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
November 1992
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