils are moved incrementally along the
survey traverse. These instruments are designed for
this procedure as they have fixed coil spacings and
employ frequencies that are sufficiently low for the
depth of penetration to be independent of frequency.
In general, the sampling depth of these instruments is
roughly three quarters to one and one-half times the
coil spacing, depending on instrument orientation. Coil
spacings range from 3 to 122 feet. In these profiling
surveys, data is collected at discrete stations, and the
resulting conductivity values are plotted as a function
of position. The resulting pattern is a map of subsurface
conductivity.
25
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TRANSMITTER
PRIMARY FIELD
COIL
GROUND SURFACE
INDUCED
CURRENT
LOOPS
SECONDARY FIELDS
FROM CURRENT LOOPS
SENSED BY
RECEIVER COIL
Figure 6: Schematic diagram of EMI method (after
Benson et al., 1982).
26
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EMI has also been used to collect sounding data. In
these investigations, the coil orientation is held fixed
while the coil separation is increased over each survey
station. (For these instruments, the frequency is also
changed at each spacing to keep the induction number con-
stant.) Usually two fixed-spacing instruments are employed
in an EMI sounding survey, and the resulting data requires
semiquantitative analysis. Unfortunately, an experienced
geophysicist is required to interpret these data, and the
data are generally inadquate for resolving more than two
or three layers (McNeill, 1980). Although the analysis
of selected soundings can provide useful information
(Grady and Haeni, 1984), the EMI method is most effective
for profiling measurements.
As with all geophysical methods, the EMI technique is
susceptible to cultural and subsurface noise. For example,
passive metallic objects may cause actual geophysical anom-
alies; power lines may interfere with EMI measurements;
and, radio signals may interfere with data acquisition.
The interpreter must account for these cultural and physical
anomalies when interpreting the data.
3.2.4 Ground-Water Applications
In many ground-water assessment studies, the EMI
method is used as a reconnaissance tool since qualitative
conductivity information can be acquired, mapped, and
interpreted in a minimum of time. The recent popularity
of EMI methods relates to instrument and data processing
advances. For many instruments, ground contact is not
required, and some instruments are even designed to make
continuous electrical conductivity measurements.
EMI reconnaissance data may be supplemented by other
surface geophysical or test hole data. Electrical resis-
tivity data are commonly used to corroborate EMI profile
data. EMI measurements are more sensitive to small-scale
lateral conductivity variations. Thus, EMI data are used to
map shallow lateral lithologic and water quality variations
while resistivity data are used to map vertical changes
or to calibrate EMI measurements.
3.2.5 Summary
The EMI conductivity method provides an efficient
means for assessing subsurface conductivity anomalies,
and extrapolating between wells or resistivity survey
lines. The EMI method however provides only limited ver-
tical resolution due to the limited amount of sounding
27
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data that can be acquired. EMI is most effective as a
profiling tool and is often applied in reconnaissance
surveys. Some investigators have used the EMI method to
map lithologic changes in unconfined aquifer systems.
Within their respective depth or resolution limitations,
both the EMI and resistivity methods can provide estimates
of hydrologic parameters and aquifer characteristics. The
careful implementation of these electrical methods can
reduce the costs of wellhead protection programs.
3.3 VERY-LOW-FREQUENCY RESISTIVITY
Many other electrical techniques have been developed
for use in both mineral and petroleum exploration as well
as ground-water studies. One technique which deserves
some mention is the very-low-frequency resistivity (VLF)
technique. VLF is similar to EMI but uses low frequency
radio waves transmitted by naval communications stations
as the source. VLF instruments consist of one EMI-type
coil which measures the horizontal magnetic field, and a
pair of electrodes which measure the orthogonal electric
field. These portable instuments provide a direct reading
of apparent resistivity values in the field. As a result,
VLF data can be acquired both rapidly and inexpensively.
Since the input frequency is constant, the penetra-
tion depth of the probing radiation is solely determined
by the subsurface resistivity. In resistive terrains,
the penetration depth can range from 100 to 1000 feet.
In conductive terrains, the penetration depth can be as
little as 10 feet. VLF has been applied in both ground-
water contamination studies (Grady and Haeni, 1984), and
aquifer mapping studies (Haeni, 1986d). Haeni used
VLF in conjunction with other geophysical techniques to
delineate hydrogeologic boundaries in glacial aquifer
systems. Thus, the VLF method is similar to the EMI
method in both data acquisition characteristics and
application potential. Both techniques are simple to use
and provide information suitable for reconnaissance
purposes.
3.4 GROUND PENETRATING RADAR
3.4.1 Introduction
Ground penetrating radar (GPR) is a reflection tech-
nique which uses high-frequency electromagnetic waves to
continuously map subsurface parameter changes. This
technique is similar to seismic reflection in that both
methods measure the time required for a wave to travel
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from the earth s surface to a reflecting horizon and back
to the surface. These travel times are used to map the
distribution of hydrologic or lithologic changes. The
probing electromagnetic radiation of the radar technique
responds to the same property changes as the electromag-
netic technique. Thus the GPR technique has acquisition
similarities to seismic reflection yet measures electro-
magnetic instead of acoustic property changes. This
technique has been available for a number of years, and
recent technological advances and environmental applica-
tions have led to the technique's increased popularity.
The principles and the applications of GPR are discussed
by Davis et al. (1984), Coon et al. (1981), Dolphin et al.
(1978), and Moffat and Puskar (1976).
3.4.2 Theory
The subsurface parameters which influence the propa-
gation of radar waves are the dielectric permittivity and
and electrical conductivity. The radar technique responds
to near-surface changes in electrical conductivity and the
dielectric constant. Dielectic permittivity is controlled
by water content, clay content and bulk density. Clay
minerals tend to increase permittivity substantially
while high salinity slightly decreases permittivity. The
result is that the depth of penetration of radar waves
is controlled by the electrical conductivity, water
content, and clay content.
3.4.3 Methodology
Ground penetrating radar devices emit electromagnetic
pulses and record the travel time of the transmitted and
reflected energy as the instrument is moved along the
earth's surface (see Figure 7). A radar device contains
a radio antenna, a signal receiver, a recorder, and
electronic circuitry to coordinate the signal input and
output. This equipment is highly sophisticated, yet
data acquisition procedures are simple. A typical radar
transducer is mounted on wheels a few inches off the
ground and either pushed or pulled across the survey
area. With this approach, data are collected continuously,
and a large amount of data can be rapidly acquired. Many
radar units are towed behind field vehicles at speeds up
to five miles per hour. The resolution of the resulting
data is determined in part by the rate of traverse.
Although data collection is a simple procedure,
operating the radar instrumentation can be challenging
since instrument parameters must be adjusted to accomodate
site characteristics. For example, the bandwidth of the
survey antenna must be selected so as to optimize the
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ANTENNA
GROUND SURFACE
TRANSMITTED
— PULSE
Figure 7: Schematic representation of Ground Penetrating
Radar method (after Costello, 1981)
30
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depth of penetration and the vertical resolution. Higher
input frequencies favor greater resolution but are subject
to increased attenuation. Penetration depth however is
the primary consideration and resolution a minor factor.
The estimated travel time required for the radar waves to
probeitthe targeted depth is also an input parameter.
This time window" is based on an assessment of the
subsurface geology. Thus, to effectively operate complex
radar instrumentation, field personnel must have a working
knowledge of geophysics and electronics.
3.4.4 Data Processing and Interpretation
Radar data are presented graphically as distance
versus two-way travel time. Each graphic profile represents
a direct vertical slice of the survey area which allows
interpreters to associate subsurface features with surface
locations. Most radar instruments contain graphic display
devices which provide a "picture" record in the field. By
checking display records, real-time analog processing para-
meters can be refined in the field.
In many surveys, the data is recorded on magnetic tape
_and is eventually reprocessed to reveal more detailed
subsurface information. This processing utilizes digital
or analog filters to eliminate background system or cultural
noise. Some survey objectives may require the application
of sophisticated digital processing algorithms which have
been developed by the petroleum industry for reflection
data.. The costs of applying these processing techniques
are not usually justified for radar surveys. Most radar
surveys are used for the rapid reconnaissance of surficial
materials.
It is often important to convert the time-distance
output plots to depth-distance plots. This can only be
accomplished by estimating the subsurface velocity of the
radar wave. The accuracy of these estimates depends on
the geologic complexity of the area and the reliability of
the data. In many cases, the moisture content of the
porous media varies with depth. As a result, the vertical
velocity distribution has a nonlinear component which is
not accounted for in time-depth conversions. This
leads to the inaccurate location of reflectors.
3.4.5 Ground-Water Applications
Since water has a high dielectric constant and bedrock
has a low conductivity, GPR can be used to identify the
water table and the alluvium/bedrock contact in some shallow
unconfined aquifer systems. This technique can also detect
some small-scale stratigraphic changes in some alluvial
deposits.
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Since radar is a reflection technique, it is useful
to compare its methodology to that of seismic reflection.
The radar technique provides extremely high-resolution data;
a seismic reflection survey with equivalent resolution
would have geophones spaced only a few inches apart. The
primary advantage of radar however relates to its data
acquisition capabilities. The radar technique combines
rapid continuous spatial sampling with qualitative in-field
interpretation. By contrast, seismic reflection data
acquisition is a complicated process which requires com-
plex equipment and a field crew. Furthermore, processing
of reflection data involves the application of sophisticated
computer algorithms, but the resulting interpretations are
quantitative. Both reflection techniques require experi-
enced personnel for successful operation.
The primary difference between radar and seismic
reflection is the effective depth of penetration. As
mentioned above, radar waves are attenuated by the electri-
cal conductivity of pore fluids and clay minerals. Pene-
tration depths are highly site-specific. By contrast,
engineering reflection surveys are most effective in areas
with a shallow water table and an overburden which exceeds
100 feet in thickness (Haeni, 1986b). Thus, in ground-water
studies, seismic reflection is used to quantitatively map
deeper horizons while radar is used as a shallow recon-
naissance tool due to its data aquisition capabilities.
3.4.6 Limitations
Like all surface geophysical techniques, radar has
its own unique capabilities and limitations. To under-
stand the applicability of this method, the operator must
understand the characteristics of the site. In general,
high subsurface conductivity results in an increased
attenuation of the probing waves and a decreased depth of
penetration. Highly-conductive media or clay-rich lith-
ologies impair the penetration depth of the technique.
A fine-grained, clay-rich, saline-saturated lithology
can lead to a penetration depth of 3 feet. In coarse-
grained sands which are either dry or saturated with
fresh water, the penetration depths can be as deep as 100
feet. Thus, interstitial water influences the radar
response in two ways: changes in dielectric properties
produce strong reflections, and high conductivity atten-
uates the radar waves, thereby reducing penetration
depths. The result is that the radar technique can
accurately map shallow water table surfaces in some
alluvial settings (Olhoeft, 1984; Wright et al., 1984).
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3.4.7 Summary
The radar technique provides continuous reflection
profiles of shallow dielectric interfaces. Radar is most
effective as a reconnaissance tool since data can be
acquired rapidly and do not require complicated processing.
The radar technique is most suited to mapping the water
table or bedrock surfaces in certain unconfined aquifers.
This application is highly site-specific since the probing
depth of the electromagnetic radiation is limited by conduc-
tive materials.
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CHAPTER 4
POTENTIAL FIELD METHODS
4.1 GRAVITY
In gravimetric studies, the local vertical component
of the acceleration due to gravity is measured at the
earth's surface using a gravimeter. These data are
processed to remove known gravitational effects (for
example, local topography) and then the resulting "gravity
anomalies" are presented as profiles or contour maps.
The spatial variation of gravity anomalies is related to
lateral subsurface density contrasts. In principle, any
geologic feature which has an associated density change
can be detected by the gravity method.
Gravity data cannot be directly related to a unique
subsurface model. Data interpretation consists of
comparing field anomaly patterns with the gravitational
patterns of theoretical models. This comparison procedure
can be done by simple graphical means or by computerized
iterative inversion routines. The measured gravitational
field is a complex function of the shape, density and
depth of the subsurface features, and quite different
subsurface features may give rise to rather similar
anomalies. This inherent nonuniqueness of data sets
is a serious problem. The use of highly-sophisticated
computerized inversion algorithms cannot reduce uncer-
tainty due to nonuniqueness factors.
In general, gravity interpretations are improved if
they are constrained by additional subsurface geologic or
geophysical information. This information is usually
derived from well data, regional geologic mapping studies,
and other geophysical data. The rudiments of gravimetric
data interpretation and acquisition are discussed by
Dobrin (1976), and Telford et al. (1976).
With the development of modern gravimeters, the
gravity method has become an important reconnaissance
tool in ground-water studies. These instruments are
portable and can be operated by relatively inexperienced
personnel. The gravity method has proved to be an
inexpensive and rapid means of determining the large
scale features of unconfined aquifer systems, at least in
areas of relatively flat topography (Stewart, 1980;
Carmichael and Henry, 1977; Zohdy et al., 1974; Ibrahim
and Hinze, 1972; Rankin and Lavin, 1970). The success of
the gravity method in these studies depends on the density
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contrast between the unconsolidated sediments and the
underlying bedrock. In general, this density contrast is
measurable, and the gravity method is often used to map
bedrock topography by defining the boundaries of these
aquifers. Many times these surveys focus on determining
aquifer thickness.
Care must be taken in gravity data collection and
processing. Results can be seriously impaired by local
geologic inhomogeneities, topographic irregularities,
gravity station location errors, and vibrational noise.
Additionally, the resolving power of the method is limited
and usually additional geophysical data are required to
formulate an interpretation. The primary advantage of
the gravity method is its facility in data acquisition.
Processing and interpretation of these data however can
be a tedious and inconclusive process. This is especially
true if the data require complicated topographic cor-
rections which can add noise that may seriously obscure
the anomaly patterns of the .aquifer. In any case, the
gravity method can provide inexpensive reconnaissance
data which can be used to supplement other geophysical
data in ground-water studies.
4.2 MAGNETICS
The magnetic method detects variations in subsurface
magnetic susceptibility through measurements taken on
or above the earth's surface. Magnetic surveys measure
either the absolute or the relative intensity of some
magnetic field component. The magnetic field at any
surface location is composed of the earth's primary mag-
netic field and the magnetic field anomalies produced by
local distributions of magnetic material. The quantitative
interpretation of magnetic data reveals the spatial magnetic
characteristics of these materials.
Magnetic surveys range from the simple and inexpensive
to the complicated and expensive. Correspondingly, mag-
netic field measuring devices range from primitive mechani-
cal instruments to airborne systems. The instruments
most commonly used are hand-held proton-precession magneto-
meters. These instruments are available in various
sensitivities. The type of instrument used in a magnetic
survey depends on the required level of precision, the
spatial sampling, and the survey resources. Data is
rapidly collected in all types of magnetic surveys and is
either plotted as profiles or as contour maps.
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The magnetic and gravity methods are similar in data
acquisition and interpretation characteristics. Both
methods sample a potential field quantity at the earth's
surface and relate these observed data to theoretical
models by curve-matching techniques. Nonuniqueness of
data sets is also inherent in magnetic data, and experi-
enced personnel are often required to provide accurate
interpretations. The interested reader should consult
Dobrin (1976), Telford et al. (1976), or Grant and West
(1965) for more detailed treatments of magnetic data
acquisition and analysis.
Sedimentary units are the most common water-bearing
deposits but are typically nonmagnetic. Thus, the magnetic
method is not generally applicable to mapping sedimentary
units. (A few coarse elastics do contain magnetic minerals
and have been mapped directly.) Special circumstances
have made the magnetic method applicable to ground-water
investigations. For instance, magnetometer surveys have
been used to estimate the thickness of unconsolidated
deposits overlying magnetic crystalline basement (Birch,
1984; Zohdy et al., 1974). Also, Wire et al. (1984) have
used magnetic data to assist in the location of wells in
bedrock formations, and Harmon (1984) used magnetics to
delineate fracturing in basalts overlain by alluvium.
Therefore, although the magnetic method has limited use
in aquifer mapping studies, innovative surveys have been
designed to reveal hydrologic boundaries which are defined
by magnetic susceptibility contrasts.
The primary advantage of the magnetic method is its
facility in data acquisition. Magnetic data can be
rapidly collected in both surface and airborne surveys.
Like gravity data, magnetic data has poor vertical
resolution so that interpretations of depth parameters
tend to be inaccurate. Both gravity and magnetic data,
however, can give good resolution of lateral parameter
changes. As a result, magnetic data is used qualitatively
for reconnaissance purposes or to support other geophysical
data.
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CHAPTER 5
METHOD COSTS
5.1 INTRODUCTION
Aquifer delineation programs are designed according
to economic and hydrogeologic constraints. Investigators
often compare the production costs of the various aquifer
delineation methods when designing a program. Many surface
geophysical surveys are conducted in conjunction with the
collection of observation well data. As a result, integrated
surveys are sometimes designed according to cost analyses
that consider the comparative utilities of surface and
subsurface data. Often, the variability of survey costs
prohibits the easy selection of one approach over the other.
In this section, cost estimates for surface geophy-
sical surveys are presented with some of their associated
operational constraints. These estimates are based on
private contractor fees for penetration depths of 100
feet (where appropriate), based on limited personal com-
munication in early 1987. Contracting costs are presented
here because the capital costs of obtaining geophysical
equipment can be expensive (see Table 2). Although these
Technique
Refraction
Reflection
Resistivity
EMI
VLF
GPR
Gravity
Magnetics
Acquisition Instrument
12 channel seismograph & acces.
same as above & digital recorder
transmitter/receiver & cables
instrument
instrument
instrument
instrument
instrument (hand-held)
Cost
$25000
$32000
$12000
$19000
$10000
$25000
$40000
$5000
Table 2: Capital cost estimates for geophysical instruments,
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estimates are in dollar values, they should be viewed
in a somewhat relative sense and not be used to plan
individual surveys. One should remember that surface
geophysical surveys are highly variable depending on who
does the work, the details of the site, and the scope of
the investigation. In this section, the cost estimates
are grouped by method according to production cost
similarities.
5.2 SEISMIC AND ELECTRICAL RESISTIVITY
The seismic refraction, seismic reflection and
electrical resistivity techniques are labor-intensive
techniques that can require complex data processing pro-
cedures. The estimated costs are presented in Table 3.
In estimating survey costs, the investigator should
account for field crew, equipment rental, and data pro-
cessing costs. In addition to these fixed costs, one must
realize that the location and the terrain of the site as
well as the depth of investigation are important factors.
The cost ranges in Table 3 reflect the variability in
production rate and processing requirements for these
techniques. Crew travel and land surveying costs are
not included.
Technique
Seismic Refraction
Seismic Reflection
Resistivity Sounding
EMI
VLF
GPR
Resistivity Profile
Gravity Profile
Magnetic Profile
Cost
$2.00
$6.00
$275
$0.25
$0.25
$0.10
$2.00
$0.75
$0.50
- $3.50/foot
- $9.00/foot
- $425/location
- $0.50/foot
- $0.50/foot
- $0.30/foot
- $3.75/foot
- $1 .25/foot
- $1 .00/foot
Table 3: Contractor cost estimates for a depth of penetra-
tion less than 100 feet and a station spacing of
50 feet (where applicable).
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5.3 GRAVITY AND MAGNETICS
Unlike the preceeding techniques, these potential
methods do not require a field crew of more than one
person, so equipment rental and data processing costs are
relatively more significant. The costs for running a
gravity survey and a magnetic survey are similar with
gravity slightly more expensive due to increased labor
and processing costs. The cost estimates in Table 3 for
the gravity and magnetic methods consider the labor of
one geophysicist, equipment rental, and data processing
with travel and land surveying costs not included. (Land
surveying is critical to the success of a gravity survey
and represents a sizable additional expense.) The cost
range presented for these methods results from variation
in production rates at a constant station spacing, here
assumed to be 50 feet.
5.4 EMI, VLF, and GPR
Electromagnetic induction, very-low-frequency, and
ground penetrating radar are electromagnetic techniques
which permit rapid and simple data collection. The
processing requirements are not expensive for each of
these techniques. Thus, linear foot costs for these
methods are relatively inexpensive (see Table 3). Survey
costs include: field crew labor, equipment rental and
data processing. As before, the range in the estimated
costs results from variability in rates of production.
5.5 CONCLUSIONS
Table 3 provides an estimate of private contractor
survey costs. Private contractors however are not the
only reliable means of collecting surface geophysical
data. The U. S. Geological Survey, State geological
surveys, and geology departments at many universities
are currently conducting geophysical studies for municipal
or State governments at somewhat different cost/timing
structures. Investigators should be aware of these
possibilities when designing an aquifer delineation
program. Table 3 can be used as a rough guide when
designing a water resources investigation. Actual survey
costs can only be determined from site reconnaissance
data and the accessability of the site.
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CHAPTER 6
WHPA AND AQUIFER DELINEATION
6.1 INTRODUCTION
In WHPA delineation programs, surface geophysical
techniques can: 1) provide aquifer information to comple-
ment well data used by some WHPA delineation methods, and
2) map the hydrogeologic flow boundary WHPA criterion.
Surface geophysical techniques are most applicable for
aquifers under water table or shallow confinement con-
ditions. In geophysical surveys, the selection of a
particular technique depends on the geology of the survey
area, the existing hydrogeologic data, and the project
resources.
6.2 AQUIFER ASSESSMENT
Except for the arbitrary fixed radius method, all
WHPA delineation methods map hydrologic WHPA criteria and
therefore require well data as input (see Guidelines for
Delineation of Wellhead Protection Areas, 1987). In
order to map a WHPA, regional or subregional hydrogeologic
information are required. As an alternative to an
extensive drilling program, decision-makers may wish to
supplement limited well data with surface geophysical or
geological data. There are many different strategies
for integrating surface and well data based on economic
and hydrogeologic considerations. In general, these
surface geophysical surveys are designed to provide a
more detailed understanding of the subsurface while
limiting the cost of drilling additional exploratory
water wells.
6.2.1 Reconnaissance
An important step in developing a WHPA program is
the assessment of hydrogeologic conditions within a
state's borders. One initial step is the review of pub-
lished and unpublished hydrogeologic data on the survey
areas. In many survey areas, well data provide the aquifer
information required to design surface geophysical surveys.
After the preliminary work has been completed, then
geophysical reconnaissance data may be collected. If, as
in many cases, there are only limited well data, in the
40
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survey area, then these reconnaissance measurements may
provide an initial assessment of subsurface aquifer
characteristics. If, however, there are extensive well
data in the survey area, then geophysical reconnaissance
data may be used to extrapolate this subsurface information
away from or between these wells. In general, the design
of a geophysical reconnaissance survey depends on the
existence and the nature of previous data sets.
The selection of a particular technique used in a
reconnaissance survey depends on the nature of the survey
area, and the availability of equipment and skilled
personnel. In reconnaissance work, the least expensive
geophysical techniques are generally applied first. For
example, the gravity method is an economical and rapid
means for determining the gross configuration of some
glacial aquifers, and this method may be applied in advance
of the more expensive seismic refraction technique (Tucci
and Pool, 1987; Frohlich, 1979). Similarly, the electro-
magnetic induction (EMI) technique is often applied in
coastal settings to estimate the location of the salt-
water/fresh-water encroachment zone without drilling
expensive observation wells (Stewart, 1982). In these
encroachment surveys, EMI data may later be supplemented
by more expensive electrical resistivity data which offer
improved vertical resolution. As a final example, seismic
refraction and electrical resistivity reconnaissance
measurements have been used to determine the saturated
thickness variations of a glacial aquifer in efforts to
site water wells in the area (Underwood et al., 1984).
Once reconnaissance work has been completed, more
detailed investigation is required to define the flow
characteristics of aquifer systems. To obtain this
information, decision-makers may decide either to site
new observation wells, or to perform more detailed geo-
physical investigation. In general, their decision is
based on a comparison of the relative cost of drilling
water wells against the cost and accuracy of the geophy-
sical methods.
6.2.2 Detailed Site Investigation
In detailed geophysical surveys, the acquisition
parameters are designed to allow maximum resolution.
These studies generally provide detailed maps of sub-
surface parameter variations. As before, the selection
of a particular technique depends on the hydrogeologic
setting. For example, in some glacial aquifers, the ground
penetrating radar (GPR) technique can be employed to map
the water table and possibly to predict ground-water flow
(Wright et al., 1984). Similarly, a seismic refraction
survey may be designed to produce a detailed map of the
alluvium/bedrock contact (Sverdrup, 1986). Also, the
41
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electrical resistivity method has been used to predict
aquifer permeability and transmissivity values (Kosinski
and Kelly, 1981); these aquifer parameters may serve as
the input to analytical ground-water modeling routines.
The final example discussed here was an integrated
survey conducted by Tucci and Pool (1987) in the alluvial
basins of Arizona. This investigation used the strategy
of collecting reconnaissance data first and later con-
ducting more detailed site investigation. The gravity
method was used as the reconnaissance tool to delineate
the basin margins and estimate the depth to bedrock.
Although the accuracy of these data are estimated to be
only within plus or minus 30 percent, they do provide a
rapid and relatively inexpensive first approximation.
This reconnaissance work was followed by a detailed
refraction/resistivity survey. The refraction survey
identified the depth to bedrock and the water table as
well as provided more detailed stratigraphic and struc-
tural information on such features as buried faults.
The resistivity survey verified the structural and
stratigraphic interpretations of the seismic method and
also provided information on the location of fine-grained
deposits. The resistivity technique was unsuccessful in
locating the water table due to the heterogeneity of the
deposits. Therefore, this integrated survey provided a
variety of basin information that could be used to guide
further ground-water flow investigation.
In addition to these cited examples, there are other
published reports which discuss detailed aquifer assessment
surveys. Most of these studies are focused on ground-water
resource evalution and are directly applicable to WHPA
delineation efforts.
6.3 WHPA DELINEATION
In the above examples, surface geophysical tech-
niques are an economical means for recovering subsurface
information related to ground-water flow. These geophy-
sical data supplement well data which are used for mapping
WHPA delineation criteria. As mentioned above, geophysical
reconnaissance surveys often map the margins of alluvial
aquifers. In some of these settings, the aquifer margins
correspond to the aquifer recharge zone and therefore
form one flow boundary of the WHPA. In the hypothetical
valley aquifer shown in Figure 8, surface geophysical
techniques are used to map the upgradient limit of the
aquifer materials. This lithology change is designated
as one WHPA boundary. Therefore, surface geophysics, as
one set of techniques within the hydrogeologic mapping WHPA
delineation method outlined in the Guidelines, may be used
to directly map the flow boundaries WHPA criterion.
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STREAM
PUMPING WELL
BEDROCK (NON-AQUIFER
MATERIAL)
ALLUVIAL AQUIFER
—•—'-~ Primary WHPA Boundary Drawn as Upgradient Limit of Aquifer Materials.
Figure 8: WHPA "flow boundary" criterion is mapped using
surface geophysical techniques.
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