We developed a new digital ocean bottom seismometer with a hydrophone using a magneto optical storage device, called MOOBS/H (Magneto-Optical Ocean Bottom Seismometer with Hydrophone) -1. This unit is designed to intend equipping a large-amount data storage, higher accuracy for the data and the random access capability, and improving the reliability of data storage; those features were not realized by the first generation. By the use of the power on-off control of the slave CPU, the SCSI interface, and the MO unit, the total power is reduced to 1/100 compared to the full use. By the MOOBS/H-1, 2000-3000 earthquakes can be stored in an MO disk.
The MOOBS/H-1 is composed of seismic sensors, amplifiers, a multiplexer, a 16-bit A/D, a watch-dog processor, a time-keeper, a data-transfer processor and a five inch MO storage. Two 8 bit CPUs are used : one for a watch-dog and the other for data I/O between the main memory and the MO unit. One 16-bit A/D device is used. Recording channels can be selected among 8 channels. Six channels out of 8 channels have amplifiers. The sampling rate of A/D can be selected among various frequencies from 1 to 800Hz. The recording mode is chosen among several modes such as continuous, time window, logical trigger, and amplitude trigger modes. The necessary parameters for each observation are set using external key-pad unit through a bus-connector. The key-pad unit can be removed after setting parameters.
The maximum amount of the data storage is 320Mb. The maximum number of earthquake events are c.a. 2, 200 if 100Hz sampling, 4 channels and 3 minutes record length are used. In this case, the maximum record length for each event can be extendible up to more than 42 minutes. When lithium batteries are used, the recording life is roughly 2 months.
The data in the MO disk are directly readable by an NEC-PC and can be transferred from the PC to a workstation for analysis. Software to use the MOOBS/H data was also developed.
The usefulness of MOOBS/H was confirmed by a number of deep-sea observations in such as the Japan Trench, the Okinawa Trough, off Japan, the Mariana Trough and the TAG area of the Mid-Atlantic Ridge. Using frequency characteristics of hydrophone, amplifier and recording unit, the very low frequency behavior of hydrothermal vent was firstly observed.

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