Abstract: Some Observations on Ground-Noise Surveys in Geothermal Exploration

R. P. Geitgey, C. B. Reynolds

Ground-noise anomalies in the range of 2-5 Hz can correlate well with local surface geology and thermal-spring activity. The presence of areas of high ground noise in this frequency range at the intersection of apparent fault systems, on the downdip side of range-marginal faults, and along anticlinal crests, in areas of known thermal activity gives a certain degree of confidence in the applicability of this method to geothermal exploration. However, ground-noise measurements are subject to many natural and cultural interferences and recordings should be made by trained observers able to determine potential interference sources and to relate ground noise to local geology. Wind noise, the most common natural interference, is dependent on local topography, type of vegetation, and wind ve ocity. Such interference is not necessarily correlative with wind velocity in the immediate area of the recording instruments. Ground noise also may be contributed by surf action, flowing streams, and animal movements. Recordings in areas of cultural activity such as vehicular traffic, agricultural machinery, railroads, aircraft, and pumping water wells, must be evaluated in terms of season, time of day, and observed sources of interferences. Data collected near population centers should be accepted with considerable caution. Local geologic conditions can be of extreme importance in recognizing sources and effects of both natural and cultural interference. Valleys filled with water-saturated alluvium can transmit seismic waves of the appropriate frequencies for distances of several miles with little apparent attenuation. Soils made rigid by freezing or by caliche deposition can behave in a similar manner. Used with proper respect for local geology, potential interferences, and limitations inherent in the method itself, ground-noise surveys still appear to be a rapid and effective method of geothermal exploration.

AAPG Search and Discovery Article #90979©1975 AAPG – SEPM Rocky Mountain Sections Annual Meeting, Albuquerque, New Mexico