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STENNIS SPACE CENTER, Mississippi Bldg. 3101
Application of Surface-Geophysical Methods to 

Investigations of Sand and Gravel Aquifers in the 

Glaciated Northeastern United States

By F.P. HAENI

REGIONAL AQUIFER-SYSTEM ANALYSISNORTHEAST GLACIAL VALLEYS

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1415-A

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

GORDON P. EATON, Director

Any use of trade, product, or firm names in this publication is for

descriptive purposes only and does not imply endorsement by the

U.S. Government

Library of Congress Cataloging in Publication Data

Haeni, F.P.

Application of surface-geophysical methods to investiations of sand and gravel aquifers in the glaciated northeastern United

States / F.P. Haeni.

p. cm. (Regional aquifer system analysis northeast glacial valleys) (U.S. Geological Survey professional paper ;

1415-A)

Includes bibliographical references. 

Supt.ofDocs.no.: I19.16:1415-A 

1. Aquifers northeastern States. 2. Prospecting Geophysical methods. I. Title. II. Series. III. Series: U.S. Geological

Survey professional paper ; 1415-A. 

GB1199.3.N77H34 1995

551.49'0974-dc20 93-13284

CIP

For sale by U.S. Geological Survey

Information Services, Box 25286 

Federal Center, Denver, CO 80225
FOREWORD

THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM

The Regional Aquifer-System Analysis (RASA) Program was started in 

1978 following a congressional mandate to develop quantitative appraisals of 

the major ground-water systems of the United States. The RASA Program 

represents a systematic effort to study a number of the Nation's most 

important aquifer systems, which in aggregate underlie much of the country 

and which represent an important component of the Nation's total water 

supply. In general, the boundaries of these studies are identified by the 

hydrologic extent of each system and accordingly transcend the political 

subdivisions to which investigations have often arbitrarily been limited in the 

past. The broad objective for each study is to assemble geologic, hydrologic, 

and geochemical information, to analyze and develop an understanding of the 

system, and to develop predictive capabilities that will contribute to the 

effective management of the system. The use of computer simulation is an 

important element of the RASA studies, both to develop an understanding of 

the natural, undisturbed hydrologic system and the changes brought about in 

it by human activities, and to provide a means of predicting the regional 

effects of future pumping or other stresses.

The final interpretive results of the RASA Program are presented in a series 

of U.S. Geological Survey Professional Papers that describe the geology, 

hydrology, and geochemistry of each regional aquifer system. Each study 

within the RASA Program is assigned a single Professional Paper number, 

and where the volume of interpretive material warrants, separate topical 

chapters that consider the principal elements of the investigation may be 

published. The series of RASA interpretive reports begins with Professional 

Paper 1400 and thereafter will continue in numerical sequence as the interpretive products of subsequent studies become available.

Gordon P. Eaton 

Director

CONTENTS VII

Page

42. Diagram showing simplified geoelectric Earth model from direct-current-resistivity data, and well and test-hole logs from

the study site in Farmington, Conn................................................................................................................. A48

43. Diagrams showing very-low-frequency apparent resistivity and phase-angle field data, and the interpreted geoelectric 

Earth model from direct-current-resistivity, seismic refraction and very-low-frequency apparent resistivity data, 

from the study site in Farmington, Conn.......................................................................................................... 49

44. Graphs showing field and computed inductive terrain-conductivity values for the 10-, 20-, and 40-meter-intercoil spacings

at the study site in Farmington, Conn. ............................................................................................................ 50

45. Diagram showing geologic logs from test holes in Farmington, Conn........................................................................... 51

46. Map showing location of the study site in Preble, N.Y., including wells, test holes, and surface-geophysical survey lines .... 52

47. Graph showing smoothed plot of field-data points from direct-current-resistivity sounding, the computed sounding curve,

and the interpreted layered geoelectric Earth model for data from the study site in Preble, N.Y. ............................. 53

48. Diagram showing simplified geoelectric Earth model from direct-current-resistivity data, and test-hole logs from the

study site in Preble, N.Y............................................................................................................................... 56

49. Diagrams showing very-low-frequency apparent resistivity and phase-angle field data, and the interpreted geoelectric

Earth model from direct-current-resistivity and very-low-frequency apparent resistivity data, from the study site in 

Preble, N.Y................................................................................................................................................. 57

50. Graphs showing field and computed inductive terrain-conductivity values for the 10-, 20-, and 40-meter-intercoil spacings

at the study site in Preble, N.Y...................................................................................................................... 58

51. Map showing location of the study site at the airport in Oxford, Maine, including wells, test holes, and surface-geophysical

survey lines................................................................................................................................................. 59

52. Graph showing smoothed plot of field-data points from direct-current-resistivity sounding, the computed sounding curve,

and the interpreted layered geoelectric Earth model for data from the study site at the airport in Oxford, Maine........ 60

53. Diagram showing simplified geoelectric Earth model from direct-current-resistivity data, and well and test-hole logs from

the study site at the airport in Oxford, Maine ................................................................................................... 61

54. Diagrams showing very-low-frequency apparent resistivity and phase-angle field data, and the interpreted geoelectric 

Earth model from direct-current-resistivity, seismic refraction, and very-low-frequency apparent resistivity data, 

from the study site at the airport in Oxford, Maine............................................................................................ 62

55. Graphs showing field and computed inductive terrain-conductivity values for the 10-, 20-, and 40-meter-intercoil spacings

at the study site at the airport in Oxford, Maine................................................................................................ 63

56. Map showing location of the esker site in Oxford, Maine, including test holes, spring, and surface-geophysical survey

lines........................................................................................................................................................... 64

57. Diagrams showing very-low-frequency apparent resistivity and phase-angle field data, and the interpreted geoelectric 

Earth model from direct-current-resistivity, seismic refraction, and very-low-frequency apparent resistivity data, 

from the esker site in Oxford, Maine ............................................................................................................... 65

58. Graphs showing field and computed inductive terrain-conductivity values for the 10-, 20-, and 40-meter-intercoil spacings

at the esker site in Oxford, Maine................................................................................................................... 66

59. Diagram showing geologic logs from test holes at the esker site in Oxford, Maine......................................................... 67

TABLES

Page

TABLE 1. General relation of hydrogeologic units in the glaciated Northeastern United States to the velocity of sound in each unit....... A3

2. Resistivity values of various materials used in the hypothetical Earth models.................................................................. 10

3. Interpreted seismic velocities in stratified-drift deposits at six field sites in the glaciated Northeastern United States ........... 48

4. Interpreted resistivity of unsaturated, stratified-drift deposits at seven field sites in the Northeastern United States............ 52

5. Interpreted resistivity of saturated, stratified-drift deposits at eight field sites in the Northeastern United States................ 53

6. Interpreted seismic velocity and resistivity of sedimentary and crystalline bedrock at seven field sites in the Northeastern

	United States...........................................................................................................^ 54

7. Comparison of depth to bedrock, determined by seismic-refraction, direct-current-resistivity, and drill-hole data.................. 54
VIII CONTENTS

CONVERSION FACTORS, ABBREVIATIONS, AND VERTICAL DATUM

Multiply

meter (m) 

kilometer (km) 

millisiemens per meter (mS/m) 

ohm-meter (ohm-m) 

kilometer per second (km/s)

By

3.281 

0.6214 

0.305 

3.281 

3,281

To obtain

foot (ft) 

mile (mi) 

millisiemens per foot (mS/ft) 

ohm-foot (ohm-ft) 

foot per second (ft/s)

Abbreviated water-quality units used in the report: jxS/cm at 25°C=microsiemens per centimeter at 25 degrees Celsius 

For temperature conversions between degrees Celsius (°C) and degrees Fahrenheit (°F), the following formula may be used:

°F=(1.8x°C)+32

Sea level: In this report "sea level" refers to the National Greodetic Vertical Datum of 1929 (NGVD of 1929) a geodetic datum derived from 

a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.
REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

APPLICATION OF SURFACE-GEOPHYSICAL METHODS TO

INVESTIGATIONS OF SAND AND GRAVEL AQUIFERS IN THE

GLACIATED NORTHEASTERN UNITED STATES

By F.P. HAENI

ABSTRACT

Reconnaissance hydrogeologic mapping and ground-water-flow modeling in the glaciated Northeastern United States require data on the 

hydrogeologic boundaries and grain-size characteristics of sand and 

gravel aquifers. The combined use of surface-geophysical methods such 

as seismic-refraction, direct-current-resistivity, very-low-frequency 

terrain-resistivity, and inductive terrain-conductivity can provide 

these data.

Forward-modeling studies of 10 hypothetical aquifer systems indicated that very-low-frequency terrain-resistivity and inductive terrainconductivity methods can be used to detect horizontal and vertical 

changes in the electrical properties of typical sand and gravel aquifers.

Surface-geophysical surveys at eight sites in Connecticut, New 

York, and Maine, have shown that the combined use of four geophysical 

methods can define the hydrogeologic boundaries and can distinguish 

between fine-grained and coarse-grained stratified drift. The seismicrefraction method could be used to determine depths to the water table 

and to bedrock at all the field sites where it was used, but could not be 

used to detect lithologic changes within the aquifer. Direct-currentresistivity methods could be used to detect large-scale resistivity 

changes that were related to either specific conductance of ground 

water or major lithologic changes in the aquifers. In some hydrogeologic settings, this method could be used to identify the water table and 

bedrock surface. Very-low-frequency terrain-resistivity and inductive 

terrain-conductivity methods used in conjunction with the other 

surface-geophysical methods can be used to identify anomalies caused 

by small-scale changes in aquifer lithology or specific conductance of 

ground water.

Comparisons of the interpreted electrical resistivity of the aquifer 

with the logs of test holes and wells indicate that for a given specific 

conductance of ground water, the bulk electrical resistivity of the 

stratified drift increases with increasing grain size. This relation is 

useful for distinguishing fine-grained drift deposits from coarse sand 

and gravel deposits.

INTRODUCTION

Sand and gravel aquifers are the principal sources of 

ground water in the glaciated Northeastern United

States. The use of surface-geophysical methods to investigate sand and gravel aquifers was evaluated as part of 

the U.S. Geological Survey's Northeast Glacial Valleys 

Regional Aquifer-System Analysis (RASA). The background and objectives of the Northeast Glacial Valleys 

RASA are described by Lyford (1986) and the overall 

objectives of the RASA program are described in the 

Foreword.

This report is one of several chapters in U.S. Geological Survey Professional Paper 1415 that describe various aspects of the geology, hydrology, and geochemistry 

of the glacial aquifers in the Northeastern United States.

Delineation of hydrogeologic boundaries and grain-size 

characteristics of sand and gravel aquifers is required for 

reconnaissance hydrogeologic mapping and groundwater flow modeling studies. Test drilling is generally 

used to obtain such data but commonly at only a few 

sites. Surface-geophysical methods can be used to rapidly obtain areal hydrogeologic data on sand and gravel 

aquifers. The geophysical data, although not as detailed 

as drill-hole data, provide broader areal coverage and 

therefore are well suited for most hydrogeologic studies.

All surface-geophysical methods measure some physical property of subsurface materials, or the fluids within 

them, from the surface of the Earth. Typical physical 

properties measured are electrical resistivity or conductivity, velocity of sound, and the strength of gravity and 

magnetic fields. Knowledge of the physical properties of 

the subsurface unit of interest and the properties of the 

surrounding units is critical for the successful application 

of surface-geophysical methods. Selection of the appropriate geophysical method is determined by the specific 

physical properties of a hydrogeologic unit or the differences in these properties between adjoining hydrogeologic units.

Al
A2 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

PURPOSE AND SCOPE

This report presents the results of a study conducted 

to show how a combination of seismic-refraction, directcurrent-resistivity (de-resistivity), very-low-frequency 

(VLF) terrain-resistivity, and inductive terrainconductivity methods can be used to map hydrogeologic 

boundaries and to determine general lithologic characteristics of sand and gravel aquifers in the Northeastern 

United States.

The first part of this study used forward-modeling 

computer programs to calculate the response of VLF 

terrain-resistivity and inductive terrain-conductivity 

methods to several hypothetical aquifer settings. The 

results were then used to determine whether these 

electromagnetic methods could detect horizontal and 

vertical resistivity variations typical of glacial aquifers. 

In this modeling process, it was assumed that (1) the 

depths to the water table and bedrock were known from 

drill-hole or seismic-refraction data, (2) the gross 

subsurface-layer resistivities were known from 

de-resistivity data, and (3) the conductivity of the ground 

water was constant. Variations in the resistivities of the 

aquifer should represent variations in the grain size of 

the aquifer material.

The second part of this study was an application of the 

surface-geophysical methods at eight field sites, representing typical sand and gravel aquifers in the glaciated 

Northeastern United States. It demonstrated that (1) 

the combined use of seismic-refraction, de-resistivity, 

and electromagnetic methods could be used to detect 

hydrogeologic boundaries and determine the resistivity 

variations within the aquifer material, and (2) the resistivity variations represented differences in the grain size 

of the aquifer material.

PREVIOUS STUDIES

A combination of surface-geophysical methods can be 

used advantageously in hydrogeologic investigations. 

Many papers have described the use of individual and 

combined surface-geophysical methods in hydrogeologic 

studies and are important sources of information for 

hydrogeologists (Lennox and Carlson, 1970; Mabey, 

1970; Ogilvy, 1970; Shiftan, 1970; Zohdy and others, 

1974; Worthington, 1975; Collett, 1979).

The two surface-geophysical methods that have been 

most widely used in hydrogeologic studies are seismicrefraction and de-resistivity methods. The references 

listed above all contain sections on the use of seismicrefraction method in hydrogeologic investigations. In 

addition, Bonini and Hickok (1958), Eaton and Watkins 

(1970), and Haeni (1986a, 1988) specifically discuss use of 

the seismic-refraction method in hydrogeologic studies.

The seismic-refraction method is used primarily to determine the boundaries of aquifers in situations where 

seismic-velocity discontinuities between hydrogeologic 

units exist. This method has been used to map depth to 

the water table and depth to bedrock in many glacialaquifer reconnaissance studies (Warrick and Winslow, 

1960; Gill and others, 1965; Lennox and Carlson, 1967; 

Watkins and Spieker, 1971; Dickerman and Johnston, 

1977; Sharp and others, 1977; Haeni and Anderson, 1980; 

Mazzaferro, 1980; Fretwell and Stewart, 1981; Grady 

and Handman, 1983; Tolman and others, 1983; Haeni and 

Melvin, 1984; Ayers, 1989; Ayotte, 1989; Moore, 1990; 

Melvin and Bingham, 1991) and in a few simulation 

studies (Birch, 1976; Haeni, 1978; Morrissey, 1983; 

Haeni, 1986a; Mazzaferro, 1986; Toppin, 1987; Tepper 

and others, 1990).

De-resistivity methods have been used successfully to 

delineate hydrogeologic boundaries that are characterized by changes in electrical properties of subsurface 

materials. Thick clay layers, the presence of coarsegrained beds within a fine-grained unit, and waterquality changes are examples of hydrogeologic conditions 

that can be detected by de-resistivity methods.

In most hydrogeologic studies where de-resistivity 

methods have been applied, high-resistivity zones are 

associated with coarse-grained aquifer material saturated with water, having low dissolved-solids concentrations. Low-resistivity zones have been interpreted as 

fine-grained materials or highly conductive water (Page, 

1968; Zohdy, 1969; Flathe, 1970; Zohdy and others, 1974; 

Worthington, 1975; Zohdy and others, 1977; Gorhan, 

1976; Rijo and others, 1977; Worthington, 1977; Martinelli, 1978; Bisdorf and Zohdy, 1979; Verma and others, 

1980; South Florida Water Management District, 1982). 

Only a few de-resistivity studies in glacial terrains have 

been reported (Kelly, 1962; Buhle and Brueckmann, 

1964; Lennox and Carlson, 1967; Frohlich, 1974, 1979). 

Several applications of this method have been reported 

where the quantitative relation between aquifer resistivity and hydraulic conductivity has been investigated. 

These studies and the subsequent discussion of their 

results show that only empirical relations, over limited 

geographical areas, can be established (Kelly, 1977a, 

1977b, 1978,1980; Sabet, 1978; Heigold and others, 1979; 

Mazac and Landa, 1979; Heigold and others, 1980; Kosinski and Kelly, 1981; Urish, 1981; Kelly and Kosinski, 

1982; Leonard-Mayer and Taylor, 1982; Biella and others, 1983; Mazac and others, 1985; Huntley, 1986).

Electromagnetic methods also can be used to map 

variations in aquifer conductivity or resistivity. These 

methods have been used successfully for the qualitative 

delineation of conductive plumes in contaminated glacial 

aquifers (Duran and Haeni, 1982; Greenhouse and Slaine, 

1982; Greenhouse and Harris, 1983; Grady and Haeni,
PRINCIPLES OF SURFACE-GEOPHYSICAL METHODS A3

TABLE I. General relation of hydrogeologic units in the glaciated 

Northeastern United States to the velocity of sound in each unit

Hydrogeologic unit

Unconsolidated, stratified-drift or alluvial deposits: 

Unsaturated ................................. 0.3-0.6

Saturated ................................... 1.2-1.8

Saturated till .................................. 2.1-2.4

Saturated sedimentary rocks .................... 3.4-4.3

Saturated crystalline rocks ...................... 4.6-6. 1

1984; Barlow and Ryan, 1985; Grady, 1989; Greenhouse 

and others, 1989). Stewart (1982) used this method to 

map the saltwater interface in coastal aquifers. Grady 

and Haeni (1984) also showed how these methods could 

be used for the quantitative delineation of a conductive 

contaminant plume in a glacial aquifer. The use of 

electromagnetic methods for aquifer assessments has 

been limited (Wynn, 1979; Sinha, 1980; Haeni, 1986b) and 

no quantitative work has been reported using these 

methods in glacial terrains.

PRINCIPLES OF SURFACE-GEOPHYSICAL 

METHODS

De-resistivity, VLF terrain-resistivity, and inductive 

terrain-conductivity methods, and data from previously 

conducted seismic-refraction studies were used in this 

study. Each method provided different hydrogeologic 

information about the subsurface. The seismic-refraction 

data were used to determine the depth to water table and 

depth to bedrock. The de-resistivity method was used to 

determine if an empirical relation between aquifer resistivity and grain size existed, and to measure the gross 

electrical properties of each subsurface unit. The VLP 

terrain-resistivity and inductive terrain-conductivity 

methods were used to map the horizontal and vertical 

resistivity variations within an aquifer or adjacent units. 

Each surface-geophysical method has unique operating 

principles, advantages, and limitations.

SEISMIC-REFRACTION METHOD

Sand and gravel aquifers in the Northeastern United 

States generally consist of unconsolidated sand and 

gravel deposits underlain by till and crystalline or sedimentary bedrock. There are substantial changes in the 

velocity of sound at the water table in unconsolidated 

material and at the till and bedrock interfaces (table 1). 

The velocity of sound in the various geologic units, which 

is called seismic velocity, generally increases with depth. 

The seismic velocity also increases as the density of the 

geologic materials increases.

In the seismic-refraction method, sound waves are 

produced by a variety of methods such as sledge hammer 

strikes on a metal plate, explosives, and weight drops, 

and travel through the subsurface. When the seismic 

velocity in the subsurface units increases with depth, 

some of the sound energy is refracted along the higher 

seismic-velocity unit. The refracted sound energy is 

eventually transmitted back to the surface where it is 

received by geophones and recorded by the seismograph. 

Measurement of the sound wave traveltimes from the 

sound source to the receiver, and the measured distance 

between source and receiver can be transformed into a 

time-distance plot (fig. 1). From this plot, seismic velocity of individual layers and depth to refracting layers can 

be calculated.

Explosive sound sources and a 12-channel commercial 

seismograph, the EG&G model 1210F, were used to 

collect the refraction data used in this study. Methods for 

the interpretation of field seismic-refraction data are well 

documented (Dobrin, 1976; Telford and others, 1976; 

Mooney, 1981; Sjogren, 1984) and many computer modeling programs are available (Scott and others, 1972; 

Scott, 1973, 1977a, 1977b; Ackermann and others, 1982). 

In this study, the method described by Scott and others 

(1972), Scott (1973), Haeni and others (1987), and Haeni 

(1988) was used and the result of the interpretation is a 

cross-sectional profile of the refracting layers. An example of a geohydrologic section based on interpreted 

results of a seismic-refraction profile conducted in Stonington, Conn, is shown in figure 2.

DIRECT-CURRENT-RESISTIVITY METHOD

The theory of de-resistivity methods is well described 

by several authors (Zohdy and others, 1974; Dobrin, 

1976; Telford and others, 1976; Mooney, 1980). The bulk 

resistance of porous earth material depends on several 

variables with complex interrelations, but principally 

depends on the electrical conductivity of the matrix 

material and the electrical conductivity of the fluid within 

the porous medium. Urish (1981) concluded from his 

laboratory packing tests and from the work of Kezdi 

(1974) that fine-grained materials tend to have higher 

porosities than coarse-grained materials. He also showed 

through theoretical calculations and field tests that, if the 

fluid conductivity remains constant, the resistivity of the 

aquifer material will generally increase with grain size. 

Resistivity methods, therefore, have the capability of 

providing a qualitative estimate of the grain size of sand 

and gravel aquifers over a limited geographic area.

A commercial de-resistivity unit, the Bison model 

2390, consisting of a transmitter, receiver, and switchbox, was used in this study. The resistivity of subsurface 

materials is measured in the field by putting a direct
A4 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

Time-distance 

curve

12

SHOT POINT

45 6 7 i 

DISTANCE 

GEOPHONES

9 10 11 12 GEOPHONES

EXPLANATION

V 1 = Velocity of sound in 

unsaturated alluvium 

(460 meters per 

second)

V 2 = Velocity of sound in 

saturated alluvium 

(1,520 meters per 

second)

DISTANCE

FIGURE 1. Seismic-refraction time-distance plot and section showing direct and refracted sound waves. (Modified from Haeni,

1988, fig. 2.)

West

METERS

20 

10-

5 

Sea level 

-5 

-10 

-15

Water table

Land surface

Stratified drift

Bedrock

500 FEET

100 METERS 

Vertical exaggeration X3

East

FIGURE 2. Interpreted seismic-refraction profile in Stonington, Conn.
PRINCIPLES OF SURFACE-GEOPHYSICAL METHODS A5

Current source Current meter

Current flow 

through Earth C-) 0,2 

PI ?2

EXPLANATION

Current flow line 

-- Line of equal voltage 

Current electrode 

Potential electrode

FIGURE 3. Principles of direct-current-resistivity method.

current or a low-frequency alternating current into the 

ground and measuring the resultant voltage drop across 

a given distance (fig. 3). The apparent resistivity then 

can be calculated, and using one of several interpretation 

procedures, the resistivity of individual subsurface units 

can be interpreted (Zohdy, 1974; Zohdy and Bisdorf, 

1975). Many geometric arrangements of the current and 

voltage electrodes have been developed (Zohdy and 

others, 1974). Schlumberger soundings were used in this 

study. The distance between the current electrodes was 

systematically increased and the distance between the 

potential electrodes was periodically increased to measure the effect on deeper geologic units. This method has 

limited horizontal resolution because the interpretation 

assumes that each subsurface layer is horizontal and 

homogeneous over the distance between the electrodes.

ELECTROMAGNETIC METHODS

VERY-LOW-FREQUENCY TERRAIN-RESISTIVITY METHOD

Very-low-frequency, 15-24 kilohertz (kHz), radio 

waves are transmitted from U.S. Navy communication 

centers located around the world. As these waves propagate over the Earth's surface, magnetic and electrical 

fields are induced in the ground (fig. 4). The magnitude of 

the horizontal component of these fields depends on the 

apparent resistivity of the subsurface materials at the 

measuring point and on the frequency of the VLF radio 

wave. The angle between the horizontal components of 

the electrical and magnetic fields (the phase angle) 

depends on the stratification of the subsurface. The 

mathematical expressions for these relations are derived
A6 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

Remote, very-low-frequency 

transmitter

Conductive 

ground (p-|) 

P1«P2

Resistive 

ground (p2 ) 

P2»P1

EXPLANATION

Ez = vertical electrical field

Apparent terrain resistivity (pa ) = i: -rpF L Hy J

and, the exploration depth ( 6) = 500 ( pi F ) 1/2 

where:

P1«P2 = relative resistivity of conductive

and resistive ground 

F = frequency of transmitter 

Ex = horizontal radial electrical field 

H y = horizontal radial magnetic field

FIGURE 4. Principles of very-low-frequency terrain-resistivity method. (Modified from Collett, 1979, fig. 6.)

by Wait (1982) and discussed by Keller and Frischknecht 

(1966). In a homogeneous Earth, the phase angle is 45° 

and the measured resistivity is the true resistivity of the 

subsurface. If the shallow subsurface material is more 

conductive than the deep subsurface material, the phase 

angle will be less than 45°. When the subsurface layering 

is reversed, the phase angle will be greater than 45°.

A commercial VLF unit, the Geonics model EM16R, 

was used in this study. Field measurements were taken 

by determining the direction to the transmitting station 

using the receiver coils in the handle of the electronic 

receiver, orienting the two probes and cable in the same 

direction as this station, and reading the apparent resistivity and phase angle.

This method is limited by the effects of cultural 

interference such as pipelines, fences, power lines, or 

other metal objects. The depth of penetration is limited 

by the frequency of the transmitting station and the

conductivity of the subsurface units. In very conductive 

units (1-100 ohm-meters (ohm-m)), the penetration 

depth is limited to a few meters or tens of meters. In 

very resistive material (100-10,000 ohm-m) penetration 

depths can be as great as 400 meters (m). An example of 

how the penetration depth of the instrument varies with 

terrain resistivity and VLF transmitting frequencies is 

shown in figure 5.

INDUCTIVE TERRAIN-CONDUCTIVITY METHOD

Inductive terrain-conductivity methods induce electrical currents in the Earth by energizing a coil of wire (the 

transmitter coil) at the surface of the Earth with an 

alternating current (fig. 6). This alternating current 

produces a magnetic field which induces current flow in 

the Earth, which, in turn, produces a secondary magnetic field. The secondary field induces a voltage in the 

instrument, which is converted to apparent conductivity
PRINCIPLES OF SURFACE-GEOPHYSICAL METHODS A7

1,000

^ 100

I

15 kilohertz 

20 kilohertz

0.1 1 10 100 1-000 

RESISTIVITY, IN OHM-METERS

10,000

FIGURE 5. Penetration depth of very-low-frequency terrainresistivity method.

and displayed. Apparent conductivity would equal true 

conductivity only if the Earth were homogeneous. The 

magnitude of the secondary field is directly proportional 

to the conductivity of the Earth. This proportion is true 

only when the coil spacing is much less than the depth at 

which the electromagnetic wave attenuates to 0.3679 of 

its original amplitude (low induction number assumption). In practice, this means that the conductivity of the 

Earth is less than 100 millisiemens per meter (mS/m) 

(McNeill, 1980b). McNeill gives a clear explanation of the 

principle of operation of this method and the importance 

of the low induction number assumption.

A commercial inductive terrain-conductivity meter, 

the Geonics model EM34-3, which measures apparent 

conductivity directly, was used in this study. It consists 

of a transmitter and a transmitting coil; a receiver and a 

receiving coil; and 10-, 20-, and 40-m intercoil connecting 

cables. The depth of penetration is dependent on the

EXPLANATION

T = Transmitter coil 

R = Receiver coil

Terrain conductivity (o ) = 4/2 71 f |a 0 s2 ( H s / H p ) 

Where f = operating frequency

\ia= permeability of free space 

s = intercoil spacing 

H s = secondary magnetic field 

H p = primary magnetic field

FIGURE 6. Principles of inductive terrain-conductivity method. (Modified from Geonics, 1979, fig. 1.)
A8 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

1.00

o

Q_ 

O> 

LLJ 

DC 

LLJ

0.75

O Q.

> 0.50

I

o 0.25

\ ^ Vertical-dipole mode 

\ 

\

Horizontal-dipole mode

______I______l_

0.5 1.0 1.5 2.0 

DEPTH -H INTERCOIL SPACING (Z)

0.5 1.0 1.5

DEPTH + INTERCOIL SPACING (Z) 

(Modified from McNeill, 1980b)

2.0

Depth -*- intercoil 

spacing (Z) 0

Coil spacing 

(in meters)

10 

20

40

0.5 1.0 1.5 2.0

Depth (in meters)

0 

0

0

5 

10

20

10 

20

40

15 

30

60

20 

40

80

0.5 1.0 1.5 2.0

10

20

40

Depth (in meters)

0

0

0

5 10

10 20

20 40

15

30

60

20

40

80

FIGURE 7. Relative and cumulative response curves for the Geonics EM34-3 inductive terrain-conductivity instrument.

(Modified from McNeill, 1980b, figs. 6 and 11.)

spacing and orientation of the coils and the conductivity 

contrast of the target; it is relatively independent of the 

resistivity of the subsurface material. The influence of 

subsurface material at different depths (in a homogeneous Earth) on the apparent conductivity (recorded by 

the instrument) is shown on the left side of figure 7 for 

different coil (dipole) positions and intercoil spacings. 

When the coils are in the horizontal-dipole mode (coils 

held vertically and coplanar), the instrument responds 

mainly to earth materials located from the surface to a 

depth of 0.75 times the intercoil spacing and is most 

sensitive to near-surface materials (fig. 7). In the 

vertical-dipole mode (coils held horizontally and coplanar), the instrument responds to mainly earth materials 

between 0.1 and 1.5 times the intercoil spacing and is 

most sensitive to layers at a depth of about 0.4 times the 

intercoil spacing. Near-surface materials have little 

effect on the instrument in this mode.

The cumulative response curves, defined as the contribution to the secondary magnetic field from all material below a given depth, are shown in the right side of

figure 7. These curves allow the computation of instrument response to any hypothetical combination of layers 

and are used in the forward-modeling programs. It is 

important to note that the measured apparent conductivity is a function of the conductivity and thickness of 

individual layers and the response characteristic of the 

instrument.

Conductivity is the reciprocal of resistivity and is 

defined by

Conductivity (in 1,000

(in ohm_m)

The change in conductivity (AC) for a given change in 

resistivity (A.R), in a known environment having a resistivity of R, can be approximated by

AC= -l,OOOAfl 

R2

Because of the inverse square relation of resistivity 

and conductivity in resistive terrains (R= 1,000 ohm-m), 

a 1-mS/m change in conductivity reflects a 1,000-ohm-m
RESPONSE OF ELECTROMAGNETIC METHODS TO HYPOTHETICAL GEOELECTRIC EARTH MODELS A9

Sedimentary 

bedrock outcrops

Contact between sedimentary bedrock 

and till

Contact between stratified drift 

and till

FIGURE 8. Relations among stratified drift, till, and bedrock in a typical glacial valley.

resistivity change, whereas the same change (1 mS/m) in 

conductive terrain (72=42 ohm-m) reflects only a 1.8- 

ohm-m change in resistivity.

Six measurements that correspond to both dipole 

positions at each intercoil spacing can be made at each 

data collection station, giving the instrument a limited 

depth-sounding capability. Horizontal conductivity 

changes within the subsurface can be detected by making 

a series of measurements along a profile across the area 

of interest.

This method is limited by electrical or cultural interference caused by power lines, pipelines, and fences; the 

depth of penetration is limited by the strength and 

frequency of the transmitter and the intercoil spacing.

RESPONSE OF ELECTROMAGNETIC METHODS

TO HYPOTHETICAL GEOELECTRIC EARTH

MODELS

Hypothetical geoelectric Earth models were used to 

determine if VLF terrain-resistivity and inductive 

terrain-conductivity methods could be used to define 

horizontal and vertical lithologic changes within typical 

sand and gravel aquifers found in the Northeastern 

United States. Sand and gravel aquifers in this region 

consist primarily of unconsolidated deposits of varying

grain sizes, often underlain by till that is thin and 

discontinuous. These units are underlain by either sedimentary or crystalline bedrock (fig. 8). In order to model 

these aquifers, the following simplifying assumptions 

were made:

1. Sand and gravel aquifer material can be unsaturated 

or saturated;

2. Till is usually thin and discontinuous, and therefore is 

assumed to have little effect on the model;

3. Resistivity of the aquifer units can be empirically 

related to the grain size of the aquifer material;

4. Quality of water in the aquifer is constant within the 

modeled area;

5. Unsaturated zone is homogeneous, with constant 

resistivity;

6. Unsaturated zone has a constant thickness;

7. Each bedrock type is homogeneous and infinitely 

thick, with a constant resistivity; and

8. All layers are homogeneous and of infinite horizontal

extent.

Using these assumptions, 10 hypothetical geoelectric 

Earth models were used in this study to represent 

conditions typical of glacial valleys: (1) coarse-grained 

stratified drift overlying sedimentary or crystalline bedrock; (2) fine-grained stratified drift overlying sedimentary or crystalline bedrock; (3-6) coarse-grained stratified drift over fine-grained stratified drift overlying
A10 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

TABLE 2. Resistivity values of various materials used in the hypothetical Earth models

Material description Resistivity 

(ohm-m)

Unsaturated, coarse-grained stratified drift. 

Unsaturated, fine-grained stratified drift... 

Saturated, coarse-grained stratified drift ... 

Saturated, fine-grained stratified drift .....

Sedimentary bedrock.....................

Crystalline bedrock ......................

2,000

300

800

50

500

2,000

sedimentary or crystalline bedrock; and (7-10) finegrained stratified drift over coarse-grained stratified 

drift overlying sedimentary or crystalline bedrock.

For each hypothetical geoelectric layer in the model, 

the resistivities were estimated from previous investigations or published tables (Collett, 1979; Telford and 

others, 1976; McNeill, 1980a; Mooney, 1980). The resistivity values used to represent the various geologic layers 

are given in table 2.

Two forward-modeling computer programs were used 

to calculate the response of VLF terrain-resistivity and 

inductive terrain-conductivity instruments over various 

hypothetical geoelectric Earth models. The VLF computer program, VLF.BAS, is based on plane-wave electromagnetic theory and is written in Basic computer 

language for an IBM or compatible personal computer. 

The computer program, documented by Grantham and 

others (1986), calculates the apparent resistivity and 

phase angle between the induced horizontal magnetic 

and electric fields that would be measured by a Geonics 

EM16R VLF terrain-resistivity instrument over a horizontally layered Earth.

The inductive terrain-conductivity computer program, 

EM34.FOR, calculates the electromagnetic potential 

generated by an oscillating magnetic dipole over a horizontally layered Earth, and from that, the apparent 

conductivity of the Earth is calculated. Calculations are 

made for the Geonics EM34-3 instrument's three intercoil spacings, three frequencies, and two dipole orientations. The program, written in Fortran for an IBM or 

compatible personal computer, is documented by 

Grantham and others (1987).

The results of the forward-modeling programs are 

presented in figures 9 through 18 for the 10 hypothetical 

geoelectric Earth models. The variation of apparent 

conductivity between the various intercoil spacings and 

dipole orientation for each model can be qualitatively 

understood by comparing the model characteristics (such 

as the thickness and conductivity of individual layers) 

with the relative and cumulative response curves (fig. 7). 

The apparent resistivity, as measured by the method, is 

a function of the product of the conductivity of individual

layers and the response function of a particular intercoil 

spacing and dipole orientation.

COARSE-GRAINED AQUIFER MATERIAL

The first hypothetical geoelectric Earth model is a 

resistive geologic setting. The model consists of a coarsegrained unconsolidated aquifer saturated with freshwater (resistivity of 800 ohm-m) and is underlain by relatively conductive sedimentary bedrock (resistivity of 500 

ohm-m) or relatively resistive crystalline bedrock (resistivity of 2,000 ohm-m) (fig. 9).

In this setting, the VLF terrain-resistivity method 

measures a high apparent resistivity (590-1,630 ohm-m), 

and penetrates to a depth of about 100 m (fig. 5). A 

change in apparent resistivity is related to the thickness 

of the aquifer material over a given bedrock type. Over 

sedimentary (conductive) bedrock, the apparent resistivity will increase as the aquifer thickness increases. 

Over crystalline (resistive) bedrock, the apparent resistivity will increase as the aquifer thickness decreases. A 

change in the measured phase angle is related to the type 

of bedrock, if the bedrock is within the penetration depth 

of the method. For a given aquifer thickness, if the 

bedrock is resistive (relative to the aquifer material; for 

example, crystalline bedrock), the phase angle would be 

less than 45°; if the bedrock is conductive (relative to the 

aquifer material; for example, sedimentary bedrock), the 

phase angle would be greater than 45°. The phase angle 

is generally independent of the aquifer thickness.

In this same geologic setting, the inductive terrainconductivity method measures a low apparent conductivity (0.6-1.7 mS/m, fig. 9). The readings of individual 

intercoil spacings and dipole modes are dependent on the 

conductivity of the subsurface layers and the response 

curves of the instrument (fig. 7). McNeill (1980b) presents a detailed discussion of the response of the method 

to subsurface conductors. In general, however, the 

response is related to the product of the layer conductivity and the instrument response for a given intercoil 

spacing and dipole orientation.

In resistive terrains, the response of the instrument to 

relatively large changes in resistivity is small because of 

the design of the conductivity meter and the units of 

measurement. A change in apparent conductivity is 

related to the thickness of the aquifer material over a 

given bedrock type. Over sedimentary (conductive) bedrock, the apparent conductivity (fig. 9) will decrease 

slightly (0.1-0.4 mS/m) as the aquifer thickness 

increases. Over crystalline (resistive) bedrock, the 

apparent conductivity will increase slightly (0.1-0.4 

mS/m) as the aquifer thickness increases. All of the 

conductivity measurements are slightly higher (0.1-1.1 

mS/m) over the conductive bedrock than over the resist-
RESPONSE OF ELECTROMAGNETIC METHODS TO HYPOTHETICAL GEOELECTRIC EARTH MODELS All

EXPLANATION 

P1,2,3 = Resistivity

Unsaturated, coarsegrained material, h-|

Saturated, coarsegrained material, rt2

VLF 

Terrainresistivity 

method

1 t t * 1

Coarse-grained material 

p-i = 2,000 ohm-meters 1 WaLcl IdUlt

Coarse-grain 

material 

p2 = 800 

ohm-meter

ed 

s

Sedimentary 

bedrock 

p3 = 500 

ohm-meters

I

Crystalline 

bedrock 

P3 = 2,000 

ohm-meters

' I

t

= 3 meters

_!

"12

1

Layer thickness, h 

(meters)

3 

62

3 3 

32 12

3 3 

12 32

Apparent resistivity 

(ohm-meters)

790 680 590 

Phase 

(deg

50 50 49

3 

62

1,630 1,210 910 

angle 

rees)

41 38 39

Apparent conductivity 

(millisiemens per meter)

Terrainconductivity

method

1 10V2

10 H

20V

20 H

40V

40 H

1.1

.9

1.2

1.0

1.3

1.2

1.1

.9

1.3

1.1

1.5

1.3

1.2

1.0

1.5

1.2

1.7

1.4

0.9

.8

.8

.8

.6

.8

1.0

.8

1.0

.9

.8

.9

1.1

.9

1.1

1.0

1.0

1.0

1 1ntercoil spacing, in meters.

2 H, horizontal-dipole mode; V, vertical-dipole mode.

FIGURE 9. Hypothetical geoelectric Earth model of a coarse-grained unconsolidated aquifer overlying sedimentary and crystalline bedrock, and the modeled response based on the very-lowfrequency terrain-resistivity and inductive terrain-conductivity methods.
A12 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

EXPLANATION 

Pi ,2,3 = Resistivity

 Sounding positions

t_.. t_. t

t^f

 Water ta

Fine-grai 

materi

ohm-me1

Fine-grained material

PI = 300 ohm-meters 

ble

ned

31

3 

ers

Sedimentary 

bedrock 

p3 = 500 

ohm-meters

Cry stall 

bedroc 

P3 = 2,C 

ohm-me

t 

h-| = 3 meters

t

ne 

* 

00 

ters

Layer thickness, h 

(meters)

Unsaturated, finegrained material, h-|

Saturated, finegrained material, hi2

VLF

Terrainresistivity 

method

Terrainconductivity

method

3

62

60

50

1 10V2 13.3

10 H 10.6

20V 14.8

20 H 13.4

40V 13.7

40 H 14.5

3

32

53

47

13.3

10.6

13.5

12.8

10.5

12.7

3

12

3

12

Apparent resistivity

(ohm-meters)

88 

Phase

102

angle

(degrees)

31 23

Apparent conductivity

(millisiemens per meter)

11.3

9.5

9.0

10.2

5.2

8.6

11.1

9.4

8.4

9.7

4.2

7.9

3

32

50

46

13.3

10.6

13.3

12.7

10.0

12.4

3

62

60

50

13.3

10.6

14.8

13.4

13.5

14.4

1 1ntercoil spacing, in meters.

2 H, horizontal-dipole mode; V, vertical-dipole mode.

FIGURE 10. Hypothetical geoelectric Earth model of fine-grained unconsolidated material overlying 

sedimentary and crystalline bedrock, and the modeled response based on the very-low-frequency 

terrain-resistivity and inductive terrain-conductivity methods.
RESPONSE OF ELECTROMAGNETIC METHODS TO HYPOTHETICAL GEOELECTRIC EARTH MODELS A13

ive bedrock and are most noticeable in the 40-m verticaldipole mode (1.1 mS/m).

FINE-GRAINED MATERIAL

The second hypothetical geoelectric Earth model is a 

conductive geologic setting (resistivity of 50 ohm-m), 

such as fine-grained unconsolidated material, an unconsolidated aquifer saturated with conductive ground 

water, or till, which is underlain by relatively resistive 

sedimentary bedrock (resistivity of 500 ohm-m) or crystalline bedrock (resistivity of 2,000 ohm-m) (fig. 10).

In this setting, the VLF terrain-resistivity method 

measures a low apparent resistivity (50-102 ohm-m), and 

penetrates to a depth of about 20 m (fig. 5). A change in 

apparent resistivity is related to the thickness of the 

fine-grained (conductive) material, as well as the type of 

bedrock, if the bedrock is within the penetration depth. 

The apparent resistivity will generally decrease as the 

thickness of the fine-grained material increases, over 

either sedimentary or crystalline bedrock (fig. 10). This 

happens because both types of bedrock are resistive 

compared to the fine-grained material. A change in the 

measured phase angle is related to the type of bedrock (if 

the bedrock is within the penetration depth) and the 

thickness of the fine-grained material. The phase angle 

measured over shallow sedimentary bedrock is higher 

than the phase angle measured over shallow crystalline 

bedrock because the crystalline bedrock is much more 

resistive than the sedimentary bedrock. Both phase 

angles are less than 45° because both types of bedrock 

are resistive compared to the fine-grained material. As 

the fine-grained material thickens, the effect of the 

bedrock diminishes because of the decreased penetration 

depth of the method, thus, the phase angle increases. 

The phase angle will exceed 45° if a relatively resistive 

(unsaturated) layer is present at the surface and the 

bedrock is below the penetration depth of the method. In 

general, VLF terrain-resistivity instrument readings 

are dominated by resistivity of the conductive material.

The inductive terrain-conductivity method measures a 

moderately high apparent conductivity (4.2-14.8 mS/m) 

in the geologic setting. The depth of penetration of this 

method is not affected by the conductivity of the subsurface layers. The apparent conductivity increases over 

both sedimentary and crystalline bedrock as the thickness of the fine-grained material increases. The 10-m and 

20-m horizontal-dipole mode readings are generally 

lower than the corresponding vertical-dipole mode readings when bedrock is deep. This difference results from 

the position of the thick fine-grained (conductive) layers 

and the response curves of the instrument. The readings 

are affected by the bedrock when the fine-grained material is thin. In this case, the 20-m and 40-m horizontaldipole mode readings are higher than the corresponding 

vertical-dipole mode readings over both bedrock types; 

four readings are higher for sedimentary bedrock than 

for crystalline bedrock.

COARSE-GRAINED AQUIFER MATERIAL OVER 

FINE-GRAINED MATERIAL

The next four hypothetical geoelectric Earth models 

are examples of geologic settings that have resistive 

units (coarse-grained, saturated aquifer material) overlying more conductive units (fine-grained, saturated 

material or till). The models had different total thicknesses of coarse-grained and fine-grained materials (thin 

or thick) over different bedrock types (sedimentary or 

crystalline) (figs. 11-14).

In these settings, the VLF terrain-resistivity readings 

of apparent resistivity and phase angle vary with the 

geometry of the site. Also, as the thickness of the 

resistive layer (aquifer material) decreases and the thickness of the fine-grained material increases, the resistivity increases and the penetration depth of the method 

decreases from 100 to 20 m (fig. 5).

Where the glacial material is thin (12 m), the apparent 

resistivity decreases from 1,630 to 102 ohm-m (over 

crystalline bedrock, fig. 13) and from 590 to 88 ohm-m 

(over sedimentary bedrock; fig. 11) as the thickness of 

the conductive material increases and the thickness of 

the aquifer material decreases. The apparent resistivity 

values are always higher over resistive bedrock than 

over conductive bedrock. The readings are affected less 

by the type of bedrock as the thickness of the conductive 

material increases and the penetration depth of the 

method decreases. The phase angle also decreases from 

41° to 23° (over crystalline bedrock) and from 49° to 31° 

(over sedimentary bedrock) as the thickness of the 

conductive material increases and the effect of bedrock is 

decreased. When bedrock is within the penetration depth 

of the method, the phase angle is higher over sedimentary bedrock than over crystalline bedrock because of the 

different resistivities of these units.

Where the glacial material is thick (62 m), the apparent 

resistivity decreases from 910 to 60 ohm-m (over crystalline bedrock, fig. 14) and from 790 to 60 ohm-m (over 

sedimentary bedrock, fig. 12) as the thickness of the 

conductive material increases and the thickness of the 

aquifer material decreases. The apparent resistivity and 

phase angles are relatively independent of bedrock type 

because the penetration depth of the method is exceeded, 

except when no fine-grained material is present. The 

phase angle does not vary much (65-69°, fig. 14) until the 

section is either all fine grained (50°) or all coarse grained 

(39°).
A14 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

EXPLANATION 

Pi, 2,3,4 = Resistivity

Unsaturated, coarsegrained material, h-|

Saturated, coarsegrained material, hi2

Saturated, finegrained material, hs

VLF 

Terrainresistivity 

method

I tit 1

Coarse-grained material I / 

p-i = 2,000 ohm-meters h i = 3 meters , X^ / 

 Water table r- -*- -^ / 

h 2 / / 

Coarse-grained 1 / I 

material V/ /

P2 - 800 ohm-meters / \ ^ 

/ P3 =

Fine-grained 

material 

300 ohm-meters

/ hs Fine-grained 

/ material 

/ Ps = 50 ohm-meters

Sedimentary bedrock 

P4 = 500 ohm-meters

Layer thickness, h 

(meters)

3 333 

12 864 

0 468

Apparent resistivity 

(ohm-metersj

590 266 192 143

Phase angle 

(degrees)

49 41 39 37

3 

0 

12

88

31

Apparent conductivity 

(millisiemens per meter)

T . 1 10V2 1.2 3.0 4.3 6.0 Terrainconductivity 10 H 1.0 1.9 2.7 3.8 

method 20V 1.5 3.7 5.0 6.4 

20 H 1.2 2.7 3.8 5.2 

40V 1.7 3.1 3.8 4.4 

40 H 1.4 3.1 4.2 5.3

11.3 

9.5 

9.0 

10.1 

5.2 

8.6

1 1ntercoil spacing, in meters.

2 H, horizontal-dipole mode; V, vertical-dipole mode.

FIGURE 11. Hypothetical geoelectric Earth model of a thin unconsolidated section consisting of a 

coarse-grained aquifer overlying fine-grained material, all of which overlies sedimentary bedrock, and the modeled response based on the very-low-frequency terrain-resistivity and inductive 

terrain-conductivity methods.
RESPONSE OF ELECTROMAGNETIC METHODS TO HYPOTHETICAL GEOELECTRIC EARTH MODELS A15

EXPLANATION 

Pi, 2,3,4 = Resistivity

Unsaturated, coarsegrained material, h-|

Saturated, coarsegrained material, (12

Saturated, finegrained material, (13

VLF 

Terrainresistivity 

method

1 tit

Coarse-grained material t 

p-i = 2,000 ohm-meters n i = 3 

 Water table -T- *-

Coarse-grained \/

material / , 

P2 = 800 ohm-meters /

/ P^ =

\

Fine-grained 

material 

300 ohm-meters

/ (13 Fine-grained 

/ material 

/ P3 = 50 ohm-meters

Sedimentary bedrock 

P4 = 500 ohm-meters

Layer thickness, h 

(meters)

3 333 

62 42 32 20 

0 20 30 42

Apparent resistivity 

(ohm-meters)

790 376 325 186 

Phase angle 

(degrees)

50 66 69 67

3 

0 

62

60

50

Apparent conductivity 

(millisiemens per meter)

... . 1 10V2 1.1 1.4 1.7 2.5 Terrainconductivity 10 H .9 1.0 1.2 1.6 

method 20V 1.2 2.2 3.1 5.1 

20 H 1.0 1.5 2.0 3.1 

40V 1.3 3.3 4.9 7.8 

40 H 1.2 2.3 3.2 5.2

13.3 

10.6 

14.8 

13.4 

13.7 

14.5

1 1ntercoil spacing, in meters.

2 H, horizontal-dipole mode; V, vertical-dipole mode.

FIGURE 12. Hypothetical geoelectric Earth model of a thick unconsolidated section consisting of a 

coarse-grained aquifer overlying fine-grained material, all of which overlies sedimentary bedrock, and the modeled response based on the very-low-frequency terrain-resistivity and inductive 

terrain-conductivity methods.
A16 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

EXPLANATION 

Pi ,2,3,4 = Resistivity

Unsaturated, coarsegrained material, h-|

Saturated, coarsegrained material, \\2

Saturated, finegrained material, hs

VLF 

Terrainresistivity 

method

I t f t A XI

Coarse-grained material T / 

pq = 2,000 ohm-meters n i = 3 ,meters^/ / 

 Water table -j- -*- ^~~^r~ 

h2 // /

Coarse-grained I/ /

material ,/ L ______

p2 = 800 ohm-meters / Fine-grained 

/ material 

x/ pi = 300 ohm-meters

/ hs Fine-grained 

/ material 

./ Ps = 50 ohm-meters

Crystalline bedrock 

P4 = 2,000 ohm-meters

Layer thickness, h 

(meters)

3 333 3 

12 864 0 

0 468 12

Apparent resistivity 

(ohm-meters)

1,630 407 258 177 102 

Phase angle 

(degrees)

41 30 29 28 23

Apparent conductivity 

(millisiemens per meter)

1 10V2 0.9 2.7 4.0 5.7 11.1 Terrainconductivity 10 H .8 1.8 2.5 3.7 9.4

method 20V .8 3.0 4.3 5.7 8.4 

20 H .8 2.4 3.5 4.9 9.7 

40V .6 2.0 2.7 3.3 4.2 

40 H .8 2.5 3.5 4.7 7.9

1 1ntercoil spacing, in meters.

2 H, horizontal-dipole mode; V, vertical-dipole mode.

FIGURE 13. Hypothetical geoelectric Earth model of a thin unconsolidated section consisting of a 

coarse-grained aquifer overlying fine-grained material, all of which overlies crystalline bedrock, 

and the modeled response based on the very-low-frequency terrain-resistivity and inductive 

terrain-conductivity methods.
RESPONSE OF ELECTROMAGNETIC METHODS TO HYPOTHETICAL GEOELECTRIC EARTH MODELS

 Sounding positions 

A17

EXPLANATION

Pi,2,3,4 = Resistivity

t t

Coarse-grained material I 

PT = 2,000 ohm-meters h i = 3 meters , / / 

 Water table r- -^ -^ / 

h2 

Coarse-grained

material 

P2 = 800 ohm-meters

1

Fine-grained

material 

Pi = 300 ohm-meters

Fine-grained

material 

Ps = 50 ohm-meters

Crystalline bedrock 

P4 = 2,000 ohm-meters

Layer thickness, h 

(meters)

Apparent conductivity 

(millisiemens per meter)

1 1ntercoil spacing, in meters.

2 H, horizontal-dipole mode; V, vertical-dipole mode.

Unsaturated, coarsegrained material, hi

Saturated, coarsegrained material, \\%

Saturated, finegrained material, (13

3

62

0

3

42

20

3

32

30

3

20

42

3

0

62

Apparent resistivity

VLF

Terrainresistivity 

method

910

39

360

65

(ohm-meters)

318 

Phase angle

(degrees)

69

180

67

60

50

Terrainconductivity

method

1 10V2

10 H

20V

20 H

40V

40 H

1.1

.9

1.1

1.0

1.0

1.0

1.4

1.0

2.1

1.5

3.0

2.1

1.7

1.2

3.0

2.0

4.7

3.1

2.5

1.6

5.1

3.1

7.5

5.1

13.3

10.6

14.8

13.4

13.5

14.4

FIGURE 14. Hypothetical geoelectric Earth model of a thick unconsolidated section consisting of a 

coarse-grained aquifer overlying fine-grained material, all of which overlies crystalline bedrock, 

and the modeled response based on the very-low-frequency terrain-resistivity and inductive 

terrain-conductivity methods.
A18 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

In summary, as the thickness of the conductive (finegrained) layer increases, and the thickness of the overlying aquifer material decreases (figs. 11 to 14), the 

depth of penetration of the method is exceeded. Because 

of this, the measured apparent resistivity and phase 

angles are independent of bedrock type and, thus, are 

dependent upon the thickness and resistivity of the 

fine-grained conductive material and the resistive aquifer material.

For the inductive terrain-conductivity method and 

where the glacial material is thin (12 m, figs. 11 and 13), 

the apparent conductivity readings all increase as the 

thickness of the conductive material increases and the 

thickness of the aquifer material decreases. In addition, 

the readings over conductive bedrock are all higher 

(0.1-1.1 mS/m) than the equivalent readings over resistive bedrock and are most noticeable in the 40-m verticaldipole mode, which is most responsive to deep layers (fig. 

7). The vertical-dipole readings (figs. 11 and 13) are 

generally higher than the horizontal readings. This difference reflects the position of the resistive over conductive glacial material in the subsurface and the response 

characteristics of the two dipole orientations (the 

horizontal-dipole mode is generally affected by shallow 

layers and the vertical-dipole mode is affected by deeper 

layers). As the aquifer material thins and the finegrained material thickens, first the 40-m horizontaldipole and then the 20-m horizontal-dipole readings 

become larger than the vertical-dipole readings. This 

change is due to the thickening conductive material and 

its effect on the relative response curve of the horizontaldipole mode (fig. 7).

Where the glacial material is thick (62 m, figs. 12 and 

14), the inductive terrain-conductivity method has readings that again all increase as the thickness of conductive 

material increases and the thickness of the aquifer material decreases. All of the readings except for the 40-m 

vertical-dipole readings are independent of bedrock 

type. The 40-m vertical-dipole readings are slightly 

higher (0.2-0.3 mS/m) over the conductive bedrock than 

over the resistive bedrock since this dipole mode has the 

greatest depth of penetration. Vertical-dipole mode 

readings are almost all higher or equal to the horizontaldipole readings reflecting the occurrence of resistive 

over conductive material. This difference is due to the 

presence of the conductive fine-grained material at the 

maximum point on the relative response curves for the 

vertical-dipole modes. The difference also results from 

the presence of the resistive unsaturated surface material or the resistive aquifer material at the maximum 

point on the relative response curve for the horizontaldipole modes (fig. 7). The one exception is the 40-m 

reading when the section consists entirely of fine-grained 

material. In this case, the relatively resistive bedrock is

affecting the 40-m vertical dipole, causing it to be less 

conductive than the horizontal dipoles which gets most of 

its response from the conductive fine-grained material.

FINE-GRAINED MATERIAL OVER COARSE-GRAINED 

AQUIFER MATERIAL

The next four hypothetical geoelectric Earth models 

are examples of geologic settings that have conductive 

units (fine-grained saturated material) overlying resistive material, such as a buried coarse-grained esker. 

These models specified either thick or thin layers of 

fine-grained and coarse-grained glacial material over 

different bedrock types (sedimentary or crystalline) 

(figs. 15-18).

In these settings, the depth of penetration of the VLF 

terrain-resistivity method is limited due to the high 

conductivity of the shallow fine-grained material. For 

example, the penetration depth of the method in material 

with a resistivity of 50 ohm-m is only 20 m (fig. 5).

Where the glacial material is thin (12 m, figs. 15 and

17), the method penetrates the entire unconsolidated 

section. The increasing thickness of the coarse-grained 

material and the decreasing thickness of the fine-grained 

material may be detected by increases in the measured 

apparent resistivity (88-590 and 102-1,630 ohm-m). The 

phase angle is independent of the relative thickness of 

the fine- and coarse-grained material, until the section 

becomes entirely coarse-grained. However, the phase 

angle is affected by the bedrock type. More resistive 

bedrock decreases the phase angle, and increases the 

apparent resistivity. 

Where the glacial material is thick (62 m, figs. 16 and

18), buried coarse-grained materials and different bedrock types beneath about 26 m of fine-grained material 

are undetectable because of the limited penetration 

depth of the method. VLF terrain-resistivity readings of 

phase angle and apparent resistivities therefore become 

independent of the resistivity of deeper layers as thickness of the upper fine-grained section increases and the 

thickness of the coarse-grained aquifer unit decreases.

For the inductive terrain-conductivity method and 

where the glacial material is thin (12 m, figs. 15 and 17), 

the apparent conductivity readings decrease (11.3-1.0 

and 11.1-0.6 mS/m) as the thickness of the conductive 

material decreases and the thickness of the coarsegrained aquifer increases. The 10-m spacing verticaldipole readings are generally greater or equal to the 

10-m horizontal dipole readings. This difference is due to 

the presence of the conductive saturated fine-grained 

materials underlying the relatively more resistant unsaturated fine-grained materials. In the 20-m and 40-m 

intercoil spacings, however, the horizontal-dipole readings are higher than the vertical-dipole readings because
RESPONSE OF ELECTROMAGNETIC METHODS TO HYPOTHETICAL GEOELECTRIC EARTH MODELS A19

EXPLANATION 

Pi, 2,3,4 = Resistivity

Fine-grained mater 

Pi = 300 ohm-mete 

 Water table 

Fine-grained 

material 

P2 = 50 ohm-meters

/ h

t 1

al t / 

rs h-] = 3 meters /

/ Coarse-grained 

material 

p-i = 2,000 ohm-meters

3 Coarse-grained 

material 

p3 = 800 ohm-meters

Sedimentary bedrock 

P4 = 500 ohm-meters

Layer thickness, h 

(meters)

Unsaturated, finegrained material, h-i

Saturated, finegrained material, hi2

Saturated, coarsegrained material, hs

VLF 

Terrainresistivity 

method

3 333 3 

12 864 0 

0 468 12

Apparent resistivity 

(ohm-meters)

88 121 159 222 590 

Phase angle 

(degrees)

31 30 30 32 49

Apparent conductivity 

(millisiemens per meter)

T . 1 10V2 11.3 9.7 8.5 6.7 1.2 Terrainconductivity 1 ° H 9 - 5 8.6 7.9 6.7 1.0 

method 20V 9.0 6.9 5.6 4.2 1.5 

20 H 10.1 8.6 7.5 6.1 1.2 

40V 5.2 3.8 3.1 2.5 1.7 

40 H 8.6 6.9 5.8 4.7 1.4

1 1ntercoil spacing, in meters.

2 H, horizontal-dipole mode; V, vertical-dipole mode.

FIGURE 15. Hypothetical geoelectric Earth model of a thin unconsolidated section consisting of a 

fine-grained material overlying a coarse-grained aquifer, all of which overlies sedimentary 

bedrock, and the modeled response based on the very-low-frequency terrain-resistivity and 

inductive terrain-conductivity methods.
A20 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

EXPLANATION 

Pi, 2,3,4 = Resistivity

Fine-grained material I 

P! = 300 ohm-meters h i = 3 meters 

 Water table i ^ -/^ / 

Fine-grained

material 

P2 = 50 ohm-meters

£

Sounding positions

t t t 1

Coarse-grained

material 

Pi = 2,000 ohm-meters

Coarse-grained

material 

= 800 ohm-meters

Sedimentary bedrock 

P4 = 500 ohm-meters

Layer thickness, h 

(meters)

Apparent conductivity 

(millisiemens per meter)

1 Intercoil spacing, in meters.

2 H, horizontal-dipole mode; V, vertical-dipole mode.

Unsaturated, finegrained material, hi

Saturated, finegrained material, h2

Saturated, coarsegrained material, (13

VLF 

Terrainresistivity 

method

3 

62 

0

3 

42 

20

3 

32 

30

3 

20 

42

3 

0 

62

Apparent resistivity 

(ohm-meters)

60

50

56

50

54 

Phase angle 

(degrees)

49

60

37

786

50

Terrainconductivity

method

1 10V2

10H

20V

20 H

40V

40 H

13.3

10.6

14.8

13.4

13.7

14.5

13.4

10.6

14.2

13.1

11.9

13.5

13.3

10.6

13.4

12.7

10.3

12.6

12.7

10.3

11.5

11.6

7.5

10.6

1.1

.9

1.2

1.0

1.3

1.2

FIGURE 16. Hypothetical geoelectric Earth model of a thick unconsolidated section consisting of a 

fine-grained material overlying a coarse-grained aquifer, all of which overlies sedimentary 

bedrock, and the modeled response based on the very-low-frequency terrain-resistivity and 

inductive terrain-conductivity methods.
RESPONSE OF ELECTROMAGNETIC METHODS TO HYPOTHETICAL GEOELECTRIC EARTH MODELS A21

EXPLANATION 

Pi, 2,3,4 = Resistivity

Sounding positions

t t t i

Fine-grained material I 

p-, = 300 ohm-meters h i = 3 ,meters / / 

 Water table -T- -*- -+ ' 

h2 

Fine-grained

material 

p2 = 50 ohm-meters / Coarse-grained

material 

Pi = 2,000 ohm-meters

Coarse-grained

material 

P3 = 800 ohm-meters

Crystalline bedrock 

P4 = 2,000 ohm-meters

Layer thickness, h 

(meters)

Apparent conductivity 

(millisiemens per meter)

1 1ntercoil spacing, in meters.

2 H, horizontal-dipole mode; V, vertical-dipole mode.

Unsaturated, finegrained material, h-]

Saturated, finegrained material, h 2

Saturated, coarsegrained material, hs

VLF 

Terrainresistivity 

method

3

12

0

102

23

333

864

468

Apparent resistivity 

(ohm-meters)

150 213 233 

Phase angle 

(degrees)

21 21 22

3

0

12

1,630

41

Terrainconductivity

method

1 10V2

10 H

20V

20 H

40V

40 H

11

9,

8,

9,

4,

7,

.1

.4

.4

.7

.2

.9

9,

8,

6.

8.

2.

6,

.4

.5

.2

.2

.7

.2

8.2

7,

4,

7,

2

5

.7

.9

.1

.1

.2

6.

6.

3.

5.

1.

4.

.4

.6

.5

,7

.5

.0

0.9

.8

.8

.8

.6

.8

FIGURE 17. Hypothetical geoelectric Earth model of a thin unconsolidated section consisting of a 

fine-grained material overlying a coarse-grained aquifer, all of which overlies crystalline bedrock, 

and the modeled response based on the very-low-frequency terrain-resistivity and inductive 

terrain-conductivity methods.
A22 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

EXPLANATION 

Pi,2,3,4 = Resistivity

i

Sounding positions

t t t 1

Fine-grained material

Pi = 300 ohm-meters

Water table 

Fine-grained

material 

P2 = 50 ohm-meters Coarse-grained

material 

Pi = 2,000 ohm-meters

Coarse-grained

material 

= 800 ohm-meters

Crystalline bedrock 

4 = 2,000 ohm-meters

Layer thickness, h 

(meters)

Apparent conductivity 

(millisiemens per meter)

1 1ntercoil spacing, in meters.

2 H, horizontal-dipole mode; V, vertical-dipole mode.

Unsaturated, finegrained material, h-|

Saturated, finegrained material, \\2

Saturated, coarsegrained material, hs

VLF 

Terrainresistivity 

method

3 

62 

0

63

50

3 

42 

20

54

50

3 3 

32 20 

30 42

Apparent resistivity 

(ohm-meters)

52 56 

Phase angle 

(degrees)

49 35

3 

0 

62

912

39

Terrainconductivity

method

1 10V2

10 H

20V

20 H

40V

40 H

13,

10,

.3

.6

14.8

13

13,

14

.4

.5

.4

13.

10.

14.

13.

,4

,6

.2

,1

11.7

13.,4

13.

10.

13.

12.

10.

12.

.3

,6

.4

.7

,1

.5

12.

10.

11,

11.

7.

10.

,7

3

.4

6

2

,5

1.1

.9

1.1

1.0

1.0

1.0

FIGURE 18. Hypothetical geoelectric Earth model of a thick unconsolidated section consisting of a 

fine-grained material overlying a coarse-grained aquifer, all of which overlies crystalline bedrock, 

and the modeled response based on the very-low-frequency terrain-resistivity and inductive 

terrain-conductivity methods.
FIELD APPLICATION OF SURFACE-GEOPHYSICAL METHODS A23

of conductive fine-grained material being near the maximum response portion of the horizontal-dipole response 

curve. Both dipole readings are also affected by the 

conductivity of the bedrock; all readings increase (0.1-1.1 

mS/m) as the conductivity of the bedrock increases. The 

largest increase caused by bedrock is in the 40-m 

vertical-dipole mode, in which most of the response is 

from the bedrock (fig. 7).

Where the glacial material is thick (62 m, figs. 16 and 

18), all of the apparent conductivity readings except the 

40-m vertical-dipole readings become independent of 

bedrock type since this is about the maximum penetration depth of the method. The 40-m vertical-dipole readings are slightly higher (0.2-0.3 mS/m) over the conductive bedrock than over the resistive bedrock. As in the 

shallow case, almost all of the apparent conductivity 

measurements increase with increasing thickness of the 

conductive material and decreasing thickness of the 

aquifer material. The 10- and 20-m vertical-dipole readings are generally higher than the horizontal-dipole 

readings a response that reflects the resistive, unsaturated fine-grained surface material overlying the conductive saturated material. The 40-m horizontal-dipole 

readings are generally higher than the vertical-dipole 

readings. This higher reading reflects the maximum 

response of the horizontal dipole located within the 

conductive fine-grained material and the maximum 

response of the vertical dipole located within the resistive coarse-grained aquifer material.

SUMMARY OF MODELING STUDIES

Results of computer models that simulate the use 

of VLF terrain-resistivity and inductive terrainconductivity methods in hypothetical aquifer settings, 

representative of the glaciated Northeastern United 

States, demonstrate that both methods can detect small 

scale horizontal and vertical electrical changes in the 

subsurface. Each method works better in some geologic 

settings than in others because of differences in the 

design and operating principles of the two instruments. 

Forward-modeling studies of specific geologic settings 

provide a basis for designing and interpreting field 

measurements.

In general, in resistive terrains composed of entirely 

coarse-grained aquifer material or in coarse-grained 

aquifer materials over fine-grained material, the VLF 

terrain-resistivity method is sensitive to moderate 

changes in the thickness of aquifer material in the 

subsurface. The method has a good depth of penetration 

(100 m in 800 ohm-m terrains) (fig. 5) and can detect 

conductive layers underlying resistive aquifer layers. If 

bedrock is within the penetration depth of the method, it 

affects the phase angle.

In these settings, the inductive terrain-conductivity 

method measures low conductivity values that may be 

difficult to accurately read on the instrument. Small 

changes in the apparent conductivity are related to the 

thickness of the aquifer material and to the type of 

bedrock.

In conductive terrains composed of fine-grained material or fine-grained material overlying coarse-grained 

aquifer material, the VLF terrain-resistivity method has 

a limited penetration depth (25 m in 50 ohm-m terrains) 

(fig. 5). Consequently, this method responds only to 

near-surface changes in resistivity. Changes in apparent 

resistivity are dependent on the thickness of the finegrained conductive material and to the type of shallow 

bedrock.

The penetration depth with inductive terrainconductivity methods depends on the intercoil spacing 

and dipole mode, and is relatively independent of the 

resistivity of the ground. It is sensitive to changes in the 

position, thickness, and conductivity of individual layers 

in the subsurface. The apparent conductivity increases as 

the thickness of the conductive fine-grained material 

increases. The 40-m vertical-dipole position is affected by 

the conductivity of the bedrock.

FIELD APPLICATION OF SURFACEGEOPHYSICAL METHODS

Eight field sites in the glaciated Northeastern United 

States were selected to (1) verify the results obtained 

with the hypothetical geoelectric Earth models; (2) show 

that the combined use of seismic-refraction, deresistivity, and two electromagnetic methods may be 

capable of determining hydrogeologic boundaries and 

resistivity changes within sand and gravel aquifers; and 

(3) establish that an empirical relation exists between 

formation resistivity and grain size. These sites, located 

in Connecticut, New York, and Maine (fig. 19), represent 

typical hydrogeologic settings of the region, and are 

similar to the settings used in the hypothetical models. 

The sites were selected because the following criteria 

were met:

1. Well or test-hole data were available;

2. Seismic-refraction data or well data that defined 

depths to water and bedrock were available;

3. Cultural features such as power lines, pipelines, and 

fences were minimal;

4. Water-quality problems were not known to exist;

5. Area was large enough to conduct Schlumberger 

dc-electrical soundings;

6. Land-surface topography was mostly flat;

7. Site had easy field access; and

8. Water-quality information was available.
A24 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

EXPLANATION

Northeastern glacial 

Regional Aquifer-System 

Analysis study area

Surface-geophysical 

survey sites

Oxford

(Regional airport and

esker sites)

SJmsbury 

x^ Farmiojton ^

PENNSYLVANIA

/'NEW 

>7 JERSEY

FIGURE 19. Location of the eight field sites in the glaciated Northeastern United States for testing surface-geophysical

methods. (Modified from Lyford and others, 1984.)

Existing information from seismic-refraction profiles, 

well and test-hole logs, water-quality data, and geologic 

maps of each site were evaluated prior to the commencement of field work.

A de-resistivity sounding, a VLF terrain-resistivity 

survey, and an inductive terrain-conductivity survey 

were conducted at each site coincident with the location 

of a seismic-refraction profile. A de-resistivity sounding 

was not conducted at one of the sites in Maine (representing an esker) because the assumption of a horizontally layered Earth was not valid.

A Bison model 2390 de-resistivity instrument was used 

to conduct Schlumberger electrical soundings; the resulting plot of apparent resistivity as a function of electrode 

spacing was plotted in the field to ensure that a smooth 

curve was obtained. The electrode spacing was increased 

until several readings in bedrock (or the deepest layer of

interest) were obtained. Since the thickness and composition of glacial materials can vary substantially across a 

valley, the de-resistivity measurements are considered 

to represent the average lithology and thickness of the 

subsurface units at a particular sounding site.

Upon completion of the de-resistivity sounding, a VLF 

terrain-resistivity survey using a Geonics model EM16R 

and an inductive terrain-conductivity survey using a 

Geonics model EM34-3 were run. Intercoil spacings of 

10 m, 20 m, and 40 m and both horizontal- and verticaldipole modes were used on the EM34-3 survey. At most 

sites, electromagnetic readings (both VLF and terrainconductivity) were taken at 30- or 60-m intervals along 

the de-sounding and seismic-refraction lines. At the 

esker site in Maine, the electromagnetic profiles were 

perpendicular to the assumed axis of the esker.
RESULTS AND INTERPRETATIONS OF FIELD STUDIES A25

RESULTS AND INTERPRETATIONS OF FIELD 

STUDIES

A geologic section showing land surface, water table, 

and depth to bedrock was constructed from seismicrefraction and test-hole data. Next, the de-resistivity 

sounding data were interpreted to obtain general resistivity values for the hydrogeologic layers at each site. The 

field-generated de-resistivity sounding curves were 

smoothed by shifting individual segments and eliminating cusps caused by lateral inhomogeneities (Zohdy and 

others, 1974). Six values of apparent resistivity for each 

log cycle of current electrode spacing were then chosen 

and entered into a computer inversion program (Zohdy, 

1974). The result was a geoelectric Earth model with a 

large number of layers based on small electrical property 

variations. Many of these layers were combined manually and used with the seismic-refraction and test-hole 

data to construct a simplified four- or five-layered geoelectric Earth model. A major limitation of this simplified 

geoelectric Earth model is the assumption that each 

subsurface layer is horizontal and homogeneous over the 

distance between the electrodes. Small-scale lateral variations in layer thickness or resistivity of individual layers 

cannot be detected using this method. The interpreted 

resistivity values for individual layers were used with 

the geologic logs of nearby wells and test holes to develop 

a relation between grain size and electrical resistivity.

The VLF forward-modeling computer program was 

used then to refine the simplified geoelectric Earth 

model so that it agreed with the field VLF data. Several 

simplifying assumptions had to be made in the VLF 

modeling process since a four-layer Earth model would 

have eight unknowns the thickness and resistivity of 

each layer. Varying all of the unknowns could lead to an 

unreasonable subsurface model. In general, depths to 

the water table and to bedrock were assumed to be 

known from the seismic-refraction data, and were not 

varied during the VLF modeling process. The resistivity 

of the unsaturated material and the coarse-grained aquifer material was assumed to be constant. Thus, the 

thickness and resistivity of the fine-grained material and 

the resistivity of the bedrock are unknowns. To match 

the apparent resistivity and phase angle determined in 

the field by the VLF survey, some or all of these 

variables were changed during the modeling process. 

The changes depended on the specific geologic setting 

and results of the hypothetical modeling process. Typically, four to six modeling runs of the program were 

needed to match the modeled VLF data to the field data.

An interpreted geoelectric Earth model resulted from 

this process, but the question of whether or not it was a 

reasonable solution remained. Because the response of 

both electromagnetic methods to lateral and vertical 

variations in conductivity or resistivity is different, the

conjunctive use of both methods would define an 

improved geoelectric Earth model. To accomplish this, 

data from the VLF model were used as input for the 

terrain-conductivity forward-modeling program. The 

output of this model then was compared with the 

EM34-3 terrain-conductivity field data. Points along the 

electromagnetic profile then were computed and compared with the measured field data. If the field data 

varied substantially, the points were noted as problem 

areas and the differences were attributed to one or more 

of the following: (1) errors in obtaining field measurements of electromagnetic data, (2) cultural interference 

with field measurements, (3) a nonreasonable VLF 

model, and (4) the presence of unknown water-quality 

problems.

COARSE-GRAINED AQUIFER MATERIAL OVERLYING 

CRYSTALLINE BEDROCK

A stratified-drift deposit located in Stonington, Conn., 

(fig. 20) represents a coarse-grained aquifer overlying 

crystalline bedrock. A de-resistivity sounding, a VLF 

terrain-resistivity profile, and an inductive terrainconductivity profile were conducted in an open field 

parallel to and 15 m from a dirt road. Power and 

telephone lines along the road service a baseball field in 

the area. Seismic-refraction data from line B-B' and a 

geologic log, water level, and water-quality data from 

well SN-164 were available from a previous study of this 

area (Bingham, 1991). The depth to the water table 

determined from seismic-refraction data agrees with the 

water level in the well, but the depth to bedrock determined from seismic-refraction data is about 4 m deeper 

than the depth determined from the well and test holes.

The smoothed plot of the field-data points from the 

de-resistivity sounding, the computed sounding curve, 

and the resulting layered geoelectric Earth model are 

shown in figure 21. This multilayered geoelectric Earth 

model was simplified into a four-layer model by using the 

seismic-refraction data for depths to the water table and 

bedrock, and by combining layers with similar resistivities. Comparison of this simplified four-layer geoelectric 

Earth model with the geologic logs from well SN-164 and 

the two test holes SN-143th and SN-144th, drilled for 

this study (fig. 22), shows that the top two geoelectric 

layers (450 and 2,000 ohm-m) can be correlated with 

unsaturated soil and unsaturated, coarse-grained stratified drift, respectively. The third geoelectric layer (500 

ohm-m) represents the saturated aquifer material that 

varies in grain size from very fine sand to gravel. The 

lowest geoelectric layer (1,600 ohm-m) represents crystalline bedrock. The specific conductance of ground 

water from the coarse-grained stratified drift (well
A26 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

72°52'30" 72°52'

nr/ Base from U.S Geological Survey, 

u Ashaway 1 24,000, 1953, photorevised 1970 

Old Mystic 1 24,000, 1958, photorevised 1970

1,000 2,000 FEET 

_I

500 METERS

CONTOUR INTERVAL 50 FEET 

Datum is Sea Level

EXPLANATION

A'

B'

Electromagnetic 

survey line

Seismic-refraction 

line

Direct-cunrentresistivity survey 

line

SN-143TH

SN-164

Test hole Number 

is town well number 

used in U.S. Geological 

Survey publications

Observation well 

Number is town well 

number used in U.S. 

Geological Survey 

publications

FIGURE 20. Location of the study site in Stonington, Conn., 

including wells, test holes, and surface-geophysical survey 

lines.

SN-164) was 102 microsiemens per centimeter at 25 

degrees Celsius (jjiS/cm at 25°C) on August 17, 1982.

This field site is similar to the hypothetical model 

shown on the right side of figure 9. The results of the 

VLF terrain-resistivity survey are shown in the upper 

part of figure 23. The data show generally high apparent 

resistivities of 1,200-1,700 ohm-m in the middle of the 

line and lower values, 300-700 ohm-m, at each end of the 

line. The phase angles are all less than 45°, which 

indicates relatively conductive material over resistive 

material (saturated, coarse-grained stratified drift over 

crystalline bedrock).

In the VLF forward-modeling interpretation, the electrical resistivities, the thicknesses of layers 1 and 2 

(unsaturated soil and stratified drift) and the thickness of 

layer 3 (saturated stratified drift) were not varied. The 

only variables in the modeling process were the electrical 

resistivities of layers 3 and 4 (saturated stratified drift 

and bedrock).

The interpreted geoelectric Earth model resulting 

from the VLF forward-modeling process is shown in the 

lower part of figure 23. The apparent resistivity and 

phase angles generated in the modeling program closely 

matched the field values. The interpreted electrical 

resistivity of layer 3 (saturated stratified drift) is highest 

in the middle of the section and decreases at both ends. 

Because the specific conductance of ground water is 

assumed to be constant across this section, this resistivity change may indicate that the coarsest material is in 

the center of the section and that the amount of fine 

material increases toward each end. The modeled resistivity of t}ie bedrock also varies along the section and is 

largely dependent on the depth of penetration of the 

VLF terrain-resistivity method and the degree and 

depth of water-filled fractures in the bedrock. The difference in the number of fractures at depth, and hence 

the bulk resistivity of the bedrock, probably accounts for 

the resistivity change of this unit.

The interpreted layer thicknesses and resistivities 

calculated by the VLF modeling process (fig. 23) were 

used as input data to the inductive terrain-conductivity 

forward-modeling program. The computed results were 

compared with the field measurements (fig. 24). The 

computed values agree with the field values with the 

exception of two values: the 20-m vertical-dipole readings at stations 60 and 120. Because only these two field 

readings are high, and because these readings differ 

substantially from adjacent values, they may reflect 

nearby electrical interference from telephone or power 

lines. The close agreement of the remaining data points 

suggests that the geoelectric Earth model (fig. 23) interpreted from the de-resistivity, terrain-resistivity, and 

seismic-refraction data is a reasonable model of the 

subsurface. Geologic logs from the two test holes and one
RESULTS AND INTERPRETATIONS OF FIELD STUDIES A27

10,000 r=

X

0 1,000

z

CO

CO 

UJ 

DC

100

i i I r i i i i i i i M

P

Layered geoelectric Earth model

I I I II

 Computed sounding curve 

O Smoothed field-data points

I I I I I I

0.1 10

ELECTRODE SPACING (AB + 2), OR 

DEPTH BELOW LAND SURFACE, IN METERS

100 1,000

FIGURE 21. Smoothed plot of field-data points from direct-current-resistivity sounding, the computed sounding curve, and the 

resulting layered geoelectric Earth model for data from the study site in Stonington, Conn.

well along this profile indicate that the center of the 

profile has slightly coarser grained material than the 

ends of the profile (fig. 22).

COARSE-GRAINED AQUIFER MATERIAL OVERLYING 

SEDIMENTARY BEDROCK

Stratified-drift deposits located in Granby, Conn., and 

Smyrna, N.Y., represent coarse-grained aquifers overlying sedimentary bedrock. At the Granby site (fig. 25), 

a de-resistivity sounding, a VLF terrain-resistivity profile, and an inductive terrain-conductivity profile, were 

conducted parallel to the axis of the valley, 200-300 m 

from a paved road. Seismic-refraction data from line 

B-B' and a geologic log and water-level data from well 

GR-330 were available at this site. Water-quality data 

were collected on February 14, 1985. The depths to 

water table and bedrock determined from seismicrefraction data are similar to data from well GR-330, 

although complete seismic-refraction coverage was not 

available.

The smoothed plot of the field-data points from the 

de-resistivity sounding, the computed sounding curve, 

and the resulting geoelectric Earth model are shown in 

figure 26. This multilayered geoelectric Earth model was 

simplified into a five-layer model by using the seismicrefraction data for depth to water table and bedrock and 

by combining layers having similar resistivities. Comparison of this simplified five-layer geoelectric Earth 

model with the geologic logs of well GR-330 and test 

holes GR-25th and GR-26th (fig. 27) shows that the top 

two geoelectric layers (425 and 7,000 ohm-m) can be 

correlated with unsaturated soil and unsaturated, 

coarse-grained stratified drift respectively. The next two 

layers (2,000 and 450 ohm-m) represent the saturated 

aquifer material, with the lower unit reflecting the 

combined effect of a thin layer of till beneath a layer of 

slightly finer-grained aquifer material. The bottom geoelectric layer was initially assumed to be all sedimentary 

bedrock. This sounding actually included two bedrock 

types on each side of a major fault. The resistivity value, 

therefore, is not indicative of either rock type. The 

specific conductance of ground water from the coarsegrained stratified drift (well GR-330) was 130 |xS/cm at 

25°C on February 14, 1985.

This field site is similar to the hypothetical model 

shown on the left side of figure 9. The results of the VLF 

terrain-resistivity survey are shown in the upper part of 

figure 28. The data show a low apparent resistivity value 

of 100 ohm-m at the southern end (A) of line A-A', and an 

increase to 200 ohm-m near the northern end of the line. 

The phase angles are all greater than 45°, which indicates 

resistive over conductive material (saturated, coarsegrained stratified drift over sedimentary bedrock).
A28

CJ

<LJ_

cc13

CO 

Q

Simplified geoelectric 

Earth model

Resistivity in ohm-meters

REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

WELL AND TEST-HOLE LOGS

SN-164 SN-143TH

20

__

450

2,000

500

1,600

Unsaturated soil 

Unsaturated, 

stratified drift

Saturated, 

stratified drift

Fill

Sand, FtoC 

and gravel

Sand, M to C

Sand, Cto VC 

and gravel

Sand, M to C

Sand, VFtoF

Till

Gneiss^

-WL

Bottom of hole at 13 meters

Sand, Fand Silt

Sand, M to C 

and gravel

Sand, C to VC 

and gravel

Sand, VF and silt

Sand, M to C 

 Till 

 Gneiss-

-WL

Bottom of hole at 14 meters

SN-144TH

Sand, VR to F 

and silt

Sand, F to C 

and gravel

Sand, F to M

Sand, C to VC

Sand, VF to silt 

 Till 

-Gneiss-

-WL

Bottom of hole at 15 meters

EXPLANATION

Crystalline bedrock

VF = Very fine 

F = Fine 

M = Medium 

C = Coarse 

VC = Very coarse 

WL = Water level Estimated from 

observation well SN-164

FIGURE 22. Simplified geoelectric Earth model from direct-current-resistivity data, and well and test-hole logs from the study site in

Stonington, Conn.

In the VLF forward-modeling interpretation, electrical resistivities and thicknesses of layers 1 and 2 (unsaturated soil and stratified drift) were held constant. 

Layers 3 and 4 were combined into one layer, representing the saturated, coarse-grained aquifer material. The 

thickness of this layer was varied in the modeling process 

only where seismic-refraction data were not available. 

Therefore, the variables in the modeling process were 

the electrical resistivities of layers 3 and 4 (saturated 

stratified drift and bedrock) and, in some areas, the 

thickness of the saturated stratified drift.

The interpreted geoelectric Earth model resulting 

from the VLF forward-modeling process is shown in the 

lower part of figure 28. The apparent resistivity and 

phase angles generated in the modeling program closely 

matched the field data. Depth to bedrock, determined 

from the modeling process where seismic-refraction data 

were unavailable, had mixed results. The depths were 

similar to those in test hole GR-25th and well GR-330. 

However, modeled depth was substantially in error at 

test hole GR-26th, near station 0. The bedrock resistivity

was generally low (35-115 ohm-m), but at stations 420 

and 480, it increased to 150 and 135 ohm-m, respectively. 

A geologic map of the Tariffville quadrangle (Schnabel 

and Eric, 1965) shows the location of a major Triassic 

boundary fault that separates sedimentary and crystalline bedrock in this area (fig. 25). The change in bedrock 

resistivity was, therefore, interpreted as a change in 

bedrock type. The change in bedrock type was later 

confirmed with rock samples from test holes GR-25th and 

GR-26th (fig. 27). The low resistivity of the crystalline 

bedrock (as compared to the Stonington and Oxford 

sites) is interpreted to be caused by a greater number of 

fractures in the crystalline bedrock associated with the 

boundary fault and the possibility of more conductive 

water in these fractures. Specific conductance of ground

FIGURE 23. Very-low-frequency apparent resistivity and phaseangle field data, and the interpreted geoelectric Earth model 

from direct-current-resistivity, seismic refraction, and very-lowfrequency apparent resistivity data, from the study site in 

Stonington, Conn.
RESULTS AND INTERPRETATIONS OF FIELD STUDIES A29

CO 

DC

3,000

2,500

2,000

A

 Apparent resistivity 

O Phase angle

> 1,500

1,000

500

10

A

METERS 

20^

15-O

10-

5-

Sea level -

5-

10

30 60 90 120 150 180 210 240 270 300 330 360 390 

DISTANCE, IN METERS

A

-^-

Coarse-grained 

stratified drift, 

2,000 ohm-meters

Land e (

Soil, 450 

ohm-meters

surface ^

,o> 2

H CD

150 280

Coarse-grained 

stratified drift

400 450 

(ohm-meters)

Water table

300 200 250

Crystalline bedrock

1,200 5,000 4,000 5,000 4,000

(ohm-meters) 

Seismic-refraction data used to interpret depth to water table and bedrock

1,100 1,400

VERTICAL EXAGGERATION X9 100 200 300 FEET

30 60 90 METERS
A30 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

A1 A

10 METER VERTICAL-DIPOLE MODE

20

16

12

FOR ALL GRAPHS: 

- o o Field values

 Computed values

10 METER HORIZONTAL-DIPOLE MODE

A'

cc.

o. 

a. 

< 20

16

12

20 METER VERTICAL-DIPOLE MODE

<JJ £\)

z

LU

2

UJ 16

C/5

_l

i 12

Z

> 8

0 4<

D ( 

Q

Z 

00 

I I I

 

"

^ __

^"^ ^^ * +

I * I "^^"^^

0

40 METER VERTICAL-DIPOLE MODE

20 METER HORIZONTAL-DIPOLE MODE

40 METER HORIZONTAL-DIPOLE MODE

60 120 180 240 300 360 0 60 120

MEASUREMENT STATION NUMBER

180 240 300 360

0 250 500 750 FEET

60 120 180 METERS

FIGURE 24. Field and computed inductive terrain-conductivity values for the 10-, 20-, and 40-meter-intercoil spacings at the study

site in Stonington, Conn.
RESULTS AND INTERPRETATIONS OF FIELD STUDIES A31

72°50'

Base from U S. Geological Survey, 

Tanffville 1 24,000, 1956, photorevised1970

1,000 

I

2,000 FEET 

_I

I 

500 METERS

CONTOUR INTERVAL 50 FEET 

Datum is Sea Level

EXPLANATION

 Approximate location 

of border fault 

(Schnabel and Eric, 

1965) A'

I Electromagnetic 

1 survey line

A'

__^ Direct-currentresistivity survey 

line

GR-25TH

Test hole Number 

is town well number 

used in U.S. Geological 

Survey publications 

I

Observation well 

Number is town well 

number used in U.S. 

Geological Survey 

publications

B'

Seismic refraction 

line

FIGURE 25. Location of the study site in Granby, Conn., including 

wells, test holes, and surface-geophysical survey lines.

water from sedimentary rocks of the Upper Connecticut 

River basin ranges from 86 to 2,150 n-S/cm at 25°C 

(Ryder and others, 1981, p. 59).

The interpreted layer thicknesses and resistivities 

calculated by the VLF modeling process (fig. 28) were 

used as input data to the inductive terrain-conductivity 

forward-modeling program. The computed results were 

compared with the field measurements (fig. 29). Data 

from stations 0 and 300 do not agree, possibly because 

field data at these points are poor or the geoelectric 

Earth model was interpreted incorrectly. In addition, all 

of the computed 40-m vertical-dipole readings are too 

high. The field values indicate that the bedrock is more 

resistive than the value obtained in the VLF modeling 

process. The causes for this observation are the oversimplification of the interpreted Earth model and the different penetration depths of the two electromagnetic methods. The penetration depth of the VLF terrainresistivity method is limited in conductive material (at 90 

ohm-m it penetrates 35 m of material), which at this site 

consists of a thin glacial till and upper fractured and 

saturated sedimentary and crystalline bedrock. The 

inductive terrain-conductivity method maintains its 

depth penetration in this setting (penetration depth of 

the vertical-dipole mode at the 40-m spacing is about 60 

m) and therefore measures deeper, more resistive bedrock layers. The detailed de-resistivity data (fig. 26) 

indicate that the bedrock is not uniform, but rather is 

layered. The top 30 m of bedrock has an interpreted 

resistivity value of 175 ohm-m and is underlain by a 400 

ohm-m layer.

A stratified-drift deposit located in Smyrna, N.Y., 

(fig. 30) represents a second example of a coarse-grained 

aquifer overlying sedimentary bedrock. A de-resistivity 

sounding, a VLF terrain-resistivity profile, and an inductive terrain-conductivity profile were conducted perpendicular to the axis of the valley. Seismic-refraction data 

from line B-B' were available from a previous study 

(Reynolds and Brown, 1984), and water-quality data 

were collected at well 13-23 on June 11,1985. A comparison of the depths to the water table and bedrock 

determined from seismic-refraction data with the log of 

test hole 14-21 is difficult at this site because the test 

hole is located 200 ft from the seismic-refraction line. 

Water-level measurement was not obtainable for this 

well. Well 13-23 did not have a log or water level because 

it is a domestic well and only water-quality information 

from the coarse-grained stratified drift was available.

The smoothed plot of the field-data points from the 

de-resistivity sounding, the computed sounding curve, 

and the resulting layered geoelectric Earth model are 

shown in figure 31. This multilayered geoelectric Earth 

model was simplified into a four-layer model by using the
A32 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

10,000

CO

DC

CO 

CO

1,000

100

 Layered geoelectric 

Earth model

 Computed sounding

curve

O Smoothed field-data 

points

i i i i 1 1 1

0.1 1 10 100 1,000

ELECTRODE SPACING (AB + 2), OR 

DEPTH BELOW LAND SURFACE, IN METERS

FIGURE 26. Smoothed plot of field-data points from direct-current-resistivity sounding, the computed sounding curve, and the 

resulting layered geoelectric Earth model for data from the study site in Granby, Conn.

seismic-refraction data for depth to bedrock and by 

combining layers with similar resistivities. Comparison 

of the simplified four-layer geoelectric Earth model with 

the geologic log of test hole 14-21 (fig. 32) shows that the 

top two geoelectric layers, 700 and 2,200 ohm-m, can be 

correlated with unsaturated soil and coarse-grained 

material and that the bottom two layers, 550 and 300 

ohm-m, represent saturated, coarse-grained material 

and sedimentary bedrock. The interpreted resistivity 

values of the saturated coarse-grained layer (550 ohm-m) 

is substantially lower than the 2,000 to 2,500 ohm-m 

determined for coarse-grained drift at Granby, Conn. 

This lower value is due primarily to the high conductivity 

of the ground water at the Smyrna site. The specific 

conductance of the ground water from the coarse-grained 

stratified drift (well 13-23) at Smyrna was 710 |xS/cm at 

25°C as compared to 130 |xS/cm at 25°C for the stratified 

drift at Granby. The more conductive ground water 

substantially lowers the bulk resistivity of the coarsegrained aquifer.

This field site is similar to the hypothetical model 

shown on the left side of figure 9. The results of the VLF 

terrain-resistivity survey are shown in the upper part of 

figure 33. These data show that the apparent resistivity 

varies from 150 to 300 ohm-m. The phase angles are all 

45° or slightly higher, reflecting slightly more resistive

material overlying slightly more conductive material (the 

unsaturated and saturated stratified drift over sedimentary bedrock).

In the VLF forward-modeling process, the electrical 

resistivities of layers 1 and 2 (unsaturated soil and 

stratified drift) and the thickness of layer 1 were held 

constant and the depths to water table and bedrock, 

determined from seismic-refraction data, were specified. 

Therefore, variables in the modeling process were the 

resistivities of layers 3 and 4 (saturated stratified drift 

and bedrock).

The interpreted geoelectric Earth model resulting 

from the VLF forward-modeling process is shown in the 

lower part of figure 33. The apparent resistivities and 

phase angles generated in the modeling program closely 

matched the field data. The resistivity of both saturated 

stratified drift (160-300 ohm-m) and bedrock (133-330 

ohm-m) did not vary greatly.

The interpreted layer thicknesses and resistivities 

calculated by the VLF modeling process (fig. 33) were 

used as input data to the inductive terrain-conductivity 

forward-modeling program. The computed results were 

compared with the field measurements (fig. 34). All 

stations show a good correlation at all intercoil spacings 

except for station 0. At this station, the field values are 

substantially greater than the predicted values. Station 0
RESULTS AND INTERPRETATIONS OF FIELD STUDIES A33

Simplified geoelectric 

Earth model

Resistivity in ohm-meters

LU

O

w

Q

Q_

LLJ

Q

425

7,000

2,000

450

250

Unsaturated soil

Unsaturated, coarsegrained stratified 

drift

Water table

Saturated, coarsegrained stratified 

drift

Saturated, coarsegrained stratified 

drift

WELL AND TEST-HOLE LOGS 

GR-25TH GR-330 GR-26TH

Soil

Sand, M to VC 

and gravel

 WL

Sand, M to VC

Sand, M to VC 

and gravel

Sand, F to C

Till

GneissSand and gravel

Sand, F to

Sand, M to

Sand, F to

 Till 

M

C

M

 WL

Bottom of hole at 13 meters Refusal at 13 meters

Bedrock

 WL

Sand, M to VC 

and gravel

TilK?)

Fractured 

red conglomerate

Specific conductance of ground water from the coarse-grained 

stratified drift aquifer in well GR-330 was 130 microsiemens 

per centimeter at 25 degrees Celsius on February 14, 1985.

1 Red siltstone '

EXPLANATION Bottom of hole at 15 meters 

F = Fine 

M = Medium 

C = Coarse 

VC = Very coarse 

WL = Water level

FIGURE 27. Simplified geoelectric Earth model from direct-current-resistivity data, and well and test-hole logs from the study site in

Granby, Conn.

is located next to a state highway that receives heavy 

applications of deicing chemicals each winter. The large 

field readings of this station could be due to the interference from power lines and guard rails along the highway 

or the presence of deicing chemicals in the ground water.

FINE-GRAINED MATERIAL OVERLYING SEDIMENTARY 

BEDROCK

A stratified-drift deposit located in Simsbury, Conn., 

(fig. 35) represents fine-grained material overlying sedimentary bedrock. A de-resistivity sounding, a VLF 

terrain-resistivity profile, and an inductive terrainconductivity profile were conducted along a dirt road 

that crosses the axis of the valley. Sources of cultural 

interference occurred at the west end of the profile from 

railroad tracks and from a construction company workshop with power and telephone lines. The remainder of 

the area, which is used partly for agricultural activities, 

was a relatively flat flood plain free from cultural interference. Seismic-refraction data from line B-B', and the 

geologic log of well SI-363 were available from an 

ongoing study of this area (Melvin and Bingham, 1991). 

Specific conductance of ground water in the fine-grained 

stratified drift and depth to the water table was measured in well SI-363 on February 11,1985. The depths to 

the water table and bedrock determined from seismicrefraction data agree with the limited data from well 

SI-363.

The smoothed plot of the field-data points from the 

de-resistivity sounding, the computed sounding curve, 

and the resulting layered geoelectric Earth model are 

shown in figure 36. This multilayered geoelectric Earth 

model was simplified into a four-layer model by using the 

depths to water table and bedrock determined from 

seismic-refraction data and by combining layers with 

similar resistivities. Comparison of this simplified fourlayer geoelectric Earth model with the geologic log of 

well SI-363 (fig. 37) shows that the top layer, with a 

resistivity of 800 ohm-m, can be correlated with the
A34 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

1,000

800

600

400

200

 Apparent resistivity 

O Phase angle

60 120 180 240 300 

DISTANCE, IN METERS

360 420

80

50

40

10

480

A

METERS

105 |

100 

( 

95 

90 

85 

80 

7K

w I / Coarse-grained 

j| vo Soil, 475 =3 « / stratified drift,

 g ^ ohm-meters ^ J, / 2,000 ohm-meters

-)H ° ^ \ ^ ° ^ / ^ Land ^ surface ^

2,

\

A1

|IJ= CM

^~\ /-

^ \ ^^ W / \-J ^J ^-S ^

/

Water table '

OOQ Coarse-grained stratified drift

2,500

v

\ 2'500 > ROD

\ I 2,500 3,000 2'5°°

\ 2,500

x. (ohm-meters) ^_______ 

Sedimentary \. ^__^. "~~ ,_

bedrock x. ^^^^ -§

^^^^^ o

GQ

80 115 115 65 65 35 80

(ohm-meters)

Seismic-refraction data used to interpret depth to water table and bedrock

j \.

 

2,500

2,5nn

Crystalline

+., bedrock

03

150 135

P

100 200 300 400 500 FEET

60

I 

120 METERS

Vertical exaggeration X12 

Datum is Sea Level

Q

FIGURE 28. Very-low-frequency apparent resistivity and phase-angle field data, and the interpreted geoelectric Earth 

model from direct-current-resistivity, seismic refraction, and very-low-frequency apparent resistivity data, from the study

site in Granby, Conn.
RESULTS AND INTERPRETATIONS OF FIELD STUDIES A35

10 METER VERTICAL-DIPOLE MODE

A A A' 

10 METER HORIZONTAL-DIPOLE MODE

20

16

12

FOR ALL GRAPHS: 

 o -o Field values

 Computed values

i i i i

20 METER VERTICAL-DIPOLE MODE

LU

tr

40 METER VERTICAL-DIPOLE MODE

n i i r

20 METER HORIZONTAL-DIPOLE MODE

\ i i i i i

iii

40 METER HORIZONTAL-DIPOLE MODE

0 60 120 180 240 300 360 420 480 0 60 120 180 240 300 360 420 480

MEASUREMENT STATION NUMBER

0 250 500 750 FEET

h

60 120 180 METERS

FIGURE 29. Graphs showing field and computed inductive terrain-conductivity values for the 10-, 20-, and 40-meter-intercoil

spacings at the study site in Granby, Conn.
A36 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

75°33'30" 75°33'

Base from U S. Geological Survey, 

Earlville 1'24,000, 1943, photorevised 1986

1,000

I

2,000 FEET 

 i f r _J

500 METERS

CONTOUR INTERVAL 100 FEET

Datum is Sea Level

EXPLANATION

A '

pi

Electromagnetic 

survey line

Seismic-refraction 

line

resistivity survey 

line

14-21 

<0> Test hole-Number

is town well number 

used in U.S. Geological 

Survey publications

13-23

0 Domestic well in 

stratified drift 

Number is town 

well number used 

in U.S. Geological 

Survey publications

FIGURE 30. Location of the study site in Smyrna, N.Y., including 

wells, test holes, and surface-geophysical survey lines.

combined unsaturated stratified drift and soil layers. The 

second layer, with a resistivity of 500 ohm-m, represents 

a thin layer of saturated coarse-grained material. The 

third layer, with a resistivity of 140 ohm-m, represents a 

very thicks section of fine-grained material, and the 

lowest layer, with a resistivity of 200 ohm-m, represents 

the sedimentary bedrock. The specific conductance of the 

ground water in the fine-grained stratified drift in the 

well was 180 jxS/cm at 25°C.

This field site is similar to the hypothetical model 

shown on the left side of figure 10, except that a shallow 

resistive coarse-grained unit is present. The results of 

the VLF terrain-resistivity survey are shown in the 

upper part of figure 38. The field measurements generally have a low apparent resistivity, 30-150 ohm-m, and 

phase angles of 45°-60°, indicating resistive over conductive material. These readings reflect the limited penetration of the VLF method in conductive terrains (approximately 25 m of penetration in 50-ohm-m material) and 

the presence of thin layers of resistive unsaturated and 

saturated, coarse-grained material above the thick 

conductive fine-grained section. At this site, the VLF 

terrain-resistivity method cannot detect resistivity 

changes in the bedrock.

The electrical resistivity determined by the deresistivity sounding, of layer 1 (soil), layer 2 (saturated, 

coarse-grained stratified drift), and layer 4 (bedrock), as 

well as the depths to water table and bedrock, as 

determined from the seismic-refraction profile, were not 

varied in the VLF forward-modeling process. Variables 

in the modeling process were therefore, thickness of 

saturated, coarse-grained material and thickness and 

resistivity of fine-grained material.

The interpreted geoelectric Earth model resulting 

from the VLF forward-modeling process is shown in the 

lower part of figure 38. A good match between field and 

calculated VLF apparent resistivities and phase angles 

was obtained. This interpretation shows a thin, discontinuous high-resistivity layer of saturated, coarsegrained material overlying a conductive (16-140 ohm-m) 

layer or section of saturated, fine-grained material. The 

resistivity of the deep sedimentary bedrock had no effect 

on the model calculations.

The interpreted layer thicknesses and resistivities 

calculated by the VLF modeling process (fig. 38) were 

used as input data to the inductive terrain-conductivity 

forward-modeling program. The computed results were 

compared with the field measurements (fig. 39). Field 

data do not agree with the computed values obtained at 

the beginning of the profile (stations 0-300) but agree 

beyond this point. Because the values measured in the 

field were anomalously high, it was inferred that either 

an unknown water-quality problem exists at this site or
RESULTS AND INTERPRETATIONS OF FIELD STUDIES A37

10,000 cr

1,000 

Layered geoelectric Earth model

 Computed sounding curve 

O Smoothed field-data points

1,000

ELECTRODE SPACING (AB + 2), OR 

DEPTH BELOW LAND SURFACE, IN METERS

FIGURE 31. Smoothed plot of field-data points from direct-current-resistivity sounding, the computed sounding curve, 

and the resulting layered geoelectric Earth model for data from the study site in Smyrna, N.Y.

cultural interference strongly affects the readings. Readings could not be made in this area at the 40-m intercoil 

spacing; therefore, the high readings are assumed to be 

caused by cultural interference.

COARSE-GRAINED AQUIFER MATERIAL OVER

FINE-GRAINED MATERIAL OVERLYING SEDIMENTARY

BEDROCK

Stratified-drift deposits located in Farmington, Conn., 

and Preble, N.Y., represent coarse-grained aquifer 

material over fine-grained material overlying sedimentary bedrock. The Farmington, Conn., site (fig. 40) is 

located in the bottom of a flat valley with no nearby 

cultural interference, and is underlain by an arkosic 

sandstone (Simpson, 1966). A de-resistivity sounding, a 

VLF terrain-resistivity profile, and an inductive terrainconductivity profile were conducted in an open field 

perpendicular to the axis of the valley. Seismic-refraction 

data from line B-B', the geologic log and water-level 

data from well F-282 and test hole F-68th, and waterquality data from well F-295 were available from a 

previous investigation (Mazzaferro, 1980). The depth to 

the water table determined from seismic-refraction data 

agrees with the test-hole and well data, and the depth to 

bedrock agrees in general with other test-hole data in the 

valley (Mazzaferro, 1980).

The smoothed plot of the field-data points from the 

de-resistivity sounding, computed sounding curve, and 

resulting layered geoelectric Earth model are shown in 

figure 41. This multilayered geoelectric Earth model was 

simplified into a five-layer model by using the seismicrefraction data for depths to water table and bedrock, 

the geologic log from test hole F-68th, and by combining 

layers with similar resistivities. Comparison of this simplified five-layer geoelectric Earth model with the geologic log (fig. 42) shows that the two top layers, with 

resistivities respectively of 900 and 1,740 ohm-m, are 

unsaturated soil and coarse-grained material. It is underlain by a saturated, coarse-grained unit with a resistivity 

of 560 ohm-m, which, in turn, is underlain by a thick, 

fine-grained unit with a resistivity of 240 ohm-m. The 

entire section is underlain by sedimentary bedrock with 

a resistivity of 140 ohm-m. The specific conductance of 

the ground water from the coarse-grained stratified drift 

(well F-295) was 162 ^S/cm at 25°C on May 17, 1977.

This field site is similar to the hypothetical model 

shown in figure 11. The results of the VLF terrainresistivity survey are shown in the upper part of figure 

43. The field data show low apparent-resistivity values, 

150-400 ohm-m, and phase angles greater than 45°, 

indicating resistive material (saturated, coarse-grained 

stratified drift) over conductive material (fine-grained 

stratified drift and sedimentary bedrock).
A38 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

bimphtied geoelectric TEST-HOLE LOG

Earth model

Resistivity in ohm-meters 14 21

0 | 

-

cr 10 -

i

LU

LU

£20-

LL

EC

D

C/5

Q

%

-> 30 -

O

LU

CO

I

Si 40-

Q

50

700

2200

550

300

 Unsaturated soil

 Unsaturated coarsegrained stratified

drift

Saturated coarsegrained stratified

drift

 Bedrock

Gravel

Till

Shale

Specific conductance of ground water from the coarse-grained stratified 

drift in well 13-23 was 710 microsiems per centimeter 

at 25 degrees Celsius on June 11, 1985.

FIGURE 32. Simplified geoelectric Earth model from directcurrent-resistivity data, and well and test-hole logs from the 

study site in Smyrna, N.Y.

In the VLF model, the top two layers of the simplified 

geoelectrical model were combined into one layer with a 

resistivity of 1,250 ohm-m. In the VLF forward-modeling 

process, the resistivity of layer 1, soil and Unsaturated 

stratified drift, was decreased from 1,250 ohm-m to 300 

ohm-m and then held constant. One reason for the 

different values of the upper layer between the two 

methods is that the low resistivity soil could have been 

absent from the area near the center of the de-resistivity 

spread. The resistivities of layer 2, saturated, coarsegrained stratified drift; and layer 4, sedimentary bedrock, were not changed. Thickness of layer 1 and depth 

to bedrock were determined from seismic-refraction data 

and were not varied. Variables, therefore, were thickness of layer 2 (coarse-grained stratified drift) and resistivity of layer 3 (fine-grained stratified drift).

The interpreted geoelectric Earth model resulting 

from the VLF forward-modeling process is shown in the 

lower part of figure 43. A good match between the 

calculated and field values for apparent resistivities and 

phase angles was obtained. The interpreted geoelectric 

Earth model shows a saturated, coarse-grained unit of

varying thickness over a fine-grained unit. As in the 

previous site at Simsbury, Conn., the low resistivity of 

the fine-grained layer prevented deep penetration by the 

VLF method; therefore, the modeling results are independent of the bedrock resistivity.

The interpreted layer thicknesses and resistivities 

calculated by the VLF modeling process (fig. 43) were 

used as input data to the inductive terrain-conductivity 

forward-modeling program. The computed results were 

compared with the field measurements (fig. 44). The 

agreement was generally very good with the exception of 

the data at station 650. At this station, the interpreted 

geoelectric Earth model does not represent the subsurface.

Upon completion of the geophysical surveys, six test 

holes were drilled along the geophysical survey line to 

confirm the interpreted Earth model. The logs of the drill 

holes are shown in figure 45 and, in general, confirm the 

interpreted geoelectric model. The one exception is the 

test hole at station 650 (F-73th) where the predicted 

contact between coarse- and fine-grained stratified drift 

is much deeper than the contact reported in the test hole. 

This confirms the previous conclusion that the interpreted geoelectric Earth model does not represent the 

subsurface at this station.

A stratified-drift deposit filling a broad valley in the 

town of Preble, N.Y., (fig. 46) is a second example of 

coarse-grained aquifer material over fine-grained material overlying sedimentary bedrock. The field site is 

located between two lakes and a major interstate highway, in a gently rolling glacial valley underlain by 

Devonian shale. The highway has power lines, guard 

rails, lights, and buried pipes associated with it. A 

de-resistivity sounding, a VLF terrain-resistivity profile, and an inductive terrain-conductivity profile were 

conducted near the shores of Green and Upper Little 

York Lakes, as far away from the highway as possible. 

Geologic logs from test holes 54-49b and 44-26b were 

available from a previous study (Randall, 1972). Waterquality data were available from well 05-50 (Buller, 1978, 

well CP38) and from well 00-01, which was sampled for 

this study on June 12, 1985.

Bedrock was penetrated at 102 m in a test hole, 4 

kilometers (km) south of the study area (Buller, 1978) 

and is assumed to be at about the same depth under the 

study area. Because bedrock is beyond the penetration 

depth of electromagnetic methods, it was not considered 

in the interpretation process.

FIGURE 33. Very-low-frequency apparent resistivity and phase- ^ 

angle field data, and the interpreted geoelectric Earth model 

from direct-current-resistivity, seismic refraction, and very-lowfrequency apparent resistivity data, from the study site in 

Smyrna, N.Y.
RESULTS AND INTERPRETATIONS OF FIELD STUDIES A39

A

500

400

300

A

METERS

360 -i

350-

340-

330-

320-

310

 Apparent resistivity 

O Phase angle

60 120 180 240 

DISTANCE, IN METERS

Coarse-grained 

stratified drift, 

2,200 ohm-meters

300

Soil, 700 

ohm-meters

275 237 

(ohm-meters)

Coarse-grained stratified drift 

165 160 243

330 280 160

Sedimentary bedrock

200 

(ohm-meters)

240 193

Seismic-refraction data used to interpret depth to water table and bedrock

360

A'

O

133

VERTICAL EXAGGERATION X6 

Datum is Sea Level

100 200 300 FEET

CO

UJ 

UJ 

DC

(D

UJ

O

UJ 

CO

30 60 90 METERS
A40 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

10 METER VERTICAL-DIPOLE MODE

20

16

12

FOR ALL GRAPHS: 

- o -o Field values

 Computed values

A' A

10 METER HORIZONTAL-DIPOLE MODE

LU 

CO

20 METER VERTICAL-DIPOLE MODE 20 METER HORIZONTAL-DIPOLE MODE

40 METER VERTICAL-DIPOLE MODE 40 METER HORIZONTAL-DIPOLE MODE

120 180 240 300 360 0 60

MEASUREMENT STATION NUMBER

0 250 500 FEET

h

120 180 240 300 360

0 60 120 METERS
RESULTS AND INTERPRETATIONS OF FIELD STUDIES A41

The smoothed plot of the field-data points from the 

de-resistivity sounding, the computed sounding curve, 

and the resulting layered geoelectric Earth model are 

shown in figure 47. The multilayered geoelectric Earth 

model was simplified to a five-layer model by assuming 

that the adjacent lake levels represent the altitude of the 

water table and combining layers with similar resistivities. Comparison of the simplified five-layer geoelectric 

Earth model with test-hole logs 44-26b and 54-49b (fig. 

48) shows that the two top layers, with resistivities of 

325 and 1,500 ohm-m, can be correlated with soil and 

unsaturated coarse-grained material, respectively. 

These units are underlain by saturated sand and gravel 

with some silt having an interpreted resistivity of 250 

ohm-m. This relatively low resistivity is due to the high 

specific conductance of the ground water in the coarsegrained stratified drift, 500 jjiS/cm at 25°C, as measured 

in wells 05-50 and 00-01. The bottom two geoelectric 

layers, having resistivities of 60 and 175 ohm-m, are 

correlated with the fine sand and silt layers that are 

present in sections of this valley.

This field site is similar to the hypothetical model 

shown in figure 12. The results of the VLF terrainresistivity survey are shown in the upper part of figure 

49. The field data show a relatively low apparent resistivity (100-250 ohm-m), which reflects the high specific 

conductance of ground water in coarse and fine-grained 

material underlying it. Phase angles from 50° to 60° 

indicate resistive material over conductive material (saturated, coarse-grained stratified drift over fine-grained 

stratified drift).

In the VLF forward-modeling process, the electrical 

resistivity of the two unsaturated zones and the saturated, coarse-grained stratified drift was held constant at 

325, 1,500, and 250 ohm-m respectively; depth to water 

table was estimated from the lake level. Bedrock was 

considered to be too deep to affect the VLF readings. 

Depth to and resistivity of the fine-grained material were 

the variables at this site.

The interpreted geoelectric Earth model resulting 

from the VLF forward-modeling process is shown in the 

lower part of figure 49. A good match between the 

calculated and field values of apparent resistivity and 

phase angle was obtained at each station. The interpreted geoelectric Earth model shows 5-15 m of saturated coarse-grained material overlying a relatively uniform fine-grained section (60-105 ohm-m). Because

FIGURE 34. Field and computed inductive terrain-conductivity 

values for the 10-, 20-, and 40-meter-intercoil spacings at the 

study site in Smyrna, N.Y.

72°48' 72°47'30"

30" Base from U S Geological Survey,

Tanffville 1 24,000, 1956, photorevised 1970

1,000 2,000 FEET 

__I

500 METERS

A'

CONTOUR INTERVAL 50 FEET 

Datum is Sea Level

EXPLANATION

B B'

__| Electromagnetic 

' survey line

A'

-A Direct-currentresistivity survey 

line

SI-363

Seismic-refraction 

line

Observation well 

Number is town well 

number used in U.S. 

Geological Survey 

publications

FIGURE 35. Location of the study site in Simsbury, Conn., including wells, test holes, and surface-geophysical survey lines.
A42 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

10,000 cr

1,000 

Layered geoelectric Earth model

 Computed sounding curve 

O Smoothed field-data points

1,000

ELECTRODE SPACING (AB + 2), OR 

DEPTH BELOW LAND SURFACE, IN METERS

FIGURE 36. Smoothed plot of field-data points from direct-current-resistivity sounding, the computed sounding curve, 

and the resulting layered geoelectric Earth model for data from the study site in Simsbury, Conn.

bedrock is beyond the penetration depth of the electromagnetic methods, it was not considered in the interpretation process.

The interpreted layer thicknesses and resistivities 

calculated by the VLF modeling process (fig. 49) were 

used as input data to the inductive terrain-conductivity 

forward-modeling program. The computed results were 

compared with the field measurements (fig. 50). The field 

readings at the north (A') end of the profile (station 480, 

which is nearest the highway) are very high for the 20-m 

horizontal-dipole mode (32 mS/m) and could not be measured for the 40-m horizontal-dipole mode or the 40-m 

vertical-dipole mode. It is inferred, therefore, that either 

the interpreted geoelectric Earth model at station 480 is 

not a true representation of the subsurface, or the field 

readings are affected by cultural interference from the 

highway.

COARSE-GRAINED AQUIFER MATERIAL OVER

FINE-GRAINED MATERIAL OVERLYING CRYSTALLINE

BEDROCK

A stratified-drift deposit in Oxford, Maine, represents 

coarse-grained aquifer material over fine-grained material overlying crystalline bedrock. The site is located on 

a flat outwash plain near the Oxford County Regional 

Airport (fig. 51) and could have cultural interference

from the airport or the utilities along nearby roads. A 

de-resistivity sounding, a VLF terrain-resistivity profile, and an inductive terrain-conductivity profile were 

conducted next to the airport access road. Geologic, 

water-level, and water-quality data were available from 

wells 0-1223 and 0-1367, as well as a detailed bedrock 

contour map from a previous study (Morrissey, 1983).

The smoothed plot of the field-data points from the 

de-resistivity sounding, the computed sounding curve, 

and the resulting layered geoelectric Earth model are 

shown in figure 52. This multilayered geoelectric Earth 

model was simplified into a four-layer geoelectric model 

by using depth to water from well 0-1223, depth to 

bedrock from the bedrock contour map, and by combining layers of similar resistivity. The upper layer (fig. 53) 

is a thin, unsaturated, coarse-grained layer, with a 

resistivity of 10,000 ohm-m. It is underlain by saturated, 

coarse-grained material, with a resistivity of 800 ohm-m, 

which overlies saturated, fine-grained material with a 

resistivity of 60 ohm-m. The lowest layer is crystalline 

bedrock with a resistivity of 1,700 ohm-m. The specific 

conductance of the ground water from the coarse-grained 

stratified drift (well 0-1367) was 35 jjiS/cm at 25°C on 

August 12, 1980.

This field site is similar to the hypothetical model 

shown in figure 14. The results of the VLF terrainresistivity survey are shown in the upper part of figure
RESULTS AND INTERPRETATIONS OF FIELD STUDIES A43

Simplified geolectric 

Earth model

Resistivity in ohm-meters

co 20 -|

o

Q

!

40 

DQ

X 

Q.

g 60

80 

140

200

Unsaturated soil 

and stratified drift 

800 ohm-meters

 Water table

^Saturated, coarsegrained stratified 

drift 

500 ohm-meters

Saturated, finegrained stratified 

drift

WELL LOG

SI-363

Sand, F to VC ~ WL

Silt and Clay

Silt and VF sand

Sand, VF and silt

Silt and Clay

Silt and VF sand

Sand, VFtoVC

and silt

(Till)

Sedimentary 

bedrock

Sandstone, 

fine-grained

Bottom of hole at 62 meters

Specific conductance of ground water from the fine-grained 

stratified drift in well SI-363 was 180 microsiemens per centimeter 

at 25 degrees Celsius on February 11, 1985.

EXPLANATION

VF = Very fine

F = Fine

VC = Very coarse 

WL = Water level

FIGURE 37. Simplified geoelectric Earth model from direct-current-resistivity data, and well and 

test-hole logs from the study site in Simsbury, Conn.

54. The low apparent resistivities of 90-200 ohm-m, 

reflect the presence of the buried fine-grained material. 

Phase angles of about 60° also indicate that resistive 

material overlies conductive material (saturated, coarsegrained material over the fine-grained material). The 

fine-grained layers prevented deep penetration by the 

VLF method; therefore, the VLF data were independent of the bedrock resistivity.

In the VLF forward-modeling interpretation, the 

resistivity and thickness of the unsaturated zone, and the 

resistivities of the saturated, coarse-grained material

and the bedrock were held constant. The only 

two variables, therefore, were thickness of the coarsegrained material and resistivity of the fine-grained 

material.

The interpreted geoelectric Earth model resulting 

from the VLF forward-modeling process is shown in the 

lower part of figure 54. A good match between the 

calculated and field apparent resistivity values and phase 

angles at each station was obtained. The interpreted 

geoelectric Earth model shows a layer of saturated, 

coarse-grained material, which varies in thickness from
A44 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

1,000

LU 

DC

 Apparent resistivity 

O Phase angle

10

630 720 810

DISTANCE, IN METERS

A

METERS

70 -i 

60 

50

40 - 

30 - 

20 - 

10 

Sea level 

10 

20 - 

30 - 

40 -

50

Soil, stratified drift 

800 ohm-meters

CO

vo

2

CD

a

/Coarse-grained

/ stratified drift, 

/ 500 ohm-meters 

/ Land surface

63

Water table

Fine-grained stratified drift 

120

105 

(ohm-meters)

115

140

120

Sedimentary bedrock 

200 ohm-meters

140

Seismic-refraction data used to interpret depth to water table and bedrock

VERTICAL EXAGGERATION X5

500 FEET 

I I

CO

LU 

LU 

DC

LU 

Q

z

LU

(D

90 METERS

FIGURE 38. Very-low-frequency apparent resistivity and phase-angle field data, and the interpreted geoelectric Earth 

model from direct-current-resistivity, seismic refraction, and very-low-frequency apparent resistivity data, from the 

study site in Simsbury, Conn.
RESULTS AND INTERPRETATIONS OF FIELD STUDIES A45

A1 A

10 METER VERTICAL-DIPOLE MODE

FOR ALL GRAPHS: 

o o Field values

Computed values

A'

10 METER HORIZONTAL-DIPOLE MODE

I I I I

Anomalous value, off 

scale, possible cultural 

interference

I__I

00

20 METER VERTICAL-DIPOLE MODE

o

3

Q

Z 

O 

o

i hi i i ii I I I I I

Anomalous value, off 

scale, possible cultural 

interference

I I I I I I

20 METER HORIZONTAL-DIPOLE MODE

Anomalous value, off 

scale, possible cultural 

interference

I I I I I I I I I I I I I

40 METER VERTICAL-DIPOLE MODE 40 METER HORIZONTAL-DIPOLE MODE

120 240 360 480 600 720 840 0 120 240 360 480 600 720 840

MEASUREMENT STATION NUMBER

0 500 1,000 FEET

i II. I. i I T I \ \

0 120 240 METERS

FIGURE 39. Field and computed inductive terrain-conductivity values for the 10-, 20-, and 40-meter-intercoil spacings at the study

site in Simsbury, Conn.
A46 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

72°50'45" 72°50'

Base from U S Geological Survey,

New Britain 1 24,000, 1966, photorevised 1972

1,000 

__I

2,000 FEET 

__I

\ 500 METERS

CONTOUR INTERVAL 10 FEET 

Datum is Sea Level

A

A

A 

B

m 

EXPLANA' 

___ I Electromagnetic 

' survey line

A'

^ nirort-nirrpntresistivity survey 

line

B' 

j Seismic-refraction

"" line

ATION

F-74TH 

<0> Test hole Number 

is town well number 

used in U.S. Geological 

Survey publications 

F-282 

0 Observation well 

Number is town well 

number used in U.S. 

Geological Survey 

publications

FIGURE 40. Location of the study site in Farmington, Conn., 

including wells, test holes, and surface-geophysical survey 

lines.

8 to 13 m. This unit overlies a fine-grained layer with 

resistivity of 47-70 ohm-m. The model is not affected by 

the resistivity of the bedrock.

The interpreted layer thicknesses and resistivities 

calculated by the VLF modeling process (fig. 54) were 

used as input data to the inductive terrain-conductivity 

forward-modeling program. The computed results were 

compared with the field measurements (fig. 55). The data 

at stations 180, 240, and 300 do not agree. This disagreement may indicate that the resistivity of the fine-grained 

layer is higher than the VLF-modeled value or that the 

terrain-conductivity measurements are affected by the 

resistivity of the bedrock.

FINE-GRAINED MATERIAL OVER COARSE-GRAINED 

AQUIFER MATERIAL

The final field site, also located in Oxford, Maine (fig. 

56), represents fine-grained material over coarsegrained aquifer material. This deposit is an esker, a 

sinuous glaciofluvial deposit of coarse sand and gravel. 

The esker occurs in the shallow subsurface embedded 

within fine-grained glaciomarine sediments (Morrissey, 

1983). There is no topographic evidence of the esker at 

the site, but a sand and gravel quarry was operated here 

and a prominent topographic ridge delineates the esker 

deposit 3 km south of the site. A VLF terrain-resistivity 

profile, an inductive terrain-conductivity profile, and a 

seismic-refraction profile were conducted between a 

trailer park and the Little Androscoggin River. Because 

the subsurface layers are not horizontal and homogeneous over a sufficiently wide distance, a de-resistivity 

sounding was not conducted at this site. The specific 

conductance of the ground water in a nearby unused 

spring (U.S. Geological Survey number 1366) was 110 

jjuS/cm at 25°C on August 24, 1980.

This field site is similar to several hypothetical models. 

In the center of the section, it is similar to figures 17 and 

18, fine-grained material overlying coarse-grained material. At the A end of the profile, it is similar to figure 14, 

a thick section consisting of coarse-grained material 

overlying fine-grained material. At the A' end of the 

profile, it is similar to the right side of figure 10, 

fine-grained material over crystalline bedrock. The 

results of the VLF terrain-resistivity survey are shown 

in the upper part of figure 57.. The data show a marked 

increase (from 150 to 1,500 ohm-m) in the apparent 

resistivity near the center of the profile, and a corresponding decrease in the phase angle. The increase in 

resistivity is due to the presence of the resistive coarsegrained esker material in the center of the profile. The 

phase angle is 30°-35° on both ends of the line, reflecting 

the thick conductive fine-grained material over the 

resistant bedrock. The phase angle decreases to 15°-25°
SUMMARY OF RESULTS OF FIELD STUDIES A47

10,000 rr

1,000

100

i \ \ n M F

 Layered geoelectric Earth model

 Computed sounding curve 

O Smoothed field-data points

0.1 10

ELECTRODE SPACING (AB 4- 2), OR 

DEPTH BELOW LAND SURFACE, IN METERS

100 1,000

FIGURE 41. Smoothed plot of field-data points from direct-current-resistivity sounding, the computed sounding 

curve, and the resulting layered geoelectric Earth model for data from the study site in Farmington, Conn.

in the center of the profile because to the presence of a 

thin layer of conductive material over very resistive, 

thick, coarse unconsolidated material and bedrock.

In the VLF forward-modeling interpretation, the 

resistivity of the unsaturated zone and the shallow, 

saturated, coarse-grained material was held constant. 

The depth to the water table and bedrock were determined from the seismic-refraction data and were not 

varied. Therefore, the variables in the modeling process 

were the thickness and resistivity of the buried coarsegrained aquifer material, the resistivity of the finegrained material, and the resistivity of the bedrock.

The interpreted geoelectric Earth model resulting 

from the VLF forward-modeling process is shown in the 

lower part of figure 57. A good match between computed 

and field apparent resistivities and phase angles was 

obtained. The interpreted geoelectric Earth model 

shows a thin, saturated, coarse-grained unit near the 

surface underlain by fine-grained material. The esker, 

shown in the center of the profile, contains as much as 20 

m of resistive coarse-grained material.

The interpreted layer thicknesses and resistivities 

calculated by the VLF modeling process (fig, 57) were 

used as input data to the inductive terrain-conductivity 

forward-modeling program. The computed results were 

compared with the field measurements (fig. 58). The 

terrain-conductivity model agrees fairly well with the

field data for all terrain-conductivity intercoil spacings 

and dipole modes, indicating that the interpreted geoelectric Earth model closely approximates the subsurface.

After the geophysical survey was completed, two test 

holes (0-1719 and 0-1720) were drilled to confirm the 

geophysical interpretation. The geologic logs, shown in 

figure 59, verify the geophysical interpretation, except 

that they do not show the thin, fine-grained layer overlying the esker.

SUMMARY OF RESULTS OF FIELD STUDIES

Seismic-refraction, de-resistivity, and electromagnetic 

surface-geophysical methods were used at eight field 

sites in the glaciated Northeastern United States. These 

sites are characterized by unconsolidated stratified-drift 

deposits underlain by crystalline or sedimentary bedrock.

Results of the field investigations show that no single 

method is capable of defining the hydrogeologic boundaries and differentiating between fine-grained and 

coarse-grained stratified drift. The combined use of all 

these methods, or the selective use of several of them, in 

combination with the specific conductance of ground 

water and geologic logs of test holes or wells, can help to 

define depth to the water table, depth to bedrock,
A48 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

Simplified geoelectric 

Earth model

WELL AND TEST-HOLE LOGS

Resistivity in ohm-meters

x Soil 900 ohm-meters

n Xx Unsaturated, coarse- "

CO

a: 20 

UJ

UJ

Z

LU

0 40_

u_ 

oc

D

CO 

a

z

<

-1 60 

£

0

_i

UJ _

00

I

t

uj 80 

a

inn

560

240

140

'/ grained stratified drift

1740 ohm-meters ^~~~ Water table

 Saturated, coarsegrained stratified

drift

^

Saturated, finegrained stratified

drift

Sedimentary bedrock

F-282

Sand and gravel

Sand, Fto C

Very fine sand, 

silt, some clay

-WL

~Soil and sand, 

FtoM

Bottom of hole at 35 meters

F-68TH

Sand and gravel

Sand, VF to F

Sand, M to C

Sand, VF to F

Sand and gravel

Sand, F to C

Sand and gravel

Sand, VFtoF, 

silt and clay

WL

-Soil and sand, 

FtoM

EXPLANATION

VF = Very fine 

F = Fine

M = Medium Bottom of hole at 76 meters 

C = Coarse 

VC = Very coarse 

WL = Water level 

^ = Coarse-grained/fine-grained 

stratified drift contact

Specific conductance of ground water from the coarse-grained stratified drift in well F-295 

was 162 microsiemens per centimeter at 25 degrees Celsius on May 17, 1977.

FIGURE 42. Simplified geoelectric Earth model from direct-current-resistivity data, and well and test-hole logs from the study site in

Farmington, Conn.

general lithologic characteristics of sand and gravel 

aquifers, and lithologic changes within these glacial 

deposits.

DEPTH TO THE WATER TABLE

Seismic-refraction surveys were capable of determining the depth to the water table in unconsolidated 

stratified-drift deposits at all field sites. This method was 

successful because of the mean difference in seismic 

velocity between unsaturated (0.32 kilometer per second 

(km/s)) and saturated (1.54 km/s) stratified drift. Table 3 

compares the seismic velocities for these two materials at 

the six field sites that have available seismic-refraction 

data. Similar seismic velocities have been reported by 

Haeni and Melvin (1984), Morrissey and others (1985), 

and Haeni (1988).

TABLE 3. Interpreted seismic velocities in stratified-drift deposits at 

six field sites in the glaciated Northeastern United States

Depth of water 

TTV i j -fo table determined 

Field Slte from seismic 

refraction data (m)

Stonington, Conn. .

Granby, Conn...... 

Smyrna, N.Y. .....

Simsbury, Conn. . . .

Farmington, Conn..

Oxford, Maine 

(esker site). .....

Mean

2.44

1.83 

1.22 

1.22

3.66

2.14

Interpreted 

seismic velocity 

in unsaturated 

material (km/s)

0.34

.30 

.34 

.21

.46

.27

.32

Interpreted 

seismic velocity 

in saturated 

material (km/s)

1.59

1.56 

1.40 

1.59

1.56

1.53

1.54
SUMMARY OF RESULTS OF FIELD STUDIES A49

1,000

CO

or 800

CO

LU

oc

 Apparent resistivity 

O Phase angle

600 (

400

200

0 60 120 180 240 300 360 420 480 540 600 660 720 780 840 900

DISTANCE, IN METERS

A a, 

METERS'0 p

70 - «S

a J

CO

£ I o P

60 

40 

30 

20 

10 

Sea level 

10 

20 

30 

40

Soil, and coarse-grained 

stratified drift 300 ohm-meters Land surface

End of hole

Coarse-grained 

stratified drift, 

560 ohm-meters

200

End of hole

Fine-grained stratified drift

160 180 190 180 200 180 

(ohm-meters)

150

Sedimentary bedrock 

140 ohm-meters

Seismic-refraction data used to interpret depth to water table and bedrock

Vertical exaggeration X6 400 FEET 

I I I

\ \

90 METERS

FIGURE 43. Very-low-frequency apparent resistivity and phase-angle field data, and the interpreted geoelectric Earth 

model from direct-current-resistivity, seismic refraction, and very-low-frequency apparent resistivity data, from the 

study site in Farmington, Conn.
A50 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

A A

A'

10 METER VERTICAL-DIPOLE MODE 10 METER HORIZONTAL-DIPOLE MODE

20

16

12

I I I

FOR ALL GRAPHS:

- o -o Field values

 Computed values

I I I I I I I I

20

^ 16

12

0_

< 20

16

12

20 METER VERTICAL-DIPOLE MODE 20 METER HORIZONTAL-DIPOLE MODE

40 METER VERTICAL-DIPOLE MODE 40 METER HORIZONTAL-DIPOLE MODE

I I I I l I I

I I I I I

0 100 200 300 400 500 600 700 800 900 0 100 200 300 400 500 600 700 800 900

MEASUREMENT STATION NUMBER

0 500 1,000 FEET

I III I

I \ I T

0 100 200 300 METERS

FIGURE 44. Field and computed inductive terrain-conductivity values for the 10-, 20-, and 40-meter-intercoil spacings at the study site

in Farmington, Conn.
SUMMARY OF RESULTS OF FIELD STUDIES A51

TEST-HOLE LOGS

5 

15 

O_j

LLJ 

CD

t

LLJ

Q 20 H

25 

F-71TH F-69TH

Silty loam, 

unsaturated

Sand, M to C 

and gravel

Sand, Fto C

Sand, C to VC 

and gravel

Sand, FtoC

Sand.VF to F

Sand, F to C

Sand, Fto M

Sand, F to VC 

and gravel

Sand,VF to M

Sand.VF to F

-^

Silty loam, 

unsatu rated

Sand, F to C 

and gravel

Sand,VF to F

Sand, 

and silt

Sand,VFtoF

-^

Bottom of hole

F-70TH F-72TH

Silty loam, 

unsatu rated

Sand, Fto C

Sand, MtoVC 

and gravel

Sand, F to M

Sand,VFtoF

Sand, VF

and silt;

some clay

Bottom of hole

Sand, Fto M, 

unsaturated

Sand, Fto C 

and gravel

Sand.VF to F

Sand, F to M

Sand, M to VC 

and gravel

Sand.VF to F

 WL

-^

Bottom of hole

Bottom of hole

EXPLANATION

VF = Very fine

F = Fine 

M = Medium 

C = Coarse 

VC = Very coarse 

WL = Water level

-^ = Contact between coarse-grained and 

fine-grained stratified drift

F-73TH F-74TH

Silty loam, 

unsaturated

Sand, M to VC 

and gravel

Sand,VFtoF

Sand, F to M

Sand.VF to F

Sand, VF 

and silt

 WL

-^

Silty loam, 

unsaturated

Sand, FtoC 

and gravel

Sand, F to C

Sand.VF to F

Sand.VF 

and silt

Bottom of hole

-^

Bottom of hole

FIGURE 45. Geologic logs from test holes in Farmington, Conn.

The de-resistivity of unsaturated and saturated 

stratified-drift deposits depends on a combination of 

factors, including porosity, moisture content, mineralogy, and specific conductance in each unit. In combination, these variables do not always result in a resistivity 

contrast at the water-table surface. The de-resistivity 

method was able to detect the water table at the 

Simsbury, Stonington, and Farmington, Conn.; the 

Smyrna, N.Y.; and the airport road in Oxford, Maine, 

field sites, but not at the Preble, N.Y., nor Granby, 

Conn., field sites.

The unsaturated layer resistivities presented in table 4 

were determined by combining several individual geoelectric layers from the interpreted de-resistivity data 

and comparing these values with the lithologic description of the layer from a well or test hole at the site. In 

general, two layers were defined: an upper soil layer, 

with resistivities ranging from 325 to 10,000 ohm-m; and 

a more resistive, unsaturated stratified-drift layer, with 

resistivities ranging from 1,500 to 10,000 ohm-m. The

saturated stratified-drift layer resistivities shown in 

table 5 were similarly determined and varied from 60 to 

2,000 ohm-m.

Electromagnetic methods depend on the resistivity 

contrast between layers to differentiate the boundaries 

between them and, therefore, are subject to the same 

limitations as the de-resistivity method.

DEPTH TO BEDROCK

Depth to bedrock was consistently defined by interpreting seismic-refraction data from all of the field sites 

where it was used. The velocity difference between 

saturated, unconsolidated materials and bedrock ranged 

from 2.13 km/s at Granby, Conn., to 3.90 km/s at 

Stonington, Conn. The seismic velocity of the saturated, 

unconsolidated materials is listed in table 3. The seismic 

velocity of the bedrock at each site is listed in table 6. 

These velocities are similar to those reported by Haeni
A52 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

76°09'30" 76°08'45"

Base from U.S. Geological Survey, 

Homer 1-24,000,1955, photorevised 1978

1,000 

I

2,000 FEET 

__I

\ 

500 METERS

CONTOUR INTERVAL 100 FEET 

Datum is Sea Level

EXPLANATION

A'

B'

Direct-currentresistivity survey 

line

Electromagnetic 

survey line

00-01

Test hole Number 

is town well number 

used in U.S. Geological 

Survey publications

Observation well 

Number is town well 

number used in U.S. 

Geological Survey 

publications

FIGURE 46. Location of the study site in Preble, N.Y., including 

wells, test holes, and surface-geophysical survey lines.

TABLE 4. Interpreted resistivity of unsaturated, stratified-drift 

deposits at seven field sites in the Northeastern United States

Interpreted

Field site

Generalized

lithologic 

description from

well or 

test-hole logs

direct currentsounding data

(ohm-m)

Stonington, Conn..

Granby, Conn.

Smyrna, N.Y. 

Simsbury, Conn.

Farmington, Conn.

Preble, N.Y.

Oxford, Maine 

(airport site)

Soil; sand fine; and

silt 0.0-0.9 450 

Sand, fine to coarse;

gravel .9-2.0 2,000

Soil, and silty loam .0-.5 425 

Sand, medium to 

coarse; gravel .5-3.6 7,000

Soil .0-.6 700 

Gravel .6-2.4 2,200

Soil .0-.5 400 

Sand, very fine to 

medium .5-1.8 1,500

Soil and sand, fine to

medium 0.0-0.8 900 

Sand and gravel .8-2.5 1,740

Soil .0-1.5 325 

Sand and gravel 1.5-5.6 1,500

Soil and sand, coarse 

to very coarse .0-1.2 10,000
SUMMARY OF RESULTS OF FIELD STUDIES 

10,000 p i | I I | |||| | | I I I Illj l I I I I III] I | I I I MiA53

1,000

100

10

Layered geoelectric Earth model

 Computed sounding curve 

O Smoothed field-data points

I i i i i i I i i i i i I I I I I I

0.1 1 10 100 1,000

ELECTRODE SPACING (AB + 2), OR 

DEPTH BELOW LAND SURFACE, IN METERS

FIGURE 47. Smoothed plot of field-data points from direct-current-resistivity sounding, the computed sounding curve, and the resulting layered geoelectric Earth 

model for data from the study site in Preble, N.Y.

TABLE 5. Interpreted resistivity of saturated, stratified-drift deposits at eight field sites in the Northeastern United States

[ , no data available]

Field site

Stonington, Conn. ......

Granby, Conn. ..........

Smyrna, N.Y. ..........

Simsbury, Conn. .......

Farmington, Conn.

Preble, N.Y. ...........

Oxford, Maine 

(airport site) .........

Oxford, Maine 

(esker site) ...........

Generalized 

lithologic description 

from well or test-hole logs

. . . Sand, medium to very coarse, and gravel

. . . Sand, medium to very coarse, and gravel

Sand, fine to coarse 

Gravel, well sorted

... Sand, fine to very coarse

Sand, very fine, and silt and clay 

Sand, fine to coarse, and some gravel

Sand, very fine to fine, 

and silt and clay

. . . Sand and gravel, some silt

Sand, fine, and some silt 

... Sand and gravel

Sand, very fine to fine, and silt and clay 

... Sand and gravel

Sand, very fine, and silt and clay

Specific conductance 

of ground water 

((juS/cm at 25°C)

102

130

180

162

550

35

110

Interpreted 

resistivity from 

direct-current 

sounding data 

(ohm-m)

500

2,000

450 

550

500

140 

560

240 

250

60 

800

60

Interpreted 

resistivity from 

very-low-frequency 

modeling 

(ohm-m)

150-450

2,000-3,000

held constant 

160-300

held constant

16-140 

held constant

75-200 

held constant

60-105 

held constant

47-70 

800-5,000

110-255

Possible road salt contamination in well.
A 54 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

TABLE 6. Interpreted seismic velocity and resistivity of sedimentary and crystalline bedrock at seven field sites in the Northeastern United States

[ , no data available]

Field site

Stonington, Conn. .......

Granby, Conn.. ..........

Smyrna, N. Y. ...........

Simsbury, Conn. ........

Farmington, Conn. ......

Oxford, Maine 

(airport site) ..........

Oxford, Maine 

(esker site) ............

Lithologic description

Gneiss, quartz-feldspar-biotite

Gneiss, muscovite-biotite-quartz

Siltstone, very fine-grained 

Shale

Sandstone, fine-grained

Sedimentary, arkose

Granite, biotite-rich

Granite, biotite-rich

Seismic 

velocity 

(km/s)

5.49

3.69 

4.42

4.39

4.09

5.03

Interpreted 

resistivity from 

direct-current 

sounding data 

(ohm-m)

1,600

250

250 

300

200

140

1,700

Interpreted 

resistivity from 

very-low-frequency 

modeling 

(ohm-m)

1,100-5,000

135-150

35-115 

133^330

held constant

held constant

2,000-10,000

TABLE 7 . Comparison of depth to bedrock, determined by seismicrefraction, direct-current-resistivity, and drill-hole data 

[ , no data available]

Depth to bedrock, in meters

Field site

Stonington, Conn. . .

Granby, Conn. ..... 

Smyrna, N.Y. ......

Simsbury, Conn. . . .

Farmington, Conn. .

Oxford, Maine 

(esker site) ......

Seismic 

refraction

14.0

16.2

17.7

12.7 

17.0 

60.0

73.2

24.0

34.0

Direct-current

resistivity

17.8

14.0 

12.4 

not detected

not detected

-

, as determined by

Drill hole

14.0 (SN-143th)

13.4 (SN-164)

14.3 (SN-144th)

12.5 

61.0

76.0 (refusal)

22.0 (0-1720) 

(refusal)

31.0 (0-1719)

(bottom of hole)
CONCLUSIONS A55

and Anderson (1980), Morrissey and others (1985), Haeni 

(1986a), and Haeni (1988) for similar geologic settings.

The de-resistivity method was able to detect the 

bedrock surface at Stonington and Granby, Conn., and 

Smyrna, N.Y. This method was not able to detect the 

bedrock at Farmington or Simsbury, Conn., because of 

the lack of electrical resistivity contrast between the 

saturated, unconsolidated material (table 5) and bedrock 

(table 6). A comparison of the depth to bedrock determined by drilling with the depth interpreted from the 

seismic-refraction and de-resistivity data are shown in 

table 7. In general, the seismic-refraction depths agree 

with the test-hole data for all of the field sites. The 

depths determined from resistivity data were not as 

accurate as the seismic-refraction data. At two sites, 

depth to bedrock could not be detected from resistivity 

data. Resistivity values for crystalline rock changed 

substantially between Granby, Stonington, and Oxford 

(table 6). Resistivity values at Granby were obtained 

near a major geologic fault separating crystalline and 

sedimentary rocks. In outcrops near the fault, both rock 

types are highly fractured. These fractures and the 

presence of more conductive ground water in the sedimentary rocks are probably the reasons for the abnormally low resistivities in the crystalline bedrock.

Electromagnetic methods were used to determine the 

depth to bedrock at the Granby site. Interpretation of 

these data was satisfactory when the geoelectric Earth 

was limited to four layers and the only variables in the 

modeling process were thickness of saturated material 

and resistivity of the bedrock. Thickness and resistivity 

of the overlying unsaturated units and the resistivity of 

the saturated units were assumed to be known and were 

held constant.

CHARACTERISTICS OF SAND AND GRAVEL AQUIFERS

Dc-resistivity soundings at each field site could be 

correlated qualitatively with the general grain-size characteristics of the aquifer material. The relations among 

lithology of stratified-drift deposits, specific conductance 

of ground water, interpreted de-resistivity, and interpreted VLF resistivity at the eight field sites is shown in 

table 5. The data presented indicate that when the 

specific conductance of ground water remains relatively 

constant, the bulk resistivity of the aquifer is generally 

representative of the aquifer's grain-size characteristics. 

Specifically, coarse-grained material generally is more 

resistive than fine-grained material. A limitation of the 

de-resistivity method is that small-scale horizontal 

changes of the resistivity of the subsurface units cannot 

be detected with this method.

Electromagnetic methods detect smaller changes of 

the subsurface units than de-resistivity methods and,

therefore, are capable of mapping hydrogeologic features 

such as eskers and variable thicknesses of coarse-grained 

units. However, independent data (drill-hole logs or 

geologic knowledge of the area) on subsurface conditions 

at any site must be used to confirm the results of these 

methods.

The seismic-refraction method cannot be used to 

detect grain-size changes in aquifer materials because 

seismic velocity does not vary greatly with changes in 

the grain size of saturated, unconsolidated materials 

(table 1).

CONCLUSIONS

Computer forward-modeling programs were used to 

calculate the response of electromagnetic methods in 

hydrogeologic settings typical of the glaciated Northeastern United States. Subsequent surface-geophysical 

field investigations were conducted at eight sites. The 

two parts of this study have shown that the combined use 

of seismic-refraction, de-resistivity, VLF terrainresistivity, and inductive terrain-conductivity methods 

can distinguish between fine-grained and coarse-grained 

stratified-drift aquifers and can be used to determine 

their hydrogeologic boundaries.

Forward-modeling studies of hypothetical systems 

demonstrated that both VLF terrain-resistivity and 

inductive terrain-conductivity methods can be used to 

detect horizontal and vertical changes in electrical properties of the subsurface layers or materials. Some 

surface-geophysical methods might work better in certain hydrogeologic settings than in others because of 

different operating principles of the individual methods. 

The forward-modeling results indicated that in resistive 

terrains, the VLF terrain-resistivity method (1) can be 

used to measure moderate horizontal and vertical 

changes in electrical properties of the subsurface, (2) has 

a penetration depth of about 100 m, and (3) can detect 

conductive material underlying resistive material. The 

inductive terrain-conductivity method (1) is insensitive 

to large changes in the resistivity of resistive layers, (2) 

maintains a constant penetration depth, and (3) can be 

used to detect conductive layers at depth. In conductive 

terrains, the VLF terrain-resistivity method is only 

sensitive to near-surface resistivity changes and has a 

limited depth of penetration. Also, the inductive terrainconductivity method (1) is sensitive to changes in the 

thickness and conductivity of individual conductive layers, (2) maintains its depth of penetration, and (3) can be 

used to detect small changes in layer conductivities.

Surface-geophysical surveys seismic-refraction, deresistivity, and electromagnetic methods at eight sites 

in Connecticut, New York, and Maine have shown that
A56 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

Simplified geoelectric TEST-HOLE LOGS

Earth model

Resistivity in ohm-meters

CO

cc

o

CO

Q

g

LU 

CD

t

u

5 

10 

15 

20 

25 

3D -

325

1,500

250

60

175

Unsaturated soil

Unsaturated, coarsegrained stratified 

drift

Water table

Saturated, coarsegrained stratified 

drift

Saturated, finenrained stratified 

drift

Saturated, finegrained stratified 

drift

44-26b 54-49b

Gravel 

and sand

Fine sand, 

some silt

Muck and marl

Silt, some clay

Gravel, some sand 

and silt

Bottom of hole

Bottom of hole

Specific conductance of ground water from the coarse-grained stratified drift 

in well 05-50 was 500 microsiemens per centimeter at 25 degrees Celsius on 

February 25, 1977 and from the coarse-grained stratified drift in well 00-01 

was 550 microsiemens per centimeter at 25 degrees Celsius on June 12, 1985.

FIGURE 48. Simplified geoelectric Earth model from direct-current-resistivity data, and well and test-hole logs from the study

site in Preble, N.Y.

FIGURE 49. Very-low-frequency apparent resistivity and phaseangle field data, and the interpreted geoelectric Earth model 

from direct-current-resistivity and very-low-frequency apparent 

resistivity data, from the study site in Preble, N.Y.
CONCLUSIONS A57

co

LLJ 

DC

ILLJ 

DC

500

400

300

200

100

A

METERS 

120 i

110 

100

90 

80 

70

 Apparent resistivity 

O Phase angle

120 180 240 300 

DISTANCE, IN METERS

360

Soil, 325 

ohm-meters

e- -e- -e- Land surface e -eCoarse-grained stratified drift, 

1,500 ohm-meters

420

-e-

_ _ _ _ Water table

Coarse-grained stratified drift, 

250 ohm-meters

480

A1

-0

85 70 75

Fine-grained stratified drift

85 70 105 80

(ohm-meters) 

Seismic-refraction data used to interpret depth to water table and bedrock

80 60

Vertical exaggeration X7 

Datum is Sea Level

100 200 300 FEET

60 METERS
A58 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

A' A

10 METER VERTICAL-DIPOLE MODE

20

16

I I I I 

FOR ALL GRAPHS: 

 o -o Field values

 Computed values

|____L

10 METER HORIZONTAL-DIPOLE MODE

1 I I I I T

J____I

A1

20 METER VERTICAL-DIPOLE MODE 20 METER HORIZONTAL-DIPOLE MODE

40 METER VERTICAL-DIPOLE MODE 40 METER HORIZONTAL-DIPOLE MODE

0 60 120 180 240 300 360 420 480 0 60 120 180 240 300 360 420 480

MEASUREMENT STATION NUMBER

0 250 500 750 FEET

I______i______|______| I I I I

0 60 120 180 METERS

FIGURE 50. Field and computed inductive terrain-conductivity values for the 10-, 20-, and 40-meter-intercoil spacings at the study

site in Preble, N.Y.
CONCLUSIONS A59

,70°29'15" 70°28 f 30"

Base from U S. Geological Survey, 

Oxford 1 24,000, 1980

1,000 

_L_

2,000 FEET 

_I

500 METERS

CONTOUR INTERVAL 10 FEET 

Datum is Sea Level

EXPLANATION

A'

B'

0-1367

Electromagnetic Domestic water-supply 

survey line well Number is

town well number

Direct-current- used in U.S. Geological 

resistivity survey Survey publications 

line 0-1223

0 Observation well 

Number is town well 

number used in U.S. 

Geological Survey 

publications

FIGURE 51. Location of the study site at the airport in Oxford, 

Maine, including wells, test holes, and surface-geophysical survey 

lines.

no single method can be used to define the hydrogeologic 

boundaries and distinguish between fine-grained and 

coarse-grained stratified drift. The combined interpretation of the seismic-refraction and de-resistivity data 

helps to delineate boundaries, distinguish between finegrained and coarse-grained deposits, and provide information on the specific conductance of ground water 

within sand and gravel aquifers.

The seismic-refraction method was used to determine 

depths to the water table and bedrock in all hydrogeologic settings. This method cannot be used to detect 

lithologic changes within saturated stratified-drift 

aquifers.

De-resistivity can be used to detect resistivity changes 

within stratified-drift material caused by water-quality 

(specific conductance) or lithologic variations. At five 

field sites, the de-resistivity method could be used to 

determine depth to water table, and at three field sites, 

it could be used to determine depth to bedrock.

The combined use of two electromagnetic methods, 

VLF terrain-resistivity and inductive terrainconductivity, further improves characterization of the 

aquifer systems. These two methods can be used conjunctively to refine the interpretation of a model based 

on seismic-refraction and de-resistivity data. These 

methods also are useful for mapping electrical anomalies 

that relate to small-scale vertical or lateral changes in 

aquifer lithology and specific conductance of ground 

water. The successful interpretation of the electromagnetic data, over an earth with more than two layers, 

requires subsurface data from seismic-refraction and 

de-resistivity methods or other sources to obtain a geologically reasonable solution.

Interpreted de-resistivities or interpreted VLF resistivities of each subsurface layer, ground-water specific 

conductance, and lithologic logs from nearby test holes or 

wells were compared at all eight field sites. The results 

indicate that for a given specific conductance of ground 

water, fine-grained stratified drift is more conductive 

than coarse-grained stratified drift. With increasing specific conductance of ground water, the same general 

relation applies, but the values of each layer's resistivity 

decreases. The resistivity, therefore, of sand and gravel 

aquifers in the Northeastern United States can be 

related empirically to the grain size of the aquifer 

material in limited geographic areas where the specific 

conductance of ground water is considered uniform.
AGO REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

100,000 F

10,000

1,000

100

10

T i^ i i 1111 i i i i i 1111 i i i i i ii n i n ITTTC

0\

 Layered geoelectric 

Earth model

 Computed sounding

curve 

O Smoothed field-data

points

i i i i i 1111 i i i i i

0.1 10 100 1,000

ELECTRODE SPACING (AB + 2), OR 

DEPTH BELOW LAND SURFACE, IN METERS

FIGURE 52. Smoothed plot of field-data points from direct-current-resistivity 

sounding, the computed sounding curve, and the resulting layered geoelectric 

Earth model for data from the study site at the airport in Oxford, Maine.
CONCLUSIONS A61

Simplified geoelectric 

Earth model

Resistivity in ohm-meters

C/) 

DC 10 

LU

LU

z

LU" 

O

cn 20 

D 

C/)

Q

<

O

m 30 

DO

t

LU

Q

40 

10,000

800

60

1,700

Unsaturated, coarsegrained stratified 

drift

Water table

Saturated, coarse-

§ rained stratified 

rift

Saturated, fine-

§ rained stratified 

rift

Crystalline bedrock

Specific conductance of ground water from the coarse-grained 

stratified drift in well 0-1367 was 35 microsiemens per centimeter 

at 25 degrees Celsius on August 12, 1980.

WELL LOG 

0-1223

Sand, C to VC

Sand, F to M

Sand, F to VF, 

and silt

Sand, VF and clay

Bottom of hole

EXPLANATION

VF = Very fine 

F = Fine 

M = Medium 

C = Coarse 

VC = Very coarse 

WL = Water level 

 ^ = Coarse-grained/fine-grained 

	stratified drift contact

FIGURE 53. Simplified geoelectric Earth model from direct-current-resistivity data, and well and test-hole logs from the study site at the

airport in Oxford, Maine.
A62 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

CO

500

400

300

 Apparent resistivity 

O Phase angle

100

60

A

METERS 

120 -

110 -

120 180 

DISTANCE, IN METERS

240 300

A

100 -

90 -

80 -

70 -

60

50 -

Land surface

Coarse-grained 

stratified drift, 

10,000 ohm-meters

CO

<N T? 

<N _§

3 8

ll

-0-

Coarse-grained stratified drift, 

800 ohm-meters

Water table

Fine-grained stratified drift

70 60 60 

(ohm-meters)

55 55 47 

End of hole

Crystalline bedrock 1,700 ohm-meters 

Well data used to interpret depth to water table and bedrock

Vertical exaggeration X3 

Datum is Sea Level

0 100 200 300 FEET

60 METERS

FIGURE 54. Very-low-frequency apparent resistivity and phase-angle field data, and the interpreted geoelectric Earth 

model from direct-current-resistivity, seismic refraction, and very-low-frequency apparent resistivity data, from the 

study site at the airport in Oxford, Maine.
CONCLUSIONS A63

A A' A

10 METER VERTICAL-DIPOLE MODE

I I I 

FOR ALL GRAPHS: 

 o -o Field values

Computed values

A

10 METER HORIZONTAL-DIPOLE MODE

20 METER VERTICAL-DIPOLE MODE

40 METER VERTICAL-DIPOLE MODE

20 METER HORIZONTAL-DIPOLE MODE

40 METER HORIZONTAL-DIPOLE MODE

180 240 300 0 60 

MEASUREMENT STATION NUMBER

0 250 500 FEET

120 180 240 300

0 60 120 METERS

FIGURE 55. Field and computed inductive terrain-conductivity values for the 10-, 20-, and 40-meter-intercoil spacings at the study

site at the airport in Oxford, Maine.
A64 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

70°30' 70°29'30"

Base from U.S. Geological Survey, 

Oxford 1 24,000, 1980

1,000 

I

2,000 FEET 

__I

I 

500 METERS

CONTOUR INTERVAL 10 FEET 

Datum is Sea Level

A'

EXPLANATION

B'

Electromagnetic 

survey line

Seismic-refraction 

line

0-1720

0-1366

Test hole Number 

is town well number 

used in U.S. Geological 

Survey publications

Unused spring 

Number is county 

number used in U.S. 

Geological Survey 

publications

FIGURE 56. Location of the esker site in Oxford, Maine, including 

test holes, spring, and surface-geophysical survey lines.
CONCLUSIONS A65

5,000

 Apparent resistivity 

O Phase angle

120 150 180 

DISTANCE, IN METERS

Coarse-grained 

stratified drift, 

1,500 ohm-meters 

800 ohm-meters

Land surface

O

200 200

_ / Water table

190 110 

(ohm-meters)

Coarse-grained 

stratified drift,

(esker) 

950 I 1,000

Fine-grained 

stratified drift

Fine-grained 

stratified drift

Bottom of hole

Crystalline bedrock

3,000 5,000 7,000

(ohm-meters) 

Seismic-refraction data used to interpret depth to water table and bedrock

Vertical exaggeration X4 

Datum is Sea Level 100 200 300 FEET

LU 

CO

60 METERS

FIGURE 57. Very-low-frequency apparent resistivity and phase-angle field data, and the interpreted geoelectric Earth 

model from direct-current-resistivity, seismic refraction, and very-low-frequency apparent resistivity data, from the 

esker site in Oxford, Maine.
A66 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHEAST GLACIAL VALLEYS

A1

10 METER VERTICAL-DIPOLE MODE

20

16

12

I I 

FOR ALL GRAPHS: 

 o -o Field values

 Computed values

-» O>

12

10 METER HORIZONTAL-DIPOLE MODE

20 METER HORIZONTAL-DIPOLE MODE

40 METER HORIZONTAL-DIPOLE MODE

120 180 240 300 0 60

MEASUREMENT STATION NUMBER 

0 250 500 FEET

120 180 240

A'

300

0 60 120 METERS

FIGURE 58. Field and computed inductive terrain-conductivity values for the 10-, 20-, and 40-meter-intercoil spacings at the esker

site in Oxford, Maine.
REFERENCES CITED A67

TEST-HOLE LOGS

0-1719 0-1720

C/5 

CC 

LLI

O

< U_

cc

D 

C/5 

Q

10

20

30

Sand, VFtoC, 

and some gravel

Silt and clay

Sand, M to VC

Sand, VF, silt 

and clay

Silt and clay, 

with sand layers

Sand, VF to M

Bottom of hole at 31 meters

Specific conductance of water in unused spring 0-1366 

was 110 microsiemens per centimeter at 25 degrees Celsius 

on August 24, 1980.

Sand, M to VC, 

and gravel

Sand and gravel

Sand Fto VC, 

and gravel

Sand C to VC

Sand, Fto VC, 

and gravel

Till

Refusal at 22 meters

EXPLANATION

VF = Very fine 

F =Fine 

M = Medium 

C = Coarse

VC = Very coarse

FIGURE 59. Geologic logs from test holes at the esker site in Oxford, Maine.

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"Publications of the U.S. Geological Survey, 1971-1981" 

may be purchased by mail and over the counter in paperback book 

form (two volumes, publications listing and index) and as a set of 

microfiche.

Supplements for 1982, 1983, 1984, 1985, 1986, and for subsequent years since the last permanent catalog may be purchased 

by mail and over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic 

and Water-Supply Reports and Maps For (State)," may be purchased by mail and over the counter in paperback booklet form 

only.

"Price and Availability List of U.S. Geological Survey 

Publications," issued annually, is available free of charge in 

paperback booklet form only.

Selected copies of a monthly catalog "New Publications of 

the U.S. Geological Survey" are available free of charge by mail 

or may be obtained over the counter in paperback booklet form 

only. Those wishing a free subscription to the monthly catalog 

"New Publications of the U.S. Geological Survey" should write to 

the U.S. Geological Survey, 582 National Center, Reston, VA 

22092.

Note Prices of Government publications listed in older catalogs, announcements, and publications may be incorrect. Therefore, the prices charged may differ from the prices in catalogs, 

announcements, and publications.