DOWNHOLE GEOPHYSICAL LOGGING

Abstract

Downhole geophysical logging is the method of making geophysical measurements in drill holes and interpreting these measurements for evaluation of geological properties of the subsurface. The downhole measurements can be made with great spatial resolution with a single sonde, containing both source and receivers, which are separated by 10 cm to tens of meters. Measurements can be repeated every few centimeters as the sonde moves upward or downward in the borehole. This technique is called well logging. It is also possible to make downhole measurements with the sensor in the borehole at depth and the source at the surface. When the sensor package is moved to a different depth, the source signal is repeated. When these measurements are made with seismic waves, the process is called vertical seismic profiling (VSP). In VSP the surface source can be any appropriate system that would generate a seismic pulse (dynamite, air gun) or a sinusoidal wave (vibroseis), and the downhole sensor can be a hydrophone or borehole geophone. Vertical seismic profiling is a rapidly expanding technique, and some of its applications are described in a series of recent books (Gal'perin 1974, Hardage 1983, Balch & Lee 1984, Toksoz & Stewart 1984). Another method of downhole measurement is possible when closely spaced drill holes are available, with the source placed in one drill hole and the detector (receiver) in another. This technique, called cross-