Downhole geophysical surveys provide localized details on soil, rock, or fluid along the length of an existing monitoring well or a borehole. The surveys can also identify permeable zones, such as sand lenses in glacial tills, weathered zones, and fractures or solution cavities in rocks.
Several downhole logging techniques are available including nuclear, electromagnetic, and acoustic or seismic as described in Keys and MacCary (1976) and the U.S. EPA desk reference guide on subsurface characterization and monitoring techniques (1993a). Some of these techniques provide measurements from inside plastic or steel casing, and some allow measurements in the unsaturated zone as well as the saturated zone. A description of the most commonly used logs follows.
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