Underground measurements of short-period seismic noise



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

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DOI: https://doi.org/10.4401/ag-5040
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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X