Noise Attenuation in Shallow Holes (I)

Abstract

For the purpose of improving the signal-to-noise ratio of seismological observations of the Japan Meteorological Agency, the attenuation of background noise in a shallow hole has been studied. Three-component seismograph of 1.5 sec in period was placed at depths of 10,20,50,100,150, and 200 m as an observation hole was being bored in the premises of the Meteorological Research Institute, Tokyo. Simultaneous observation was made on the surface by seismograph of the same characteristics, and the comparison between surface and hole was made with respect to frequency and depth (see Figs.1 and 2).
The results are summarized in t he following:
1) Attenuation of background noise is more rapid with higher frequencies (see Figs.5 and 6).
2) Low frequency noises of less than 0.5 cps hardly attenuate with depth (Fig.5). This is also the case with very deep holes of 3000 m t e sted in the United States.
3) As far as observations of local earthquakes with seismograph of 1 sec in period are concerned, significant improvements in signal-to-nois e ratio can be obtained near a depth of 50 m. No improv e m ent of the ratio will probably be expected by any deeper hole, and the optimu m depth both for the signal-to-noise ratio and instrumental op e rations seems to be in this vicinity. Such depth, however, may d e pend to some extent on the geological structure near the surface, and furthe r studies will be made at different places.
4) Short-time noises due to vehicles running near-by or to the dropping weight become insignificant at a depth of 50 m or even less (F ig.8).