| Title | Development of a Full-Automatic Monitoring System of Microearthquakes for Geothermal Development |
|---|---|
| Authors | Ikawa, T., Kuroda, T., Hashida, I., Nakagome, O., |
| Year | 1988 |
| Conference | Japan International Geothermal Symposium |
| Keywords | |
| Abstract | The observation of microearthquakes is required for the environmental monitoring in the geothermal development, while it is useful for the detection of the subsurface structure in terms of seismic velocity and fracture distribution. For such a purpose, an accurate recording of earthquake waveform is necessary in a wide range of their magnitude, say, -2 to 4. After WATANABE(1971}, the relationship between magnitude (M) and maximum velocity amplitude (A) of ground motion at the epicentral distance (R) is expressed by 0.85 M-Z.50 log A + 1.73 log R within a limited R «ZOO Km). Using this equation, it is found that at least 102 db of dynamic range of recording' is required for the earthquakes between the above magnitudes. Therefore, a very rapid auto-gain-ranging digital amplifier, that is Instantaneous Floating Point (IFP) amplifier, was adopted in the new system. The concept of IFP is very common in the seismic exploration for petroleum but is hardly used in the observation of earthquakes. The IFE' amplifier works so as to fill AID converter bits over 90 % changing its gain which can vary from 0 to 84 db with 6 db step at each digitizing instance. Consequently, the IFP amplifier system achieves a wide dynamic range recording of 120 db or more without any human intervention, where one digital sample is expressed by 4 gain bits and signed 16 bits of AID converter output. The data from three seismometers from every station are digitized at every 5 msec and transmitted through telephone communication line to the central computer on real time basis. The computer is always scanning the input data from all the stations so that all the earthquake events are detected, analyzed and filed automatically without missing (Fig.2). In addition to the aboves, the system has self diagnostic functions such as geophone test, filter test etc., and has auto-adjustment function of transmission skew. Consequently, the developed new system will provide a full range of automation of the microearthquake observation and will minimize the running cost for its long term operation. Fig.3 and 4 are showing typical outputs from the system. |