U.S. Department of the Interior 

U.S. Geological Survey

Use of Azimuthal Square-Array Direct-Current 

Resistivity to Locate a Geologic Structure 

at Fort Detrick, Maryland

by Dorothy H. Tepper and Daniel J. Phelan

Open-File Report 01-285

Prepared for the

U.S. Army Corps of Engineers,

Baltimore District

Baltimore, Maryland 

2001
U.S. Department of the Interior

GALE A. NORTON, Secretary

U.S. Geological Survey

Charles G. Groat, Director

The use of trade, product, or firm names in this report is for 

descriptive purposes only and does not imply endorsement 

by the U.S. Government.

For additional information contact:

District Chief

U.S. Geological Survey, WRD 

8987 Yellow Brick Road 

Baltimore, MD 21237

Copies of this report can be purchased from:

U.S. Geological Survey 

Branch of Information Services 

Box 25286 

Denver, CO 80225-0286

CONTENTS

Abstract................................................................................................................................!

Introduction..........................................................................................................................2

Purpose and scope...........................................................................................................2

Description of the field site .............................................................................................2

Geologic setting..............................................................................................................5

Stratigraphy ................................................................................................................5

Structural geology ......................................................................................................5

Description of the azimuthal square-array direct-current resistivity method..................6

Acknowledgments...........................................................................................................?

Methods................................................................................................................................?

Field methods..................................................................................................................7

Resistivity profiles......................................................................................................?

Resistivity soundings..................................................................................................9

Electromagnetic terrain-conductivity survey .............................................................9

Analytical methods. ........................................................................................................9

Results of resistivity profiles, resistivity soundings, and terrain-conductivity survey.......11

Resistivity profiles......................................................................................................... 11

Profiles 1 and 2.........................................................................................................11

Profiles 3 and4.........................................................................................................19

Resistivity soundings ....................................................................................................19

Sounding 1................................................................................................................19

Sounding 2................................................................................................................31

Sounding 3................................................................................................................31

Terrain-conductivity survey ..........................................................................................31

Interpretation of resistivity profiles and resistivity soundings ...........................................31

Resistivity profiles.........................................................................................................31

Resistivity soundings ....................................................................................................33

Summary and conclusions .................................................................................................34

References cited.................................................................................................................36

FIGURES

1. Map showing location of Fort Detrick, Frederick, Maryland, and locations

of the square-array direct-current resistivity profiles 1-4 and soundings 1-3 .......3

2. Geologic map and lithologic descriptions for the vicinity of Area B ........................4

3. Diagram showing electrode configurations for square-array direct-current

resistivity measurements.......................................................................................8

4. Schematic of square-array layouts used along profiles 1-4 .....................................10

5. Plot of mean geometric resistivity plotted against distance along profiles 1-4........12

6. Plot of apparent anisotropy plotted against distance along profiles 1-4 ..................13

IV
FIGURES - Continued

7-10. Plots of apparent resistivity of:

7. non-rotated squares plotted against distance along profiles 1 and 2........ 14

8. rotated squares plotted against distance along profiles 1 and 2...............15

9. non-rotated squares plotted against distance along profiles 3 and 4........ 16

10. rotated squares plotted against distance along profiles 3 and4..............17

11. Plot of apparent resistivity anomalies along profiles 1 and 3 ...........................18

12-20. Plots of azimuth and resistivity, in ohm-meters, for:

12. sounding 1, squares 1-3...........................................................................22

13. sounding 1, squares 4-6...........................................................................23

14. sounding 1, squares 1-6...........................................................................24

15. sounding 2, squares 1-3...........................................................................25

16. sounding 2, squares 4-6...........................................................................26

17. sounding 2, squares 1-6...........................................................................27

18. sounding 3, squares 1-3...........................................................................28

19. sounding 3, squares 4-6...........................................................................29

20. sounding 3, squares 1-6...........................................................................30

21. Diagram showing terrain-conductivity measurements across the

gas pipeline and road to the west of sounding 1 ................................................32

TABLES

1. Azimuthal apparent resistivity for soundings 1-3, Area B,

Fort Detrick, Maryland .......................................................................................20

2. Fracture-strike directions for soundings 1-3 determined by use of

graphical methods and program FITELLIPSE ...................................................21

APPENDIX

A. Use of a square-array direct-current resistivity method to 

detect fractures in crystalline bedrock in New Hampshire 

(Lane and others, 1995) ......................................................................................38
CONVERSION FACTORS AND VERTICAL DATUM

Multiply By To obtain

meter (m) 3.281 foot 

kilometer (km) 0.6214 mile

Vertical datum: In this report, "sea level" refers to the National Geodetic Vertical Datum of 

1929 a geodetic datum derived from a general adjustment of the first-order level nets of the 

United States and Canada, formerly called Sea Level Datum of 1929.

VI
Use of Azimuthal Square-Array Direct-Current 

Resistivity to Locate a Geologic Structure at Fort 

Detrick, Maryland

By Dorothy H. Tepper and Daniel J. Phelan 

Abstract

This report presents the results of the azimuthal square-array direct-current resistivity surveys 

performed by the U.S. Geological Survey at Fort Detrick in Frederick, Maryland, from July 8 

through 13, 1998. The purpose of this investigation was to determine if there was geophysical 

evidence of a suspected geologic structure crossing Area B near the eastern base boundary. If this 

structure is present, there is some concern that contaminants may be migrating along it. If so, this 

structure might provide an ideal site to install an extraction well to intercept contaminants before 

they can migrate beyond the base boundary.

The report includes a discussion of the following topics: geologic setting, the square-array 

direct-current resistivity method, methods used to perform the surveys, and results and 

interpretation of the surveys. A total of four profiles and three soundings were performed. 

Because resistivity along two of the profiles showed little change and was probably affected by 

buried debris, these profiles were not interpreted. Soundings were interpreted on the basis of 

graphical results and results from the software program FITELLIPSE.

The area where the resistivity surveys were performed was selected because it is within 0.4 

kilometers of the intersection of an earlier mapped fault trending 131 degrees with a fault trending 

66 degrees. A fault or unconformity also has been mapped in this area by the U.S. Army Corps of 

Engineers and in an earlier study. In addition, the field site is adjacent to the base boundary near 

Robinson Spring.

The data from the profiles and soundings are consistent with the presence of a fault (or 

unconformity) that trends 66 degrees between two of the soundings. In addition, a calculated 

trend of 11-11 degrees or the offset on the anomalies on two profiles is consistent with the 66 

degree trend. On the basis of some assumptions concerning the actual depth of the soundings, the 

dip of the fault would range from 6.7 to 13.3 degrees, which is relatively close to the dip of 20 

degrees estimated in an earlier study on the unconformity.
Introduction

A suspected geologic structure crosses Area B near the eastern boundary of Fort Detrick, near 

Frederick, Maryland (figs. 1 and 2). If this structure is present, there is some concern that groundwater contaminants may be migrating along it. If so, this structure might provide an ideal site to 

install an extraction well to intercept contaminants before they can migrate beyond the base 

boundary. The U.S. Geological Survey (USGS), in cooperation with the Baltimore District of the 

U.S. Army Corps of Engineers, performed azimuthal square-array direct-current (d.c.) resistivity 

surveys at Fort Detrick from July 8 through 13,1998, to determine if there was geophysical 

evidence of the suspected structure.

Purpose and Scope

This report presents the results of the azimuthal square-array d.c. resistivity surveys performed 

by the USGS at Fort Detrick from July 8 through 13, 1998. The azimuthal square-array d.c. 

resistivity technique was used to answer the following questions:

 What is the fracture orientation in the near-surface bedrock?

 Is there an apparent change in fracture orientation in the near-surface bedrock and with depth?

 Is there evidence of a geologic structure on the basis of changes in primary and secondary 

fracture orientations?

 If a structure is present, can preliminary information on its depth be determined?

 If a structure is present, how closely can it be located?

The report includes a discussion of the following topics: geologic setting, the square-array d.c. 

resistivity method, methods used to perform the surveys, and results and interpretation of the 

surveys.

Description of the Field Site

The area where square-array d.c. resistivity profiles 1-4 and soundings 1-3 (fig. 1) were 

performed was selected because it is near the intersection of faults trending 66° (degrees) and 

131° that were mapped by Jonas and Stose (1938) (fig. 2). A fault or unconformity also has been 

mapped in this area by the U.S. Army Corps of Engineers (1983) and ICF Kaiser Engineers 

(1998). In addition, the field site is adjacent to the base boundary near Robinson Spring (fig. 1). 

Because the area is an open grassy field with minimal cultural interferences such as powerlines or 

buried utilities, it was suitable for working with the electrodes and electrical cables necessary for 

this geophysical technique. A utility check determined that a polyvinyl chloride (PVC) gas 

pipeline is buried along the shoulder of the road, but no other buried utilities are in the area. 

Building demolition debris is reportedly buried along the southernmost extent of profiles 2 and 4 

according to the base personnel performing the utilities checks.
Figure 1. Location of Fort Detrick, Frederick, Maryland, and locations of the square-array direct-current resistivity profiles 1 -4 and soundings 1 - 3 

(modified from ICF Kaiser Engineers, 1998).
New Oxford formation

( Hed nftflte and gray tn red arkotte; Tile, basal limeitonf conglomerate 

of crystalline limestone peblilen in red and prat/ calcarrou* matrix: 

txixnl quartztae conglomerate. Tiqc, rounded quartz pfbblen in red 

nanny matrix; baked shale at border nf dialtnite rtikr* nhnnti hy 

rrd ruling)

UNCONFORMITY

(irove limestone

{Thirk-beddfd dark-prat/ la iightjlnveJtne-arainfd pure limentane.

fxtmxlvelv qunrritd, and tfrlrk-brddrd glutening dolomite with

aUun/quartz aroint, Oaq of bane, thick-bedded dolomite and limeetvne containing manv rmtnd (raartz aroint, which irramer to

paroru fiauMtmeand loose tawl. duff for building Kind, fourili

Jtrmu in vlnce*; thiftnrja 600 - fret)

Frederick limestone

( Thin-bedded Wile limetfane. tiahbv granular limestone vith many 

argUlarema partings, buffearthv Itmettnne, and blv.e mottled argillofftmt limestone; thtn dnrlt shale, Cls, in Itnrer part efformatttm, 

mapped lorallv; fovrilijerou» iti placet: thickneia 600 - feet. Vncimjormablv merliet Anlietam quarttttein eattern part of Frederick 

Valleu)

Figure 2. Geologic map and lithologic descriptions for the vicinity of Area B, Ft. Detrick, Maryland 

(from Jonas and Stose, 1938). Faults trending 66°and 131°are identified in the margin of 

the geologic map.
Geologic Setting

Fort Detrick is near the western edge of the Piedmont Physiographic Province, which is 

characterized by rolling topography. An early geologic map of Frederick County was produced by 

Jonas and Stose (1938). The Frederick County part of the State geologic map (Cleaves and 

others, 1968) and revisions of other mappers were included in a report by Duigon and Dine (1987, 

plate 1). Fort Detrick lies within the north-south trending Frederick Valley, which is underlain 

primarily by the Frederick and Grove Limestones (fig. 2).

Stratigraphy The Frederick Limestone (Upper Cambrian) is a dark gray, thin-bedded, 

argillaceous limestone that contains numerous small fractures filled with secondary calcite, and 

the total stratigraphic thickness is about 150 m (meters) (Nutter, 1973). The Upper Cambrian and 

Lower Ordovician Grove Limestone, which is approximately 180 m thick, is a thick-bedded, nearly pure limestone with massive beds of fine-grained dolomite in the lower part and highly quartzose limestone at the base (Nutter, 1973).

The Paleozoic rocks are overlain by Triassic sediments that include conglomerates, sandstones, siltstones, and shales. The Triassic New Oxford Formation, which crops out in the Fort 

Detrick area (fig. 2), overlies Paleozoic rocks in the western and northern parts of the Frederick 

Valley and to the south in Virginia (Reinhardt, 1974). The New Oxford Formation includes a basal limestone conglomerate member consisting of limestone pebbles and cobbles in a fine-grained 

matrix containing quartz grains (Nutter, 1973). This basal conglomerate can be mistaken for the 

Frederick Limestone in areas where the limestone clasts are predominant and where little or no 

matrix material is present. It grades to a quartz-pebble conglomerate north of Frederick (Jonas 

and Stose, 1938). The Frederick Valley apparently was once completely covered by Triassic sedimentary rocks (Nutter, 1973).

The following discussion of the stratigraphy at Area B is condensed from the Draft Remedial 

Investigation Report for Area B (ICF Kaiser Engineers, 1998, p. 2-4 - 2-5). The Rocky Springs 

Member of the Frederick Limestone underlies the southern part of Area B. The strike is 

approximately 35° and the dip is to the southeast at approximately 50°. The Rocky Springs 

Member in this area is a thin-bedded argillaceous limestone with numerous calcite-filled 

fractures. Solution cavities frequently were encountered at depths ranging from 3 to 57 m bis 

(below land surface). In the northern part of Area B, the Rocky Springs Member is 

unconformably overlain by the Triassic New Oxford Formation. According to Nutter (1973), the 

New Oxford Formation near Area B strikes approximately 35° and dips 20° to the northwest; 

however, this unit is thought to be an alluvial fan deposit and as such, does not have true strike and 

dip. Although this formation contains conglomerate, shale, sandstone, and siltstone, the northern 

part of Area B primarily is underlain by the conglomerate and there is a small area of shale, 

sandstone, and siltstone in the northwestern section. Solution cavities up to 5 m long have been 

encountered at depths ranging from 5 to 52 m bis in the New Oxford Formation.

Structural Geology The Triassic rocks in Carroll and Frederick Counties typically strike 

northeast and dip to the northwest at an average of 20°, but the dip ranges from 5° to 40° (Nutter, 

1975). Of the three predominant joint sets in the Triassic rocks of the Frederick Valley, the most 

predominant set consists of joints that parallel the strike and are steeply dipping; joint sets that
strike 315° and 300° and that dip steeply to the east are somewhat less prominent; and a joint set 

that strikes east-west is least prominent (Nutter, 1975).

The Frederick Valley is an asymmetrical synclinorium that is bounded on the west by Catoctin 

Mountain (Reinhardt, 1974), which is a limb of the South Mountain Anticlinorium (Cloos, 1941, 

1947; Whitaker, 1955). The western edge of the valley is formed by a Triassic high-angle reverse 

fault (Nutter, 1973). The Frederick Valley is bordered on the east by the Martic Line, which separates the phyllites of the western Piedmont from the rocks of the Frederick Valley (Reinhardt, 

1974). Jonas and Stose (1938) mapped a fault that separates the Frederick Limestone (Cambrian) 

from conglomerates of the Triassic New Oxford Formation. The fault offsets several Cambrian 

strati graphic units, beginning approximately 1.6 km (kilometers) southwest of the site, with the 

area of offset continuing more than 3.5 km farther to the southwest along the fault. Cambrian and 

Precambrian units are offset along this fault to the northeast, beginning approximately 8.8 km 

northeast of the site. A fault with the same general orientation and position as the one mapped by 

Jonas and Stose (1938) is shown in Nutter (1975), but is not discussed in that report. That fault 

offsets the Triassic border fault along Catoctin Mountain and extends eastward across the Frederick Valley at a trend of approximately 77°.

The field site is near the faults trending 66° and 131° that were mapped by Jonas and Stose 

(1938) and is within 0.4 km of the mapped intersection of these faults. ICF Kaiser Engineers 

(1998, p. 2-5 and Exhibit 2-4) does not mention these faults, but instead refers to an unconformity 

in Area B that separates the Triassic New Oxford conglomerates from the Cambrian Frederick 

Limestone. Because the unconformity was interpreted to be a zone of higher permeability, ICF 

Kaiser Engineers (1998) mapped its surface trace on the basis of surface-drainage features. This 

trace corresponds in shape to the area on the Jonas and Stose (1938) map where the Triassic New 

Oxford conglomerate is truncated near the intersection of the faults trending 66° and 131° (fig. 2). 

The true dip of the unconformity is estimated by ICF Kaiser Engineers (1998) to be about 20°, 

which corresponds to the dip of the Triassic rocks (20° to the northwest, according to Nutter, 

1973) that were deposited on the surface of the unconformity. In the area where the resistivity 

profiles were run, the unconformity shown by ICF Kaiser Engineers (1998) is most likely related 

to the faults mapped by Jonas and Stose (1938), but it is beyond the scope of this report to 

determine the type and relation of these structures.

Description of the Azimuthal Square-Array Direct-Current Resistivity Method

Direct-current, or d.c. resistivity methods have been used by a number of investigators to map 

bedrock fractures, but most of these studies have used colinear arrays, such as Wenner or 

Schlumberger arrays, that have been rotated about a common centerpoint to measure azimuthal 

variations in apparent resistivity that are related to fracture sets (Lewis and Haeni, 1987). The 

azimuthal square-array d.c. resistivity method has been successfully used to detect fracture strike 

in bedrock and it is more sensitive to anisotropy than the more commonly used Wenner or 

Schlumberger arrays (Lane and others, 1995). Another advantage of this method is that it requires 

about 65 percent less surface area than an equivalent survey using a Schlumberger or Wenner 

array (Lane and others, 1995). Habberjam (1979) presents a detailed discussion of the squarearray method and techniques for data analysis. A recent paper by Lane and others (1995), which 

is cited throughout this report, is included in appendix A.
In the azimuthal technique, squares of increasing side length are expanded symmetrically 

about a common centerpoint, which is the location of the apparent resistivity measurements taken 

for each square. Measurements are taken in three directions (alpha, beta, and gamma) for each 

square (fig. 3). The alpha and beta measurements are taken perpendicular to each other and 

provide data on the directional variation of the apparent resistivity. The gamma measurement can 

be used to check the accuracy of the alpha and beta measurements (Lane and others, 1995). The 

concentric squares then are progressively rotated clockwise, typically at 15-degree intervals, 

about the common centerpoint so that directional variations in apparent resistivity with depth can 

be determined.

Acknowledgments

The authors thank the following people for their important contributions to this 

investigation. Thomas Meyer of the U.S. Army Corps of Engineers provided project support. John 

Lane and Eric White of the USGS Office of Ground Water, Branch of Geophysical Applications 

and Support are thanked for their assistance with field work and data compilation.

Methods

The following sections describe field methods for performing the square-array profiles and 

soundings, the electromagnetic terrain-conductivity survey, and methods used to analyze and 

interpret the data.

Field Methods

All data for the square-array profiles and soundings were collected using an ABEM Multimac 

d.c. resistivity system. This computer-controlled data acquisition and storage system allowed a 

complete sounding at a given azimuth to be collected automatically through remotely accessed 

addressable switchers, which connected electrodes for a given measurement (Lane and others, 

1995). Electrode positions for the alpha, beta, and gamma directions of measurements are shown 

in figure 3. Software provided with the system was modified by Lane and others (1995) for the 

square array to control the measurement sequence.

Resistivity Profiles Two sets of parallel resistivity profiles were completed (fig. 1). The 

orientation of these profiles was 25°-205°. Profiles 1 and 2 were the westernmost of the two sets. 

Profile 1 extended to the main entrance road. Profile 2 was an extension of profile 1 that began on 

the southwest side of the main entrance road. Profile 3 was parallel to profile 1 and profile 4 was 

parallel to profile 2. The distance between the two sets of profiles was 25 m on center. Positions 

of the profiles were marked in the field with arrows painted on the road and fences so that points 

along the profiles could easily be located afterwards. These arrows pointed to the origin for 

profiles 1 and 3, to the points where these profiles crossed the road, and to the points on the other 

side of the road where profiles 1 and 3 continued as profiles 2 and 4, respectively.
p, c,

Direction of measurement

C,, C2 = Current electrodes

P1? P2 = Potential electrodes

p,

Alpha Beta Gamma

Figure 3 . Electrode configurations for square-array direct-current resistivity measurements 

(modified from Lane and others, 1995).
Each profile consisted of overlapping squares that were 10 m in length along a side (fig. 4). 

Adjacent squares overlapped by 5 m to provide some data redundancy. In addition to a series of 

squares that were oriented 25°-205° (referred to herein as "non-rotated squares"), another series 

of squares (referred to as "rotated squares") were oriented 45° to the 25°-205° trend (2 parallel 

sides of the rotated square trended 70°-250° and the other 2 parallel sides trended 160°-340°) 

(fig. 4).

Resistivity Soundings Three resistivity soundings were performed after the profiles were 

completed and the data were reviewed. Soundings 1 and 2 were conducted between profiles 1 and 

3 (fig. 1). The centerpoint for sounding 1 was located at the midpoint between profiles 1 and 3, 

along the origin line. Sounding 2 also was located at a midpoint between profiles 1 and 3 that was 

60 m southwest of the centerpoint for sounding 1. Sounding 3 was in a field approximately 760 m 

southwest of the location of sounding 1 (fig. 1). This site was selected to collect background 

information on the Frederick Limestone at some distance away from the suspected structure. 

Each sounding consisted of a set of 6 concentric squares with the following side lengths: 4.24, 

7.07, 9.90,14.14, 19.80, and 28.28 m. These squares initially were oriented at 350°, along the 

line between the potential electrodes in the alpha electrode configuration (fig. 3). After the initial 

set of readings was completed, the squares progressively were rotated clockwise about the array 

centerpoint in 15-degree increments to azimuths of 5°, 20°, 35°, 50°, and 65°. A complete set of 

readings was taken at each orientation; alpha measurements were made between 350° and 65° and 

beta measurements were made between 80° and 155°.

Electromagnetic Terrain-Conductivity Survey A Geonics EM-34 electromagnetic terrainconductivity meter was used to determine if the PVC gas pipeline buried along the shoulder of the 

road would have any effect on the square-array resistivity readings. A short survey approximately 

32 m long was performed. This survey began in the field where profiles 1 and 3 were completed, 

crossed the road, and ended in the field on the west side of the road (fig. 1).

Analytical Methods

The resistivity profiles and soundings 1 and 2 were interpreted on the basis of methods 

described in Lane and others (1995), which are based largely on a method described by 

Habberjam (1972). Field data were entered into a spreadsheet and apparent resistivity was 

calculated on the basis of a geometric factor for the array, the voltage potential difference 

(measured between electrodes PI and P2; fig. 3), and the current (measured between electrodes 

Cl and C2; fig. 3). For the purposes of this report, an anomaly was considered to be a point or 

cluster of points that was clearly outside the trend of nearby points, and was determined on the 

basis of observation rather than defined by quantitative criteria.
Non-rotated squares Rotated s

Measuring points (MP)

Figure 4. Square-array layouts used along profiles 1-4. Non-rotated squares are shown on the left and rotated squar
For all squares in soundings 1-3, the fracture strike was determined graphically by plotting the 

apparent resistivity for each of the square arrays against the azimuth of that measurement. The 

principal fracture strike is interpreted to be perpendicular to the direction of maximum resistivity 

(Lane and others, 1995). In cases in which there were some high values of apparent resistivity for 

a given square, trends in nearby data points also were considered. That is, the highest apparent 

resistivity value was not selected if it was an obvious spike rather than an identifiable trend in the 

data from adjacent points. The few data points that were erroneous due to poor electrical 

connections have been removed from the plots and data table. Because sounding 3 showed little 

anisotropy, it was difficult to use this graphical technique to determine the fracture strike 

direction. The program FITELLIPSE by Hart and Rudman (1997) also was used. This program 

calculates the orientation and values of the major and minor axes of a best-fit ellipse to the data. 

This program was used on all squares in sounding 3 and also was used to compare the graphical 

solutions in all squares in soundings 1 and 2.

Results of Resistivity Profiles, Resistivity Soundings, and TerrainConductivity Survey

Results of the resistivity profiles, resistivity soundings, and the terrain-conductivity survey are 

presented in the following sections.

Resistivity Profiles

The mean geometric resistivity along profiles 1-4 is shown in figure 5. Apparent anisotropy for 

profiles 1-4 is shown in figure 6. For profiles 1 and 2, the apparent resistivity for non-rotated 

squares is shown in figure 7 and for rotated squares is shown in figure 8. For profiles 3 and 4, the 

apparent resistivity for non-rotated squares is shown in figure 9 and for rotated squares is shown 

in figure 10. A plot of the anomalies along profiles 1 and 3 is presented in figure 11.

Profiles 1 and 2 Although there are strong upward spikes in the interval between 40 and 55 m 

and an increasing trend near the road, the mean geometric resistivity generally decreases from 

northeast to southwest along profiles 1 and 2 (fig. 5). Pronounced changes in apparent anisotropy 

along profiles 1 and 2 occur between 35 to 65 m and between 75 to 100 m (fig. 6). The alpha, 

beta, and gamma readings for profiles 1 and 2 for the non-rotated squares are shown in figure 7. 

In the alpha configuration, there were changes in apparent resistivity in the interval between 40 

and 55 m, particularly between 40 and 45 m, where the apparent resistivity increased by 31 ohmmeters. The gamma reading also showed changes (indicating anisotropy) in the interval between 

35 and 65 m, particularly between 55 and 60 m and between 60 m and 65 m (fig. 7). Between 90 

and 95 m, the beta reading dropped 28 ohm-meters, and the gamma showed 43 ohm-meters of 

difference between the alpha and beta readings (fig. 7). On the rotated squares (fig. 8), the alpha 

readings showed changes at 25 m, in the interval between 35 and 55 m, at 80 m, and at 95 m; the 

beta readings showed changes between 50 and 55 m and at 95 m; and pronounced differences 

were observed in the gamma readings between 50 to 55 m, at 85 m, and at 95 m.

11
Northeast Southwest

180

160140120

100

1

IV)

40 60 80 100 

Distance, In meters

120 140 160 180

Figure 5. Mean geometric resistivity plotted against distance along profiles 1 - 4, Fort Detrick, Maryland.
Northeast 

2.5

Southwest

CO

0 20 40 60 80 100 120 140 160 180

Distance, in meters

EXPLANATION

Profiles 1 and 2 Profiles 3 and 4

Figure 6. Apparent anisotropy plotted against distance along profiles 1 - 4, Fort Detrick, Maryland.
Northeast 

140

Southwest

20 40 60 80 100 

Distance, in meters

120 140 160 180

EXPLANATION 

Electrode configurations: - -Alpha -»-Beta -*-Gamma

Figure 7. Apparent resistivity of non-rotated squares plotted against distance along profiles 1 and 2, Fort Detrick, Maryland.
Northeast 

200

Southwest

-100

0 20 40 60 80 100 120 140 160 180

Distance, in meters 

EXPLANATION

Electrode configurations: Alpha - - Beta *- Gamma 

Figure 8. Apparent resistivity of rotated squares plotted against distance along profiles 1 and 2, Fort Detrick, Maryland
Northeast 

200

Southwest

-100

0 20 40 60 80 100 120 140 160 180

Distance, in meters

EXPLANATION

Electrode configurations: Alpha Beta A Gamma 

Figure 9. Apparent resistivity of non-rotated squares plotted against distance along profiles 3 and 4, Fort Detrick, Maryland.
Northeast Southwest

250

-100

20 40 60 80 100

Distance, in meters

EXPLANATION 

Electrode configurations: * Alpha

120 140

Beta * Gamma

160 180

Figure 10. Apparent resistivity of rotated squares plotted against distance along profiles 3 and 4, Fort Detrick, Maryland.
00

PROFILE 1

Distance, in meters

PROFILE 3

Distance, in meters

EXPLANATION

NON-ROTATED 

SQUARES

10 20 30 40_______50 60 70 80 90 100 110

ROTATED SQUARES

NON-ROTATED 

SQUARES

10___ 20 30 40 50 60 70 80 90 100______110

ROTATED SQUARES

alpha anomaly 

beta anomaly gamma anomaly

apparent anisotropy spike

mean geometric resistivity spike

Figure 11. Apparent resistivity anomalies along profiles 1 and 3, Fort Detrick, Maryland
In summary, the mean geometric resistivity showed an overall decrease along profiles 1 and 2. 

Anomalies in apparent resistivity were observed in the non-rotated squares in the intervals 

between 35 to 65 m, between 90 to 95 m, and at 95 m. In the rotated squares, anomalies were 

observed at 25 m, in the interval between 35 to 55 m, and at 80 m, 85 m, and 95 m.

Profiles 3 and 4 As in profiles 1 and 2, the mean geometric resistivity generally decreased 

from northeast to southwest along profiles 3 and 4 (fig. 5). The most pronounced change was a 

drop of 62 ohm-meters in the interval between 95 to 110 m. Smaller changes were observed 

between 10 and 15m and at 75 m. The most pronounced changes in apparent anisotropy along 

profiles 3 and 4 were observed between 15 to 25 m and between 105 to 110 m (fig. 6). The alpha, 

beta, and gamma readings for profiles 3 and 4 for the non-rotated squares are shown in figure 9. 

Alpha anomalies were observed between 10 and 15m, and at 75 m and 95 m. Beta anomalies 

were observed at 30 m and between 105 to 110 m. Gamma anomalies were observed at 75 m and 

between 105 to 110 m. Alpha, beta, and gamma values all decreased fairly sharply in the interval 

between 100 to 110 m. On the profiles for the rotated squares (fig. 10), there was an anomaly in 

the alpha reading between 15 to 35 m, anomalies in the beta readings at 75 and between 100 to 

110m, and in the gamma readings from 15 to 30 m, at 60 m and at 75 m, and between 105 to 110 

m.

In summary, the mean geometric resistivity generally decreased from northeast to southwest 

along profiles 3 and 4, with the largest decline between 95 to 110m. Anomalies in apparent resistivity were observed in the non-rotated squares in the interval between 10 to 15 m, at 30 m, at 

75 m, at 95 m, and from 105 to 110 m. In the rotated squares, anomalies were observed at 15 to 

35 m, at 60 m, at 75 m, and from 95 to 110 m.

Resistivity Soundings

Data for the soundings are presented in table 1 and are plotted as apparent resistivity with 

respect to azimuth in figures 12-20. Fracture strike directions for soundings 1-3 determined by 

use of graphical methods and the FITELLIPSE approach (Hart and Rudman, 1997) are shown in 

table 2.

The maximum depth of penetration of a given sounding would be equivalent to the length of 

the side of the square (the A-spacing), but the depth of penetration is usually less and is affected 

by the resistivities of the various layers, which are affected by porosity, water content, water 

quality, and the rock matrix (John Lane, U.S. Geological Survey, oral commun., 1998).

Sounding 1 Plots of apparent resistivity and azimuth are shown for squares 1-3 in figure 12, 

for squares 4-6 in figure 13, and for squares 1-6 in figure 14. In squares 1 and 2, the primary 

fracture strike based on the graphical method was 110° and 95°, respectively, and the interpreted 

highest apparent resistivity decreased with depth from 263 to 204 ohm-meters (fig. 12). In square 

3, the primary fracture strike based on the graphical method was 50°. In squares 4-6, the primary 

fracture strike was 65°, 65°, and 50°, respectively, and the interpreted highest apparent resistivity 

increased with depth from 250 ohm-meters in square 4 to 423 ohm-meters in square 5, but 

decreased to 300 ohm-meters in square 6 (fig. 13; table 1). The shapes of the plots are appreciably 

different between squares 1-3 and 4-6, but both show strong directional trends.

19
Table 1. Azimuthal apparent resistivity for soundings 1-3, Area B, Fort Detrick, Maryland

[Location of soundings are shown in figure 1; m, meters; --, data points missing because of poor electri

ro 

o

Sounding 1

A-spacing

(m)

4.24

7.07

9.90

14.14

19.80

28.28

Sounding 2 

A-spacing 

(m)

4.24

7.07

9.90

14.14

19.80

28.28

Sounding 3 

A-spacing

(m)

4.24

7.07

9.90

14.14

19.80

28.28

Geometric

Factor

45.51

75.84

106.18

151.69

212.37

303.38

Geometric 

Factor

45.51

75.84

106.18

151.69

212.37

303.38

Geometric

Factor

45.51

75.84

106.18

151.69

212.37

303.38

Mean

208.8

180.5

175.1

193.4

231.7

249.8

Mean

83.1

111.6

135.3

163.8

209.2

267.2

Mean

518.0

320.2

278.9

252.7

277.0

381.0

350

251.7

203.3

182.6

200.2

278.2

273.0

350

90.6

110.0

132.7

177.5

237.8

291.2

350

-

275.3

225.1

219.9

265.5

312.5

5

259.8

204.0

160.3

185.1

210.2

285.2

5

80.1

111.5

140.2

120.0

223.0

303.4

5

446.9

291.2

231.5

263.3

--

20

262.6

203.3

171.0

169.9

212.4

282.1

20

111.5

140.2

171.4

206.0

282.1

20

480.6

321.6

243.2

227.5

250.6

321.6

35

225.3

169.1

157.2

166.9

169.9

248.8

35

87.8

112.3

140.2

166.9

191.1

260.9

35

572.0

377.7

292.0

276.1

299.4

270.0

Azimuth, in degrees from true north

50 65 80 95

Apparent resistivity, in ohm-meters

180.7 251.7 170.7 172.5

150.9 216.9 157.0 169.9

152.9 203.9 172.0 178.4

172.9 183.5 191.1 185.1

182.6 216.6 208.1 201.7

260.9 257.9 248.8

Azimuth, in degrees from true north

50 65 80 95

Apparent resistivity, in ohm-meters

92.4 81.9 89.2 96.0

106.9 103.1 111.5 110.0

129.5 125.3 128.5 128.5

154.7 150.2 147.1 154.7

180.5 167.8 163.5 189.0

233.6 227.5 224.5 242.7

Azimuth, in degrees from true north

50 65 80 95

Apparent resistivity, in ohm-meters

505.1 502.9 450.5 498.8

358.7 289.0 354.2 293.5

321.7 250.6 319.6 268.6

309.4 229.1 277.6 285.2

259.1 259.1 278.2 303.7

263.9 336.8 348.9 358.0
Table 2. Fracture-strike directions for soundings 1 -3 determined by use of graphical

methods and program FITELLIPSE1 , Fort Detrick, Maryland

[m, meters; °, degrees; azimuth directions are shown in degrees from true north]

Square 

number

1

2

3

4

5

6

Square Sounding 1 Sounding 2 Sounding 3

Size(m) Graphical FITELLIPSE Graphical FITELLIPSE Graphical FITELLIPSE

4.24

7.07

9.90

14.14

19.80

28.28

110°

95°

50°

65°

65°

50°

107°

97°

19°

50°

62°

62°

65°

65°

65°

65°

65°

65°

29°

58°

76°

64°

63°

73°

125°

125°

140°

140°

20°

20°

130°

114°

19°

6°

12°

29°

1 Hart and Rudman, 1997

21
335,

320,'

50

j, 65

275 

ro ro

- 95

260 '

245

230

215

EXPLANATION 

Square-array side length, in meters (A-spacing)

-4.24 -»-7.07 -Hk- 9.90

Figure 12. Azimuth and resistivity, in ohm-meters, for sounding 1, squares 1-3, Fort Detrick, Maryland.
350

335.

65

290

80

CO

EXPLANATION

Square-array side length, in meters (A-spacing) 

- -14.14 - -19.80 -28.28

Figure 13. Azimuth and resistivity, in ohm-meters, for sounding 1, squares 4-6, Fort Detrick, Maryland.
350 0

335. -.450- -.20

320,

305 50

65

80

155

185 " v/0

-4.24

EXPLANATION 

Square-array side length, in meters (A-spacing)

- -7.07 -9.90 -*^14.14 -*-19.80 -28.28

Figure 14. Azimuth and resistivity, in ohm-meters, for sounding 1, squares 1-6, Fort Detrick, Maryland.
350 0

CJi

-4.24

EXPLANATION

- -7.07 -9.90

Square-array side length, in meters (A-spacing) 

Figure l!r. Azimuth and resistivity, in ohm-meters, for sounding 2, squares 1-3, Fort Detrick, Maryland.
O>

EXPLANATION 

Square-array side length, in meters (A-spacing)

-4-14.14 -*-19.80 -*- 28.28

Figure 16. Azimuth and resistivity, in ohm-meters, for sounding 2, squares 4-6, Fort Detrick, Maryland.
N>

335..

320.

\50

65

140

155

EXPLANATION

-4.24 -7.07 9.90 14.14 -28.28 -28.28

Figure 17. Azimuth and resistivity, in ohm-meters, for sounding 2, squares 1-6, Fort Detrick, Maryland.
350

335. ,600 .-,20

320-

, 50

80

N> 

00

EXPLANATION 

Square-array side length, in meters (A-spacing)

-»-4.24 -*-7.07 9.90

Figure 18. Azimuth and resistivity, in ohm-meters, for sounding 3, squares 1-3, Fort Detrick, Maryland.
35

,,50

N) CD

EXPLANATION 

Square-array side length, in meters (A-spacing)

- -14.14 - -19.80 -28.28

Figure 19. Azimuth and resistivity, in ohm-meters, for sounding 3, squares 4-6, Fort Detrick, Maryland.
350 Q

335,- ., 20

320*"'

65

CO O

125

140

EXPLANATION 

Square-array side length, in meters (A-spacing)

-4.24 -»-7.07 -- 9.90 -*-14.14 -*-19.80 - 28.28

Figure 30. Azimuth and resistivity, in ohm-meters, for sounding 3, squares 1-6, Fort Detrick, Maryland.
Sounding 2 Plots of apparent resistivity and azimuth are shown for squares 1-3 in figure 15, 

for squares 4-6 in figure 16, and for squares 1-6 in figure 17. The highest interpreted apparent resistivity values increased with depth from a minimum of 80 in square 1 to a maximum of 309 

ohm-meters in square 6 (table 2). In squares 1-3, the primary fracture strike based on the graphical method was somewhat poorly defined but probably was about 65°. In squares 4-6, the primary 

fracture strike determined graphically was better defined and also was 65°. The plot shapes are 

somewhat similar, but the trends are better defined in the plots of squares 4-6, which are more elliptical and, therefore, more anisotropic, indicating stronger trends in the fracture orientation.

Sounding 3 Plots of apparent resistivity and azimuth are shown for squares 1-3 in figure 18, 

for squares 4-6 in figure 19, and for squares 1-6 in figure 20. As stated previously, because sounding 3 indicated little anisotropy, the program FITELLIPSE by Hart and Rudman (1997) was used 

to calculate the direction of highest apparent resistivity and the primary fracture strike for each 

square. Data points that clearly were in error, most likely because of poor electrical connections 

(shown as dashes in table 1), were dropped from the squares oriented at 350° (square 1) and 5° 

(squares 4 and 6). In squares 1 and 2, the primary fracture strike is 130° and 114°, respectively. 

In squares 3-6, the primary fracture strike is 19°, 6°, 12°, and 29°, respectively. The interpreted 

highest apparent resistivity generally decreased with depth from squares 1-3, but increased from 

squares 4-6.

Terrain-Conductivity Survey

Data from the terrain-conductivity survey are presented in figure 21. The terrain-conductivity 

readings decreased as the pipeline and road were crossed, but these changes were within 5 to 6 m. 

of the pipeline and road. It was concluded from this survey that the presence of the road and the 

PVC pipeline would not affect the resistivity profiles and soundings anywhere except within that 

5- to 6-m distance. This interference, in addition to being unable to block traffic to place 

electrodes in the entrance road, is the reason for the break in profile lines 1-2, and 3-4 shown in 

figs. 5 through 10.

Interpretation of Resistivity Profiles and Resistivity Soundings

A comparison of the types and abundance of anomalies along profiles 1 and 3 is presented in 

figure 11. Because resistivity along profiles 2 and 4 showed little change and probably was 

affected by buried debris, these profiles were not interpreted. Soundings were interpreted on the 

basis of graphical results and results from the program FITELLIPSE.

Resistivity Profiles

The pattern of anomalies near 95 m on profile 1 was similar to those at 75 m on profile 3 (fig. 

11). In addition, the zone from 35 to 55 m on profile 1 was similar to the zone from 15 to 30 m on 

profile 3. The offset of these anomalies between profiles 1 and 3 ranged from 20 to 25 m. In 

addition, on the plot of apparent anisotropy versus distance along the profiles (fig. 6), there were 

pronounced peaks on profile 1 at 45 m, 80 m, and at 90 m, with a less pronounced peak at 60 m. 

The peaks on profile 3 were at 20 m, 35 m, 60 m, and 110m. There may be a structure that

31
Southeast (near sounding 1)

8

CO

0 2 4 6 8 10 12 14 16 18 20 22 24 26 

Distance, in meters

Figure 21. Terrain-conductivity measurements across the gas pipeline and road to the west of sounding 1, Fort 
crosses profiles 1 and 3 at an angle, causing an offset of 20-25 m in the observed peaks on these 

profiles, assuming that on profile 1, the peaks at 45 m, 60 m, and 80 m matched the peaks on 

profile 3 at 20 m, 35 m, and 60 m, respectively. There was apparently no match on profile 3 for 

the peak on profile 1 at 90 m. The peak on profile 3 at 110 m may correlate with a feature in 

profile 2, where the data are inconclusive. On the basis of a trigonometric solution, an offset of 20 

m between profiles 1 and 3 would trend at 77° and an offset of 25 m would trend at 71°. This 

range of trends for the offset on the anomalies on the profiles fits reasonably closely with the trend 

of 66° for the fault mapped by Jonas and Stose (1938). As previously described, several 

Cambrian stratigraphic units are offset along this fault, beginning approximately 1.6 km 

southwest of the site, with the area of offset continuing over 3.5 km to the southwest along the 

fault. Cambrian and Precambrian units are offset along this fault to the northeast, beginning 

approximately 8.8 km northeast of the site.

Resistivity Soundings

At sounding 2, the dominant strike in squares 1-6 was 65° based on the graphical results. The 

interpreted highest apparent resistivity increased with depth in both sets of squares, ranging from 

99 to 309 ohm-meters. This same fracture trend of 65° also was observed in sounding 1, in 

squares 4-5, based on the graphical results. A possible interpretation is that the fault (66°) is at or 

very close to the surface at sounding 2, the fault plane dips gently towards sounding 1, and was 

intersected between squares 2 and 4 on sounding 1, where the most pronounced change in the 

fracture strike direction occurred (based on graphical and FITELLIPSE results, table 2).

Another fault that trends 131° was mapped by Jonas and Stose (1938) (fig. 2) in the vicinity of 

profiles 1-4 and soundings 1 and 2. The Frederick Limestone (Cambrian) and the New Oxford 

Formation (Triassic; red shale and gray to red arkose, with basal limestone conglomerate) contact 

each other on opposite sides of this fault. This 131° fault trend is close to the graphical trend of 

110° observed in sounding 1, square 1. The field site is within 0.4 km of the mapped intersection 

of this fault with the fault that trends 66°. A possible interpretation based on the graphical 

solution is that the fracture orientation in the shallower bedrock observed in square 1 of sounding 

1 may be influenced by the fault that trends 131° and the deeper bedrock observed in squares 4-6 

of sounding 1 may be influenced by the fault that trends 66°. The FITELLIPSE results indicate 

that the major change in fracture strike orientation occurred between squares 2 and 4. Soundings 

1 and 2, therefore, may have been very close to the intersection of these faults.

The data are consistent with the presence of a fault (or unconformity) that trends 66° between 

soundings 1 and 2. Based on the graphical solution, the dominant fracture direction in all 6 

squares on sounding 2 was 65°. On sounding 1, the graphical solution for square 1 was that the 

dominant fracture direction was 110°, which may have been influenced by the nearby fault 

trending at 131°. The FITELLIPSE program showed the dominant fracture direction in square 1 

at 107°. The graphical solution for square 2 was 95° and the FITELLIPSE solution was 97°. The 

graphical solution showed squares 4-6 at 65°, 65°, and 50°, respectively. The FITELLIPSE 

solution showed square 3 at 19° and it showed squares 4-6 at 50°, 62°, and 62°, respectively. This 

solution indicates that soundings 1 and 2 may have been near the intersection of the faults 

trending 66° and 131° shown on figure 2. It also is likely that the fault trending 66° dips from 

sounding 2 towards sounding 1, where the fault was encountered at depths represented between

33
squares 2 and 4. The dip on the surface of the unconformity in the area is estimated to be about 

20° (ICF Kaiser Engineers, 1998), which corresponds to the dip of the Triassic units (Nutter, 

1975) deposited on the surface of the unconformity.

According to John Lane (U.S. Geological Survey, oral commun., 1998), the maximum depth of 

penetration for a square is equal to the length of its side, but the depth of penetration usually is 

less and can be affected by the resistivities of the various layers, which are affected by porosity, 

water content, water quality, and the rock matrix. Square 2 on sounding 1 would have a maximum 

depth of penetration of 7.07 m. Square 4 on sounding 1 would have a maximum depth of 

penetration of 14.14 m. The distance between the centerpoints of sounding 1 and 2 was 60 m. 

Assuming that the fault is at the surface at sounding 2, the dip would range from 6.7° to 13.3°, 

which is relatively close to the true dip of 20° estimated by ICF Kaiser Engineers (1998).

Based on the FITELLIPSE results, sounding 3 in squares 1 and 2 showed a fracture strike direction of 130° and 114°, respectively. Squares 3 through 6 showed fracture strike directions of 

19°, 6°, 12°, and 29°, respectively. In addition, the interpreted highest apparent resistivities 

ranged from 275 to 572 ohm-meters, which is a slightly broader range (297 ohm-meters) than that 

observed at soundings 1 (229 ohm-meters) and 2 (210 ohm-meters). The site for sounding 3 was 

selected to collect background information on the Frederick Limestone away from the suspected 

fault or unconformity. Using a program by Zohdy and Bisdorf (1989) that uses mean resistivity 

values to calculate the resistance of the bedrock, it was determined that soundings 1 and 2 were on 

resistive bedrock (greater than 1,000 ohm-meters) and sounding 3 was on less resistive bedrock 

(200 ohm-meters). The site for sounding 3 was in an area where solution cavities have been 

encountered. If solution channels are present in the area, they may have caused the slightly 

broader range observed in the apparent resistivity and may have influenced the observed fracture 

strike directions.

Summary and Conclusions

A suspected geologic structure crosses Area B near the eastern boundary of Fort Derrick, near 

Frederick, Maryland. If this structure is present, there is some concern that ground-water 

contaminants may be migrating along it. If so, this structure might provide an ideal site to install 

an extraction well to intercept contaminants before they can migrate beyond the base boundary. 

The U.S. Geological Survey, in cooperation with the Baltimore District of the U.S. Army Corps of 

Engineers, performed azimuthal square-array direct-current resistivity surveys at Fort Detrick to 

determine if there was geophysical evidence of the suspected structure. Four square-array profiles 

and three soundings were performed from July 8 through 13, 1998.

The area where the profiles and soundings were performed was selected because geologic 

structures have been mapped in the vicinity by several previous investigators. In addition, the 

field site is adjacent to the base boundary near a large spring. Because the area is an open grassy 

field with minimal cultural interferences such as powerlines or buried utilities, it was suitable for 

working with the electrodes and electrical cables necessary for this geophysical technique. The 

resistivity surveys were performed to answer specific questions about the suspected geologic 

structure. These questions and their answers are summarized below.

34
What is the fracture orientation in the near-surface bedrock?

The dominant fracture orientation in sounding 1, square 1 (maximum depth of penetration is 

4.24 m (meters)) was 110° (degrees) based on the graphical solution. For sounding 1, square 2 

(maximum depth of penetration is 7.07 m), it was 95°, also based on the graphical solution. In 

sounding 2, the dominant fracture orientation in squares 1 and 2 using the graphical solution 

was 65°.

Is there an apparent change in fracture orientation in the near-surface bedrock and with depth?

In sounding 1, based on the graphical solution, the fracture orientation in the shallower bedrock 

observed in squares 1 and 2(110° and 95°, respectively) shifted to a range of 50° to 65° in 

squares 3-6. On the basis of the graphical solution, no change in fracture orientation in the nearsurface bedrock or with depth was observed among the squares in sounding 2.

Is there evidence of a structure based on changes in primary and secondary fracture orientations?

The field site is within 0.4 kilometers of the intersection of an earlier mapped fault trending 

131° with a fault trending 66°. The shift in fracture orientation between squares 1 and 2 on 

sounding 1 (110° and 95°, respectively) to a range of 50° to 65° in squares 3-6 indicates that the 

uppermost 2 squares may have been affected by the fault trending 131°, and the deeper squares 

(3-6) may have been affected by the fault trending 66°.

If there is a structure, can preliminary information on its depth be determined?

The FITELLIPSE program results indicate that the major change in strike occurred between 

squares 2 and 4 on sounding 1, at maximum depths of approximately 7 to 14 meters.

If there is a structure, how closely can it be located?

Soundings 1 and 2 may have been near the intersection of the faults trending 66° and 131°. It is 

likely that the fault (unconformity) trending 66° dips from sounding 2 towards sounding 1, 

where it was encountered at depths represented between squares 2 and 4. Assuming the fault is 

at the surface at sounding 2, and that the maximum depth of penetration of square 2 was 7.07 

meters and of square 4 was 14.14 meters, the dip of the fault would range from 6.7° to 13.3°, 

which is relatively close to the dip of 20° estimated in an earlier study of the unconformity. In 

addition, the trend of 71°-77° for the offset on the anomalies on profiles 1 and 3 is consistent 

with the trend of 66° for the earlier mapped fault.

35
References Cited

Cleaves, E.T., Edwards, J.R., and Glaser, J.D., compilers, 1968, Geologic map of Maryland: 

Maryland Geological Survey, 1 sheet, scale 1:250,000.

Cloos, Ernst, 1941, Flowage and cleavage in Appalachian folding: N.Y. Academy of Science 

Transactions, series 2, v. 3, p. 185-190.

___, 1947, Oolite deformation in the South Mountain fold, Maryland: Bulletin of the

Geological Society of America, v. 58, p. 843-917.

Duigon, M.T., and Dine, J.R., 1987, Water resources of Frederick County, Maryland: 

Maryland Geological Survey Bulletin 33, 106 p., 2 pis.

Habberjam, G.M., 1972, The effects of anisotropy on square array resistivity measurements: 

Geophysical Prospecting, v. 20, p. 249-266.

___, 1979, Apparent resistivity observations and the use of square array techniques, in Saxov,

S. and Flathe, H. (eds.): Geoexploration Monographs, series 1, no. 9, p 1-152.

Hart, D., and Rudman, A.J., 1997, Least-squares fit of an ellipse to anisotropic polar data: 

application to azimuthal resistivity surveys in karst regions: Computers and Geosciences, 

v.23,p. 189-194.

ICF Kaiser Engineers, 1998, Draft document: Fort Detrick remedial investigation of Area B, 

volumes 1, 2, and 3: Edgewood, Maryland, ICF Kaiser Engineers, [variously paged].

Jonas, A.I., and Stose, G.W., 1938, Geologic map of Frederick County and adjacent parts of 

Washington and Carroll Counties: Maryland Geological Survey, scale 1:62,500, 1 sheet.

Lane, J.W., Jr., Haeni, P.P., and Watson, W.M., 1995, Use of a square-array direct-current 

resistivity method to detect fractures in crystalline bedrock in New Hampshire: Ground 

Water, v. 33, no. 3, p. 476- 485.

Lewis, M.R., and Haeni, PP., 1987, The use of surface geophysical techniques to detect fractures 

in bedrock an annotated bibliography: U.S. Geological Survey Circular 987, 14 p.

Nutter, L.J., 1973, Hydrogeology of the carbonate rocks, Frederick and Hagerstown Valleys, 

Maryland: Maryland Geological Survey Report of Investigations No. 19, 70 p., 2 pi.

_____, 1975, Hydrogeology of the Triassic rocks of Maryland: Maryland Geological Survey

Report of Investigations No. 26, 37 p.

Reinhardt, Juergen, 1974, Stratigraphy, sedimentology and Cambro-Ordovician paleogeography 

of the Frederick Valley, Maryland: Maryland Geological Survey Report of Investigations

No. 23,74 p, 2 pi.

36
U.S. Army Corps of Engineers, 1983, Study of proposed landfill Area B, Fort Detrick, Maryland: 

Baltimore, Maryland, U.S. Army Corps of Engineers Baltimore District, 18 p.

Whitaker, J.C., 1955, Geology of Catoctin Mountain, Maryland and Virginia: Bulletin of the 

Geological Society of America, v. 66, p. 435-462

Zohdy, A.A.R., and Bisdorf, R.J., 1989, Programs for the automatic processing and interpretation 

of Schlumberger sounding curves in QuickBASIC 4.0: U.S. Geological Survey Open-File 

Report 89-0137-A, 64 p.

37
APPENDIX A

Reprinted with permission of the National Ground Water Association, copyright 1995,

all rights reserved.

38
Use of a Square-Array Direct-Current Resistivity

Method to Detect Fractures in Crystalline

Bedrock in New Hampshire

by J. W. Lane, Jr., F. P. Haeni, and W. M. Watson"

Abstract

Azimuthal square-array direct-current (dc) resistivity soundings were used to detect fractures in bedrock in the Mirror 

Lake watershed in Grafton County, New Hampshire. Soundings were conducted at a site where crystalline bedrock underlies 

approximately 7 m (meters) of glacial drift. Measured apparent resistivities changed with the orientation of the array. 

Graphical interpretation of the square-array data indicates that a dominant fracture set and (or) foliation in the bedrock is 

oriented at 030° (degrees). Interpretation of crossed square-array data indicates an orientation of 027° and an anisotropy 

factor of 1.31. Assuming that anisotropy is due to fractures, the secondary porosity is estimated to range from 0.01 to 0.10.

Interpretations of azimuthal square-array data are supported by other geophysical data, including azimuthal seismicrefraction surveys and azimuthal Schlumberger de-resistivity soundings at the Camp Osceola well field. Dominant fracture 

trends indicated by these geophysical methods are 022° (seismic-refraction) and 037° (de-resistivity). Fracture mapping of 

bedrock outcrops at a site within 250 m indicates that the maximum fracture-strike frequency is oriented at 030°.

The square-array de-resistivity sounding method is more sensitive to a given rock anisotropy than the more commonly 

used Schlumberger and VVenner arrays. An additional advantage of the square-array method is that it requires about 65 

percent less surface area than an equivalent survey using a Schlumberger or VVenner array.

Introduction

Research on the application of surface-geophysical 

methods to detect bedrock fractures and to estimate hydraulic properties of fractured bedrock is being conducted by the 

U.S. Geological Survey (USGS) as part of a bedrock 

research investigation in the Mirror Lake watershed of the 

Hubbard Brook Experimental Forest in Grafton County, 

New Hampshire (Figure 1).

One part of the study assesses the ability of directcurrent (dc) resistivity methods to detect bedrock fractures. 

De-resistivity methods have been successfully used by a 

number of investigators to detect bedrock fractures (Risk, 

1975; McDowell, 1979; Palacky et al., 1981; Soonawala and 

Dence, 1981; Taylor, 1982; Mallik et al., 1983; LeonardMayer, 1984a, 1984b; Ogden and Eddy, 1984; Taylor, 1984; 

Taylor and Fleming, 1988; Lieblich et al., 1991, 1992; Ritzi 

and Andolsek, 1992). Most of these investigations have used 

collinear arrays (Wenner or Schlumberger) rotated about a 

fixed centerpoint to measure azimuthal variations in apparent resistivity that are related to sets of similarly oriented, 

steeply dipping fractures (Lewis and Haeni, 1987).

aU.S. Geological Survey, Room 525,450 Main St., Hartford, 

Connecticut 06103.

Received December 1993, revised June 1994, accepted August 

1994.

Habberjam (1972) showed that a square array is more 

sensitive to anisotropy in the subsurface and requires less 

surface area than collinear arrays. Recently, de-resistivity 

surveys using a square array have been conducted to detect 

productive fracture zones in crystalline bedrock for groundwater supply (Darboux-Afouda and Louis, 1989; Sehli, 

1990). These studies verified Habberjam's earlier work, 

showing that the square array has a greater sensitivity to a 

given bedrock anisotropy and requires less surface area than 

collinear arrays.

The square array was tested at the Mirror Lake 

research site. This paper describes the square-array deresistivity method, outlines a simplified method of data 

analysis to determine fracture strike and secondary porosity, and presents the results of the field test.

The Square-Array Method

The square array was originally developed as an alternative to Wenner or Schlumberger arrays when a dipping 

subsurface, bedding, or foliation was present (Habberjam 

and Watkins, 1967). A complete discussion of the square 

array and methods of data analysis is provided by Habberjam 

(1979). Techniques for analyzing directional-resistivity data 

provided by the square-array method have been developed 

(Habberjam, 1972), but the method has not been widely 

used. Few case studies or interpretive methods specifically 

applied to the square array are found in the literature, 

although commercial software is available for layered earth 

interpretations.

476

3f\

Vol. 33, No. 3 GROUND WATER May-June 1995
U.S. GEOLOGICAL SURVEV WATER RESOURCES DIVISION 

MIRROR LAKE BASE MAP - STATE PLANE PROJECTION ZONE 4676 

NATIONAL GEODETIC VERTICAL DATUM OF 1929 

Q IQOO ZOOO FEET

7 1041' iff 71° 4 l' 23*

5OO METERS

43°56'26" -

43°56'22" 

Center of azimuthal. 

DC resistivity and 

refraction surveys

CAMP OSCEOLA WELL FIELD

500 FEET

IOOM6TPKS

Fig. 1. Location of U.S. Geological Survey fractured bedrock research site, showing square-array test area, Mirror Lake, Grafton 

County, New Hampshire. (Modified front Haeni et al., 1993, fig. 1.)

Field Measurements

A de-resistivity survey using the square-array method is 

conducted in a manner similar to that for traditional collinear arrays. The location of a measurement is assigned to 

the centerpoint of the square. The array size (A) is the length

of the side of the square. The array is expanded symmetrically about the centerpoint, in increments of A(2)!/2 

(Habberjam and Watkins, 1967), so that the sounding can be 

interpreted as a function of depth.

For each square, three measurements are made two
N M

M A

N M

B A

Alpha Beta Gamma

Electrode configurations per Square array

AB = Current electrodes

MN = Potential electrodes

Fig. 2. Electrode positions for square-array measurements.

Starting orientation

Second orientation

7 rotated 45 degrees

about center point

Current electrode configuration 

for Crossed Square array

Alpha = Pa. Alpha' 

Beta

Beta'

Gamma

Gamma'

1 - 4 

5-8 

1 - 2

5-6

1 - 3

5-7

perpendicular measurements (alpha, a; and beta, /S) and one 

diagonal measurement (gamma, -y) (Figure 2). The a and 0 

measurements provide information on the directional variation of the subsurface apparent resistivity (pa ). The azimuthal orientation of the a and /? measurements is that of the line 

connecting the current electrodes. The y measurement 

serves as a check on the accuracy of the a and 0 

measurements.

In an isotropic medium,

P*o P*/}, therefore, pay = 0, and (1) 

in a homogeneous anisotropic medium,

P*y = paa pa/} , (2)

where p, = apparent resistivity, in ohmmeters.

Apparent resistivity is determined using the equation:

KAV

I

(3)

where pa = apparent resistivity; K = geometric factor for the 

array; AV = potential difference, in volts; and I = current 

magnitude, in amperes. 

For the square array,

, (Habberjam and Watkins, 1967) (4) 2 - (2) 1

where A = square-array side length, in meters. In practice, 

multiple square-array data are collected at small angular 

intervals. For example, six square arrays separated by a 

rotational angle of 15° will provide three independent 

crossed square-array data sets (two square arrays separated

478

by an angle of 45°; Figure 2) for analysis, as well as sufficient 

data for graphical display and interpretation of individual 

square-array data on a rosette diagram using Taylor and^ 

Fleming's methods (1988).

Data Analysis

The data were analyzed using a method described by 

Habberjam (1972). The following discussion is a brief summary of the method and outlines the potential uses of the 

square-array method for hydrologic investigations.

Depth Sounding

Habberjam and Watkins (1967) demonstrated that 

apparent-resistivity data obtained with a square array can 

be interpreted using published methods for Wenner or 

Schlumberger soundings. This is done by translating 

apparent-resistivity data obtained from the square array to 

those of equivalent Wenner or Schlumberger arrays.

First, the square-array resistivity measurements are 

reduced to a single measurement (pm ) by

P". = [(P»«) (Pa0)]'/2 » Habberjam and Watkins, 1967 (5)

where pm mean geometric resistivity.

The more rigorous relation between the size of a square 

array and the size of an equivalent Wenner or Schlumberger 

array is given by

A A r

s)

2r

(6)

where A = square-array side length, in meters; r = AM = 

current electrode and nearest potential electrode separation;
Square array 

side length = A

Equivalent 

Colinear array

Wenner array

r=s

Schlumberger array 

r=45s 

A M N B lr. MN/2=5s

Fig. 3. Common collinear arrays equivalent to a square array of 

side length A.

and s MN = potential electrode separation (Figure 3).

Later work (Habberjam, 1972) showed that an azimuthal-independent value for that spacing can be obtained by 

calculating the geometric mean of pm obtained from two 

square arrays separated by a rotational angle of 45° (about 

the centerpoint) (the crossed square array). However, reduction of the square-array resistivity measurements to yield a 

single directionally stable measurement for layered earth 

interpretation removes the information about directional 

resistivity differences contained within the individual squarearray measurements.

Azimuthal Resistivity

Variations in azimuthal resistivity can be caused by 

many factors such as slope of the bedrock surface, or dip of 

bedding or foliation. Experiments by Sauck and Zabik 

(1992) have shown that azimuthal resistivity data can be 

affected by overburden thickness. The following discussion 

assumes that the resistivity differences are only caused by 

similarly oriented, steeply dipping fractures (Figure 4).

Habberjam (1972) derived the expression for the variation of apparent resistivity with square-array orientation 

over a homogeneous anisotropic earth. For fractured rock 

that approximates such a medium, the predicted squarearray apparent resistivity in a given orientation is

pa =

2- [\+ (N' - 1) coslT]2-11/2

1

[2 4- (N2 - 1

fj i

!)(! 4-sin20)] 1/2 ' 

1

fW2 n (\ cinlWl 1 /2 (7)

where pm = [(pa ,) (pai)] ^; pa, apparent resistivity perpendicular to fractures; pai = apparent resistivity parallel to 

fractures; B = angle measured from azimuth of current 

electrodes to fracture strike; N = effective vertical anisotropy, = [(1 + (X2 1) sina2 )] 172; X = coefficient of anisotropy; X = (pa ,/pai) 1/2 ; a = dip of fractures; N = X for a =

7T/2.

When variations in azimuthal resistivity are detected 

over an anisotropic earth, and the variations are caused by 

fractures, the interpretive methods of Habberjam (1972; 

1975) and Taylor (1984) can be used to determine fracture 

strike and to estimate secondary porosity.

Determination of Fracture Strike

The fracture strike can be determined graphically or 

analytically. To interpret strike graphically, the apparent

resistivity for azimuthal square arrays is plotted against the 

azimuth of that measurement. The principal fracture strike 

direction is perpendicular to the direction of maximum 

resistivity.

Crossed square-array data can be interpreted analytically in order to yield fracture strike (Habberjam, 1975):

1

0 = - tan

2

,r 

L

(D 2 -C 2 ) 

(A'2 - B-2 )

1 J

(8)

1/2

where B fracture strike, measured from pa) ;

A = [(pa3 4- 3pai )/2 4- (pa, 4- pa2)/(2)|/2 ]/[(2 + (2)^];

B = [(pa | + 3pa3 )/2 4- (pa2 4- pM)/(2) ]/[(2 4- (2) /2 ];

C = [(PM + 3Pa2)/2 4- (pal 4- Pa3 )/(2)I/2 ]/[(2 4- (2)1/2 ];

and

D = [(pa2 + 3p»4 )/2 4- (pa3 4- pa ,)/(2) ]/[(2 4- (2)1/2 j;

Pai, Pa2 , Pa3 , and p»4 are constituent resistivity measurements

from a crossed square array (Figure 2).

Estimation of Secondary Porosity

Using the crossed square-array measurements, the secondary porosity (</>) can be estimated by modifying Taylor's 

method developed for collinear arrays in saturated, clayfree, nonshale rocks (1984). To calculate secondary porosity, 

it is first necessary to calculate the anisotropy (N) from the 

field data, using Habberjam's method (1975):

where T = A"2 + B"2 4- C 2 4- D"2 ; S = 2[(A"2 - B"2 )2 + 

(D"2 - C"2 )2 ] 1/2; and A, B, C, and D are defined previously. 

The secondary porosity is then estimated by:

3.41 X 104 (N - 1) (N2 - 1)

N

(10)

where </> = secondary porosity; p, * = maximum squarearray apparent resistivity; pmin minimum square array 

apparent resistivity; and C=specific conductance of ground 

water in microsiemens per centimeter.

EXPLANATION 

p j = resistivity parallel to fracturing

"l = resistivity perpendicular to fracturing

-0- = angle from fracture strike to current electrodes AB

OC = angle of fracture dip

Fig. 4. Model of homogeneous anisotropic Earth. (Modified and 

reprinted from Habberjam, 1972, fig. 2, and published with 

permission.)
Comparison of Square Array and Collinear Array

Sensitivity to Anisotropy

To correctly interpret azimuthal de-resistivity data over 

fractured rock, the bedrock must behave as an anisotropic 

medium. Satisfying this requires the electrode spacings to be 

large with respect to the fracture spacing (Taylor, 1982). The 

square array has a ratio of potential electrode to current 

electrode spacing of 1:1. The Wenner array ratio is 1:3, and 

the Schlumberger ratio is generally less than 1:10. As the 

above arrays are expanded, the square array, with the largest 

electrode-spacing ratio, will most rapidly satisfy the above 

condition.

The square array has been shown to be more sensitive 

to anisotropy than the Schlumberger or Wenner array 

(Habberjam, 1972; LeMasne, 1979; Darboux-Afouda and 

Louis, 1989). For the square array, the ratio of apparent 

resistivity measured perpendicular to fracture strike (pa,) to 

apparent resistivity parallel to fracture strike (pa) ) is called 

the apparent anisotropy (Xa ) and is given by the ratio:

Pat N[(N'

Pa, [(N< + 1)'" - N]

(Darboux-Afouda and Louis, 1989) (11)

The apparent anisotropy for the Schlumberger or 

Wenner array is given by:

X* a = XT 

 N

Pa,

(12)

where N = effective vertical anisotropy.

The apparent anisotropy of the square array and the 

Schlumberger array for a true rock anisotropy is shown in 

Figure 5. The higher apparent anisotropy measured by the

15

CL,

I

as

CL,

10

5

3

1

0

Square array

Schlumberger array

1

BEDROCK ANISOTROPY, L

Fig. 5. Anisotropy measured by the square array and the 

Schlumberger array for a given bedrock anisotropy. (Reprinted 

from Darboux-Afouda and Louis, 1989, fig. 2 and published 

with permission.)

480

square array is an advantage, because the anisotropy is less 

likely to be obscured by heterogeneities in bedrock or overburden, bedrock relief, cultural noise, electrode placement 

errors, or other sources of noise.

Survey Area Requirements

The square-array geometry is more compact than 

Schlumberger or Wenner arrays for azimuthal surveys. The 

square array requires 65 percent less surface area than the 

equivalent collinear arrays (Habberjam and Watkins, 1967). 

This is an advantage in an area with significant lateral 

heterogeneities or when the area available to conduct a 

survey is limited.

Maximum Resistivity in Relation to Fracture Strike

The direction of maximum apparent resistivity measured by the square array will be perpendicular to fracture 

strike. This is a function of the cosine term in the denominator of equation (7).

The direction of maximum apparent resistivity measured by the collinear array will be parallel to the fracture 

strike (Habberjam, 1972). This is a consequence of the 

"paradox of anisotropy" (Keller and Frischknecht, 1966) 

and is determined by the sine term in the denominator of the 

following equation showing apparent resistivity in relation 

to orientation:

Pa(0) =

KAV

I [[1 + (N 2 - 1) sin02 ]] 1/2 (13)

where K = geometric factor for the array; N = effective 

vertical anisotropy; and pm = mean geometric resistivity.

Use of Square Array to Detect Fractures 

at Graft on County, New Hampshire

The square-array de-resistivity method was field tested 

in Priest Field, near the Camp Osceola (CO) well cluster, 

located at the Mirror Lake research site (Figure 1). At this 

site, approximately 7 m (meters) of stratified glacial drift is 

underlain by crystalline bedrock.

The square-array survey was conducted during the 

Summer of 1992. The survey consisted of six square-array 

soundings separated by a 15° rotational angle about the 

array centerpoint. The A-spacings of the arrays were 

expanded from 5 m to 50 m [in increments of (2) 1/2 ] for each 

sounding. The data were collected using an ABEM Multimac 

de-resistivity system. (Use of tradenames is for identification 

purposes only and does not constitute endorsement by the 

U.S. Geological Survey.) This computer-controlled data 

acquisition and storage system (Figure 6) allowed a complete sounding at a given azimuth to be collected automatically through remotely accessed addressable switchers, 

which connected electrodes for a given measurement. Software provided with the system was modified for the square 

array to control the measurement sequence.

Interpretation of Square-Array Data and Crossed 

Square-Array Data

Data collected in Priest Field show a significant variation of apparent resistivity for different azimuthal array
ATable 1. Azimuthal Apparent Resistivities Collected at Priest Field, Mirror Lake Watershed, New Hampshire

Geometric

Square-array 

azimuthal apparent resistivities, in ohmmeters

u^y»*v.*» £

(meters)

5

7.1

10

14.1

20

28.3

40

50

(K)

53.6

75.8

107.3

151.7

214.5

303.4

429.0

536.3

Mean

7,380

7,604

7,030

5,149

3,628

2,479

1,987

1,911

0

6,790

7,023

5,859

4,885

3,696

2,555

1,707

1,341

75

6,870

6,932

6,073

4,475

3,389

2,348

1,167

1,148

30

7,417

7,538

6,320

4,369

3,087

2,112

1,347

1,132

45

7,044

7,504

6,218

4,392

2,969

2,040

1,418

1,279

60

7,129

7,561

6,824

4,809

3,179

2,057

1,700

1,877

75

7,514

8,183

7,275

5,067

3,314

2,148

1,806

1,845

90

7,991

8,388

7,854

5,476

3,376

2,209

2,068

1,866

105

8,141

8,449

7,758

6,129

3,700

2,558

2,561

2,944

120

7,535

7,546

7,146

5,810

4,030

2,894

2,831

3,040

755

7,385

7,197

6,985

5,810

4,232

3,004

2,664

2,660

150

6,975

5,613

4,138

3,167

2,561

2,328

165

7,374

7,326

6,374

4,961

3,762

2,661

1,913

1,480

 No data available.

orientations for all A-spacings. Apparent-resistivity data 

collected at the site are shown in Table 1. The data from the 

largest arrays were analyzed to minimize possible overburden effects. The apparent resistivities plotted against 

azimuth are shown for the 40- and 50-m A-spacings, which 

are least affected by the overburden (Figure 7). For both 

A-spacings shown, the maximum resistivity measured is 

parallel to a trend of 120°. A graphical interpretation of the 

data is that a primary fracture set is present with a strike of 

030°. The 50-m data show a secondary trend of high resistivity at 060°. Assuming that these data do not reflect heterogeneities in the bedrock, this peak could indicate the presence of a secondary fracture set oriented at 150°.

Because data for six square arrays separated by 15° 

were collected, there are three independent crossed square 

arrays available for analysis. Using the analytical methods 

described in the section "Field Measurements "for the 50-m 

data, a strike estimate of 027° and an anisotropy value of 

1.31 was obtained.

Using these analytical results for the strike and anisotropy values, the apparent resistivity values predicted using 

equation (6) were calculated and the results superimposed

Array expanded to 

limits of sounding

Fig. 6. Block diagram of direct-current resistivity data-collection 

system.

on the field data (Figure 7). This shows that the field data 

closely approximate the data for an isotropic homogeneous 

earth.

Using the anisotropy of 1.31, a maximum resistivity of 

3,040 ohm-m (ohmmeters), a minimum resistivity of 1,132 

ohm-m, and a range of specific conductance for ground 

water in the CO well field of 30 to 315 /uS/cm (P. T. Harte, 

U.S. Geological Survey, written commun., 1991), and equation (10), the secondary porosity due tox fracturing is estimated to range from 0.01 to 0.10. This demonstrates the 

sensitivity of the secondary porosity estimate to the specific 

conductance of ground water.

Comparison of Interpreted Data and Other 

Supporting Data

Fracture Strike

An outcrop located on the center median of Interstate 

93 (within 250 m of the site) consists of schist, massive 

granitic dikes, granitic dikes with schist xenoliths, and pegmatite (C. C. Barton, U.S. Geological Survey, written 

commun., 1989). The graphically interpreted fracture-strike 

from the square-array data of 030° and the crossed squarearray data of 027° correlates with fracture-strike frequency 

data measured at the 1-93 outcrop. Analysis of the measured 

fracture-strike data yields a mean fracture-strike frequency 

maximum of 030° with major peaks at 005° and 040°. Most 

of the fractures are steeply dipping (C. C. Barton, U.S. 

Geological Survey, written commun., 1993; Figure 8). The 

secondary resistivity maximum seen in the 50-m squarearray data indicates the possible presence of another fracture set oriented at 150°. This might correlate with a minor 

fracture strike maximum at the outcrop oriented at 135° or 

indicate a change in foliation orientation.

Azimuthal p-wave seismic-refraction surveys have been 

conducted at the square-array test site (Lieblich et al., 1991). 

Data were collected every 22.5° about the same centerpoint 

used in the square-array survey. The contrast between the 

measured seismic velocity and the refraction-line orientation (Figure 9) was interpreted as indicating a fracture and 

(or) foliation oriented at 022.5° (Lieblich et al., 1991).

An azimuthal de-resistivity survey using the 

Schlumberger array was conducted at the CO well field 100 

m from the square-array test site (Haeni et al., 1993). Data 

were collected every 45° with a Bison 2390 resistivity system,

H
A = 40 m

000

EXPLANATION

Line of equal apparent resistivity, 

in ohm meters

Interpreted primary fracture strike

Possible secondary fracture strike 

Forward modeled data for homogeneous 

anisotropic earth

Data points showing apparent resistivity 

at indicated azimuthal directions

Compass azimuth, in degress

Fig. 7, Square-array apparent resistivity plotted against azimuth for the 40- and 50-meter A-spacings, collected at Priest Field, U.S. 

Geological Survey fractured bedrock research site, Mirror Lake, New Hampshire. (Modified from Haeni et al., 1993, fig. 5).

and the AB/2 spacing was expanded from 3 m to 30 m. 

Qualitative analysis of these data support a fracture set and 

(or) foliation oriented at 037° (Figure 10); however, the 

anomalously large anisotropy indicates some cultural interference (for example, well casing or buried metal) within the 

data.

Two azimuthal surveys using the Schlumberger array 

also were conducted at the square-array test site about the 

same centerpoint as the square array. One set of soundings

was conducted every 22.5° using a Bison 2390 resistivity 

system. The AB/2 spacings were expanded from 3 m to 40 

m. A second set of soundings was conducted every 15° with 

the ABEM Multimac system. The AB/2 spacings were 

expanded from 3 m to 40 m. The fracture orientation interpreted from each of these data sets is 352° (Figure 11).

Inspection of the data, field site, and survey design may 

explain the discrepancy between the Schlumberger and 

square-array data, and illustrate possible weaknesses of

482
EXPLANATION

 1.2- Line of equal fracture intensity 

Contour interval is 1.0 percent 

N=687

Fig. 8. Equal area plot of fractures mapped from Route 1-93 

outcrops near the U.S. Geological Survey fractured bedrock 

research site, West Thornton, New Hampshire. (From C. C. 

Barton, U.S. Geological Survey, written commun., 1993.)

azimuthal Schlumberger array surveys for fracture detection at this site. Because the large current electrode separation of the Schlumberger array required a large surface area, 

a buried phone line was included within the array. This 

phone line was not located within the square array survey 

area. The largest MN/2 potential-electrode separations 

used for the Schlumberger array was 4 m, which may not 

have satisfied the theoretical requirement of large electrode 

spacing as a function of fracture spacing. In addition, heterogeneities present near potential electrodes may have affected 

the Schlumberger data. In view of the above, the squarearray results are interpreted to indicate fracture conditions 

at the test site more accurately than the Schlumberger 

array results.

Secondary Porosity

The secondary porosity estimate interpreted from the 

50-m data ranges from .01 to .1. This range encompasses 

values higher than average secondary porosities for crystalline rock, which are generally less than 0.02 (Freeze and 

Cherry, 1979). An estimate of the fracture porosity at well 

CO-1, made by measuring the number of fractures and the 

average fracture aperture, is 0.001 (P. T. Harte, U.S. Geological Survey, oral commun., 1992). A reason for this difference is that secondary porosity estimates obtained from 

wells are sensitive to subhorizontal fractures and the deresistivity method is more sensitive to vertical fractures. 

Another explanation is that foliation could be a significant 

contributor to anisotropy. The secondary porosity calculations from the square array assumed that fracturing is the 

only cause of anisotropy.

N Azimuth ^

0 S

kilomleters/second C. A'

5.0.

EXPLANATION

Line of equal seismic velocity 

in kilometers per second

Interpreted primary fracture strike 

Possible secondary fracture strike

Data points showing interpreted 

seismic velocity through bedrock at 

indicated azimuthal directions

014.5 Compass azimuth, in degress

Fig. 9. Seismic compressional wave velocity plotted against azimuth at Priest Field, U.S. Geological Survey fractured bedrock research 

site, Mirror Lake, New Hampshire. (From Lieblich and others, 1991, fig. 6.)

H-&
Azimuth

000

EXPLANATION

Line of equal apparent resistivity, 

in ohm meters

Interpreted primary fracture strike

Data points showing apparent resistivity 

measured at MN/2 = 2 meters 

at indicated azimuthal directions

Data points showing apparent resistivity 

measured at MN/2 = 4 meters 

at indicated azimuthal directions

Compass azimuth, in degrees

Fig. 10. Schiumberger array apparent resistivity plotted against azimuth for the 40-meter AB/2 spacing, collected at the Camp Osceola 

well field, U.S. Geological Survey fractured bedrock research site, Mirror Lake, New Hampshire. (From Haeni et al., 1993, fig. 4.)

Azimuthal de-resistivity methods are sensitive to a 

sloping bedrock interface (Habberjam, 1975), and a reliable 

method of correcting for the slope is not available. Results 

from seismic-refraction surveys at Priest Field indicate that 

the bedrock surface slopes at less than 5° (D. A. Lieblich, 

U.S. Geological Survey, oral commun., 1992). This should 

have a minimal effect on the anisotropy measurement and 

the interpreted secondary porosity.

Summary

The square-array de-resistivity method was used to 

determine the orientation of bedrock fractures at the USGS 

bedrock research site in the Mirror Lake watershed of the 

Hubbard Brook Experimental Forest in Grafton County, 

New Hampshire. A primary fracture strike orientation of 

030° was interpreted from a graphical analysis of squarearray data and an orientation of 027° was interpreted from

Azimuth

037

EXPLANATION

Line of equal apparent resistivity, 

in ohm meters

Interpreted primary fracture strike

Data points showing apparent resistivity 

measured at MN/2 = 2 meters 

at indicated azimuthal directions

Data points showing apparent resistivity 

measured at MN/2 = 4 meters 

at indicated azimuthal directions

Compass azimuth, in degrees

Fig. 11. Schiumberger array apparent resistivity plotted against azimuth for the 40-meter AB/2 spacing, collected at Priest Field, ILS. 

Geological Survey fractured bedrock research site, Mirror Lake, New Hampshire.

484
the analysis of the crossed square-array data. These values 

closely match the orientation of fracture trends mapped in 

bedrock (030°) near the sites of surface-geophysical surveys. 

Fracture-strike orientation interpreted from square-array 

data is supported by other geophysical data, including 

azimuthal p-wave seismic refraction (022.5°) and azimuthal 

Schlumberger de-resistivity (037° at Camp Osceola). The 

estimated secondary porosity attributed to fracturing ranged 

from 0.01 to 0.10, which was higher than the porosity estimated at well CO-1 (0.001). Further comparison of secondary porosity estimates determined from square-array surveys with porosities determined from other sources is 

needed to assess the square-array secondary porosity 

calculations.

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