Underground measurements of short-period seismic noise

Abstract

Seismic noise in the frequency range of S-33 cycles/sec<br />has been recorded in three mines (Haggruvan, Idkerberget, Stripa) in central<br />Sweden at different levels down to about 750 m. As an average it is found<br />that the noise amplitude decreases to 25 percent of its surface value at a<br />depth of 50 m, to 13 percent at 100 m, to 6 percent at 200 m depth, and<br />is less than 1 percent of the surface value at depths exceeding 500 m. Individual<br />variations in these numbers amount to about 100 percent in average,<br />which could be explained by variations in rock properties, wave periods<br />and distance and direction to noise sources. The source of the noise studied<br />was various surface disturbances, mainly traffic. The obtained amplitudedepth<br />diagrams suggest an interpretation in terms of body waves rather<br />than surface waves, and the depth effect is mainly to be explained as due<br />to increasing distance from the noise source