of geological interpretations beyond the limits of physical data locations (drillholes, outcrops,
soil gas survey points, aerial photography, satellite imagery), provided that sufficient
confidence is established between the interpreted geologic and geophysical models. Applied
in this complementary manner, the geophysical and physical sampling data provide the means
to optimize and direct the site investigation. The U.S. EPA's "Geophysical Expert Advisor
System, Version 1.0" (1989) software is a tool for assisting in the selection of appropriate
site specific geophysical techniques.
Surface geophysical techniques include resistivity, electromagnetic induction, ground
penetrating radar, seismic refraction and reflection, and gravimetry. The precise physical
location and elevation (land survey) of the geophysical measurement points, transects, or grids
in site or other coordinate systems are integral to conducting successful geophysical surveys
in the field that can be readily interpreted with other site data. Information regarding surface
and borehole geophysical surveys and their applications to hydrogeologic investigations is
abundant in the literature. Several general references include Driscoll (1986); Schlumberger
(1989); Ellis (1987); Benson et al. (1982); Telford et al. (1976); and Zhody et al. (1974).
Borehole geophysical techniques are conventionally applied as a suite of tool
measurements that, when used in combination, allow the interpreter to determine physical
properties of the formation. Borehole surveying is advantageous in that it provides a means
for continuous measurement of in situ parameters and provides elements for the development
of a three dimensional site model when combined with other site data. A wide array of tools
are available that measure formation neutron and gamma ray attenuation, natural gamma ray
radiation, sonic wave propagation and formation imaging, formation resistivity and
conductivity, spontaneous potential, downhole/crosshole detection of seismic sources, and
borehole size and direction. Formation properties that can be interpreted from the measured
log data include: formation porosity, density, resistivity, conductivity, and spontaneous
potential; clay content estimation; water saturation and water quality estimation; permeability
estimation; formation dynamic elastic moduli; and fracture detection (Schlumberger, 1989).
General limitations in the application of surface geophysical techniques are related to
the resolution of the surveys and to the non unique interpretation of the measured data. The
capacity of a surface geophysical method to resolve (detect) small scale, isolated sources is
not typically a goal of a large scale geologic or hydrogeologic investigation. However,
location of buried containers, voids, trenches or other smaller scale objects is a primary goal
of investigations at many hazardous waste sites. Because surface geophysical techniques are
commonly conducted along transect lines that intersect to form a grid over the area of
interest, the resolution for a particular survey target can be enhanced by careful planning and
adjustment of the survey transects. More closely spaced transect lines will provide more data
points over the same area of interest. Attendant with the collection of more data, however, is
the increased level of effort required for data collection and processing. The ability of a
specific geophysical instrument to adequately measure details of the geology at a specific site
is also contingent on the selection of the proper technique for the application. Techniques
November 1992
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