Microseisms in geothermal exploration: studies in Grass Valley, Nevada

Frequency-wavenumber (f-k) spectra of seismic noise in the bands 1 less than or equal to f less than or equal to 10 Hz in frequency and parallel bar k parallel bar less than or equal to 35.7 cycles/km in wavenumber, measured at several places in Grass Valley, Nevada, exhibit numerous features which can be correlated with variations in surface geology and sources associated with hot spring activity. Exploration techniques for geothermal reservoirs, based upon the spatial distribution of the amplitude and frequency characteristics of short-period seismic noise, are applied and evaluated in a field program at a potential geothermal area in Grass Valley, Nevada. A detailed investigation of the spatial and temporal characteristics of the noise field was made to guide subsequent data acquisition and processing. Contour maps of normalized noise-level derived from carefully sampled data are dominated by the hot spring noise source and the generally high noise levels outlining the regions of thick alluvium. Major faults are evident when they produce a shallow lateral contrast in rock properties. Conventional seismic noise mapping techniques cannot differentiate noise anomalies due to buried seismic sources from those due to shallow geological effects. The noise radiating from a deep reservoir ought to be evident as body waves of high phase velocity with time-invariant source azimuth. A small two-dimensional array was placed at 16 locations in the region to map propagation parameters. The f-k spectra reveal local shallow sources, but no evidence for a significant body wave component in the noise field was found. With proper data sampling, array processing provides a powerful method for mapping the horizontal component of the vector phase velocity of the noise field. In Grass Valley, and probably in most areas, the 2 to 10 Hz microseismic field is predominantly fundamental mode Rayleigh waves controlled by the very shallow structure