Search for geothermal seismic noise in the East Mesa area, Imperial Valley, California

Abstract

The U. S. Geological Survey made seismic noise measurements in the East Mesa area of Imperial Valley, California, to find out if a noise anomaly was associated with the Mesa thermal anomaly. Thirty-three locations were occupied in the area using slow-speed tape-recording seismic systems. One of the stations (CEN) was operated close to where a geothermal test well was subsequently drilled by the U. S. Bureau of Reclamation. Several sources of cultural noise are present in the area. Large fluctuations in noise level, superposed on a constant high level of noise, occur from traffic on a freeway to the south of the region. There is noise generated by canals to the west and south and agricultural activity to the west of the region. Noise at 2.5 Hz frequency generated by a small waterfall (power drop) on the All American Canal propagates as far as 10 km. Average noise levels were computed at each station using several quiet samples selected from 4-hour sections of data recorded at night and contoured. Spatial distribution of 2-3 Hz noise show noise radiating from the power drop. Noise in 0-2, 3-5, and 5-10 Hz bands show high levels extending along the freeway to the south and East High Line Canal to the west of the area. The Mesa thermal anomaly is centered about 2.5 km from the freeway and canal and does not seem to have any anomalous noise amplitudes associated with it. Additional results using data from two arrays of closely-spaced instruments extending from the freeway to the Mesa thermal anomaly also show no indications of high noise levels over the anomaly. This conclusion differs from the results of two previous surveys in the area (Douze and Sorrells, 1972; Geothermal Staff of Teledyne-Geotech, 1972) which show well defined noise anomalies in the 0-2, and 3-5 Hz frequency bands. A search was also made for anomalous features in noise spectra and for coherent wave trains indicating the presence of discrete sources of noise. The only predominant feature in the spectra is the 2.5 Hz peak seen at most stations, from noise generated by the power drop. Records from several three-element, L-shaped arrays with 0.3 km instrument spacing show very little visual coherence. Measurements using a cross-spectral method, however, show that highly coherent waves are found at 0.4 and 2.5 Hz; coherence is less, though significant, at 3.4 and 4.4 Hz; direction studies using phase delays from the arrays give many non-unique solutions, as the array spacing is larger than the wavelengths involved. Using the knowledge that 2.5 Hz waves are generated by the power drop, the velocity of seismic noise waves in the East Mesa area is estimated to be 0.5 km/sec.