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).
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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