STUDIES OF HIGH-FREQUENCY SEISMIC NOISE IN EASTERN KAZAKHSTAN

Abstract

Three seismographic stations have been established in the region of the principal underground nuclear weapons test site in the republic of Kazakhstan in the Soviet Union. Instrumentation includes both surface and borehole seismometers with a bandwidth of 100 Hz and a system dynamic range of 180 dB. Power spectra of the regional ambient ground acceleration peak between 4 and 6 Hz, and are, on average, frequency independent above 10 Hz, where levels are approximately equal to 10−15 to 10−16 (m sec−2)2Hz−1. Kazakh noise levels are roughly equivalent to those observed by the Regional Seismic Test Network between 1 and 20 Hz, and about 10 to 20 dB higher than the very low noise model for Lajitas, Texas, in the same band. Noise observed by the borehole sensors is about 10 dB less than that observed by the surface instruments above about 20 Hz under low wind conditions at two of the three Kazakh sites; borehole improvement is greater under high winds. The third Kazakh site, Karasu, is characterized by much higher surface noise levels (15 to 20 dB higher than the borehole levels from 4 to 40 Hz) under both low or high wind conditions, which is attributed to amplification by local site conditions and the exposed terrain. Power spectra of individual runs of ambient vertical ground acceleration measured down the borehole show that high-frequency slopes (between 10 and 80 Hz) vary between f−0.60 and f0.22, depending on wind and site conditions.