Poor Interpretation   Possible causes include an inadequate interpretation

method, insufficient background information, or insufficient or noisy data;

Insufficient Data   Possible causes include a lack of understanding of methods

and/or site conditions and objectives, operator inexperience, or lack of

up to date plotted data in the field (some contractors gather data but do not plot

it or look at it until they are back in the office).

4.2.5.1

Geophysical Surveys   Surface Techniques

Surface geophysical methods, as previously noted, are useful in mapping subsurface

conditions over a broad area of interest. The measurements are particularly useful when they

are integrated into an overall site investigation program where they can be interpreted along

with other available information for the site. The techniques are useful both as a means of

rapid site reconnaissance that can provide information for planning subsequent field activities,

and also in extrapolation of existing data to previously uninvestigated areas    provided that

sufficient site specific correlations have been established between the physical feature being

extrapolated and the geophysical survey. Survey data should be collected, processed, and

interpreted by a qualified geophysicist, geologist, or ground water scientist familiar with the

theory, application, interpretation, and limitations of the applied geophysical techniques.

Direct Current (DC) Electrical Resistivity Methods

The direct current (DC) resistivity method is used to measure the bulk resistivity of

soil or rock volumes occurring between the measuring electrodes. This technique utilizes

electric currents that are introduced into the ground through electrodes or long line contacts.

The apparent resistivity of the subsurface volume is determined by measuring the potentials at

other electrodes in the vicinity of the current flow (Telford et al., 1976). The objective,

through the use of inverse modeling and curve matching, is to obtain the true resistivities and

layer thicknesses of the subsurface geologic strata from the apparent resistivities measured at

the ground surface. In ground water studies, DC resistivity techniques can be used to model

the geoelectric response of the bulk formation and to estimate ground water quality (Zhody,

1974; Stollar and Roux, 1975; Van Dam, 1976; Rogers and Kean, 1980; Urish, 1983).

The electrical resistivity technique is used for lateral profiling or vertical electric

sounding. Through application of both techniques, a vertical geoelectric cross section of the

subsurface along the survey transect can be obtained. Lateral profiling techniques enable the

scientist to map lateral changes in subsurface electrical properties along a transect line to a

finite investigation depth related to the spacing of the measurement electrodes and the applied

current. Vertical electrical sounding measures vertical changes in subsurface resistivity as the

measuring electrode is moved various finite distances away from a stationary electrode at the

center of the measurement array. Qualified interpretation of sounding data can provide an

estimate of the depth and thickness of subsurface layers having contrasting apparent

November 1992

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