Interferences due to overhead power lines, known subsurface utilities, and metal
objects such as fences, above ground oil tanks, and cars are noted when conducting an EM
survey. Readings obtained in the vicinity of such instrument interferences are either
discarded or regarded as suspect during the interpretation of the data. In areas of large power
lines, instrument overloading can occur. To ensure that measurements are consistent and that
instrumental overloading is not present, readings are typically obtained at two different
sensitivity scales in areas of such interferences. The survey also may be operated
perpendicular rather than parallel to linear sources.
Gravity Methods
Gravity measurements are useful for estimating depth to bedrock, for locating voids
and fault zones, for estimating the ground water volume in alluvial basins (Hinze, 1988). The
observed density contrasts between rock, air, water, and soil make gravity measurements a
useful mapping tool. A low value of gravity indicates an anomalously low density subsurface
mass, which might be due to a subsurface void, a cavity in rock filled with lighter density
material, a thickening of the soil layer overlying bedrock, a decrease in soil density, or a
variation of ground water volume (Hinze, 1988). Gravity measurements alone are not
sufficient to uniquely determine the cause of a gravity anomaly; however, an experienced
interpreter can often define the source of the anomaly when gravity methods are used in
conjunction with knowledge of the local geologic setting and a soil/rock boring program.
Borehole Geophysical Techniques
Borehole geophysical logging is used to obtain continuous vertical profiles of
subsurface conditions at resolutions that cannot be obtained economically from the physical
drilling, sampling, and testing of subsurface formations. Borehole geophysical methods
measure the responses of subsurface rock, soils, and fluids to various logging tools and utilize
the measurements to ascertain physical characteristics of the subsurface formations and their
contained fluids. Available logging tools include electrical, visual, thermal, acoustic (sonic),
magnetic, nuclear (radioactive), fiber optic, and mechanical sensors. Some tools that are
available to measure the physical properties of the borehole include borehole calipers,
borehole deviation surveying tools, temperature measurement tools, and downhole video
surveying cameras. Borehole geophysical measurements can be obtained in open boreholes or
cased wells, however all tools are not functional in both environments. Generally only
nuclear and sonic tools are applied to cased hole logging. In either instance, the application
of a specific tool to a borehole or cased well may require that the borehole or well be fluid 
filled and that the composition or clarity of the fluid be constrained within the tool's limits
for optimum performance.
Borehole geophysical logs can be utilized to correlate formation properties between
boreholes and to refine surface geophysical interpretations. Geophysical logs obtained with
equipment that is properly calibrated and standardized can provide objective and consistent
November 1992
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