4.2.5.3

Surface to Borehole, Cross Borehole Geophysical Methods

Surface to borehole and cross borehole geophysical methods combine the use of

electrodes or geophones in boreholes with surface electrodes or sources to affect surface to

borehole and cross borehole measurements (Dobecki and Romig, 1985). Application of both

surface and borehole geophysical techniques increases the resolution of targets because

borehole probes can be positioned close to the target of interest (Van Nostrand and Cook,

1966). Geophysical techniques applied within and between boreholes include vertical seismic

profiling, geotomography (utilizing both seismic and EM waves) and DC resistivity. Cross

borehole EM techniques have been used by Lytle et al. (1979, 1981) to locate high contrast

electrical anomalies (e.g., tunnels) and to monitor the direction and flow rate of injected

fluids. Butler and Curro (1981) have described cross borehole procedures for obtaining

accurate seismic velocity profiles.

Cross borehole and surface to borehole methods provide a greater lateral radius of

investigation than can be achieved through single borehole logging, thereby providing

measurement over a larger formation volume. The region surveyed is a path between the

energy source and the detector, but it is not necessarily the straight line path between the two

points. The probability that the sampled region is along a straight line path between the

source and detector increases as the distance between the source and detector decreases. The

surface to borehole and cross borehole techniques are limited by many of the factors affecting

most geophysical surveys such as non uniqueness of results, and therefore, require other

integrated data for verification of results.

4.3

Characterizing Ground Water Flow Beneath the Site

In addition to characterizing site geology, the owner/operator should characterize the

hydrology of the uppermost aquifer and its confining layer(s) at the site. The owner or

operator should install wells and/or piezometers to assist in characterizing site hydrology.

The owner/operator should determine and assess:

The direction(s) and rate(s) of ground water flow (including both horizontal

and vertical components of flow);

Seasonal/temporal, natural, and artificially induced (e.g., off site production

well pumping, agricultural use) short term and long term variations in ground 

water elevations and flow patterns; and

The hydraulic conductivities of the stratigraphic units at the site, including

vertical hydraulic conductivity of the confining layer(s).

Section 4.3.1 provides a brief introduction to ground water flow in porous media and

conduits; Section 4.3.2 provides a discussion of the Agency's definition of "uppermost

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