Analysis of high-frequency seismic noise in the western United States and eastern Kazakhstan

Seismographic stations were operated for several months during 1987 at three sites within 250 km of the Kazakh Test Site in the U.S.S.R. and through most of 1988 and early 1989 at three sites within 250 km of the Nevada test site in the United States. We have analyzed surface and borehole noise levels at these sites as a function of wind speed, using nearly 2000 recordings of ambient ground noise collected at the U.S. stations and several hundred noise measurements from the U.S.S.R. sites. At the surface, wind did not become a noticeable source of noise until a minimum speed of 4 to 5 m/sec. In the boreholes, noise levels were affected by wind conditions to a much smaller degree than at the surface. Between 3 and 10 Hz, surface and borehole noise levels were typically comparable, and western U.S. sites were generally quieter than Kazakh sites. In the 10 to 80 Hz band, surface noise in the western United States was much higher than in Kazakhstan, but downhole noise levels in both regions were similar, and greatly reduced compared to surface noise. We suspect that this is due to differences in surface vault quality between the two sets of stations.