that are dependent on subsurface contrasts in density, velocity, or salinity, for example, will

not adequately resolve details of the geology in formations where these physical contrasts are

minimal.

Limitations associated with borehole geophysical surveys are generally related to

individual tool response in different environments. As with surface geophysical techniques,

the proper tools should be selected for the individual application formation or borehole

conditions. Typical borehole geophysical surveying requires specialists trained in tool

operation and handling and data collection. Collected data will routinely require corrections

for borehole conditions prior to interpretation.

The potential for multiple interpretations of geophysical data results from the large

number of potential combinations of subsurface conditions that can occur to produce the

measured response. The limits in resolution and non uniqueness in interpretations of

geophysical methods should be recognized. Isolated surveys with no supporting information

should be carefully interpreted. Information from geophysical surveys should be utilized in

conjunction with other physical data to verify the initial interpretations of the geophysical

methods and provide constraints to remove some of the non uniqueness. In more complex

areas, surface to borehole and cross borehole geophysical methods may be considered to

delineate subsurface structure (Dobecki and Romig, 1985). However, application of multiple

geophysical methods at a site is not a guarantee that one survey will resolve the ambiguities

of another survey.

Equipment for performing many surface geophysical surveys is available from a

variety of sources, and include modern, computerized microprocessors and electronics.

Although the equipment can generally be operated by trained technicians, all aspects of data

collection, processing, and interpretation will require the oversight of a qualified geophysicist,

geologist, or ground water scientist having extensive experience with the equipment operation

and data interpretation. Borehole geophysical equipment is highly specialized and will

require a qualified contractor to obtain the logs.

Johnson and Johnson (1986) discuss some of the problems that are commonly

encountered when using geophysical techniques to investigate the shallow subsurface. These

problems include:

Incorrect Method Applied   Possible causes include lack of understanding of

geophysical technology, site conditions or survey objectives;

Poor Data Quality   Possible causes include high ambient noise, poor field

procedures, improper use of equipment, faulty equipment, adverse geologic

conditions, or inexperienced operators;

November 1992

4 23