resistivities. Information generated from multiple resistivity profiling and sounding arrays can

be used to produce two  and three dimensional geoelectric models.

Resistivity techniques are dependent on resistivity contrasts in subsurface materials and

predictably will not be useful at sites where measurable contrasts do not exist. The accuracy

of resistivity methods is limited by several factors including: heterogeneity in surface and

subsurface conditions, proximity of human made sources of electrical interference, departure

of the subsurface structure from a horizontally layered model, and the inherent lack of a

unique data interpretation (Mooney, 1980; Mooney and Wetzel, 1956; Urish, 1983; and

Telford et al., 1976). Field procedures for conducting electrical resistivity surveys are

relatively more tedious than other applicable techniques such as electromagnetics.

Seismic Methods

Seismic survey methods of subsurface exploration are based on the principle that

seismic waves, consisting of compressional and shear pulses, emanate from a seismic source

(e.g., hammer blow, large weight drop, explosion, pipe gun, vibratory source) and travel

through subsurface soil and rock at velocities that vary with the elastic properties of the

materials. Two surface geophysical methods commonly applied to hydrogeologic

investigations include seismic refraction and seismic reflection. Seismic surveys are typically

conducted along intersecting traverses to provide a grid of measurements over the survey area

and to allow for two dimensional contouring of velocity, depth, or thickness at each geophone

location. The geometry of the established grid is related to the goals of the survey, known

data, cultural features, and surface obstructions.

Seismic refraction is used to determine the thickness and depth of subsurface geologic

layers having contrasting seismic velocities. In the presence of sufficient subsurface contrasts,

refraction techniques can be used to map depths to specific horizons, including bedrock

surfaces, clay layers, and the water table.

Equipment necessary for conducting seismic refraction surveys includes a seismic

source, geophones, a seismograph, and a qualified operator. Equipment for conducting

seismic surveys has become sophisticated in recent years so that high quality data are

accessible for most applications. The following technological developments have greatly

improved the quality of collected refraction data: relatively low cost, multi channel

seismographs; increased geophone sensitivity and improvements in multi shot pattern

surveying; signal enhancement; and digital signal processing.

The seismic source typically consists of a sledgehammer blow or explosive detonation

at or slightly below the ground surface. The use of explosives is warranted in many

applications where extensive loose material is present at the surface or when a higher energy

source pulse is needed. The seismic source transmits elastic waves traveling at different

velocities into the subsurface where they are refracted at the interfaces between layers having

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