Published: 01 January 1990
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Cite this document
Borehole-geophysical logs can be used to obtain information on water quality and water chemistry. Water quality characteristics that normally are measured directly in waterfilled holes or wells include chloride, dissolved oxygen, pH, temperature, and conductivity. In-situ ground water may exist at some distance adjacent to the borehole, and estimates of water quality or water chemistry can be made by measuring the resistivity or specific electrical conductivity of the water in pore spaces. Most geophysical logs, however, are made in test holes filled with drilling fluid. In this environment, logs enabling estimates of water resistivity (Rw) are useful. Relations among Rw and dissolved solids, sodium chloride solutions, and temperature are well established for saline waters; for freshwater, however, the activities of other dissolved ions also need to be considered. The spontaneous potential (SP) is a function of activity of the mud filtrate and water resistivity and, thus, can be used to estimate Rw.
Two methods of estimating Rw are useful: the spontaneous potential (SP) method, which uses data from a spontaneous potential log and a resistivity log, and the cross-plot method, which uses log-derived porosity and resistivity-log data. The application of SP logs depends upon the water quality contrast between the water in the pores (Rw) and the mud filtrate on the borehole wall. Both methods estimate Rw to about a half order of magnitude. However, the accuracy of both methods can be greatly improved if additional data, such as a chemical analysis, can be correlated to a log.
geophysical logging, spontaneous potential, resistivity, ground-water quality
Hydrologist, U.S. Geological Survey, Denver, CO