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.

4.2.5.2

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

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