The base map on the front cover shows geophysical survey locations overlaying a geologic map of 

U.S. Geological Survey, Windham, New Hampshire, 1:24,000-scale quadrangle. Geology is by 

G.S. Walsh and S.F. Clark, Jr. (1999) and lineaments are from Ferguson and others (1997) and 

R.B. Moore and Garrick Marcoux, 1998.

The photographs and graphics overlying the base map are showing, counterclockwise from the 

left, a USGS scientist using a resistivity meter and surveying equipment (background) to survey the 

bedrock beneath the surface using a geophysical method called azimuthal square-array directcurrent resistivity. In the lower left, this cross section is showing the results of a survey along 

line 3 in Windham, N.H., using another method called two-dimensional direct-current resistivity. In 

the lower right, the photograph is showing a bedrock outcrop located between red lines 3 and 4 

(on base map) at Windham, in which the fractures and parting parallel to foliation have the same 

strike as the azimuthal square-array direct-current resistivity survey results, and remotely sensed 

lineaments (purple and green lines on base map). The upper right graphic shows a polar plot of 

the results of an azimuthal square-array direct-current resistivity survey at Windham for array 1 

(red circle on base map). 
Geophysical Investigations of Well Fields 

to Characterize Fractured-Bedrock Aquifers 

in Southern New Hampshire

U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 01-4183

By James R. Degnan, Richard Bridge Moore, and Thomas J. Mack

Pembroke, New Hampshire

2001

In cooperation with the

NEW HAMPSHIRE DEPARTMENT OF ENVIRONMENTAL SERVICES
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes

For additional information write to:

District Chief

U.S. Geological Survey

New Hampshire/Vermont District

361 Commerce Way

Pembroke, NH 03275-3718

or through our Web site at

 http://nh.water.usgs.gov

U.S. Geological Survey

Branch of Information Services

Box 25286

Federal Center

Denver, CO 80225

Copies of this report can be purchased from:

only and does not constitute endorsement by the U.S. Government.
Contents III

CONTENTS

Abstract.................................................................................................................................................................................. 1

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

Purpose and Scope....................................................................................................................................................... 2

Previous Investigations ................................................................................................................................................ 2

Site Selection ............................................................................................................................................................... 3

Geohydrologic Settings ............................................................................................................................................... 3

Acknowledgments ....................................................................................................................................................... 4

Approach and Methods.......................................................................................................................................................... 4

P-Wave Seismic Refraction ......................................................................................................................................... 5

Ground-Penetrating Radar........................................................................................................................................... 6

Magnetics..................................................................................................................................................................... 6

Very-Low-Frequency Electromagnetics ...................................................................................................................... 6

Inductive Electromagnetic Terrain Conductivity......................................................................................................... 7

Two-Dimensional Direct-Current Resistivity.............................................................................................................. 7

Azimuthal-Square Array Direct-Current Resistivity ................................................................................................... 8

Borehole Geophysical Surveys.................................................................................................................................... 8

Analysis and Results of Geophysical Investigations of Well Fields...................................................................................... 9

Site 1, Bedford, New Hampshire................................................................................................................................. 10

Geophysical Surveys and Interpretation............................................................................................................ 10

Integration of Results......................................................................................................................................... 14

Site 2, Windham, New Hampshire .............................................................................................................................. 16

Geophysical Surveys and Interpretation............................................................................................................ 18

Integration of Results......................................................................................................................................... 19

Site 3, Pelham, New Hampshire.................................................................................................................................. 25

Geophysical Surveys and Interpretation............................................................................................................ 27

Integration of Results......................................................................................................................................... 29

Site 4, Goffstown, New Hampshire ............................................................................................................................. 30

Geophysical Surveys and Interpretation............................................................................................................ 32

Integration of Results......................................................................................................................................... 35

Site 5, Goffstown, New Hampshire ............................................................................................................................. 35

Geophysical Surveys and Interpretation............................................................................................................ 37

Integration of Results......................................................................................................................................... 39

Site 6, Salem, New Hampshire.................................................................................................................................... 39

Geophysical Surveys and Interpretation............................................................................................................ 41

Integration of Results......................................................................................................................................... 44

Summary and Conclusions .................................................................................................................................................... 47

Selected References............................................................................................................................................................... 48

Appendix 1. Graphs Showing Borehole Geophysical Logs of Three Sites in New Hampshire........................................... 51
IV Contents

FIGURES

1-2. Maps showing:

1. Location of the geophysical study area in the Pinardville, Windham, and Salem Depot 7.5-minute 

quadrangles in southern New Hampshire...................................................................................................... 2

2. Geophysical survey locations, bedrock geology, and lineaments at site 1, Bedford, N.H. ............................... 11

3-4. Graphs showing:

3. Processed ground-penetrating radar profile at site 1 from line 1, Bedford, N.H............................................... 12

4. Magnetic and electromagnetic surveys at site 1 from line 1.............................................................................. 13

5. Magnetic and electromagnetic surveys at site 1 from line 2.............................................................................. 13

6-7. Cross sections showing inverted resistivity sections of two-dimensional, direct-current resistivity data at:

6. Site 1 from line 1, Bedford, N.H. ...................................................................................................................... 14

7. Site 1 from line 2 ............................................................................................................................................... 15

8. Polar plots showing azimuthal square-array direct-current resistivity at site 1 for arrays 1 and 2, Bedford, N.H..... 16

9. Map showing geophysical survey locations, bedrock geology, and lineaments at site 2, Windham, N.H................. 17

10-14. Graphs showing:

10. Processed ground-penetrating radar profile at site 2 from line 1, Windham, N.H. ........................................... 19

11. Magnetic and electromagnetic surveys at site 2 from line 1.............................................................................. 20

12. Magnetic and electromagnetic surveys at site 2 from line 2.............................................................................. 20

13. Magnetic and electromagnetic surveys at site 2 from line 3.............................................................................. 21

14. Magnetic and electromagnetic surveys at site 2 from line 4.............................................................................. 21

15-18. Cross sections showing inverted resistivity sections of two-dimensional, direct-current resistivity data at:

15. Site 2 from line 1, Windham, N.H..................................................................................................................... 22

16. Site 2 from line 2 ............................................................................................................................................... 23

17. Site 2 from line 3 ............................................................................................................................................... 23

18. Site 2 from line 4 ............................................................................................................................................... 24

19. Polar plot showing azimuthal square-array direct-current resistivity at site 2 for array 1, Windham, N.H. .............. 24

20. Map showing geophysical survey locations, bedrock geology, and lineaments at site 3, Pelham, N.H. ................... 26

21-22. Graphs showing magnetic and electromagnetic surveys at:

21. Site 3 from line 1, Pelham, N.H......................................................................................................................... 27

22. Site 3 from line 2 ............................................................................................................................................... 28

23-24. Cross sections showing inverted resistivity sections of two-dimensional, direct-current resistivity data at:

23. Site 3 from line 1, Pelham, N.H......................................................................................................................... 28

24. Site 3 from line 2 ............................................................................................................................................... 29

25. Polar plots showing azimuthal square-array direct-current resistivity at site 3 for arrays 1 and 2, Pelham, N.H...... 30

26. Map showing geophysical survey locations, bedrock geology, and lineaments at site 4, Goffstown, N.H. .............. 31

27-28. Graphs showing magnetic and electromagnetic surveys at:

27. Site 4 from line 1, Goffstown, N.H.................................................................................................................... 32

28. Site 4 from line 2 ............................................................................................................................................... 33

29-30. Cross sections showing inverted resistivity sections of two-dimensional, direct-current resistivity data at:

29. Site 4 from line 1, Goffstown, N.H.................................................................................................................... 33

30. Site 4 from line 2 ............................................................................................................................................... 34

31. Polar plot showing azimuthal square-array direct-current resistivity at site 4 for array 1, Goffstown, N.H.............. 34

32. Diagram showing lower hemisphere equal-area nets from bedrock well GNW 406 at site 4, Goffstown, N.H........ 35

33. Map showing geophysical survey locations, bedrock geology, and lineaments at site 5, Goffstown, N.H. .............. 36

34-35. Graphs showing magnetic and electromagnetic surveys at:

34. Site 5 from line 1, Goffstown, N.H.................................................................................................................... 38

35. Site 5 from line 2 ............................................................................................................................................... 38

36-37. Cross sections showing inverted resistivity sections of two-dimensional, direct-current resistivity data at:

36. Site 5 from line 1, Goffstown, N.H.................................................................................................................... 40

37. Site 5 from line 2 ............................................................................................................................................... 40

38. Polar plot showing azimuthal square-array direct-current resistivity at site 5 for array 1, Goffstown, N.H.............. 41

39. Diagram showing lower hemisphere equal-area nets from bedrock well GNW 408 at site 5, Goffstown, N.H........ 41

40. Map showing geophysical survey locations, bedrock geology, and lineaments at site 6, Salem, N.H. ..................... 42
Contents V

41-43. Graphs showing electromagnetic surveys at:

41. Site 6 from line 1, Salem, N.H........................................................................................................................... 43

42. Site 6 from line 2 ............................................................................................................................................... 43

43. Site 6 from line 3 ............................................................................................................................................... 43

44-46. Cross sections showing inverted resistivity sections of two-dimensional, direct-current resistivity data at:

44. Site 6 from line 1, Salem, N.H........................................................................................................................... 45

45. Site 6 from line 2 ............................................................................................................................................... 46

46. Site 6 from line 3 ............................................................................................................................................... 46

47. Polar plot showing azimuthal square-array direct-current resistivity at site 6 for array 1, Salem, N.H..................... 47

48. Diagram showing lower hemisphere equal-area nets from bedrock well SAW 272 at site 6, Salem, N.H................ 47

1A-C. Appendix graphs showing borehole geophysical logs of:

A. Well GNW 406 at site 4, Goffstown, N.H. ........................................................................................................ 52

B. Well GNW 408 at site 5..................................................................................................................................... 53

C. Well SAW 272 at site 6, Salem, N.H. ................................................................................................................ 54

 CONVERSION FACTORS, ABBREVIATIONS, AND VERTICAL DATUM

Sea Level: In this report “sea level” refers to the National Geodetic 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.

ABBREVIATIONS USED IN THIS REPORT

m ohmmeter

mmho/m millimohos per meter

MHz megahertz

kHz kilohertz

ft/s foot per second

mS/m millisiemens per meter

° degree

m meter

nT nanotesla

Multiply By To obtain

Length

inch (in.) 25.4 millimeter

foot (ft) 0.3048 meter

Flow

gallon per minute (gal/min) 0.06308 liter per second

Hydraulic Conductivity

foot per day (ft/d) 0.3048 meter per day

Temperature in degrees Celsius (oC) can be converted to degrees Fahrenheit (oF) as follows: oF = (oC ÷ 5/9) + 32.

Ω
Abstract 1

Abstract

Bedrock-fracture zones near high-yield 

bedrock wells in southern New Hampshire well 

fields were located and characterized using seven 

surface and six borehole geophysical survey 

methods. Detailed surveys of six sites with 

various methods provide an opportunity to 

integrate and compare survey results. Borehole 

geophysical surveys were conducted at three of 

the sites to confirm subsurface features. 

Hydrogeologic settings, including a variety of 

bedrock and surface geologic materials, were 

sought to gain an insight into the usefulness of the 

methods in varied terrains. Results from 

15 survey lines, 8 arrays, and 3 boreholes were 

processed and interpreted from the 6 sites.

The surface geophysical methods used 

provided physical properties of fractured bedrock. 

Seismic refraction and ground-penetrating radar 

(GPR) primarily were used to characterize the 

overburden materials, but in a few cases indicated 

bedrock-fracture zones. Magnetometer surveys 

were used to obtain background information 

about the bedrock to compare with other results, 

and to search for magnetic lows, which may result 

from weathered fractured rock. Electromagnetic 

terrain conductivity surveys (EM) and very-lowfrequency electromagnetic surveys (VLF) were 

used as rapid reconnaissance techniques with the 

primary purpose of identifying electrical 

anomalies, indicating potential fracture zones in 

bedrock. 

Direct-current (dc) resistivity methods were 

used to gather detailed subsurface information 

about fracture depth and orientation. Twodimensional (2-D) dc-resistivity surveys using 

dipole-dipole and Schlumberger arrays located 

and characterized the overburden, bedrock, and 

bedrock-fracture zones through analysis of data 

inversions. Azimuthal square array dc-resistivity 

survey results indicated orientations of conductive 

steep-dipping bedrock-fracture zones that were 

located and characterized by previously applied 

geophysical methods.

Various available data sets were used for 

site selection, characterizations, and interpretations. Lineament data, developed as a part of a 

statewide and regional scale investigation of the 

bedrock aquifer, were available to identify 

potential near-vertical fracture zones. Geophysical surveys indicated fracture zones coincident 

with lineaments at 4 of the sites. Geologic data 

collected as a part of the regional scale investigation provided outcrop fracture measurements, 

ductile fabric, and contact information. Dominant 

fracture trends correspond to the trends of 

geophysical anomalies at 4 of the sites. Waterwell drillers’ logs from water supply and environmental data sets also were used where available to 

characterize sites. Regional overburden information was compiled from stratified-drift aquifer 

maps and surficial-geological maps.

Geophysical Investigations of Well Fields to 

Characterize Fractured-Bedrock Aquifers in 

Southern New Hampshire

By James R. Degnan, Richard Bridge Moore, and Thomas J. Mack
2 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Figure 1. Location of the geophysical study area in the Pinardville, Windham, and 

Salem Depot 7.5-minute quadrangles in southern New Hampshire. Numbers on the 

quadrangles refer to sites.

INTRODUCTION

Many towns and communities in New 

Hampshire have limited amounts, or an absence of, 

sand and gravel aquifers, which are favorable for 

constructing high-yield wells. These towns must look 

for additional water resources in crystalline bedrock. 

The average bedrock well yield in New Hampshire is 

about 6 gal/min. An adequate municipal or commercial well typically requires tens to hundreds of gallons 

per minute. The U.S. Geological Survey (USGS), in 

cooperation with the New Hampshire Department of 

Environmental Services (NHDES), has done a 

statewide assessment of ground-water resources in the 

bedrock aquifers of New Hampshire (R.B. Moore and 

others, U.S. Geological Survey, written commun., 

2001), which provides regional and statewide 

information regarding bedrock aquifer areas that are 

likely to be favorable for resource investigation. In 

identified potential high-yield bedrock aquifers, 

individual boreholes must be targeted to intercept a 

fracture or fracture zone that could be from 5-10 ft to 

less than 1 ft wide. The water-resources professional 

needs additional, site-specific information, to 

precisely locate boreholes to intercept specific 

bedrock-fracture zones. If such fractures are steeply 

dipping, the “target” surface area overlying the highyield bedrock aquifer can be small. As part of the 

statewide bedrock-aquifer assessment, the USGS, in 

cooperation with the NHDES, assessed the use of 

geophysical methods to identify high-yield bedrockfracture zones at six sites in New Hampshire (fig. 1). 

Purpose and Scope

This report describes the results of surface and 

borehole geophysical investigations of bedrock 

aquifers at selected well-field sites in New 

Hampshire. Included in this report are analyses of the 

data from various types of geophysical surveys to 

provide bedrock-fracture zone locations and characteristics and specifically to characterize fracture zones 

at high-yield well sites. The area of study includes six 

well field sites that were surveyed with surface 

methods from February to December 1999. Three 

sites were surveyed with borehole geophysical 

methods in December 2000. Geologic and lineament 

data were used to ensure that a variety of sites were 

selected, and were considered during the interpretation and discussion of the results of surface and 

borehole geophysical surveys.

Previous Investigations

The use of geophysical techniques are well 

documented for water-supply (Haeni, 1995) and 

contaminant investigations in unconsolidated environments. Whereas fracture zones were correlated with 

photolinear features in some settings by geophysical 

methods, for example in karst environments in Florida 

(Spratt, 1996) and in sandstones in West Africa 

(Taylor and others, 1999), few publications document 

the use of geophysical techniques for investigation of 

water supply in fractured crystalline rock, particularly 

in the northeastern United States. Previous investigations using various geophysical methods to study 

high-yield crystalline bedrock aquifers include those 

of Chapman and Lane, 1996; Mack and others, 1998; 

and Johnson and others, 1999. Direct-current 

resistivity and borehole radar techniques were used by 

Chapman and Lane (1996) to determine the orientations of fracture zones in a crystalline bedrock aquifer 

in Lawrenceville, Ga. Advanced borehole techniques 

were used in Rye, N.H., to identify fractures in wells 

and in the surrounding area. Strikes of fracture sets in 

the wells in Rye, identified by Johnson and others 

(1999), were coincident with remotely sensed 

lineaments identified by Ferguson and others (1997). 

Complex fracture patterns emerged between two wells 

using radar tomography results in Seabrook, N.H.,
INTRODUCTION 3

one of which is one of the highest yielding bedrock 

wells in the State (greater than 560 gal/min). The 

trend of a lineament at this site (Ferguson and others, 

1997) correlated with results of borehole geophysical 

log interpretations and results of aquifer tests (Mack 

and others, 1998).

Site Selection

Geophysical investigation sites for this study 

were selected in or near two 7.5-minute quadrangles 

(Pinardville and Windham, N.H.) where detailed 

geology (including bedrock-outcrop fracture data) was 

mapped as a part of this project by Walsh and Clark 

(1998), and T.R. Armstrong and W.C. Burton 

(U.S. Geological Survey, written commun., 1999). 

These quadrangles are the first two quadrangles in 

New Hampshire mapped in detail by the USGS at the 

1:24000 scale. Sites were selected within these 

quadrangles to provide a more complete geohydrologic setting for the geophysical investigations. 

Additional lineament data were identified and 

correlated with fractures in bedrock outcrops within 

the two quadrangles (R.B. Moore and others, U.S. 

Geological Survey, written commun., 2001). 

Lineaments are straight line features observed on the 

Earth’s surface that may represent bedrock fracture 

zones (Clark and others, 1996). Wells within 500 ft of 

a lineament were chosen so these data could be 

included in site characterizations and interpretations. 

Sites with bedrock-well yields greater than 75 gal/min 

were selected to ensure the presence of high-yield 

fractured rock.

Sites were assessed for potential cultural noise 

and were avoided if the noise potential was high. A 

total of 17 sites initially were selected for reconnaissance investigations. Techniques that allowed for 

rapid data collection (ground-penetrating radar (GPR), 

electromagnetic terrain conductivity surveys (EM) and 

very-low-frequency electromagnetic surveys (VLF)) 

were used at the initial sites. Six of the 17 sites were 

selected for detailed investigation, representing a 

variety of geologic and physiographic settings, and are 

the subject of this report. 

Geohydrologic Settings

Physiographic settings for the study sites ranged 

from wetland valleys to mountainsides. Over the twoquadrangle region, elevations of the sites ranged from 

150 to 930 ft. The maximum relief between survey 

lines at a site is 80 ft (at site 4 on the side of a 

mountain). Two sites had little to no relief between 

survey lines; a flat field on the side of a hill at site 5 

and a wet lowland setting at site 6 (fig. 1). 

At all of the sites the crystalline bedrock was 

covered with unconsolidated materials, which are 

glacial and glacial fluvial in origin. Four of the sites 

were covered in till that ranged in thickness from 

inches to tens of feet. Till generally is an unsorted 

mixture of clay, silt, sand, pebbles, cobbles, and 

boulders. Stratified drift overlays bedrock at site 3 and 

site 6 (fig. 1). It is not known if till is present beneath 

stratified drift at these sites. Stratified drift at these 

sites ranged from inches to tens of feet thick. 

Generally, stratified drift is deposited in streams or 

quiet water bodies fed by meltwater flowing from 

glaciers and consists of sorted and layered unconsolidated material. 

The bedrock geology of the Pinardville 

quadrangle includes a suite of metamorphosed 

intrusives, metamorphosed layered extrusives, and 

interlayered metasediments (T.R. Armstrong and 

W.C. Burton, U.S. Geological Survey, written 

commun., 1999). The rocks range in age from 

Permian to Late Proterozoic. Sites 1, 4, and 5 were in 

the Massabesic Gneiss and Rangely Formations. 

Rocks in the Massabesic Gneiss Complex are coarse 

grained and include well-foliated felsic and mafic 

gneiss and weakly foliated to well-layered migmatitic 

gneiss. Rocks of the Rangely Formation have a strong 

parallel bedding ductile fabric composed of welllayered pelitic metasediments.

Bedrock in the Windham quadrangle consists of 

Ordovician and Silurian metasedimentary rocks of the 

Merrimack Trough, with intrusive rocks as young as 

Mesozoic (Walsh and Clark, 1998). Sites 2, 3, and 6, 

in the Windham and neighboring Salem quadrangles 

are in the Berwick Formation, with site 2 on a contact 

between the Berwick Formation and the Ayer 

Granodiorite. The Berwick Formation is a biotiteplagioclase-quartz granofels schist with interbedded 

calc-silicate rocks and feldspathic quartzite. The Ayer 

Granodiorite is a Silurian-age intrusive rock. The Ayer 

Granodiorite is mapped as two phases at site 2, one 

phase being porphoritic granite to granodiorite, and a 

second phase of granodiorite (Walsh and Clark, 1999). 
4 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Brittle (fracture) geologic data from Walsh and 

Clark (1999) and T.R. Armstrong and W.C. Burton 

(U.S. Geological Survey, written commun., 1999)

were analyzed on a site-by-site basis to define fracture 

families. All measurements within a 4,000-ft radius of 

each study site with a dip greater than 45° were 

compiled from a geographic information system (GIS) 

database. Fracture families were defined for each site 

by plotting azimuth-frequency (rose) diagrams in the 

Structural Data Integrated System Analyzer (DAISY 

2.19) by Salvini (2000). The DAISY software uses a 

Gaussian curve-fitting routine for determining peaks 

in directional data (Salvini and others, 1999). Peak 

orientations, error ranges, and normalized fracture 

peak heights were compiled for site characterization 

and comparison with geophysical data. The error 

range indicates the range of trends associated with the 

peak. The normalized fracture-peak height indicates 

how large the peak is, in percent, in relation to the 

largest (100 percent) peak at a site.

Hundreds of remotely sensed lineaments are 

present in the study area (Clark and others, 1997; 

Ferguson and others, 1997). The lineament data used 

in the analysis for this study were observed based on 

the methods of Clark and others (1996). Lineaments 

associated with the sites were observed from the 

following observation platforms: side looking 

airborne radar (SLAR), satellite photography 

(Landsat), low-altitude black and white aerial photography (LOWALT), high-altitude black and white aerial 

photography (HIGHALT), color infrared aerial 

photography (CIR), and 1:24,000-scale topographic 

map (TOPO). Several of the remotely sensed 

lineaments at the sites have been correlated with 

physiographic features seen in the field at ground 

level. Many lineaments were correlated with fractures 

measured in outcrop; these lineaments are noted in this 

report when observed at the study sites. Domainanalysis and 1,000-ft buffer analysis fracturecorrelation techniques and the full data set are 

described by (R.B. Moore and others, U.S. Geological 

Survey, written commun., 2001), a companion report 

for this project.

High-yield wells at the study sites are used for 

irrigation, and domestic and public water supply. The 

reported water yield from the wells ranged from 75 to 

630 gal/min. The depth of the wells ranged from 150 

to 500 ft. The maximum depth to bedrock was 

estimated at 22 ft from drilling logs (casing length 

minus 12 ft). Water-table depth in the bedrock ranged 

from at the surface (0 ft) to 64 ft deep.

Well-yield probabilities, throughout the Pinardville and Windham quadrangles, were estimated by 

(R.B. Moore and others, U.S. Geological Survey, 

written commun., 2001) for a grid of cells, 98.4 x 

98.4 ft per cell. Probabilities of obtaining at least 

40 gal/min were estimated for theoretical wells drilled 

400 ft deep. These estimates were based on a large 

database of actual bedrock well yields of wells with 

varying depths and site characteristics. Well-site 

characteristics were derived from Statewide databases 

and quadrangle-scale topographic, geologic, and 

lineament maps. Probabilities at the geophysical sites 

ranged from 5 to 38 percent. Locations of the highyield wells at the geophysical sites had probabilities 

ranging from 12 to 38 percent based on the well-site 

characteristics.

Acknowledgments

The authors thank the State, Federal, and 

municipal officials, residents, and well contractors that 

provided data for this study. Appreciation is expressed 

to the members of the Geophysical Advisory 

Committee for providing insight and guidance for the 

study plans. Members of the Geophysical Advisory 

Committee include Richard Chormann, James Vernon, 

Joseph Ingari, Garrick Marcoux, John Jemzeck, and 

John Brooks. Dr. James H. Vernon provided data 

examples from surveys at a site just north of the 

Windham quadrangle. Surface geophysical surveys 

and analysis by George Willard contributed significantly to this study. Special thanks are extended to the 

private and corporate landowners and managers who 

volunteered access to their land and wells making this 

study possible. 

APPROACH AND METHODS

Surface geophysical survey methods are useful 

in water-resource investigations of surficial aquifers 

(Haeni, 1995). Some of these methods can be applied 

to fractured-bedrock settings. For this study, surface 

geophysical techniques were selected that can yield 

interpretable anomalies if fracture zones are present, 

and can be detected on the basis of background 

geologic and cultural conditions. Borehole-
APPROACH AND METHODS 5

geophysical techniques were selected to provide the 

location and orientation of bedrock fractures at depth 

for comparison with the results of surface-geophysical 

surveys. Processed and interpreted geophysical data 

were compared to geologic outcrop and remotely 

sensed data. 

Many geophysical-survey methods take 

advantage of the electrical anomaly associated with 

the large electrical contrast between fractures and the 

host rock. In general, the electrical conductivity of 

crystalline bedrock (such as granites, gneiss, and 

shists) in the State is low relative to other subsurface 

materials, and a fluid-filled fracture zone is more 

electrically conductive than the host rock. This 

electrical contrast creates a dielectric permittivity 

contrast (Beres and Haeni, 1991), for example, that 

makes it possible to image fluid-filled fractures with 

ground-penetrating radar. Sufficiently large fracture 

zones may have a slower average seismic velocity 

than competent rock; therefore, average bedrock 

seismic velocity can be high parallel to the dominant 

fracture strike. Low magnetic anomalies can indicate 

weathered fracture zones, which lowers the presence 

of magnetic minerals in the host rock. 

Many other factors can cause geophysical 

anomalies and must be considered when interpreting 

fracture zones. Bedrock foliation, geologic contacts, 

or intrusions may produce electric or magnetic 

contrasts or anomalies depending on mineral constituents of the rocks. Bedrock and overburden type, 

ground-water saturation, ground-water chemistry, 

bedrock and surface topography, cultural, and 

atmospheric conditions all may have an effect on 

geophysical data. Variations in electrical properties 

result from different rock and overburden materials, 

pore-water chemistry, porosity, and degree of saturation of bedrock or overburden. 

Up to seven surface-geophysical survey 

methods were used to characterize the subsurface at 

the sites—Primary wave (P-wave) seismic refraction, 

ground-penetrating radar, magnetics, very-lowfrequency electromagnetics, inductive electromagnetic 

terrain conductivity, two-dimensional direct-current 

electrical resistivity, and azimuthal square-array 

electrical resistivity. Technique application was 

limited on the basis of the natural and cultural 

conditions at each site. Borehole-geophysical logs 

also were collected at three sites and included caliper, 

fluid temperature and resistivity, electromagnetic 

induction, natural-gamma radiation, and optical 

televiewer. The following sections describe the 

surface and borehole surveys.

P-Wave Seismic Refraction

Seismic refraction uses refracted seismic waves 

to characterize the acoustic or seismic-velocity distribution of layered earth materials. A compression or 

primary (P) wave is generated at the Earth’s surface, 

travels into the earth and is refracted and reflected 

back to the surface. The resulting waves are recorded 

by geophones. Only layers increasing in seismic 

velocity with depth can be accurately detected with 

seismic refraction. Thin, intermediate velocity layers 

are not detectable. A thorough description of theory 

and interpretation of seismic refraction data is given 

by Haeni (1988). 

Variations in average bedrock seismic velocity 

were examined between survey lines with different 

orientations at the sites. Variations in bedrock seismic 

velocity can be attributed to vertical and sub-vertical 

fracture zones in which maximum velocity, the 

average of a line, is along the strike of these fracture 

zones (Hansen and Lane, 1995). Seismic surveys also 

were used to search for depressions in the bedrock 

surface, which might be indicative of a weathered 

fracture zone.

Seismic-refraction surveys were done at three of 

the sites primarily to identify seismic-velocity 

variations, but also to survey the depth to the water 

table and the depth to bedrock. Nine survey lines were 

collected at five locations using a geophone spacing of 

10 ft. Four of the locations contained two orthogonal 

lines sharing a common center point. One line was 

oriented normal and one parallel to the suspected 

strike of a fracture zone on the basis of previously 

collected geophysical data. First arrivals of P-waves 

from 5 shot points on each survey line were recorded 

using a 24-channel signal enhancement seismograph. 

A sledgehammer and a metal plate were used to 

produce the seismic waves. The relative locations and 

elevation of each geophone and shot point were 

surveyed for data analysis. 

Seismic data were processed using a computer 

program developed by Scott (1971) that uses time 

delay and ray-tracing methods. Where possible, 

identification of the water-table surface and 

underlying bedrock topography was made during 

processing to help interpret information gained from 

other techniques. Variations in seismic velocities and 

bedrock surface topography were compared with other 

geophysical data. 
6 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Ground-Penetrating Radar

Ground-penetrating-radar (GPR) surveys done 

with a transmitting and receiving antenna were used to 

image the depth to bedrock and fracture zones. The 

antenna generates and detects electromagnetic (EM) 

waves at a 300 MHz frequency. The radar-wave 

propagation is affected by differences in electromagnetic properties of the medium. These properties 

include dielectric permittivity, electrical conductivity, 

and magnetic susceptibility (Beres and Haeni, 1991), 

which are affected by water content, overburden type, 

and lithology. Features identified in this study are 

bedrock-overburden interfaces and sub-horizontal 

bedrock fractures. Hansen and Lane (1995) used GPR 

to identify bedrock-fracture zones and overburden 

interfaces. The utility of GPR is limited at sites with 

electrically conductive clay-rich overburden (such as 

till) because the EM wave can be attenuated before it 

reaches bedrock (Ayotte and Dorgan, 1995; Ayotte and 

others, 1999). Where conditions are conducive to 

successful data collection, GPR provides a rapid 

means of providing detailed insight into subsurface 

conditions.

GPR survey design and resulting data presentation for this study differed at each site. Where the land 

surface was flat and open, the surveys were done in a 

continuous data-collection mode. Continuous data 

collection requires that the antenna be pulled at a 

constant speed while radar pulses are transmitted into 

the earth. A point-survey mode was used at sites that 

were heavily wooded or had rugged terrain. During a 

point survey, the antenna is placed at regular intervals 

along a line. A 5-ft data-collection interval was used 

for all point surveys. Repeated measurements at each 

point are stacked to filter out noise. Continuous and 

point profiles were adjusted for topographic relief. 

The results of the GPR surveys for two lines where 

anomalies indicate features in bedrock are presented in 

this report. Other GPR surveys are not included.

Magnetics

Magnetometer surveys measure slight variations 

in the Earth’s total magnetic field. Changes in the 

magnetic field can result from varying types and 

amounts of magnetic minerals present in the bedrock. 

Magnetic anomalies also can differ as a result of the 

sensor and bedrock separation caused by variations in 

overburden thickness. Magnetic anomalies related to 

fracture zones could be low if the magnetic minerals 

of the host rock have been weathered. Magnetic lows 

were associated with fracture zones in an investigation 

by Frohlich (1989) of crystalline rocks across New 

England. 

Total field surveys for each line were measured 

with a proton magnetometer at a measurement interval 

of 10 ft. Surveys done for this study were completed 

in less than an hour; therefore, diurnal corrections 

were not made. Results are reported in nanoteslas 

(nT) subtracting the regional base of roughly 

54,000 nT. 

Very-Low-Frequency Electromagnetics

Very-low-frequency electromagnetic surveys 

(VLF) use very-low-frequency radio (3-30kHz) waves 

generated by distant transmitters. Measurements of a 

tilt-angle of the long axis of the primary magnetic field 

ellipse are made, which are affected by secondary 

magnetic fields. The secondary fields are a product of 

electrical galvanic currents induced in conductive 

media in the Earth from the primary magnetic field. 

This study used the VLF transmitter in Cutler, Me., 

that broadcasts at a frequency of 24 kHz at 

1,000 kilowatts power. This transmitter provided a 

consistent and strong signal. Alternate transmitters in 

Jim Creek, Wa., and Aguada, P.R., were assessed but 

the signal strength was too weak for use here. 

The tilt-angle mode of operation was used to 

detect conductive features in the bedrock. Measurements were taken every 10 ft along a line. Fracture 

zones that are fluid filled can produce high electrical 

conductivity anomalies. VLF surveys are best for 

detecting conductive anomalies when the feature is 

linear or elongated and oriented less than a 45° angle 

from the measurement point to the transmitter. The 

response of the tilt-angle percent when passing over a 

conductive feature is a high reading followed by a low 

reading. The inflection point between the high and 

low anomalies indicates the location of the feature 

(Iris Instruments, 1993). The width of the anomaly 

from peak to trough is proportional to one half of the 

depth to the top of the feature (Wright, 1994). Surface 

topography can cause subtle changes in the tilt-angle 

and must be considered. Ionospheric activity can 

affect VLF signal strength. VLF was used previously, 

in conjunction with other techniques, to identify 

fracture zones at a site in the Mirror Lake area, 

Grafton County, New Hampshire (Powers, Singha, 

and Haeni, 1999).
APPROACH AND METHODS 7

Inductive Electromagnetic Terrain 

Conductivity

Inductive electromagnetic terrain conductivity 

(EM) surveys responsed to induced electromagnetic 

signals to measure the electrical conductivity of 

subsurface media. A portable transmitter and receiver 

kept at a fixed distance (coil spacing) were used for 

EM surveys from point to point along a survey line. 

The transmitter emits an electromagnetic field by 

energizing a coil of wire with alternating current (AC). 

The resulting magnetic field (primary) induces an 

electrical current in the ground. A secondary magnetic 

field, caused by the induced current, is measured as a 

voltage difference from the primary field signal in the 

receiving coil. This induced voltage is proportional to 

the apparent conductivity of the Earth. The apparent 

conductivity measured is that of a hemisphere of all 

Earth materials between the coils where the effective 

measurement point is the mid-point between the coils.

The vertical dipole survey mode (VD) was used 

primarily in this study because it is better at detecting 

vertical conductive features and has a deeper range of 

sensitivity than the horizontal dipole survey mode. In 

the VD mode, the plane of the coils is held horizontally, with the axis of the coils oriented vertically. The 

VD survey is optimized to be most sensitive at depths 

of 0.4 times the coil spacing, and measures within a 

depth range of 0.1 to 1.5 times the coil spacing 

(McNeil, 1980). The modeled response of a vertically 

conductive feature detected with a VD survey, consists 

of a below background (sometimes negative) apparent 

conductivity measurement centered over a feature 

positioned between two above-average apparent 

conductivity measurement peaks. The distance 

between the inner limbs of the conductivity peaks 

must be equal to the coil spacing to be able to identify 

a vertical conductor (McNeil, 1980). The relative 

height of the above-average measurement peaks to 

each other can indicate the dip direction of the feature. 

The dip of a planar conductive feature is towards the 

higher conductivity peak.

The horizontal dipole (HD) mode is used to 

obtain a measurement of the near-surface conductivity. 

In the HD mode, the plane of the coils is held 

vertically, with the axis of the coils oriented horizontally in the plane of the survey line. The HD is more 

sensitive to electrical properties close to the surface; a 

measurement depth range extending to 0.75 times the 

coil spacing. HD measurements were used to provide 

a qualitative indication of relative changes in 

overburden thickness, assuming the overburden is 

more electrically conductive than the underlying 

bedrock. Since bedrock generally is much more 

electrically resistive, a HD high can represent a 

thickening of the overburden, or a filled bedrock 

trough. This technique was used by Taylor and others 

(1999) to indicate depressions in a bedrock surface.

EM measurements were made using a 20-m coil 

spacing at most of the sites for this study. At sites 

where electromagnetic noise was a problem, a 10-m 

coil spacing was used. VD measurements were made 

every 10 ft and HD measurements were made every 

20 ft along a survey line.

Two-Dimensional Direct-Current 

Resistivity

Two-dimensional direct current (dc) resistivity 

surveys, termed 2-D resistivity, measure the electrical 

resistivity of the subsurface. Direct current is induced 

in the ground by two current electrodes and the 

voltage is measured at two potential electrodes. A 

resistance value is obtained by dividing the measured 

voltage by the induced current. The apparent 

resistivity is calculated from the resistance value and 

geometric factors that are different for each array type 

(arrangement of current and potential electrodes in 

relation to each other) and takes into account the 

electrode spacing. Dipole-dipole and Schlumberger array (Zohdy and others, 1974) survey configurations were used. A combination of 28 electrodes 

and addressable switches were used at a time to collect 

resistivity measurements. When needed, electrodes 

were moved from one end of the line to the other end 

to collect additional measurements. The relative 

elevation of the land surface at each electrode was 

surveyed and accounted for in processing of the data. 

The apparent resistivity values collected in the 

field were inverted. Results are adjusted, during the 

processing, for topographic relief along a survey line. 

Field and model data sets were processed using 

RES2DINV version 3.42 (Loke, 1997) to produce 

inverted resistivity sections from the apparent 

resistivity data. Inversion gives a more realistic 

resistivity value projected to a relative elevation. 

Model cross sections of the subsurface 

resistivity distribution were created and data collection 

was simulated using RES2DMOD version 2.2 
8 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

(Loke, 1999). The input model cross sections were 

created on the basis of the known geology and the 

results and interpretations of the inverted field data. 

Synthetic apparent-resistivity data were calculated 

from the model cross-sections and inverted for 

comparison to the inverted field data. A model 

solution is reached after numerous iterations, each 

with a modified model, when the inverted resistivity 

section from the field data and inverted synthetic 

resistivity section from the model data approximately 

match. The model solutions are not unique but, with 

inclusion of known information to the model, such as 

depth to water table or depth to bedrock, the solutions 

represent a likely interpretation. 

Azimuthal-Square Array Direct-Current 

Resistivity

Azimuthal square-array dc-resistivity surveys, 

termed square-array resistivity, measure the subsurface resistivity in various orientations and allow for 

the determination of the strike of a conductive 

anomaly with depth (Habberjam and Watkins, 1967). 

To determine the strike of near-vertical conductive 

anomalies in the bedrock, a horizontal-layered 

overburden must be assumed. This technique cannot 

correct for bedrock or surface topography; therefore, 

surveys (arrays) were collected at areas without these 

conditions

Electrodes are set in square arrays, direct current 

is produced in the ground by two current electrodes on 

one side of the square and a potential difference is 

measured at two electrodes on the other side. The 

length of the side of the square is termed the Aspacing. From these four electrodes, apparent 

resistivity is calculated from electrode spacing and a 

geometric factor. For each square, the current and 

potential electrode connections are switched 90° to 

measure resistivity in another orientation using the 

same electrode locations. Making a measurement by 

placing the current and potential electrode connections 

diagonally on the square facilitates an error check.

Resistivity represents an average resistance of 

subsurface materials between the electrodes. The midpoint of resistivity can be projected to a specified 

depth and compass direction on the basis of the side 

length of the square, defined by A-spacing and the 

array orientation. The effective survey depth is 

approximately equal to the A-spacing. For each 

survey, data were collected with array “squares” 

oriented 15° apart and with a number of different 

A-spacings. The size of the survey and effective depth 

of penetration depended on the amount of 

unobstructed terrain available. After preliminary 

evaluations of 2-D resistivity data, square array 

locations were chosen.

Graphical interpretations of the data were made 

by plotting the resistivity with radial orientation. 

Resistivity data were collected and interpreted 

according to the techniques described by Lane and 

others (1995). Primary conductive strikes are orthogonal to the resistivity maximum. Secondary conductive strikes are orthogonal to the second largest 

resistivity measurements. If a range of high resistivity 

measurements is observed, then a conductive range is 

orthogonal to the range of measurement orientations.

Borehole Geophysical Surveys

Six borehole geophysical logs were collected 

including caliper (hole diameter), fluid temperature, 

fluid resistivity, electromagnetic induction, naturalgamma radiation, and optical televiewer. The first five 

logs can help identify water-bearing fractured zones, 

whereas the optical televiewer provides the fracture 

orientation. Borehole geophysical logs were 

interpreted together to characterize borehole fractures.

The caliper log was used to generate a continuous profile of the borehole diameter. This log shows 

the mechanically measured diameter of the borehole 

as a spring-loaded, three-arm caliper tool is pulled up 

the well. The arms open as they pass borehole 

enlargements. Increases in the borehole diameter 

generally are related to fractures, but also can be 

caused by changes in lithology or well construction. 

The profile indicates the roughness of the borehole 

wall. Some enlargements were larger than the caliper 

diameter (18 in.).

The electromagnetic induction (EM) log 

provides a profile of the electrical conductivity of the 

rocks and fluid in the materials surrounding the 

borehole. The conductivity changes measured by the 

EM log are caused by variations in the electrical 

conductance of the fluids in the formation, alteration 

of minerals, and increases in porosity and borehole 

enlargements. The log can be used to delineate 

changes in lithology and electrical properties of 

water in the formation. In crystalline rock in 
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 9

New Hampshire, increases in conductivity were 

associated with fractured zones (Mack and others, 

1998), primarily the result of increased water content 

in the fractures.

The fluid-temperature log displays a continuous 

measurement of fluid temperature in the borehole. In 

the absence of ground-water flow, the temperature 

gradually increases with the geothermal gradient, 

which is 0.6o C per 100 ft of depth (Keyes, 1988). A 

continuous plot of the fluid temperature with depth is 

used to identify zones that deviate from the expected 

geothermal gradient. Deviations from the gradient 

indicate locations where ground water enters or exits 

the borehole. 

The fluid-resistivity log records the electrical 

resistance of the fluid in the borehole. Changes in the 

electrical resistance of the water in the borehole 

indicate differences in the total dissolved solid concentrations in borehole water. These differences typically 

indicate sources of water that have contrasting 

chemistry and have come from alternate water-bearing 

zones. Similar to the fluid-temperature profile, fluidresistivity deviations from a straight-line gradient 

indicate locations where ground water enters or exits 

the borehole.

The natural gamma log measures the naturalgamma radioactivity of the formation surrounding the 

borehole. Gamma radiation is a natural product of the 

radioactive decay of potassium-40, and a daughter 

product of uranium and thorium decay. The gamma 

log used in this investigation does not differentiate 

between the sources of the gamma radiation. The 

gamma log is a count of total gamma-radiation 

emissions, which may be correlated with the rock type 

or with fracture infillings. Potassium-40 is abundant 

in potassium feldspar (microcline and orthoclase), 

which alters to sericite and clay. In the alteration 

process, potassium-40 is concentrated in the clay by 

processes of adsorption and ion exchange. Deviations 

in the gamma log trace indicate changes in the rock 

type or the presence of mineralized fractures. Clay 

minerals, which sometimes form in the fractures, 

generally have an elevated concentration of 

potassium-40 minerals from areas away from the 

fractures and cause an increase in the gamma values. 

An optical televiewer (OTV) log was used to 

map the location and orientation of fractures that 

intersect a well. The OTV log collects oriented digital 

pictures of the borehole wall in 360° concurrently with 

borehole deviation. The product is a high-resolution, 

digital picture of the borehole wall that can be used to 

determine the location and orientation (strike and dip) 

of fractures, lithologic contacts, or other borehole 

features. Boreholes drilled into crystalline rock 

frequently deviate from vertical because of variations 

in rock properties, the fabric of the bedrock, 

fracturing, or as a result of the drilling process or 

technique. Deviation is measured by a magnetometer 

and inclinometer, and is recorded as an azimuthal 

direction (0-360° from magnetic north) and the 

inclination of the borehole (0-90° from vertical) with 

depth. Measurements of borehole deviation are used 

to correct the apparent strike and dip of a feature in a 

deviated borehole to its true orientation.

Results of the OTV log for a borehole can be 

summarized in a fracture stereogram for comparison 

with surface or remote analyses. The stereogram, a 

lower-hemisphere, equal-area projection of poles to 

planes, was used to plot the orientation of fractures 

and contacts and foliation. Fractures include transmissive fractures, open fractures, contact fractures and 

cracks. A stereogram reduces each features plane to a 

point that represents the intersection of a pole, perpendicular to a features plane, with a lower hemisphere. 

For example, a horizontal fracture would be indicated 

by a point in the center of the stereogram, whereas a 

fracture striking 215° with a dip of 89° W would be 

indicated by a point towards the right (eastern) edge of 

the outer circle. The orientation of the fracture plane 

is reported as 215°, 89°, which in the right-hand-rule 

format implies that a fracture that dips west (89° dip, 

to the right of the 215° bearing). 

ANALYSIS AND RESULTS OF 

GEOPHYSICAL INVESTIGATIONS OF 

WELL FIELDS

Six sites in southern New Hampshire were 

selected for geophysical surveying. Detailed site maps 

and graphics of the data are used to describe survey 

results. Surface-geophysical surveys were done along 

survey lines, and around array centers at each of the 

six sites. The results are in terms of distance along 

each line or a trend for an array. Trends are reported in 

terms of azimuth degrees from true north. Borehole 

geophysical logs were collected at selected sites where 

wells were accessible (no pumps were installed), and 

(or) permission was obtained to remove water-supply 

pumps. Borehole-geophysical logs provide actual 
10 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

fracture measurements for confirmation or comparison 

with analyses of surface and remotely sensed surveys.

Numerous anomalies were detected in the 

survey lines and arrays by various methods. Locations 

on the survey lines displaying anomalies from 

multiple methods, or strong anomalies, likely are 

related to actual features in the bedrock. Multiple 

geophysical methods, and geologic and remotely 

sensed data, were used to locate and characterize 

subsurface features to provide indication of bedrockfracture zones. Square-array resistivity results, 

geologic data, and lineament locations and orientations provided information to determine the strike of 

likely fracture zones. 

Site 1, Bedford, New Hampshire

Site 1 on State Route 101 in Bedford, N.H., is a 

wooded hillside at an elevation of about 360 to 400 ft 

in the area of the surveys. T.R. Armstrong and 

W.C. Burton (U.S. Geological Survey, written 

commun., 1999), mapped the bedrock geology of this 

area as two variations of the Massabesic Gneiss 

Complex, specifically a migmatite gneiss and a 

layered paragneiss and orthogneiss (fig. 2). The 

bedrock is exposed at the surface on the west end of 

line 1 and in the central part of the site. The 

overburden is mapped as a till, which is unsorted to 

poorly sorted clay silt, sand, pebbles, cobbles and 

boulders with some gravel (Koteff, 1970). Mapped 

lineaments at the site were observed from SLAR, 

Landsat (Clark and others, 1997), and TOPO 

platforms trending 18°, 33°, and 22° (fig. 2). The 

SLAR and Landsat lineaments were fracture 

correlated using the 1,000-ft buffer analysis technique 

(R.B. Moore and others, U.S. Geological Survey, 

written commun., 2001). Fracture data within a 

4,000-ft radius of the site have three peak orientations: 

27°±7° (100 percent, normalized height), 295°±10° 

(25 percent, normalized height), and 301°±11° 

(23 percent, normalized height). Fractures in an 

outcrop between line 1 and line 2 have a strike and dip 

of 20° and 82°. Lineaments are visible at the site as 

swales at line 1 trending 5° and 20°.

Drilled to a depth of 485 ft, through approximately 24 ft of overburden, well BIW 889 (fig. 2) has 

a reported yield of 150 gal/min and a static water level 

depth of 30 ft. The drillers’ log indicates that the highyielding water-bearing zone is between 420 and 485 ft 

deep. Probabilities of exceeding a yield of 40 gal/min 

from a 400-ft deep well at this site ranged from 5 to 

29 percent. A 14-percent probability is calculated for 

the 98.4-ft (30-m) square cell that well BIW 889 is 

located in (R.B. Moore and others, U.S. Geological 

Survey, written commun., 2001). Variations in 

probability at the site appear to reflect lithologic 

contacts, topography, and lineaments.

Three geophysical survey lines were selected to 

bisect lineament locations on either side of the well. 

Line 1 is in the woods to the south of the well, just east 

of State Route 101 and extending for 480 ft to the east. 

Line 2 is 900 ft long and begins in an open field east of 

State Route 101, crosses through a wooded area and 

into another open field to the north of the well. Two 

array center locations were placed on line 2 at 

locations where geophysical anomalies were detected 

along the survey line. Array 1 was centered at 660 ft 

along line 2. Array 2 was centered at 450 ft along 

line 2 (fig. 2).

Geophysical Surveys and Interpretation

Seven geophysical methods were used to 

characterize site 1. Overburden thickness or physical 

properties were derived from the (P)-Wave SeismicRefraction, GPR, EM, and 2-D resistivity survey 

results (figs. 3-7). Bedrock characteristics and 

anomalies that could be caused by bedrock fractures 

are observed in the seismic-refraction, GPR, 

magnetometer, VLF, EM, 2-D resistivity, and squarearray resistivity-survey results.

Seismic-refraction (P)-wave data were collected 

along line 2 between 335 and 565 ft. The line was 

interpreted based on a three-layer model: unsaturated 

till, saturated till, and bedrock. The bedrock surface 

ranges from 15-40 ft below the ground surface, 

depending on the velocity chosen for the saturated till. 

The water table is about 8 ft below the ground surface. 

Three troughs are indicated in the bedrock surface. 

The deepest troughs are centered at 375 and 425 ft, 

with 10-20 ft of relief in the bedrock surface. A minor 

trough is centered at 525 ft with 5-10 ft of relief. 

Bedrock seismic velocity normal to the lineament was 

calculated to be approximately 8,000 ft/s. This 

velocity is significantly lower than bedrock velocities 

(10,000 to 20,000 ft/s) typically seen in New 

Hampshire (Medalie and Moore, 1995).

Ground-penetrating radar (GPR) data were 

collected along the entire lengths of line 1 and line 2 

(fig. 2). The GPR profile of line 1 indicates subhorizontal reflectors at about 10-20 ft below the 

interpreted bedrock surface. These reflectors likely
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 11

Figure 2. Geophysical survey locations, bedrock geology, and lineaments at site 1, Bedford, N.H. Site location is shown on figure 1.
12 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

are sheeting fractures (fig. 3). The GPR profile along 

line 2 indicates a reflector at approximately 10 ft deep 

from 600 to 900 ft, which is likely the water table in 

the overburden above bedrock. 

Magnetometer measurements were made along 

line 1 and line 2 (figs. 4 and 5). The average magnetic 

field measurement (after subtracting the regional trend 

of 54,000 nT) at this site during the surveys is about 

500 nT. Results at line 1 indicate an anomalous 

magnetic low of 445 nT between 270 and 310 ft; a 

magnetic low of 470 nT occurs at about 410 ft 

(fig. 4a). The survey results at line 2 (fig. 5a) indicate 

a low of 480 nT between 0 and 200 ft. The total field 

increases to a high of 575 nT at 455 ft. Results at line 

2 indicate a low of 525 nT between 640 and 690 ft, 

and a low of 515 nT at 845 ft (fig. 5a). 

VLF tilt-angle surveys at line 1 and line 2 

indicate anomalies. Overhead power lines nearby 

affected results along approximately the first 100 ft of 

line 1. Line 1 had inflection points at 110, 195, 240, 

and 310 ft (fig. 4b). Inflections were detected at line 2 

at 130, 300, 375, 530, 640, 800, and 880 ft; overlapping anomalies are observed between 300 and 375 ft 

(fig. 5b). 

EM surveys were collected on line 1 and line 2 

at site 1. Nearby power lines caused signal noise. To 

avoid this noise, the survey coil spacing was shortened 

from 20 (65.6 ft) to 10 m (32.8 ft). Anomalies that 

likely are associated with vertical conductors were 

observed on line 1 at 200, 290, 360, and 440 ft (fig. 

4c). Similar anomalies on line 2 are at 85 ft and 170 ft 

(fig. 5c). A combination of deepening overburden 

interpreted from the HD and potential vertical conductors are noted at 440 ft and 610 ft (fig. 5c). 

2-D resistivity surveys were collected at line 1 

and line 2. Models were created to check interpretations for both lines. Line 1 survey interpretations 

indicate conductive anomalies in resistive bedrock that 

could be a result of near-horizontal sheeting fractures. 

A vertical conductive anomaly also is found on line 1 

at 355 ft (fig. 6). Four major resistivity units from line 

2 likely represent an unsaturated, and conductivesaturated overburden, and resistive-competent and 

conductive-saturated bedrock (fig. 7). Survey results 

indicate that near horizontal fractures in the bedrock 

could be present at the southeastern end of line 2. 

Near vertical, and dipping, conductive anomalies are 

indicated at 55-95 ft (not modeled), 460-500 ft, and 

630-660 ft (fig. 7). 

Figure 3. Processed ground-penetrating radar profile at site 1 from line 1, Bedford, N.H. Site and line locations are shown on figures 1 and 2, respectively.
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 13

Figure 4. Magnetic and electromagnetic surveys at site 1 from line 1, 

Bedford, N.H. (A) magnetometer survey; (B) very low frequency (VLF) 

electromagnetic survey; (C) electromagnetic (EM) terrain conductivity 

survey with a 10-meter (32.8-foot) coil spacing. Site and line locations are 

shown on figures 1 and 2, respectively.

Figure 5. Magnetic and electromagnetic surveys at site 1 from line 2, 

Bedford, N.H. (A) magnetometer survey; (B) very low frequency (VLF) 

electromagnetic survey; (C) electromagnetic (EM) terrain conductivity 

survey with a 10-meter (32.8-foot) coil spacing. Site and line locations 

are shown on figures 1 and 2, respectively.
14 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Figure 6. Cross sections showing (A and B) inverted resistivity sections 

of two-dimensional, direct-current resistivity data at site 1 from line 1, 

Bedford, N.H.; (C) model based on field data from A and B; and (D and E) 

synthetic resistivity output data from Model C. Site and line locations are 

shown on figures 1 and 2, respectively.

Square-array resistivity data were collected at 

two arrays centered on line 2. At the largest Aspacing of 10 m, array 1 (fig. 8a) shows a prominent 

primary and secondary conductive strike of 15° and 

60°. At the largest A-spacing of 40 m, array 2 (fig. 

8b) indicates a weak primary conductive strike of 

345°. Measurements from array 2 show a decrease in 

the average resistivity from the 5-m A-spacing to the 

10-m A-spacing, and an increase in resistivity from 

10- through 40-m A-spacing. This sounding (fig. 8b) 

indicates three layers, resistive at the surface (unsaturated overburden), a conductive layer (saturated 

overburden), and a resistive lower layer (bedrock).

Integration of Results

EM and VLF anomalies, indicative of conductive features in bedrock, appear along line 1 at 200 ft. 

A magnetic low, a conductive 2-D resistivity anomaly, 

and VLF and EM anomalies indicative of conductive 

features in bedrock are found along line 1 between 

310 and 390 ft. Line 2 has near vertical, conductive 

EM and 2-D resistivity anomalies coincident with a 

magnetometer peak value between 460-500 ft. 

Fractured bedrock could be bisecting the line as 

indicated by the low bedrock seismic velocity 

between 335-565 ft. Conductive anomalies between 

610 and 660 ft along line 2 from VLF, EM, and 2-D 

resistivity coincide with a magnetic low. Conductive 

features along lines have consistent responses, where 

the magnetic response varies between high and low 

with some features, along line 2.

Conductive strikes from square-array resistivity 

results with the same orientation as fractures identified in outcrop, or remotely sensed lineaments, likely 

are related to fracture zones. Interpretation of squarearray resistivity surveys at array 1 indicate a primary 

conductive strike of approximately 15°±7.5° at the 

largest A-spacing, which is the same as the orientations of TOPO and SLAR lineaments (fig. 2). This 

strike also corroborates with the maximum fracture 

trend from geologic data. The peak fold axis trends 

65° and is correlated with the deepest secondary strike 

anomaly trends from the square-array resistivity 

survey at array 1. Mapped lineament orientations at 

this site of 22° and 33° coincide with the maximum 

fracture peak from geological mapping data of 

27°±7°. These include a 25,054-ft long Landsat 

lineament striking 33°, and a 10,900-ft long TOPO 

lineament striking 22°.
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 15

Figure 7. Cross sections showing (A and B) inverted resistivity sections of two-dimensional, direct-current resistivity data at 

site 1 from line 2, Bedford, N.H.; (C) model based on field data from A and B; and (D and E) synthetic resistivity output dataf

from Model C. Site and line locations are shown on figures 1 and 2, respectively.
16 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Figure 8. Polar plots showing azimuthal square-array direct-current resistivity at site 1 for arrays 1 and 2, Bedford, N.H. Apparent resistivity in 

ohm meters (Ω m), is plotted as a function of azimuth, in degrees east of true north; (A) resistivity of square array 1, center at 700 Ω m; (B) 

resistivity of square array 2, center at 150 Ω m. Site and array locations are shown on figures 1 and 2, respectively.

Electromagnetic (EM and VLF) surveys 

indicate electrically conductive anomalies that are 

consistent with the presence of fractured bedrock. 2-

D and square-array resistivity surveys, and geologic 

information corroborate to support the presence of a 

fractured-bedrock zone. These surveys indicate that 

lineaments are close to near-vertical conductive 

features, dipping southeast, identified with geophysical methods. These features may represent fracturedbedrock zones, which likely transmit water. Nearhorizontal features in bedrock, interpreted as sheeting 

fracture zones, were identified with GPR and 2-D 

resistivity geophysical methods.

Site 2, Windham, New Hampshire

Site 2 on Marblehead Road in Windham, N.H. 

(fig. 9), is a mostly wooded, valley-wetland setting, 

and ranges in elevation from about 170 to 210 ft in the 

area of the surveys. Walsh and Clark (1999) mapped 

the bedrock geology of this area with a contact 

between the Berwick Formation, and the Ayer 

Granodiorite (fig. 9). The bedrock is exposed at the 

surface on topographic highs at this site. The 

overburden generally is less than 20 ft thick and is 

mapped as a till, which is unsorted to poorly sorted 

clay, silt, sand, pebbles, cobbles and boulders, with 

some gravel (Larson, 1984). Three mapped 

lineaments at the site were observed from a 

LOWALT platform trending 71°, 74°, and 355°

(Ferguson and others, 1997), and one with a CIR 

platform, trending 77° (fig. 9). The 71°, 74°, and 77° 

trending lineaments were fracture correlated using 

the 1,000-ft buffer analysis technique (R.B. Moore 

and others, U.S. Geological Survey, written 

commun., 2001). Lineament criteria are visible at the 

site as a swale on line 1, trending 350°, and a shallow 

elongated valley trending 70°. Fracture data in a 

4,000-ft radius of the site have three peak orientations: 310°±9° (100 percent, normalized height), 

68°±11° (14 percent, normalized height), and 

25°±34°(9 percent, normalized height). Fractures in
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 17

Figure 9. Geophysical survey locations, bedrock geology, and lineaments at site 2, Windham, NH. Site location is shown on figure 1.

SOb

SOb

SOb

SOb

Sag

Sag

Sagd

Sagd

Sagd

Sagdg

Md

Sagd

WPW 133
18 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

an outcrop between line 3 and line 4 have a strike and 

dip of 70° and 19° dipping to the south, parting along 

foliation, and have a strike and dip of 67° and 35° 

dipping to the north.

Well WPW 133 (fig. 9) is drilled to a depth of 

300 ft with a reported yield of 100 gal/min. Approximately 7 ft of overburden is present above the 

bedrock, and the static water level in the well is at 

10 ft below landsurface. Probabilities of exceeding a 

yield of 40 gal/min from a 400-ft-deep well at this site 

ranged from 8 to 15 percent. A 14-percent probability 

is calculated for the 98.4-ft (30-m) square cell that 

well WPW 133 is in (R.B. Moore and others, U.S. 

Geological Survey, written commun., 2001). 

Variations in probability at the site most likely are 

caused by lithologic contacts and topography.

Four geophysical survey lines were located to 

bisect lineament locations on each side of well 

WPW 133 (fig. 9). Line 1 extends 570 ft from west to 

east and is on a trail in the woods to the south of 

WPW 133. It is on a topographic high with outcrops 

of the Ayer Granodiorite. Lines 2, 3, and 4 are in an 

east-west trending (70°) shallow valley. Logs from 

two monitoring wells reveal that the valley is filled by 

a 15-20-ft thick sequence of outwash and till. Line 2 

extends 440 ft from west to east and is set in the 

woods, parallel to a monitoring-well access road to the 

north of well WPW 133. Line 3 is parallel to Marblehead Road, and extends 440 ft from north to south in a 

wooded area west of well WPW 133. Line 4 is 

parallel to line 3 in a wooded area to the east of well 

WPW 133, and extends 440 ft from north to south. An 

array center was chosen on the basis of availability of 

flat terrain and the location of anomalies from other 

techniques. The center of array 1 is at 200 ft along 

line 2, 90 ft to the south in the center of the valley 

(fig. 9).

Geophysical Surveys and Interpretation

Seven geophysical surveys were used to characterize site 2. Overburden thickness and physical 

properties were derived from results of seismic refraction, GPR, EM, and 2-D resistivity surveys. Bedrock 

characteristics and anomalies that could be caused by 

bedrock fractures are seen in the seismic refraction, 

GPR, magnetometer, VLF, EM, 2-D resistivity and 

square-array resistivity survey results (figs. 11-19).

Seismic-refraction data were collected along 

line 1 and line 2 and at 90° to each line. These data 

were examined only with respect to average seismic 

wave velocity in bedrock along each line, to look at 

variations in relation to orientation to a suspected 

fracture zone. Seismic refraction on line 1 was 

centered at 350 ft. The seismic velocity in the bedrock 

along line 1 is 11,500 ft/s, whereas the velocity normal 

to line 1 is 15,000 ft/s. Seismic refraction along line 2 

is centered at 213 ft. The seismic velocity in bedrock 

along line 2 is 11,000 ft/s, and the velocity normal to 

line 2 is 9,500 ft/s. 

GPR data were collected on lines 1, 2, 3, and 4; 

data from line 1 indicate features below the bedrock 

surface. A reflector below land surface from 0 to 

160 ft along line 1 ends at a bedrock outcrop, which 

indicates that it is likely caused by the bedrock 

surface. A bedrock outcrop from 370 to 390 ft 

indicates that subhorizontal reflectors are below the 

bedrock surface from 375 to 500 ft along line 1. These 

reflectors are interpreted to be sheeting fractures 

(fig. 10). 

Magnetometer measurements were made along 

all four lines at site 2 (figs. 11-14). The average 

magnetic field measure at this site during the surveys 

is 71 nT. Line 1 survey results indicate a magnetic low 

of 51 nT centered at 55 ft (fig. 11a). Data collected 

along line 4 indicate a magnetic low of 33 nT between 

230-280 ft, and another low of 36 nT between 330- 

340 ft (fig. 14a). 

VLF tilt-angle surveys at lines 1, 2, 3, and 4 

indicate anomalies. Inflections along line 1 were 

detected at 70, 100, 140, 370, 405, and 430 ft 

(fig. 11b). Line 2 tilt-angle measurements have inflection points at 110, 240, 340, 380, and 415 ft (fig. 12b). 

Line 3 results indicate weak inflection anomalies at 

255, 305, and 395 ft (fig. 13b). The VLF data from 

line 4 has a tilt-angle inflection point at 235 ft 

(fig. 14b).

EM surveys were collected on all lines at site 2. 

Along line 1, a VD anomaly at 380 ft indicates a nearvertical conductor (fig. 11c). Results of the VD survey 

from line 2 indicate a near-vertical conductor anomaly, 

possibly dipping east, at 180 ft (fig. 12c). Line 3 VDsurvey results indicate a vertical conductor anomaly 

centered at 220 ft (fig. 13c). The results from survey 

line 4 indicate the bedrock is more conductive in the 

north than in the south (fig. 14c). The dip of features 

on lines 1-4 were not readily apparent from the EM 

data.

2-D resistivity was measured at lines 1, 2, 3, and 

4. Models were created to verify interpretations of the 

data. A near-vertical fracture on line 1 is interpreted at 

110 ft and an eastward-dipping fracture is interpreted 

as intersecting the bedrock surface at 345 ft (fig. 15). 
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 19

Figure 10. Processed ground-penetrating radar profile at site 2 from line 1, Windham, N.H. Site and line locations are shown 

on figures 1 and 9, respectively

Near-horizontal conductive features also can be 

interpreted from the dipole-dipole array at 345-450 ft 

that were not modeled (fig. 15a). A horizontal 

conductor at depth from 0 to 440 ft also was 

interpreted based on the Schlumberger array (fig. 

15b). The model, created to check interpretations of 

the data from line 2 (fig. 16), displays the effect of 

fractures intersecting from different orientations. 

Based on the results from lines 3, 4, array 1, and 

lineament data, a fracture zone with a strike close to 

the strike of line 2 intersects the line from 190 to 440 

ft. A conductive zone striking roughly perpendicular 

to line 2 intersects the bedrock surface at 110 ft along 

the line (fig. 16). Interpretations of line 3 indicate a 

fracture zone dipping to the south, and intersecting the 

surface of the bedrock at 200 ft (fig. 17). For line 4, a 

conductive south-dipping feature intersects the 

bedrock surface at 150 ft along the line (fig. 18). 

Square-array resistivity data were collected at 

array 1. The primary conductive strike determined 

graphically is 75× with a range of 60× to 105× at the 

largest A-spacing of 20 m (fig. 19). Increases in 

resistivity from the 5-m A-spacing through the 20-m 

A-spacing indicate a two layer model; conductive 

overburden and resistive bedrock (fig. 19). 

Integration of Results

Line 1 at site 2 (fig. 11) has two locations 

containing anomalies from multiple techniques. 2-D 

resistivity and VLF anomalies indicative of conductive features in bedrock were found between 100 and 

115 ft along the line. EM, VLF, and 2-D resistivity 

anomalies indicative of conductive features in 

bedrock are between 350 and 375 ft along line 1, 

whereas 2-D resistivity data indicate an eastward dip. 

Near-horizontal fractures begin in the GPR record at 

375 ft and extend to at least 500 ft. The seismicrefraction velocity of bedrock parallel to line 1 is 

approximately 3,500 ft/s slower than the velocity 

normal to line 1 centered at 350 ft. This decrease in 

velocity is consis-tent with dominant fracture trends 

near parallel to the low-altitude lineament (fig. 9), 

striking roughly towards line 2 and the well. 

The line 2 survey is near-parallel to a suspected 

fracture zone, which bisects line 3 and line 4 

intersecting line 2 at 270 ft. Interpretation of the data 

is difficult because line 2 may cross two bedrockfracture zones at different orientations. EM and 2-D 

resistivity anomalies indicative of conductive features 

in bedrock are roughly normal to the line at 180 ft. 2-

D resistivity indicated a possible eastward dip to the 
20 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Figure 11. Magnetic and electromagnetic surveys at site 2 from line 1, 

Windham, N.H. (A) magnetometer survey; (B) very low frequency (VLF) 

electromagnetic survey; (C) electromagnetic (EM) terrain conductivity 

survey with a 20-meter (65.6-foot) coil spacing. Site and line locations are 

shown on figures 1 and 9, respectively.

Figure 12. Magnetic and electromagnetic surveys at site 2 from line 2, 

Windham, N.H. (A) magnetometer survey; (B) very low frequency (VLF) 

electromagnetic survey; (C) electromagnetic (EM) terrain conductivity 

survey with a 20-meter (65.6-foot) coil spacing. Site and line locations are 

shown on figures 1 and 9, respectively.
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 21

Figure 13. Magnetic and electromagnetic surveys at site 2 from line 3, 

Windham, N.H. (A) magnetometer survey; (B) very low frequency (VLF) 

electromagnetic survey; (C) electromagnetic (EM) terrain conductivity 

survey with a 20-meter (65.6-foot) coil spacing. Site and line locations are 

shown on figures 1 and 9, respectively.

Figure 14. Magnetic and electromagnetic surveys at site 2 from line 4, 

Windham, N.H. (A) magnetometer survey; (B) very low frequency (VLF) 

electromagnetic survey; (C) electromagnetic (EM) terrain conductivity 

survey with a 20-meter (65.6-foot) coil spacing. Site and line locations are 

shown on figures 1 and 9, respectively.
22 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Figure 15. Cross sections showing (A and B) inverted resistivity sections of two-dimensional, 

direct-current resistivity data at site 2 from line 1, Windham, N.H.; (C) model based on field data 

from A and B; and (D and E) synthetic resistivity output data from Model C. Site and line locations 

are shown on figures 1 and 9, respectively.
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 23

Figure 16. Cross sections showing (A and B) inverted resistivity sections 

of two-dimensional, direct-current resistivity data at site 2 from line 2, 

Windham, N.H.; (C) model based on field data from A and B; and (D and E) 

synthetic resistivity output data from Model C. Site and line locations are 

shown on figures 1 and 9, respectively.

Figure 17. Cross sections showing (A and B) inverted resistivity sections 

of two-dimensional, direct-current resistivity data at site 2 from line 3, 

Windham, N.H.; (C) model based on field data from A and B; and (D and E) 

synthetic resistivity output data from Model C. Site and line locations are 

shown on figures 1 and 9, respectively.
24 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Figure 18. Cross sections showing (A and B) inverted resistivity sections 

of two-dimensional, direct-current resistivity data at site 2 from line 4, 

Windham, N.H.; (C) model based on field data from A and B; and (D and E) 

synthetic resistivity output data from Model C. Site and line locations are 

shown on figures 1 and 9, respectively.

Figure 19. Polar plot showing azimuthal square-array direct-current 

resistivity at site 2 for array 1, Windham, N.H. Apparent resistivity in ohm 

meters (Ω m), is plotted as a function of azimuth, in degrees east of true 

north, and resistivity center is at 150 Ω m. Site and array locations are 

shown on figures 1 and 9, respectively.

line 2 anomaly. The seismic-refraction velocity of 

bedrock along line 2 is approximately 1,500 ft/s faster 

than the velocity normal to line 2, which indicates that 

the dominant fracture trend is nearly parallel to line 2. 

Line 3 has a steeply dipping conductive 2-D 

resistivity anomaly (fig. 17) that correlates with the 

location of an EM anomaly indicative of a conductive 

feature in bedrock (fig. 13b). Line 4 has a steeply 

dipping conductive 2-D resistivity anomaly (fig. 18) 

that correlates with the location of a VLF anomaly, 

indicative of a conductive feature in bedrock (fig. 14). 

A magnetic low identified along line 4 coincides with 

the deep, down-dip portion of the 2-D resistivity 

feature. The anomalies bisecting line 3 and line 4 

appear to be the same continuous feature based on the 

primary conductive strike from the square-array 

resistivity and the location and orientation of 

lineaments. 
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 25

Conductive strikes identified by square-array 

resistivity, with the same orientation as fractures 

identified in outcrop, or remotely sensed lineaments, 

likely are related to fracture zones. Array 1 has a 

primary strike direction that is the same as the three 

lineaments striking 71×, 74×, and 77×. These 

lineaments cross line 3 and line 4 on the location of 

steeply dipping anomalies identified with 2-D 

resistivity. Array 1 has a primary bedrock strike of 75× 

that has the same orientation as a small peak in the 

geologic-fracture data analysis (68×±11×), and both 

fractures dipping south and parting along foliation 

dipping north in an adjacent outcrop. 

Electromagnetic (EM and VLF) surveys 

indicate electrically conductive anomalies that are 

consistent with fractured bedrock. DC-resistivity 

surveys and arrays, and geologic information also 

indicate fractured bedrock. These surveys show that 

the lineaments are close to dipping planar features that 

may represent fractured-bedrock zones. The possible 

fracture zones, indicated by lineaments LOWALT 74, 

LOWALT 71, and CIR 77, probably are a southward 

dipping fracture zone that allows for transmission of 

water to well WPW 133 (fig. 9). Near-horizontal 

conductive features in bedrock, that cannot be identified with a lineament analysis, were identified on line 

1 with GPR and 2-D resistivity surveys. These nearhorizontal features are interpreted as sheeting fracture 

zones and may serve to connect near-vertical fracture 

zones. 

Site 3, Pelham, New Hampshire

Site 3 on State Route 128 in Pelham, N.H., is set 

in grassy lots in a shallow valley containing Beaver 

Brook. The elevation ranges from 150 to 170 ft where 

the data were collected. Walsh and Clark (1999) 

mapped the bedrock geology of this area as the 

Berwick Formation (fig. 20). Overburden material at 

the site is mapped as a fine-grained (clay to fine sand) 

stratified drift. Transmissivity of the stratified-drift 

aquifer is less than 1,000 ft2/d (Stekl and Flanagan, 

1992). Lineaments mapped at the site were SLAR 

trending 329° and 325° (Ferguson and others, 1997), 

and CIR trending 303° (fig. 20). The 303° trending

lineament was fracture correlated using the domainanalysis technique (R.B. Moore and others, U.S. 

Geological Survey, written commun., 2001). These 

lineaments have identifying criteria visible at the site 

such as a small wetland, and a valley trend of 300° at 

survey line 1, and a straight reach of stream with a 

trend of 320° south of line 2 (fig. 20). Fracture data in 

a 4,000-ft radius of the site has four peak orientations: 

282°±8° (100 percent, normalized height), 304°±6° 

(90 percent, normalized height), 28°±4° (17 percent, 

normalized height), and 325°±13° (24 percent, 

normalized height).

At site 3, well PAW 131 was drilled to a depth of 

240 ft and has a reported yield of 120 gal/min. The 

drilling log indicates that a fracture zone was 

intersected between 225 and 240 ft deep. The static 

water level in the well is at 12 ft and bedrock is at 11 ft 

below land surface. Well PAW 420 was drilled at 

115 ft along line 2 at site 3 during this study. The 

overburden consisted of 7 ft of poorly sorted fine to 

coarse sands, gravels, and cobbles. The well was 

completed at 300 ft below land surface when a 

sufficient supply for domestic use was obtained at a 

reported yield of 30 gal/min. At 19 ft below the land 

surface, the well yield was 2 gal/min from fractures 

between soft and hard variations of the Berwick 

Formation. The high-producing zone for the well is at 

277 ft in an open fracture. The open fracture is below 

an approximately 1-ft thick silicified zone with quartzbiotite black schist and quartzite above and Berwick 

granofels below (S.F. Clark, Jr., U.S. Geological 

Survey, written commun., 1999). 

Probabilities of exceeding a yield of 40 gal/min 

from a 400-ft deep well at this site ranged from 12 to 

19 percent. A 12-percent probability is calculated for 

the 98.4-ft (30-m) square cell that well PAW 131 is in 

(R.B. Moore and others, U.S. Geological Survey, 

written commun., 2001). Well PAW 420 is within a 

cell with a 13-percent probability. Variations in 

probability at the site appear to be caused by proximity 

to surface water and topography.

Two geophysical survey lines were located to 

cross lineaments on each side of well PAW 131. Line 

1, which extends 440 ft from southwest to northeast, is 

along the property line of two developed lots and in a 

vacant lot to the northwest of well PAW 131. Line 2, 

southeast of PAW 131, is in a channel of Beaver 

Brook, and extends 440 ft from southwest to northeast. 

Well PAW 420 was drilled directly adjacent to line 2 at 

approximately 100 ft along the line. Two array 

locations were sited on electrically conductive 

anomalies after a preliminary analysis of other 

geophysical data; array 1 was set at 215 ft on line 1, 

array 2 was set at 200 ft on line 2 (fig. 20).
26 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Figure 20. Geophysical survey locations, bedrock geology, and lineaments at site 3, Pelham, N.H. Site location is shown on figure 1.
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 27

Geophysical Surveys and Interpretation

Six geophysical surveys were used to characterize site 3. Overburden thickness and physical 

properties were derived from the GPR, EM, and 2-D 

resistivity survey results. Bedrock properties were 

determined by magnetometer, 2-D resistivity, and 

square-array resistivity. Anomalies that could be 

caused by bedrock fractures are seen in the magnetometer, VLF, EM, 2-D resistivity and square-array 

resistivity survey results (figs. 21-25). 

GPR data were collected on lines 1 and 2. The 

GPR record from line 1 indicates bedded sands, with 

attenuation of the record before the bedrock is 

detected. Reflectors from the bedded sands dip 

towards the center of the cross-section. The GPR 

record from line 2 has a reflector that is interpreted to 

be the bedrock surface. From 195 to 205 ft along line 

2, this reflector drops from 7 to 14 ft in depth, 

indicating a depression in the bedrock surface. The 

GPR record did not return any clear reflectors in the 

bedrock and is not presented in this report. 

Magnetometer measurements were made along 

line 1 and line 2 (figs. 21 and 22). The average 

magnetic field measure at this site during the surveys 

is 109 nT. Line 2 survey results indicate anomalous 

lows of ñ35 nT at 120 ft, and 50 nT at 300 ft (fig. 22a). 

The steel-well casing at 115 ft along line 2 from well 

PAW 420 could affect the anomaly at 120 ft, considering the magnetic high just before it along the line. 

VLF tilt-angle measurements were made along 

lines 1 and 2. VLF data from line 1 indicates a weak 

inflection at 210 ft, but was dominated by power-line 

noise at its northeastern end (fig. 21b). An anomalous 

inflection is at 175 ft along line 2 (fig. 22b). The 

suspected fracture zone orientations at site 2 in 

relation to the transmitter are not ideal. 

EM surveys were collected on lines 1 and 2 at 

site 3. The survey along line 1 was shortened because 

of interference from power lines (fig. 21c), and the 

remaining data may be affected. The EM results from 

line 2 indicate a near-vertical conductor anomaly at 

155 ft along the line (fig. 22c). 

2-D resistivity surveys were run at lines 1 and 2. 

Four primary resistivity units from line 1 and line 2 

can be represented by resistive unsaturated and 

conductive saturated sediments, and resistive 

competent and conductive fractured bedrock. Below 

the interpreted bedrock surface at 230 ft along line 1 is 

a conductive anomaly penetrating into the bedrock. 

This anomaly was interpreted with an apparent dip to 

the southwest (fig. 23). Near horizontal conducive 

Figure 21. Magnetic and electromagnetic surveys at site 3 from line 1, 

Pelham, N.H. (A) magnetometer survey; (B) very low frequency (VLF) 

electromagnetic survey; (C) electromagnetic (EM) terrain conductivity 

survey with a 20-meter (65.6-foot) coil spacing. Site and line locations are 

shown on figures 1 and 20, respectively.
28 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Figure 22. Magnetic and electromagnetic surveys at site 3 from line 2, 

Pelham, N.H. (A) magnetometer survey; (B) very low frequency (VLF) 

electromagnetic survey; (C) electromagnetic (EM) terrain conductivity 

survey with a 20-meter (65.6-foot) coil spacing. Site and line locations are 

shown on figures 1 and 20, respectively.

Figure 23. Cross sections showing (A and B) inverted resistivity sections 

of two-dimensional, direct-current resistivity data at site 3 from line 1, 

Pelham, N.H.; (C) model based on field data from A and B; and (D and E) 

synthetic resistivity output data from Model C. Site and line locations are 

shown on figures 1 and 20, respectively.
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 29

Figure 24. Cross sections showing (A and B) inverted resistivity sections 

of two-dimensional, direct-current resistivity data at site 3 from line 2, 

Pelham, N.H.; (C) model based on field data from A and B; and (D and E) 

synthetic resistivity output data from Model C. Site and line locations are 

shown on figures 1 and 20, respectively.

features just below the surface of the bedrock could be 

the reason conductivity is high along line 2. A trough 

in the bedrock surface and a conductive bedrock 

anomaly is at 165-205 ft along line 2. The conductive 

anomaly in the bedrock is interpreted with an apparent 

dip to the southwest (fig. 24). Square-array resistivity 

surveys were run at array 1 and array 2 (fig. 25). At the 

largest A-spacing (10 m) array 1 shows a primary and 

secondary conductive strike of 345× and 90×, with a 

low resistivity value at 330×. Measurements made 

with small A-spacings from array 1 show a decrease in 

resistivity from the 5-m A-spacing to the 7.1-m Aspacing. This decrease indicates three layers; resistive 

(unsaturated overburden) at the surface, a conductive 

middle layer (saturated overburden), to a resistive 

lower layer (bedrock). At the largest A-spacing (10 

m), array 2 results indicate a primary conductive strike 

of 15× and a secondary strike of 330(fig. 25). 

Integration of Results

A 2-D resistivity and VLF anomaly are at about 

210 ft on line 1. Line 2 has a conductive southwestdipping 2-D resistivity anomaly in bedrock that is 

bounded at the surface of the bedrock by a VLF 

anomaly at 175 ft and an EM anomaly at 155 ft along 

the line. A magnetic low coincides with the bottom the 

modeled 2-D resistivity conductive feature at 120 ft.

Conductive strikes from square-array resistivity 

surveys with the same orientation as fractures identified in outcrops, or remotely sensed lineaments, likely 

are related to fracture zones. The secondary conductive strike from array 2 square-array resistivity results 

of 330× has the same orientation as the SLAR 

lineaments that cross lines 1 and 2, and corresponds 

the analysis of geologic-fracture data. Array 1 does not 

have a graphically determined primary or secondary 

conductive strike matching the lineaments, but the 

resistivity low for the largest A-spacing at 330does 

match. 

Electromagnetic (EM or VLF) and 2-D 

resistivity surveys indicate electrically conductive 

anomalies that are consistent with the presence of 

fractured bedrock along lines 1 and 2. These surveys 

indicate that the 325× and 330× striking lineaments 

detected with SLAR are electrically conductive and 

could be steeply dipping features. These features may 

represent water bearing fractured-bedrock zones.
30 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Figure 25. Polar plots showing azimuthal square-array direct-current resistivity at site 3 for arrays 1 and 2, Pelham, N.H. Apparent resistivity in ohm 

meters (Ω m), is plotted as a function of azimuth, in degrees east of true north; (A) resistivity of square array 1, center at 200 Ω m; (B) resistivity of square 

array 2, center at 200 Ω m. Site and array locations are shown on figures 1 and 20, respectively.

Near-horizontal features in bedrock that cannot be 

identified with a lineament analysis were indicated by 

2-D resistivity surveys. These near-horizontal 

features are identified as sheeting fracture zones, 

which may enhance the transmissivity of near-vertical 

fracture zones.

Site 4, Goffstown, New Hampshire

Site 4 on Mountain Road in Goffstown, N.H., is 

set in a grassy field and wooded terrain. The site is on 

the south side of North Uncanoonuc Mountain, and 

ranges in elevation from about 850 to 930 ft between 

survey lines. Bedrock geology of this area consist of 

two variations of the Rangeley Formation, biotite 

schist and granofels, and rusty biotite-muscovite 

schist (fig. 26) (T.R. Armstrong and W.C. Burton, 

U.S. Geological Survey, written commun., 1999). 

The bedrock is exposed at the surface on the northern 

part of the study area. The overburden at this site is 

mapped as a till, which is unsorted to poorly sorted 

clay silt, sand, pebbles, cobbles and boulders, with 

some gravel (Koteff, 1970). Lineaments at the site 

were identified from LOWALT and HIGHALT aerial 

photography (Ferguson and others, 1997) trending 27°

(fig. 26). These lineaments were fracture correlated by 

domain-analysis techniques (R.B. Moore and others, 

U.S. Geological Survey, written commun., 2001). 

Fracture data in a 4,000-ft radius of the site has five 

peak orientations: 55°±9° (100 percent, normalized 

height), 315°±8° (87 percent, normalized height),

272°±11° (75 percent, normalized height), 24°±7° 

(57 percent, normalized height), and 335°±5° 

(64 percent, normalized height).

Well GNW 406 (fig. 26), drilled to a depth of 

500 ft, has a reported yield of 75 gal/min. The static 

water level was at a depth of approximately 10 ft after 

drilling in April 1997, and was at a depth of 64 ft 

during the borehole-geophysical surveys in December 

2000. The drillers’ log indicates that the high-yield 

water-bearing zone is between 420 and 500 ft deep. 

Probabilities of exceeding a yield of 40 gal/min from a 

400-ft deep well at this site ranged from 10 to 

38 percent. A 38-percent probability is calculated for 

the 98.4-ft (30-m) square cell that well GNW 406 is in 

(R.B. Moore and others, U.S. Geological Survey,
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 31Figure 26. Geophysical survey locations, bedrock geology, and lineaments at site 4, Goffstown, N.H. Site location is shown on figure 1.
32 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

written commun., 2001). Variations in probability at 

the site are caused by fracture-correlated lineaments 

and topography. Two geophysical-survey lines were 

located to bisect lineaments on either side of the well. 

Line 1, extending 440 ft from west to east is in the 

field to the north of the well. Line 2 is in the woods to 

the south of the well, extending 440 ft from west to 

east. Array 1 was set on line 1 centered at 190 ft 

(fig. 26).

Geophysical Surveys and Interpretation

Six surface and six borehole-geophysical 

surveys were used to characterize site 4 (figs. 27-31). 

Overburden thickness and physical properties were 

derived from GPR, EM, and 2-D resistivity survey 

results. Magnetometer, VLF, surface EM, 2-D 

resistivity, square-array resistivity, and boreholegeophysical survey results were used to determine 

bedrock properties, and identify anomalies that could 

be caused by bedrock fractures. Borehole-geophysical surveys including caliper, fluid temperature and 

resistivity, and EM borehole logs were used to characterize and help identify bedrock features seen in the 

OTV logs.

GPR was collected on lines 1 and 2. Line 1 was 

surveyed using a continuous profile method, taking 

advantage of the open field. Subsurface nearhorizontal reflectors are seen throughout the line up to 

20 ft in depth. Line 2 was collected using a point 

survey and did not yield consistent reflectors. 

Magnetometer measurements were made along 

line 2 at site 4 (fig. 28a). The average magnetic field 

measured at this site during the survey is 352 nT. The 

magnetic field along the line 2 gradually rises, except 

for a low anomaly of approximately 340 nT between 

250 and 330 ft (fig. 28a).

VLF tilt-angle surveys were made at lines 1 and 

2. VLF tilt-angle inflections from line 1 are seen at 

125, 225, and 335 ft (fig. 27a). VLF tilt-angle survey 

results from line 2 are obscured by interference from 

near-by overhead power lines and are not useful 

(fig. 28b).

EM surveys were collected on both lines at site 

4. Line 1 VD anomalies (fig. 27c) that could indicate 

near-vertical conductors were seen at 160, 230, and 

310 ft. VD survey results from line 2 (fig. 28c) 

indicated anomalies centered at 195 and 380 ft.

Models were created to check interpretation of 

the 2-D resistivity survey data at lines 1 and 2. 

Resistivity data from line 1 indicate there are three

Figure 27. Magnetic and electromagnetic surveys at site 4 from line 1, 

Goffstown, N.H. (A) very low frequency (VLF) electromagnetic survey; (B) 

electromagnetic (EM) terrain conductivity survey with a 20-meter (65.6-

foot) coil spacing. Site and line locations are shown on figures 1 and 26, 

respectively.

primary units, which likely represent unsaturated 

overburden and resistive competent and conductive 

fractured bedrock. A model fit with the field data was 

obtained by placing the water-table surface in the 

fracture zones below the top of bedrock at a depth 

between 40 and 50 ft (relative elevation between 

80 and 90 ft). Near-vertical conductive features 

below the interpreted bedrock surface are interpreted 

at 190 and 315 ft. Several horizontal conductive 

features also were interpreted in the bedrock (fig. 29). 

2-D resistivity results from line 2 indicate four 

units that likely represent resistive unsaturated and 

conductive saturated overburden, resistive competent
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 33

Figure 28. Magnetic and electromagnetic surveys at site 4 from line 2, 

Goffstown, N.H. (A) magnetometer survey; (B) very low frequency (VLF) 

electromagnetic survey; (C) electromagnetic (EM) terrain conductivity 

survey with a 20-meter (65.6-foot) coil spacing. Site and line locations are 

shown on figures 1 and 26, respectively.

Figure 29. Cross sections showing (A and B) inverted resistivity sections 

of two-dimensional, direct-current resistivity data at site 4 from line 1, 

Goffstown, N.H.; (C) model based on field data from A and B; and (D and E) 

synthetic resistivity output data from Model C. Site and line locations are 

shown on figures 1 and 26, respectively.
34 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Figure 30. Cross sections showing (A and B) inverted resistivity sections 

of two-dimensional, direct-current resistivity data at site 4 from line 2, 

Goffstown, N.H.; (C) model based on field data from A and B; and (D and E) 

synthetic resistivity output data from Model C. Site and line locations are 

shown on figures 1 and 26, respectively.

bedrock, and conductive fractured bedrock. Nearvertical conductive anomalies below the bedrock 

surface are interpreted with a steep apparent dip to the 

west at 100, 210, and 360 ft (fig. 30). Horizontal 

conductive zones below the bedrock surface are 

interpreted, which are corrobo-rated by the model 

results (fig. 30c). 

Square-array resistivity data were collected at 

array 1. The primary conductive strike at the deepest 

A-spacing (7.1-m) was 60×, with a range of 30× to 

60×. The shallow conductive strike measured with a 5-

m A-spacing was oriented at 45× with a 15× to 45× 

range (fig. 31). 

Caliper and EM conductivity logs for well 

GNW406 (appendix 1a) were used to identify fracture 

zones on OTV logs. Transmissive-fracture zones were 

identified by fluid-temperature and fluid-resistance 

logs (appendix 1a) at approximately 150, 295, and 330 

ft. The orientations of several open fractures were

Figure 31. Polar plot showing azimuthal square-array direct-current 

resistivity at site 4 for array 1, Goffstown, N.H. Apparent resistivity in 

ohm meters (Ω m), is plotted as a function of azimuth, in degrees east of 

true north, and resistivity center is at 3,000 Ω m. Site and array locations 

are shown on figures 1 and 26, respectively.
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 35

Figure 32. Lower hemisphere equal-area nets from bedrock well GNW 

406 at site 4, Goffstown, N.H., showing (A) borehole fractures and (B) 

borehole contacts and foliation. Site and well locations are shown on 

figures 1 and 26, respectively.

determined using OTV logs. The OTV did not identify 

all of the fracture zones indicated with the other 

borehole techniques. Rock that is dark in color limits 

fracture identification with the OTV. Twenty-three 

fractures, and 28 contacts and foliation trends, were 

identified with the OTV log, with a broad range of 

orientations lacing definitive groups (fig. 32). In the 

high-yield zone between 420-500 ft below land 

surface, only two fractures were identified from the 

OTV log. At the bottom of the well, at 497 ft, the 

caliper-measured anomaly indicates a fracture zone 

that is not measurable with the OTV since the 

scanning window is above the bottom of the tool. 

Integration of Results

Line 1 has VLF and EM anomalies indicative of 

conductive-bedrock features at about 225 ft (fig. 27). 

2-D resistivity and EM anomalies, also indicative of 

conductive features in bedrock, are found at about 310 

ft (figs. 27 and 29). Line 2 has 2-D resistivity and EM 

anomalies indicative of conductive bedrock features 

located at about 195 ft (figs. 28 and 30). The magnetic 

low from the magnetometer-survey results 

corresponds with the interpreted sheeting fracture 

zones from the 2-D resistivity (figs. 28 and 30). 

Array 1 has a primary bedrock strike of 60°, 

with essentially the same strike (55°±9°) as the 

maximum peak in the geologic-data analysis. The 

range of orientations from the square-array resistivity 

shallow conductive strike of 15° to 45° correlates with 

mapped lineament orientations between lines 1 and 2 

of 27°, and the strike of a fracture peak in the geological data of 24×±7× (57 percent, normalized height). A

 fracture identified in well GNW 406 at 90.6 ft depth, 

having a strike and dip of 209× (209×-180×=29×) and 

65×, also correlates with the trends of the square-array 

resistivity, lineament, and geologic data peak. 

Borehole-geophysical surveys identified 

transmissive fractures in well GNW 406. A wide 

scattering of fracture orientations were identified in 

the analysis of the borehole data (fig.32a). The 

average of all of the racture trends (75°) and contact 

and folation trends (85°) are close (when normalized 

for the right hand rule, indicating that fractures often 

follow contact and foliation in this borehole. The 

average dip of the fractures, contacts (38°) and 

foliation (36°) seen in the borehole are less than the 

dip of fracture zones that sometimes are indicated by 

lineaments. A negative, vertical-fracture-sampling 

bias must be considered because the borehole is near 

vertical. Fracture orientations could not be collected 

at the bottom of the well where a water-producing 

fracture may be located.

Electromagnetic (EM and VLF) surveys 

indicate electrically conductive anomalies that are 

consistent with fractured bedrock but were limited, by 

cultural noise. DC-resistivity surveys and arrays and 

geologic information indicated that the 27°-trending 

lineaments at this site overlie near-vertical conductive 

features that likely represent water-transmitting 

bedrock-fracture zones. Near-horizontal features in 

bedrock, which cannot be identified with a lineament 

analysis, were identified with 2-D resistivity and 

borehole-survey analysis. These near-horizontal 

features are identified as sheeting-fracture zones, 

which may transmit water to vertical fracture zones. 

Square array, geologic-outcrop fractures, and 

lineaments support a dominant northeast-trending 

water-bearing fracture zone at site 4 in Goffstown.

Site 5, Goffstown, New Hampshire

Site 5 on Shirley Hill Road in Goffstown, is a 

relatively flat grassy field on the side of a hill at an 

elevation of about 730 to 750 ft in the surveyed area. 

The bedrock geology of this area is composed of the 

Rangeley Formation (T.R. Armstrong and 

W.C. Burton, U.S. Geological Survey, written 

commun., 1999). Three different variations of the 

Rangeley Formation have contacts at the site—biotite 

schist and granofels, migmatite schist and gneiss, and 

rusty biotite-muscovite schist (fig. 33). Bedrock is 
36 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Figure 33. Geophysical survey locations, bedrock geology, and lineaments at site 5, Goffstown, N.H. Site location is shown on figure 1.
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 37

exposed in the northern most part of the surveyed area. 

The overburden at this site is till, which is unsorted to 

poorly sorted clay, silt, sand, pebbles, cobbles and 

boulders, with some gravel (Koteff, 1970). Mapped 

lineaments at the site were observed from LOWALT 

(Ferguson and others, 1997), and TOPO platforms 

trending 347° and 346°, respectively (fig. 33). These 

lineaments are visible at the site as swales trending 

about 340°. Fracture data in a 4,000-ft radius of the 

site has four peak orientations: 34°±7° (100 percent, 

normalized height), 301°±6° (32 percent, normalized 

height), 5°±7° (24 percent, normalized height), and 

340°±11° (15 percent, normalized height). Fractures 

measured in an outcrop just south of the site on the 

side of Shirley Hill Road, have a strike and dip of 355°

and 80°.

Well GNW 263 drilled to a depth of 150 ft has a 

reported yield of 80 gal/min. The static water level 

and bedrock are at a depth of 10 ft according to the 

drillers’ log. Well GNW 408 was installed for 

domestic use during this study and provided an 

opportunity for logging by a geologist during its 

drilling and a survey with borehole-geophysical tools. 

The depth of the till, at GNW 408, was 21 ft and the 

well was completed at 200 ft into bedrock. The well 

was completed shortly after the demand for domestic 

use was met and exceeded in this case. A 20-gal/min 

producing zone was identified during drilling between 

170 and 175 ft, and is described as a contact between a 

muscovite-quartz-feldspar granofels and a muscovite 

biotite granofels (S.F. Clark, Jr., U.S. Geological 

Survey, written commun., 1999). 

The probability of exceeding a yield of 

40 gal/min from a 400-ft-deep well at this site ranged 

from 12 to 14 percent. A 14-percent probability is 

calculated for the 98.4-ft (30-m) square cell that well 

GNW 263 is in (R.B. Moore and others, U.S. Geological Survey, written commun., 2001). Well GNW 408 

is in a cell with a 12-percent probability. Variations in 

probability at the site appear to be caused by 

topography. Two geophysical-survey lines bisect 

lineament locations. Line 1 is in a field to the 

southeast of the GNW 263 and line 2 is in a field to the 

east of the well. A center location for array 1 was set 

in the swale at 230 ft on line 1 (fig. 33). 

Geophysical Surveys and Interpretation

Seven surface and six borehole geophysical 

surveys were used to characterize site 5. Overburden 

thickness and physical properties were derived from 

GPR, EM, and 2-D resistivity survey results. Bedrock 

properties and anomalies that could be caused by 

bedrock fractures were observed in seismic-refraction, 

magnetometer, VLF, EM, 2-D resistivity and squarearray resistivity, and borehole-geophysical survey 

results. Caliper, EM borehole, and a drillers’ log was 

used to characterize and help identify bedrock 

fractures measured with the OTV logs.

Seismic-refraction data were collected in two 

orientations separated by 90° at lines 1 and 2. The 

bedrock seismic-wave velocities were compared at 

each orientation. Seismic-refraction-data collection at 

lines 1 and 2 were centered at 220 and 135 ft along the 

lines. The average velocity of the seismic wave in 

bedrock along, and normal to, line 1 is 13,500 and 

14,000 ft/s respectively, and do not indicate a large 

velocity contrast. The average velocity of the bedrock 

along line 2 is 14,000 ft/s, and the average velocity 

normal to line 2 is 11,500 ft/s, which are indicative of 

a contrast.

GPR data were collected at the two lines at site 

5 using a continuous profile method. Electrically 

conductive till attenuated the radar signal and useful 

radar records could not be identified.

The average magnetic field measured at site 5 

during the survey was 374 nT. Low magnetic 

anomalies of 250 nT between 30 and 60 ft, and of 

350 nT between 200 and 250 ft were identified along 

line 1 (fig. 34a). Metal farm machinery parked on line 

2 prevented a magnetic survey from being conducted.

VLF tilt-angle surveys were made at lines 1 and 

2. Line 1 has VLF inflections at 130 and 155 ft 

(fig. 34b). Line 2 has an inflection centered at 165 ft 

(fig. 35a).

EM surveys also were collected on lines 1 and 2 

at site 5. Apparent conductivity had a range from 26 

to -28 mmho/m. Along line 1, an anomaly in the VD 

measurement that could be related to a near-vertical 

conductor was observed at 180 ft (fig. 34c). An 

additional EM survey done 50 ft to the south and 

parallel to line 1 produced similar results. The height 

of the peaks of the anomaly indicates a dip to the west. 

Along line 2, an anomaly in the VD measurement with 

a 20-m (65.6-ft) coil spacing that could be related to a 

near-vertical subsurface conductor was centered at 

160 ft (fig. 35b). The height of the peaks of the 

anomaly is consistent with the line 1 anomalies, 

indicating a dip to the west. EM measurements with a 

40-m (131.2 ft) coil spacing also were made along 
38 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Figure 34. Magnetic and electromagnetic surveys at site 5 from line 1, 

Goffstown, N.H. (A) magnetometer survey; (B) very low frequency (VLF) 

electromagnetic survey; (C) electromagnetic (EM) terrain conductivity 

survey with a 20-meter (65.6-foot) coil spacing. Site and line locations are 

shown on figures 1 and 33, respectively.

Figure 35. Magnetic and electromagnetic surveys at site 5 from line 2, 

Goffstown, N.H. (A) very low frequency (VLF) electromagnetic survey; (B) 

electromagnetic (EM) terrain conductivity survey with a 20-meter (65.6-

foot) coil spacing; (C) electromagnetic (EM) terrain conductivity survey 

with a 40-meter (131.2-foot) coil spacing. Site and line locations are 

shown on figures 1 and 33, respectively.
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 39

line 2 (fig. 35c). The 40-m-coil spacing surveys at a 

greater depth than the 20-m-coil spacing and indicates 

that a west-dipping anomaly continues at depth.

2-D resistivity surveys were done at lines 1 and 

2. Models were used to verify interpretations of the 

data. Resistivity data from lines 1 and 2 show four 

resistivity units, likely representing resistiveunsaturated, and conductive-saturated sediments, and 

resistive-competent and conductive-fractured bedrock. 

Below the interpreted bedrock surface, at around 

190 ft along the line, is a conductive anomaly 

penetrating into the bedrock (fig. 36). Resistivity data 

from line 2 (fig. 37) indicate a horizontal conductive 

feature between 140 and 180 ft at a depth of about 

20 ft. A steep, west-dipping conductive feature in 

bedrock is interpreted at 160 ft (fig. 37). 

Square-array resistivity data were collected at 

array 1. A-spacings of 5-40 m were used. Resistivity 

data from array 1 surveyed with a 40-m A-spacing has 

a primary conductive strike of 0°, and a secondary 

strike of 30°. The anomaly forming the secondary 

strike ranged from 15° to 45°. The surveys with small 

A-spacings indicate a primary conductive strike of 30° 

and a secondary conductive strike of 0° (fig. 38).

Caliper and EM conductivity logs were used in 

well GNW 408 to identify fracture zones on OTV logs 

(appendix 1b). A total of 12 fractures, including 6 

open-fracture orientations were determined using the 

OTV log. Two groups of fractures (fig. 39a) that were 

identified on the OTV log, have a range of strikes and 

dips between 317-342° and 25-42°, dipping NE, and 

202-225° and 22-51°, dipping NW. A group of 

contacts and foliation trends were identified that have 

a range of strikes and dips of 180-210° and 21-58°, 

dipping NW (fig. 39b). These trends indicate that the 

large group of fractures are coincident with foliation or 

contacts. A fracture on a contact that was identified as 

producing 20 gal/min during drilling from the OTV 

log has a strike and dip of 180° and 21°. Strong 

borehole EM-conductivity anomalies are identified 

above and below this water-bearing zone. 

Integration of Results

Line 1 has conductive 2-D resistivity, EM, and 

VLF anomalies from 160-180 ft. Conductive 

anomalies on line 2 appear near 160 ft along the line 

from the results of 2-D resistivity, EM (20- and 40-m 

coil spacing) and VLF surveys. At site 5, a number of 

geophysical anomalies (VLF, EM, and 2-D resistivity) 

were particularly distinct; the EM anomaly was 

especially large and distinct.

Conductive-striking bedrock features 

interpreted from square-array resistivity results have 

the same orientation as fractures identified in outcrop 

and remotely sensed lineaments. These features likely 

are related to fracture zones. The deep primary strike 

from array 1 of 0°±7.5 is less than 6.5° of the 

orientation of the lineament it is centered on, which 

has a strike of 347°, and is the same as fractures 

measured in outcrops adjacent to the site (355°, 80°). 

The secondary strike from array 1 has the same 

orientation as the axis of foliation seen in a 

neighboring outcrop, and coincides with the orientation of the largest geologic-data fracture peak for the 

site. Secondary strikes in the square-array resistivity 

data, at depth, range from 15° to 45° and may be 

explained by a fold-axis trend of 43°. The LOWALT 

lineament between lines 1 and 2 strikes 347° and 

correlates with a small fracture peak from geological 

mapping data. This peak has a strike of 340°±11° 

(15 percent, normalized height). 

Borehole-geophysical surveys identified a group 

of fractures with an average strike (330°) that matches 

a peak in the geologic-outcrop fracture data. This 

peak has a strike of 340°±11° (15 percent, normalized 

height). 

Electromagnetic (EM and VLF) and DCresistivity (2-D and square-array) surveys and 

geologic information indicate strong electrically 

conductive anomalies that are consistent with 

fractured bedrock. These surveys indicate that 

lineaments likely represent steep-westward-dipping 

conductive bedrock features. These features may 

represent fractured-bedrock zones likely to transmit 

water.

Site 6, Salem, New Hampshire

Site 6 on Brady Road in Salem, N.H., is a flat, 

and often wet, grassy field at an elevation of about 

130 ft. The water table in the overburden at the site 

ranges in depth from 0-2 ft below the land surface. 

Overburden material at the site is mapped as a coarsegrained (medium sand to cobble gravel) stratified drift. 

Saturated thickness ranged from 20 to greater than 

40 ft; transmissivity of the stratified-drift aquifer is 

between 1,001 and 4,000 ft2

/d (Stekl and Flanagan, 

1992). Lyons and others (1997) mapped the bedrock
40 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Figure 36. Cross sections showing (A and B) inverted resistivity sections 

of two-dimensional, direct-current resistivity data at site 5 from line 1, 

Goffstown, N.H.; (C) model based on field data from A and B; and (D and E) 

synthetic resistivity output data from Model C. Site and line locations are 

shown on figures 1 and 33, respectively.

Figure 37. Cross sections showing (A and B) inverted resistivity sections 

of two-dimensional, direct-current resistivity data at site 5 from line 2, 

Goffstown, N.H.; (C) model based on field data from A and B; and (D and E) 

synthetic resistivity output data from Model C. Site and line locations are 

shown on figures 1 and 33, respectively.
41 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Figure 38. Polar plot showing azimuthal square-array direct-current 

resistivity at site 5 for array 1, Goffstown, N.H. Apparent resistivity in 

ohm meters (Ωm), is plotted as a function of azimuth, in degrees east of 

true north, and resistivity center is at 100 Ωm. Site and array locations 

are shown on figures 1 and 33, respectively.

geology of this area as the Berwick Formation of the 

Merrimack Group. Mapped lineaments at the site 

were observed from LOWALT and HIGHALT 

platforms (Ferguson and others 1997), trending 44°

and 353°, respectively (fig. 40). Fracture data inside a 

4,000-ft radius of the site has three peak orientations: 

37°±6° (100 percent, normalized height), 294°±7° 

(67 percent, normalized height), and 278°±4° 

(50 percent, normalized height). The closest fracture 

measured in outcrop is more than 3,000 ft away.

Well SAW 207 was drilled to a depth of 533 ft at 

site 6 (fig. 40) and has a reported yield of 630 gal/min. 

The depth to bedrock at well SAW 207 is approximately 18 ft. A nearby test well SAW 272 was 

available for borehole-geophysical surveys. Well 

SAW 272 was drilled through 17 ft of overburden to a 

depth of 345 ft in bedrock and has a reported yield of 

150 gal/min. Sediment accumulation or rock 

fragments at the bottom of the well SAW 272 likely 

account for the borehole tools being unable to reach a 

depth greater than 335 ft. Three geophysical survey 

lines bisected lineaments (fig. 40). All of the lines are 

to the south of the wells in the open field. Line 1 

extends 750 ft from northwest to southeast. Line 2 

extends 440 ft from northwest to southeast. Line 3 

extends 440 ft from west to east. Array 1 was 

centered between the western ends of lines 2 and 3 

(fig. 40).

Geophysical Surveys and Interpretation

Six surface and six borehole geophysical 

surveys were used to characterize site 6. Overburden 

thickness and physical properties were derived from 

GPR, EM, and 2-D resistivity survey results. 

Seismic-refraction, EM, 2-D resistivity, and squarearray resistivity surveys were used to determine 

bedrock properties. Anomalies that could be caused 

by bedrock fractures are seen in the VLF, EM, 2-D 

resistivity and square-array resistivity survey results. 

Caliper, fluid temperature and resistivity, and EM 

borehole logs were used to characterize and help 

identify bedrock fractures measured in the OTV logs. 

Seismic-refraction modeling was used to 

identify a bedrock seismic velocity of 9,800 ft/s 

parallel to line 1 (K.J. Ellefsen, U.S. Geological 

Survey, written commun. 1997). This velocity is just

Figure 39. Lower hemisphere equal-area nets from bedrock well 

GNW 408 at site 5, Goffstown, N.H., showing (A) borehole fractures 

and (B) borehole contacts and foliation. Site and well locations are 

shown on figures 1 and 33, respectively.
42 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Figure 40. Geophysical survey locations, bedrock geology, and lineaments at site 6, Salem, N.H. Site location is shown on figure 1.
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 43

below the low end of the range of bedrock velocities 

(10,000 to 20,000 ft/s) typically found in New 

Hampshire (Medalie and Moore, 1995). A low trough 

in the bedrock was noted beneath the LOWALT 

lineament.

GPR was collected on all lines at site 6. Lines 2 

and 3 were collected using a continuous profile 

method, line 1 was collected using the point-survey 

method. Reflectors were identified in the overburden 

but the signal was attenuated before reaching bedrock. 

VLF tilt-angle surveys were made at all lines 

(figs. 41-43). Line 1 has inflections at 100, 130, 395, 

515, and 580 ft (fig. 41a). Line 2 tilt-angle results 

indicated inflection anomalies at 50 and 295 ft 

(fig. 42). Inflection anomalies are at 65, 120, 155, and 

395 ft (fig. 43) from the survey of line 3. 

Figure 41. Electromagnetic surveys at site 6 from line 1, Salem, N.H. (A) 

very low frequency (VLF) electromagnetic survey; (B) electromagnetic (EM) 

terrain conductivity survey with a 20-meter (65.6-foot) coil spacing. Site 

and line locations are shown on figures 1 and 40, respectively.

Figure 42. Very low frequency (VLF) electromagnetic survey at site 6 

from line 2, Salem, N.H. Site and line locations are shown on figures 

1 and 40, respectively.

Figure 43. Very low frequency (VLF) electromagnetic survey at site 6 

from line 3, Salem, N.H. Site and line locations are shown on figures 

1 and 40, respectively.
44 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

EM surveys were collected along line 1 

(fig. 41b). Vertical-conductor anomalies with the VD 

survey results were at 545 and 675 ft.

2-D resistivity surveys were used to characterize 

lines 1, 2, and 3. Models were created to support 

interpretations of the data. Resistivity data from line 

1, 2, and 3 indicate three resistivity units: thin 

resistive unsaturated zone, conductive saturated zone, 

and resistive bedrock. Line 1 also has a fourth 

resistivity unit interpreted as fractured bedrock. 

Below the interpreted bedrock surface, at about 270 ft 

along the line, is a conductive anomaly with an 

apparent dip to the southeast (fig. 44). Line 2 data 

indicates a topographic high in the bedrock surface at 

230 ft along the line (fig. 45). Anomalies in the 

bedrock were not identified with survey results from 

line 2 or 3 (fig. 46), only changes in the elevation of 

the bedrock surface were identified, with the bedrock 

being the deepest at the west end of line 3.

Square-array resistivity data were collected at 

array 1. Surveys were made with A-spacings of 

5-20 m. Resistivity data from array 1 has a primary 

conductive strike of 75° when surveyed with the 

largest A-spacing (20 m). The secondary strike from 

array 1 at the 20-m A-spacing is 300° with a range of 

285° to 330° (fig. 47). 

Caliper, fluid temperature, fluid resistivity, and 

EM conductivity logs for well SAW 272 were used to 

identify and confirm fracture zones indicated on the 

OTV logs (appendix 1C). A cluster of fractures were 

identified on the lower hemisphere equal area net that 

have a range of strikes and dips of 201-211° and 

46-85°, dipping NW (fig. 48). A group of contacts and 

foliation trends were identified, which have a range of 

strikes and dips of 191-229° and 38-78°, dipping NW. 

Fractures in the group fall within the range of strikes 

for the group of contacts and foliations. There also 

were widely scattered fractures and contacts and 

foliation outside of the identified groups (fig. 48). The 

largest fracture zone, at 264.5 ft, was identified as 

transmissive with fluid-temperature and resistivity 

logs from ambient and pumping borehole conditions. 

Caliper and EM logs correlate with this fracture zone, 

which was observed in the OTV log on a contact, with 

a strike and dip of 240° and 64°. 

Integration of Results

2-D resistivity results from line 1 indicate a 

prominent conductive east-dipping feature in the 

bedrock at 270 ft; however, results form lines 2 and 3 

did not indicate major anomalies. Locations of 

anomalous results from different surface-geophysical 

methods could not be correlated at this site. The 

closest anomalies using VLF and EM were 35 ft apart 

on line 1 at 545 and 580 ft along the line. Thick, 

saturated overburden may obscure the VLF and EM 

survey results at this site.

Conductive strikes from square-array resistivity 

results that have the same orientation as fractures 

identified in outcrop, or remotely sensed lineaments, 

likely are related to fracture zones. The strike of the 

20-m A-spacing secondary anomaly from array 1 

(300°) has the same orientation as a fracture peak in 

the mapped geologic data at this site striking 294°±7° 

(67 percent, normalized height). The LOWALT 

lineament at the site with an orientation of 44° is just 

outside the error range for the maximum fracture peak 

at 37°, with a ±6° error range.

Borehole surveys from well SAW 272 show that 

the strikes and dips (average, 206° and 60°) of a group 

of fractures are generally coincident with a group of 

foliation and contacts (average 214° and 62°). The 

largest transmissive fracture zone characterized in the 

borehole data, (at a depth of 264.5 ft), dips towards, 

and if projected, would intersect well SAW 207 and 

may contribute to its yield. This fracture, which is 

coincident with a contact, is within 7.5° of the orientation of the primary strike from the square-array 

resistivity survey.

Although the locations of anomalies detected by 

electromagnetic (EM and VLF) surveys do not 

correlate spacially at site 6, they do indicate fractured 

bedrock. The lack of agreement between techniques 

may be caused by thick conductive overburden 

obscuring bedrock signatures. Because the 

overburden covering bedrock is thick, the geologic 

data used at this site may represent more regional 

rather than site specific conditions. DC-resistivity 

surveys and arrays and borehole data more clearly 

indicate fractured bedrock than other geophysical 

surveys at site 6 in Salem.
ANALYSIS AND RESULTS OF GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS 45

Figure 44. Cross sections showing (A and B) inverted resistivity sections of two-dimensional, direct-current resistivity data at 

site 6 from line 1, Salem, N.H.; (C) model based on field data from A and B; and (D and E) synthetic resistivity output data from 

Model C. Site and line locations are shown on figures 1 and 40, respectively.
46 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

Figure 45. Cross sections showing (A and B) inverted resistivity 

sections of two-dimensional, direct-current resistivity data at site 6 from 

line 2, Salem, N.H.; (C) model based on field data from A and B; and (D 

and E) synthetic resistivity output data from Model C. Site and line 

locations are shown on figures 1 and 40, respectively.

Figure 46. Cross sections showing (A and B) inverted resistivity sections 

of two-dimensional, direct-current resistivity data at site 6 from line 3, 

Salem, N.H.; (C) model based on field data from A and B; and (D and E) 

synthetic resistivity output data from Model C. Site and line locations are 

shown on figures 1 and 40, respectively.
SUMMARY AND CONCLUSIONS 47

SUMMARY AND CONCLUSIONS

Bedrock aquifer ground-water resources in 

New Hampshire have been assessed statewide by the 

U.S. Geological Survey, in cooperation with the New 

Hampshire Department of Environmental Services, to 

identify areas that are favorable for more intensive 

investigation. This study identified site-specific 

anomalies in geophysical-survey results from sites 1, 

4, and 5 in the Pinardville 7.5-minute quadrangle, and 

sites 2, 3, and 6 in the Windham 7.5-minute 

quadrangle that indicate the location of bedrockfracture zones that are potentially water bearing.

At four of the sites, geophysical anomalies were 

closely correlated with geologic-fracture data and 

lineament locations and orientations. High-yielding 

bedrock wells at all of the sites indicate highly 

transmissive fracture zones in those areas. Surfacegeophysical methods used in this study were able to 

identify the locations of fracture zones at these sites. 

Seismic-refraction and ground-penetrating radar 

were used primarily to characterize the overburden, 

and provided limited bedrock characteristics. At some 

site locations, velocities of seismic waves through 

bedrock indicated a dominant fast trend near parallel 

to a specific fracture orientation. Where seismic wave 

velocities were slow, measurements often were nearly 

perpendicular to an interpreted fracture zone. Where 

overburden was thin or absent, GPR results located 

near-horizontal bedrock fractures zones that geologic 

mapping and lineament analysis could not identify. 

Conductive overburden sediments, particularly till, 

generally obscured GPR penetration to bedrock.

VLF and EM surveys provide a rapid means to 

locate conductive features such as water-filled 

fractures. VLF surveys identified several likely 

fracture zones, however, these surveys were susceptible to cultural interference and were often difficult to 

interpret. In addition to providing qualitative information about the thickness and conductivity of the 

overlying formations, EM surveys identified several 

fracture zones, and in some cases, their dip direction. 

The EM surveys can be done relatively quickly and 

are easy to interpret. Whereas other techniques, for 

example a lineament analysis, may indicate the surface 

expression of a fracture zone, geophysical methods 

sometimes help identify the dip direction of that zone. 

Dip direction is the next most valuable piece of 

information required to target a well through a fracture 

zone.

Figure 47. Polar plot showing azimuthal square-array direct-current 

resistivity at site 6 for array 1, Salem, N.H. Apparent resistivity in ohm 

meters (Ω m), is plotted as a function of azimuth, in degrees east of true 

north, and resistivity center is at 50 Ω m. Site and array locations are 

shown on figures 1 and 40, respectively.

Figure 48. Lower hemisphere equal-area nets from bedrock well 

SAW 272 at site 6, Salem, N.H., showing (A) borehole fractures and 

(B) borehole contacts and foliation. Site and well locations are shown 

on figures 1 and 40, respectively.
48 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

The collection of various layers of data and 

inversion processing of 2-D resistivity surveys yielded 

results that indicate overburden types and saturation, 

depths to bedrock, and most importantly, depths and 

dips of fracture zones. Modeling was used to back up 

interpretations of resistivity data and incorporate 

known information from well data and surface 

observations of overburden materials and bedrock 

outcrop into the analysis. Incorporating multiple 

pieces of information increased the confidence in 

2-D resistivity interpretations. Of the seven surface 

geophysical methods investigated, analysis of 2-D 

resistivity surveys provided the most quantitative 

information on fracture-zone location and dip 

direction.

The orientation of conductive-geophysical 

anomalies identified with square-array resistivity 

showed varying agreement between geologic fracture 

and lineament data. At some sites, available indicators 

(outcrop fracture measurements and lineaments) to 

strikes of features were confirmed, whereas at other 

sites, they were not. At arrays where conductivefracture zones were not interpreted, other features 

could cause the azimuthal-square-array resistivity 

anomalies if the horizontal layer assumption (bedrock 

surface and overburden) of the model was violated.

Borehole-geophysical data identified transmissive-fracture zones at the three sites surveyed. 

Borehole-survey data reinforce interpretations drawn 

from surface geophysical, geological outcrop, and 

remote-sensing surveys. Two sites had agreement 

between orientations of anomalies from surface 

geophysics, borehole-geophysical-survey feature 

orientations, and geologic data. At site 6, with no 

outcrops nearby, a large transmissive-fracture zone 

located with borehole geophysics was projected 

towards another high-yielding well that was not 

accessible for logging. 

The various geophysical surveys described in 

this report illustrate how geophysical methods can be 

integrated to help define the hydrogeology at different 

sites in crystalline rock. These survey results were 

analyzed in conjunction with other data, such as 

geologic outcrop, well logs, and remotely sensed data 

to interpret the location of subsurface fracture zones at 

high-yield well sites.

SELECTED REFERENCES

Ayotte, J.D., and Dorgan, T.H., 1995, Geohydrology of the 

Flints Pond aquifer, Hollis, New Hampshire: U.S. 

Geological Survey Open-File Report 95-363, 22 p.

Ayotte, J.D., Mack, T.M., and Johnston, C.M., 1999, 

Geophysical surveys of Country Pond and adjacent 

wetland, and implications for contaminant-plume 

monitoring, Kingston, New Hampshire: U.S. Geological Survey Open-File Report 99-51, 16 p.

Beres, Milan, Jr., and Haeni, F.P., 1991, Application of 

ground-penetrating-radar methods in hydrogeologic 

studies: Ground Water, v. 29, no. 3, p. 375-386. 

Bisdorf, R.J., 1985, Electrical techniques for engineering 

applications: Bulletin of the Association of 

Engineering Geologists, v. XXII, no. 4, p. 421-433.

Chapman, M.J., and Lane, J.W., 1996, Use of directional 

borehole radar and azimuthal square-array D.C. 

resistivity methods to characterize a crystalline 

bedrock aquifer, in Symposium on the application of 

Geophysics to Environmental and Engineering 

Problems, Keystone, Colo., April 28 through May 2, 

1996, Proceedings: Keystone, Colo., p. 833-842.

Clark, S.F., Jr., Ferguson, E.W., Moore, R.B., 1997, 

Lineament map of area 2 of the New Hampshire 

bedrock aquifer assessment, southeastern New 

Hampshire: U.S. Geological Survey Open-File Report 

96-490, 1 sheet, scale 1:48,000.

Clark, S.F., Jr., Moore, R.B., Ferguson, E.W., and Picard, 

M.Z., 1996, Criteria and methods for fracture-trace 

analysis of the New Hampshire bedrock aquifer: 

U.S. Geological Survey Open-File Report 96-479, 

12 p. 

Dahlin, Torleif, 1996, 2-D Resistivity surveying for 

environmental and engineering applications: First 

Break, v. 14, no. 7, July 1996, p. 275-283.

Drew, L.J., Karlinger, M.R., Armstrong, T.R., and Moore, 

R.B., 1999, Relations between igneous and metamorphic rock fracture patterns and ground-water yield 

from the variography of water-well yields, Pinardville 

Quadrangle, New Hampshire: Natural Resource 

Research, v. 8, no. 2, 1999.

Ferguson, E.W., Clark, S.F., Jr., and Moore, R.B., 1997, 

Lineament map of area 1 of the New Hampshire 

bedrock aquifer assessment, southeastern New 

Hampshire: U.S. Geological Survey Open-File Report 

96-489, 1 sheet, scale 1:48,000.

Frohlich, R.K., 1989, Magnetic signatures of zones of 

fractures in igneous metamorphic rocks with an 

example from southeastern New England: Tectonophysics, no. 163, p. 1-12.

Habberjam, G.M., and Watkins, G.E., 1967, The use of a 

square configuration in resistivity prospecting, in 29th 

European Association of Exploration Geophysicists 
SELECTED REFERENCES 49

Meeting, Stockholm, June 1967, Proceedings: 

Stockholm, European Association of Exploration 

Geophysicists, 23 p.

Haeni, F.P., 1988, Application of seismic-refraction 

techniques to hydrologic studies: U.S. Geological 

Survey Techniques of Water-Resources Investigations, 

book 2, chap. D2, 86 p.

———1995, Application of surface-geophysical methods 

to investigations of sand and gravel aquifers in the 

glaciated northeastern United States: U.S. Geological 

Survey Professional Paper 1415-A, 70 p. 

Haeni, F.P., Lane, J.W., Jr., and Lieblich, D.A., 1993, Use of 

surface-geophysical and borehole-radar methods to 

detect fractures in crystalline rocks, Mirror Lake Area, 

Grafton County, New Hampshire, in Hydrogeology of 

Hard Rocks, International Association of Hydrologists, 

XXIVth Congress, Oslo, Norway, June 1993, Proceedings: Oslo, Norway, International Association of 

Hydrologists, 11 p. 

Hansen, B.P., and Lane, J.W., 1995, Use of surface and 

borehole geophysical surveys to determine fracture 

orientation and other site characteristics in crystalline 

bedrock terrain, Millville and Uxbridge, Massachusetts: U.S. Geological Survey Water-Resources 

Investigations Report 95-4121, no. 25, 11 p. 

Hansen, B.P., Stone, J.R., and Lane, J.W., Jr., 1999, Characteristics of fractures in crystalline bedrock determined 

by surface and borehole geophysical surveys, Eastern 

Surplus Superfund Site, Meddybemps, Maine: 

U.S. Geological Survey Water-Resources Investigations Report 99-4050, 55 p. 

Iris Instruments, 1993, T-VLF Operating manual (Release 

1.0): Orleans, France, 32 p.

Johnson, C.D., Dunstan, A.H., Mack, T.J., and Lane, J.W., 

Jr., 1999, Borehole-geophysical characterization of a 

fractured-bedrock aquifer, Rye, New Hampshire: U.S. 

Geological Survey Open-File Report 98-558, 61 p. 

Keyes, W.S., 1988, Borehole geophysics applied to groundwater investigations: U.S. Geological Survey OpenFile Report 87-539, 305 p. 

Koteff, Carl, 1970, Surficial geologic map of the Milford 

Quadrangle, Hillsborough County, New Hampshire: 

U.S. Geological Survey, 1 sheet, scale 1:24,000.

Lane, J.W., Jr., Haeni, F.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. 

Larson, G.T., 1984, Surficial geologic map of the Windham 

Quadrangle, Rockingham and Hillsborough Counties: 

New Hampshire Department of Resources and 

Economic Development, 1 sheet, 1:24,000 scale.

Loke, M.H., 1999, Electrical imaging surveys for environmental and engineering studies, A practical guide to 

2-D and 3-D Surveys: Penang, Malaysia, 

http://www.agiusa.com/literature.shtml, 57 p.

1999, RES2DMOD vers. 2.2 Rapid 2-D resistivity forward 

modelling using the finite-difference and finiteelement methods: Penang, Malaysia, 

http://www.agiusa.com/files/pub/res2dmodmanual.pdf, 

22 p.

Lyons, J.B., Bothner, W.A., Moench, R.H., and 

Thompson, J.B., Jr., 1997, Bedrock geologic map of 

New Hampshire: U.S. Geological Survey State 

Geologic Map, 2 sheets, scale 1: 250,000 and 

1:500,000. 

Mack, T.J., Johnson, C.D., and Lane, J.W., Jr., 1998, 

Geophysical characterization of a high-yield fracturedbedrock well, Seabrook, New Hampshire: U.S. 

Geological Survey Open-File Report 98-176, 22 p.

McNeill, J.D., 1980, EM-34 Survey interpretation 

techniques: Geonics limited technical note TN-8, 

rev. January 1983, 16 p.

Medalie, Laura, and Moore, R.B., 1995, Ground-water 

resources in New Hampshire: Stratified-drift aquifers: 

U.S. Geological Survey Water-Resources Investigations Report 95-4100, 31 p.

Moore, R.B., and Clark, S.F., Jr., 1996, Assessment of 

ground-water supply potential of bedrock in New 

Hampshire: U.S. Geological Survey Fact Sheet 

FS 95-002, 2 p.

Powers, C.J., Singha, Kamini, and Haeni, F.P., 1999, 

Integration of surface geophysical methods for fracture 

detection in bedrock at Mirror Lake, New Hampshire, 

in Toxic Substances Hydrology Program Meeting, 

March 8-12, 1999, Proceedings: Charleston, S.C., 

U.S. Geological Survey, p. 757-768.

Powers, C.J., Wilson. Joanna, Haeni, F.P., and Johnson, 

C.D., 1999, Surface-geophysical characterization of 

the University of Connecticut Landfill, Storrs, 

Connecticut: U.S. Geological Survey WaterResources Investigations Report 99-4211, 34 p.

Salvini, Francesco, 2000, The Structural Data Integrated 

System Analyzer software (DAISY 2.19): Rome, 

Italy, Universita degli Studi “Roma Tre,” Dipartimento 

di Scienze Geologiche.

Salvini, Francesco, Billi, Andrea, and Wise, D.U., 1999, 

Strike-slip fault propagation cleavage in carbonate 

rocks: the Mattinata Fault Zone, Southern Apennines, 

Italy: Journal of Structural Geology, v. 21, p. 1731-

1749.

Scott, H.J., 1971, Seismic refraction modeling by computer, 

in Annual International Society of Engineering 

Geology Meeting, 41st, Houston, Tex., November 9, 

1971, Proceedings: Houston, Tex., Society of 

Engineering Geology, p. 271-284.

Spratt, J.G., 1996, Application of surface geophysical 

methods to delineate fracture zones associated with 
50 Geophysical Investigations of Well Fields to Characterize Fractured-Bedrock Aquifers in Southern New Hampshire

photolinear features in West-Central Florida in

Bell, R.S., and Cramer, M.H., eds., Symposium on the 

application of geophysics to engineering and environmental problems, Keystone, Colo., April 28 - May 2, 

1996: Keystone, Colo., Environmental and 

Engineering Geophysical Society, p. 907-916. 

Stekl, P.J., and Flanagan, S.M., 1992, Geohydrology and 

water quality of stratified-drift aquifers in the Lower 

Merrimack and Coastal River Basins, southeastern 

New Hampshire: U.S. Geological Survey WaterResources Investigations Report 91-4025, 137 p.

Taylor, K.C., Minor, T.B., Chesley, M.M., Matanawi, 

Korblaah, 1999, Cost effectiveness of well site 

selection methods in a fractured aquifer: Groundwater, 

v. 37, no. 2, March-April 1999, p. 271-274.

Walsh, G.J., and Clark, S.F., Jr., 1999, Bedrock geologic 

map of the Windham Quadrangle, Rockingham and 

Hillsborough Counties, New Hampshire: 

U.S. Geological Survey Open-File Report 99-8, 

1 sheet, 1:24,000 scale.

2000, Contrasting methods of fracture trend characterization in crystalline metamorphic and igneous rocks of 

the Windham Quadrangle, New Hampshire: 

Northeastern Geology and Environmental Sciences, 

v. 22, no. 2, 2000, p. 109-120.

Wright, J.L., 1994, VLF Interpretation Manual: Terraplus 

U.S.A., Inc., 83 p.

Zohdy, A.A.R., Eaton, G.P., and Mabey, D.R., 1974, 

Application of surface geophysics to ground-water 

investigations: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 2, chap. D1, 

86 p.
APPENDIX 1. Graphs showing borehole geophysical logs of 

three sites in New Hampshire
52 Geophysical Investigations of Well Fields to Characterize Fractured Bedrock Aquifers in Southern New HampshireFigure 1A. Borehole geophysical logs of well GNW 406 at site 4, Goffstown, N.H. Site and line locations are shown on figures 1 and 26, respectively.
APPENDIX 53 Figure AB. Borehole geophysical logs of well GNW 408 at site 5, Goffstown, N.H. Site and line locations are shown on figures 1 and 33, respectively.
54 Geophysical Investigations of Well Fields to Characterize Fractured Bedrock Aquifers in Southern New HampshireFigure 1C. Borehole geophysical logs of well SAW 272 at site 6, Salem, N.H. Site and line locations are shown on figures 1 and 40, respectively.
GEOPHYSICAL INVESTIGATIONS OF WELL FIELDS TO CHARACTERIZE FRACTURED-BEDROCK AQUIFERS IN SOUTHERN NEW HAMPSHIRE—

Degnan, J.R., Moore, R.B., and Mack, T.J. USGS Water-Resources Investigations Report 01-4183