ABSTRACT

Activities in geothermal energy, radioactive and industrial waste disposal, mineral exploration, engineering geology, and lunar and deep crustal exploration have increased the need for borehole geophysics in igneous and metamorphic rocks. Because there is a lack of knowledge and experience in the application of borehole geophysics to these rocks, the U. S. Geological Survey is conducting research in this area. Igneous and metamorphic rocks usually present different petrophysical properties, such as porosity, density, resistivity, and mineralogy, than do sedimentary rocks. For this reason all conventional geophysical logging techniques may not be useful and petrophysics must be used to develop appropriate methods of log analysis. Core analyses plotted against log response and cross plotting of various logs in the computer are particularly useful for identifying the various rock types in an igneous complex. Hydrothermal alteration and fluid movement through fractures are very important parameters that are needed from logs. Hydrothermal alteration may be recognized on well logs through the use of crossplots and borehole gamma spectrometry. The acoustic televiewer supported by other logs has been highly successful for identifying fractures that are transmitting either hot or cold water.

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