A preliminary report on infrasonic waves as a source of long-period seismic noise

Abstract

Earth motion in response to atmospheric pressure changes is a major contributor to the ambient seismic background in the 20- to 100-s period range. During windy intervals at the surface the quasi-static deformations caused by wind-generated pressure changes are often quite large in comparison to ambient vibrations from other sources. Recent studies [Sorrells et al., 1971; Ziolkowski, 1973] have demonstrated that the wind-related noise can be eliminated by installing the sensors at depths of the order of several hundred meters. When such action is taken, it is found that the noise levels obtained approximate those observed at the surface during calm intervals. The calm interval noise level thus constitutes the current minimum threshold. The calm interval noise level in the 20- to 100-s period range is largely the result of earth motion from atmospheric pressure sources of undetermined origin [Savino et al., 1972]. If noise levels are to be reduced below the currently realizable minimums, it will be necessary to establish the origin of the atmospheric pressure variations with some precision. In this paper, evidence is presented that supports the hypothesis that much of the earth noise observed during calm intervals in the 20- to 100-s period range is the result of atmospheric infrasonic waves. If this hypothesis is borne out by additional studies, then efforts to reduce further the current long-period noise threshold by increasing installation depths will not yield satisfactory results unless depths of the order of 1–3 km are reached. The relatively high costs of installing systems at such depths would rule out this particular method for obtaining further reductions in the long-period seismic noise threshold. Thus if the current noise threshold is largely the result of infrasonic waves, then the practical limits of using depth of installation for the suppression of seismic noise have already been reached.