In cooperation with the NEW HAMPSHIRE DEPARTMENT OF ENVIRONMENTAL SERVICES
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.
Abstract
Introduction
Purpose and Scope
Previous Investigations
Site Selection
Geohydrologic Settings
Acknowledgments
Approach and Methods
P-Wave Seismic Refraction
Ground-Penetrating Radar
Magnetics
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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District Chief
U.S. Geological Survey
New Hampshire/Vermont District
361 Commerce Way
Pembroke, NH 03275-3718
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