A seismic noise survey was carried out from 1982 to 1983 in the Takinoue (Kakkonda) geothermal area, Iwate prefecture. The variation of spectra and amplitudes of seismic noise before and after the closure operation of wellhead valve (“build-up test”) were investigated.
The instruments used for in-situ measurements consisted of three component seismometers, amplifier and tape recorder. The system has flat response to ground velocity in the frequency range of 1 to 40Hz. The data were analyzed in the laboratory with a microcomputer. The original analog data were degitized at 100/sec. Fourier spectra of 2048 selected samples were computed using the fast Fourier transform.
The results are summarised as follows:
1) When the production wells are closed, the amplitudes of seismic noise in the survey points near the geothermal reservoir decrease significantly and the spectral characteristics are not constant.
2) When the production wells are closed, a large decrease of spectral amplitudes in the frequency range of 4 to 12Hz included commonly in the survey points of Base C1 and C2 near the geothermal reservoir is recognized, but that for frequencies higher than 8Hz in the Base 3 and 6, far from the reservoir, is not so large.
3) The depth of origin of seismic noise is estimated to be in the range of 200 to 500m during the operation of production wells.
4) The dominant frequency 4-12Hz did not change after the closure of the production wells. This means that the geothermal seismic noise in the Takinoue area is nature in origin.
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