Seismic array noise studies at Roosevelt Hot Springs, Utah geothermal area

Abstract

As part of a study of a geothermal steam field at Roosevelt Hot Springs in Utah, the background seismic noise was investigated using seismometer arrays. Roosevelt Hot Springs is located on the Wasatch front in southern Utah, an area characterized by Tertiary intrusions. Arrays of vertical seismometers with spacings of 50 to 100 m were installed at selected locations. The data from three arrays were studied in detail; one was located directly above the geothermal reservoir, one was in the valley to the west, and one was in the Mineral Mountains to the east of the reservoir. The principle analysis technique consisted of computing frequency‐wavenumber spectra of the noise in the frequency range from 0.5 to 10 Hz. For frequencies less than 1.0 Hz, the arrays were too small for adequate resolution, and at frequencies higher than 10.0 Hz, the noise showed no spatial organization. The theoretical response of seismometer arrays to waves arriving from numerous directions, such as would be the case directly above a geothermal reservoir, is developed to allow comparison with the experimental results. During times of cultural activity (mainly road traffic and trains in the valley), the noise was highly variable in amplitude and consisted almost exclusively of Rayleigh waves from the cultural sources. At quiet times, the noise field was more complex. For the arrays at the reservoir and in the valley, the results of frequency‐wavenumber computations indicate that the noise consists of isotropic Rayleigh waves. No waves were detected that could be related to the geothermal reservoir. The array in the Mineral Mountains, farther away from the cultural noise sources, showed a completely different noise field. The noise consisted of high‐velocity P‐waves from a source to the east of the known geothermal area.