data that can be used in: the interpretation of stratigraphy, thickness, and extent of aquifers

and confining units; relative permeability, porosity, bulk density, resistivity, moisture content,

and specific yield of aquifers and confining layers; borehole deviation; casing integrity;

subsurface temperature; formation resistivity factors; and the source, movement, and

chemical/physical nature of ground water (Keys, 1988). Sources of information on the use

and interpretation of geophysical logs include Keys (1988); Keys and MacCary (1971); Labo

(1986); Telford et al. (1976); Ellis (1987); Schlumberger (1989); and Taylor et al. (1990).

In many instances, different tools (such as radioactive or sonic tools) are used to

determine the same formation property (such as porosity) by measuring the response of the

formation to the specific tool. The electrode, coil, transmitter/receiver, or source/detector

spacings of the method used reflect the properties of the formation by integrating the data

gathered over a fixed distance as a function of the source/detector spacings. These spacings

vary the depth of investigation of a particular tool into an unaltered formation. Factors such

as these emphasize the importance of having knowledgeable and experienced operators obtain

borehole geophysical logs and having qualified log analysts interpret the data.

Downhole measurements are recorded in the field using portable field equipment or

(more routinely) a service company logging truck. The service company logging truck

generally provides all downhole measurement tools, electrical cables, a winch, and extensive

truck mounted surface instrumentation for controlling tool operations and acquiring response

data. Office processing of the log data may include making corrections for borehole

conditions, mud cake, and tool standoff, and calculating formation mechanical properties,

permeability, or mineralogy. Variations in the physical environment where the geophysical

sondes operate make it necessary to correct the measured values for the borehole effects.

Corrections commonly applied to the measured data include compensation for borehole

diameter, sonde eccentricity, drilling fluid invasion, bed thickness, and mudcake formation.

By the nature of the tool design, many modern logging tools are dual detector, compensating

devices that provide a large degree of correction for the borehole environment.

Electrical Methods

Electrical logging methods that are applicable to the borehole environment include

resistivity/conductivity and spontaneous potential measurements. Borehole resistivity and

conductivity methods are analogous to surface resistivity/conductivity techniques in that the

measurements are obtained using fixed spaced electrodes or coils, and electrical currents are

passed through the formation across the fixed spaced electrodes or transmitter/receiver arrays.

The voltage is measured between the electrodes and is proportional to the formation

resistivity. Because of the need for electrical coupling among the tool electrodes, borehole

fluid, and formation, electrical resistivity curves are obtained from uncased boreholes. The

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