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

U.S. Geological Survey Water-Resources Investigations Report 01-4183

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


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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-low-frequency 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. Two-dimensional (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. Water-well 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.





Purpose and Scope

Previous Investigations

Site Selection

Geohydrologic Settings


Approach and Methods

P-Wave Seismic Refraction

Ground-Penetrating Radar


Very-Low-Frequency Electromagnetics

Inductive Electromagnetic Terrain Conductivity

Two-Dimensional Direct-Current Resistivity

Azimuthal-Square Array Direct-Current Resistivity

Borehole Geophysical Surveys

Analysis and Results of Geophysical Investigations of Well Fields 9

Site 1, Bedford, New Hampshire

Geophysical Surveys and Interpretation

Integration of Results

Site 2, Windham, New Hampshire

Geophysical Surveys and Interpretation

Integration of Results

Site 3, Pelham, New Hampshire

Geophysical Surveys and Interpretation

Integration of Results

Site 4, Goffstown, New Hampshire

Geophysical Surveys and Interpretation

Integration of Results

Site 5, Goffstown, New Hampshire

Geophysical Surveys and Interpretation

Integration of Results

Site 6, Salem, New Hampshire

Geophysical Surveys and Interpretation

Integration of Results

Summary and Conclusions

Selected References

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


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U.S. Geological Survey

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