Underground measurements of short-period seismic noise

M Bâth M Bâth

doi:10.4401/ag-5040

Abstract

Seismic noise in the frequency range of S-33 cycles/sechas been recorded in three mines (Haggruvan, Idkerberget, Stripa) in centralSweden at different levels down to about 750 m. As an average it is foundthat the noise amplitude decreases to 25 percent of its surface value at adepth of 50 m, to 13 percent at 100 m, to 6 percent at 200 m depth, andis less than 1 percent of the surface value at depths exceeding 500 m. Individualvariations in these numbers amount to about 100 percent in average,which could be explained by variations in rock properties, wave periodsand distance and direction to noise sources. The source of the noise studiedwas various surface disturbances, mainly traffic. The obtained amplitudedepthdiagrams suggest an interpretation in terms of body waves ratherthan surface waves, and the depth effect is mainly to be explained as dueto increasing distance from the noise source

Highlights

The problem to select sensitive sites for the location of seismograph stations has always been of great significance in seismology
The source of seismic noise is to be found on the earth's surface, and this is true for the whole period range of mieroseisms, from those of periods above 20 sec down to local noise with periods of around 0.1 sec or less
We find that the surface noise amplitudes are reduced to 25 percent at a depth of 50 m, to 13 percent at 100 m, to 6 percent at 200 m, to 4 percent at 300 m, to 3 percent at 400 m, and to around 1 percent at 500 m

Summary

INTRODUCTION

The problem to select sensitive sites for the location of seismograph stations has always been of great significance in seismology. (see e.g. Broding and Hearn 1961, Carder 1963, Yesiac Staff 1961) In most cases, these other measurements concern lower frequencies and sedimentary rock, i.e. two facts which have to be taken into account in any comparisons of the results. The instruments used were a 12-channel seismic refraction equipment, described by Bath (1960) This permitted us to place the seismometers at a number of levels and to get simultaneous recordings from all these levels. Out of the total number of 46 records, each running for a few seconds, we selected 28 isolated waves for amplitude and period measurement. Such cases were measured where one and the same wave could be identified on all or most seismograph traces. Two conclusions are immediately obtained, unanimously supported by all investigated cases, SITV

MARKUS BATH

Noise source

Findings

Depth interval