抄録
In recent years, ocean bottom seismometers (OBSs) have been used to obtain crustal and upper mantle seismic structures and/or distributions of earthquakes. Most of OBSs used in these experiments are analog recording type. One of the advantages which analog recording OBSs have has been that they can record data continuously for more than two weeks. On the other hand, some disadvantages of analog OBSs can be pointed out; for example, the dynamic range per recording channel is narrow.
We have developed a digital recording OBS, which has the same recording length of time as analog OBS's, to obtain higher quality data. The new digital OBS (ORI-OBS) consists of one pressure vessel, an acoustic release system, a transmitter, and a flasher. The pressure vessel (glass sphere) contains sensors, recorder, and battery cells. The design of ORI-OBS except the digital recorder is the same as the up-to-date analog recording OBS. We use a commercial portable digital audio tape (DAT) recorder to store seismic data for long period recording, low power, and low cost. The DAT recorder newly developed has a 16-bit A/D converter and an 8-bit CPU. About 1 GB of data can be stored on a DAT: a continuously recording period is about 12 days on 4 ch at 100 Hz sampling. A real-time clock (RTC) with an accuracy of 5×10-7 controls the timing of the recorder system: the timing of A/D conversion, starting data acquisition at a predetermined time, etc. The electronic setup and monitoring of the recorder, e.g., adjustment of the RTC inside the sealed vessel, can be performed by connecting another computer using the electrical penetrators of the pressure vessel. A 10-second cycle pulse is output to the outside of the pressure vessel to calibrate the RTC. Data on DAT are copied to EXABYTE (8 mm video) tape using the playback system on shore. The EXABYTE system is widely used as a data storage system for UNIX computers. We have used the digital OBS in four experiments, and obtained good quality data from earthquakes, airguns, and explosions.
