Microseisms in geothermal exploration; studies in Grass Valley, Nevada

Frequency(f)-wavenumber(k) spectra of seismic noise in the bands 1 < or = f < or = 10 Hz in frequency and k < or = 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 this potential geothermal area.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 judiciously 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 geologic 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 (2-D) array was placed at 16 locations in the region to map propagation parameters. The f-k spectra reveal shallow local 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 wavenumber of the noise field. This information, along with the accurate velocity structure, will allow ray tracing to locate a source region of radiating microseisms. In Grass Valley, and probably in most areas of sedimentary cover, the 2-10 Hz microseismic field is predominantly fundamental-mode Rayleigh waves controlled by the very shallow structure.