LINEAMENT and SURFACE GEOPHYSICAL EVIDENCE of BEDROCK-FRACTURE ZONES in SOUTHERN NEW HAMPSHIRE

National Ground Water Association
March 13-15, 2002, Denver, Colo.

James R. Degnan, U.S. Geological Survey
Stewart F. Clark, Jr., U.S. Geological Survey
Richard Bridge Moore, U.S. Geological Survey

Abstract

Remotely sensed lineaments were classified as fracture-correlated by comparison to field-measured fracture orientations in southern New Hampshire. The purpose of this research was to identify lineaments that may be related to potential water-bearing fracture zones. The study areas included the Great Bay area of southeastern New Hampshire and the Pinardville and Windham 7.5-minute quadrangles in south-central New Hampshire. Geophysical surveys were used to identify anomalies indicating fracture zones at sites with fracture-correlated lineaments in each of these three study areas.

Remotely sensed lineaments were evaluated (correlated to fractures) by two primary techniques -- buffer and domain. Bedrock outcrops within a 305 meter “buffer” around lineaments, or inside a square “domain” cell (3300 meters on a side), were examined to compare fracture and lineament orientations. The orientations of fractures in outcrops within buffers were compared to the orientations of the lineaments to identify fracture-correlated lineaments. Two types of domain analyses, discrete-measurement and spacing-normalized analyses, were applied to fractures in a domain cell to statistically determine peak-fracture orientations. Within each cell, discrete-measurement analysis uses each orientation once per fracture or fracture set per outcrop. Regularly and closely spaced fractures were normalized by the spacing-normalized technique in which fracture spacing was used to give weight to fracture orientations. The orientation of the fracture families, defined by principal peaks, was used to identify fracture-correlated lineaments in all three study areas; however, the spacing-normalized technique was unique to the Great Bay study.

Near Great Bay, analysis of 5 types of remote-sensing imagery identified 927 lineaments. Four-hundred six, or 44 percent of the 927 lineaments, were found near bedrock outcrops (within a buffer zone or domain cell containing outcrops). Of the 406 lineaments, 190 (47 percent) were identified as fracture correlated, but only 15 percent were fracture correlated by more than one of three correlation techniques. Similar results were found in the Pinardville and Windham study areas.

Surface-geophysical surveys indicated fracture zones related to fracture-correlated lineaments and high-yielding bedrock wells at all three study areas. Direct current (DC) resistivity surveys showed electrically conductive anomalies in the bedrock near fracture-correlated lineaments. Two types of DC-resistivity surveys were used to provide subsurface information that suggests fracture depth and orientation. Two-dimensional DC-resistivity surveys were used to indicate the location and dip of bedrock-fracture zones. Azimuthal-square-array DC-resistivity surveys were used to indicate the orientations of conductive, steeply dipping bedrock fractures.

Introduction

The U.S. Geological Survey (USGS), in cooperation with the Civil Engineering Department at the University of New Hampshire, and the Cooperative Institute for Coastal and Estuarine Environmental Technology, is assessing ground-water flow through fractured bedrock to Great Bay. The Windham and Pinardville, N.H. quadrangles are located in south-central N.H. and were studied by the USGS, in cooperation with the New Hampshire Department of Environmental Services (NHDES), as part of a statewide assessment of ground-water resources in the bedrock aquifers of the State (Moore and others, in press). This article summarizes the techniques of lineament identification, outcrop-fracture mapping, and geophysical investigations of the bedrock near Great Bay, and the Windham and Pinardville, N.H. quadrangles (fig. 1). Lineament, fracture and geophysical data are related to indicate potential discrete zones of fracturing in the bedrock (referred to as fracture zones in this manuscript), which may serve as ground-water-flow paths. Certain types of fracture-correlated lineaments have been correlated with high bedrock well yields in fractured-bedrock aquifers (Moore and others, in press).

Fracture-Correlated Lineaments

Bedrock-fracture measurements and remotely sensed lineaments were analyzed to identify fracture-correlated lineaments. Three types of analysis that allow for fracture correlation by orientation were used to analyze lineament and fracture data. The buffer analysis correlates fractures observed in bedrock outcrops with individual lineaments (Walsh, 2000). Domain analysis, including discrete-measurement and spacing-normalized analyses (presented here) identifies statistical fracture trends in a given region or domain (Burton and others, 2000) and correlates lineaments if they match the statistical trends (Moore and others, in press). Fractures with dips of 45 ° or greater were selected for analyses because straight-line lineaments crossing considerable relief are assumed to be formed by vertical or steeply dipping features.

Bedrock Outcrop Fracture Measurements

Fracture data were collected using representative mapping techniques that have been demonstrated by Walsh and Clark (2000). These techniques compare well with detailed station fracture mapping. Isolated fractures and fracture sets were selected by inspecting each outcrop in a process that is referred to as "subjective" by Spencer and Kozak (1976). Orientation measurements were made when looking down-strike, with the down-dip direction to the right (right-hand-rule). Fracture spacing, the perpendicular distance between parallel and subparallel joints (Segall and Pollard, 1983), was measured for all fracture sets, up to the scale of the observed outcrop (less than 10 meters).

Lineament Identification

Remotely sensed lineaments seen on the surface of the Earth are potentially related to structural features and high water-yielding zones within the bedrock (Mabee and others, 1994; Moore and others, in press). Coincident and confirmed lineaments were identified throughout N.H. by independent observers from stereo high- and low-altitude black-and-white air photographs, side-looking airborne radar, and satellite-imagery platforms using criteria published by Clark and others (1996), Brown (1961), and Lattman (1958). Criteria include straight-line patterns on the land surface formed from gaps in ridges, streams, valleys, tonal variations in soil, and anomalous vegetation patterns. Additional lineaments, from independent observers, from 1:58,000-scale color-infrared (CIR) stereo photographs were added in the Great Bay study area (Degnan and Clark, in press) and in the Pinardville and Windham study areas including additional CIR and topographic lineaments (Moore and others, in press).

Buffer Analysis

The buffer analysis (Walsh, 2000) is a technique by which a lineament is considered to be fracture correlated when fractures in bedrock outcrops within a specified “buffer” zone, (in this case, 305 meters around each lineament) (fig. 2) have orientations similar to the lineament. Those lineaments whose buffers contain at least one steeply dipping fracture (45 ° or greater) and have a strike within ± 5° (after Mabee and others, 1994) of the fracture are classified as fracture correlated.

Domain Analysis

Study areas are divided by domain cells that are square regions (3,300 m on a side) drawn in such a way as to contain as even a distribution of outcrops as possible (fig. 3). Discrete-measurement analysis gives an equal weight to each fracture orientation at an outcrop regardless of fracture spacing, or number of fractures at an outcrop. The second technique, spacing-normalized analysis, multiplies a fracture-set observation by a spacing factor if an outcrop has regularly spaced parallel fractures, and was only used at the Great Bay study area where fracture sets are common. Both techniques of domain analysis are based on the statistical identification of fracture families. Fracture families (principal trends of fractures) were defined for each domain cell by plotting normalized azimuth-frequency (rose) diagrams with software (DAISY) by Salvini (2000) that uses a Gaussian curve-fitting routine. Lineaments in the domain are classified as fracture correlated by domain analysis if their orientation is within one standard deviation of a family peak (Moore and others, in press).

Closely spaced fractures may potentially provide a greater number of flow paths for water per unit cross-sectional area of a bedrock aquifer than more widely spaced fractures (Mabee and Hardcastle, 1997). Spacing-normalized analysis requires a unit cross-sectional length which is equal to the largest spacing observed among all fracture sets in the domain cell. The unit cross-sectional length divided by the fracture spacing determines fracture frequency for a given fracture set. To normalize the number of fractures for spacing, the number of orientations analyzed with DAISY for a given region was multiplied by the calculated fracture frequency.

Surface Geophysics

Direct-current resistivity surveys indicated bedrock fracture locations, orientation, and dip directions in the vicinity of fracture-correlated lineaments. Two-dimensional (2D) direct-current (DC) resistivity surveys including dipole-dipole and Schlumberger arrays (Zohdy and others, 1974) were used to characterize the electrical resistivity of overburden, bedrock, and bedrock-fracture zones through analysis of data inversions (Loke, 1999) at all of the sites (fig. 1). In the crystalline bedrock of New Hampshire, variations in electrical resistivity are likely related to water content and water quality of fracture zones (fig. 4).

Azimuthal square-array dc-resistivity surveys (collected only at sites 1, 2, and 4, fig. 1), termed square-array resistivity, measure the subsurface resistivity in respect to azimuth and allow for the determination of the strike of a conductive anomaly with depth (Habberjam and Watkins, 1967). The effective survey depth is approximately equal to the electrode-spacing (5 to 40 meters in these study areas). Resistivity data were collected and interpreted according to the techniques described by Lane and others (1995). Primary conductive strikes, indicating fracture strikes are orthogonal to the resistivity maximum in polar plots of the data.

Results and Conclusions

Analysis of 5 types of remote-sensing imagery identified 927 lineaments near Great Bay, N.H., of which 406, or 44 percent were found near bedrock outcrops. Thirty-seven percent of the lineaments (53 of 143 lineaments) within 305 meters of an outcrop were identified as fracture correlated by use of the buffer analysis. The percent of lineaments identified as fracture correlated in the domain cells were 30 (120 of 399), and 19 (77 of 399) by domain discrete-measurement and domain-spacing-normalized analysis respectively. One-hundred and ninety (47 percent) of the 406 lineaments near Great Bay were identified as fracture correlated, but only 15 percent were fracture correlated by more than one of three correlation techniques.

Similar results were found in the Pinardville and Windham study areas. In the two quadrangles in south-central New Hampshire, 46.6 percent of the lineaments (by lineament length) were fracture correlated. The domain-analysis results indicate that, by lineament length, 30.5 percent of the lineaments in the two quadrangles match up with fracture trends, and 14.6 percent correlate with the primary fracture direction. The buffer analysis results indicate that, by lineament length, 30.9 percent of the lineaments were identified as fracture-correlated by this technique. Only 14.8 percent were identified by both techniques in the two quadrangles.

Geophysical surveys at sites 1, 2, 3, 4, and 5 (fig. 1) indicate fractured rock near fracture-correlated lineaments, further refining our understanding of the hydrogeology. The sites also have high-yield bedrock wells (greater than 40 gallons per minute) within 150 meters of the fracture-correlated lineaments. Two-dimensional dc-resistivity indicates the location and dip of fracture zones. At sites 1, 2, and 4, (fig. 1) square-array resistivity data were collected and indicated conductive strikes coincident with the orientations of fracture-correlated lineaments. Fracture correlations were derived from one to all three of the techniques, suggesting one technique should not be solely relied upon.

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