ミニコンピュータによる, 長時間海底地震記録テープの自動再生処理システム

抄録

An automatic playback system for direct analogue tape recordings from an ocean bottom seismograph (OBS) network was developed. The system utilizes a mini-computer equipped with a high speed A-to-D converter. In the direct analogue recording method, ground motion is recorded continuously for 10 to 25 days in a compact-cassette tape and is reproduced at several hundred times recording speed.
First, the output of the analogue tape from an OBS is rectified, low-pass-filtered, then digitized continuously at a relatively slow sampling rate of 13Hz (samples per seismic second) and stored in a magnetic disk. In the digitized record, the events satisfying a specified criterion are detected and put on a list for each OBS. Non-seismic events are eliminated by cross-referencing the lists of all OBS's in the network. Secondly, portions of the listed earthquakes are digitized again from the analogue tapes at a fast sampling rate of 60 to 150Hz. The re-digitizing is done automatically according to the schedule made from the lists of earthquakes. The sampling rates of the digitized earthquake data files are unified with reference to each OBS clock by numerical resampling. The earthquake data files from all the OBS's are assembled and edited for each earthquake to make a multi-station data file in which the records of all the stations are referred to a common time axis like telemetered network data. It is visualized by a D-to-A converter and a chart recorder. This format of visual records facilitate picking up earthquake phases for hypocenter determination because the correlation of the waveforms among those obtained at different stations are apparent. This leads to more accurate measurement of arrival times than by the conventional method; reading a single-station record. The digitized data files are preserved in magnetic tapes.
Although the system is intended primarily to process earthquakes network data, it is also applicable to other kinds of data such as those by explosion experiments. This system has automated and speeded up the OBS data processing significantly.