Seismic Noise Measurements in Takinoue (Kakkonda) Geothermal Area (2)

Effects of steam supply system, activity of fumaroles and stream of river on seismic noise

Abstract

A seisminc survey was carried out from 1980 to 1983 in Takinoue (Kakkonda) geothermal area, Iwate prefecture. Effects of steam supply system, activity of fumaroles and stream of river on seismic noise were investigated.
The instruments used for in-situ measurements consist 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 was analyzed in the laboratory with microcomputer. The original analog data were digitized at 100/sec. Fourier spectra of 2048 selected samples were computed using the fast Fourier transform.
The results are summarized as follows.
1) The seismic waves produced by the stream of the Kakkonda river have spectral characteristics of high frequency above 15 Hz, and the noise amplitude drops to background level within a distance of about 40 m from the riverside.
2) The seismic waves generated by the activities of the steam supply system, mainly production well, re-injection well and separator, have spectral characteristics of high frequency above 20Hz. The radius of influence is about 150m for the production wells and about lOm for the steam gathering pipeline.
3) Fumaroles and hot springs seem to be the source of high frequency seismic noise which may locally have high amplitude. The seismic noise in the frequency band 5-15 Hz is dominant in the wide area around fumaroles and hot springs. It may be assumed that low frequency waves are generated by deeper convection of steam and hot water.