GEOTHERMAL GROUND‐NOISE SURVEYS

Abstract

In recent years there has been increasing interest in the role of geothermal steam as a source of energy. Only recently has geophysical exploration begun to play a major role in the exploration for geothermal resources; heat flow, gravity, and resistivity measurements have been the techniques most used. One recent development is the discovery that high surface‐noise levels are associated with the presence of geothermal reservoirs below the surface. Field surveys using short‐period seismographs have been conducted in the Imperial Valley of California in areas where heatflow measurements or drilling have indicated the presence of a geothermal deposit. In all three surveys abnormally high noise levels were found above the reservoir in the frequency range of 0.5 to 5.0 hz. The ground‐noise anomalies show a more complex pattern than the associated heat flow and gravity measurements. A theoretical model has been developed in which small, random pressure variations in a convecting geothermal reservoir are suggested as the source of the noise. Using this model, the noise level above one of the anomalies was duplicated, using a reasonably sized reservoir with pressure variations of less than 1 millibar.