acterization of groundwater. Several electromagnetic logging techniques are available including induction logs, microwave-sensing logs, nuclear magnetic resonance logs, and surface-borehole logs. Induction logs are probably the most common electromagnetic methods used in ground-water studies.
The probe in an induction log contains a transmitter coil on the upper part, which induces eddy current in the formation around the borehole, and a receiver on the lower part. Engineers measure conductivity using the same principles as the EM methods. Because the response of the log is a function of the specific conductance of the pore fluids, it is an excellent indicator of the presence of inorganic contaminants (Benson 1991). Variations in conductivity with depth also indicate changes in clay content, permeability of a formation, or fractures.
Several other types of logging techniques are useful for characterizing lithology and hydrogeology inside a well or a borehole. Examples of these logs include caliper logs, temperature logs, fluid-flow logs, and borehole television logs.
A caliper log provides information on the diameter, lithology, fractures, and construction details of an open borehole. Many types of caliper logs are available including mechanical, electric, and acoustic. The mechanical caliper is the most commonly used. The probe in a mechanical caliper consists of spring-loaded arms which extend from the logging tool so that they follow the sides of the borehole. Mechanical caliper tools have from one to six arms and can measure variations as small as ]/4 inch in borehole diameter.
A temperature log can provide a continuous record of the temperature of the fluid inside the borehole or well. Environmental engineers can use changes in temperature to identify leaks in the casing where damage or corrosion has occurred.
Fluid-flow logs measure the fluid flow within a borehole or well (Keys and MacCary 1976). Examples of such logs include thermal and electromagnetic borehole flowmeters that sense water movement either vertically or horizontally (or both) at low velocities. Fluid-flow measurements can locate zones of high permeability (fractures) and areas of leakage in artisan wells.
Borehole television camera logs allow visual inspection of a borehole or well for fractures or casing defects (Morahan and Dorrier 1984).
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