高精度デジタル海底地震計ＭＯＯＢＳ‐２４の開発

Abstract

We made significant improvements for the previous digital OBS/H called MOOBS/H-I. We call the new OBS MOOBS/H-24 (Magneto Optical Ocean Bottom Seismometer and Hydrophone-24). The most significant improvements are use of a 1Gb Magneto Optical (MO) disk unit and 24 bit resolution A/D converters. The previous system uses a 320 Mb magneto optical disk unit and a 16 bit A/D converter. The 24 bit A/D converter is delta-sigma type.Mainly due to the limitation on the power consumption, we obtained 120 dB dynamic range and 20 bit effective resolution. We also increased the buffer memory size from 2 Mb to 4 Mb. Using this buffer memory, we can continuously obtain a seismic record during 1 hour and 14 minutes if we choose 100 Hz sampling rate and 3 ch. inputs. The loss time during the 4Mb-data transfer from the buffer memory to the MO disk is 1 minute and 14 seconds. If we choose careful time schedules for explosions in the seismic experiment, it might be a reasonable specification. The instrument has also logical and/or level trigger modes. It can be operated during a specified time window. The parameters for initial setting is input through a serial or parallel interface unit. Using the serial interface unit, we can start the instrument after it is enclosed inside of a glass sphere pressure case.We used ten MOOBS/H-24 units in the field experiments off Cape Muroto and off Tokai, Japan and obtained satisfactory records. When a 15 kg-dynamite explosion was detonated just at the sea surface above-an ocean bottom instrument at 2 km water depth, the acoustic water wave showed high amplitude unsaturated waveforms.