A direct-recording ocean-bottom seismograph

A direct-recording ocean-bottom seismograph featuring small size and low cost has been constructed and operated in the deep ocean. The design consists of a cylindrical instrument package 15 cm in diameter and 53 cm long. Signals from a gimbal-mounted 2-Hz vertical geophone and a hydrophone are amplified and direct-recorded at different gain levels on three channels of 6.4-mm wide magnetic tape. BCD-coded time pulses, generated from a crystal oscillator signal, are recorded on the fourth channel. The instrument has its maximum displacement response at 17 Hz. Tape consumption is 22 m/day (0.01 ips) from 12.7-cm reels permitting continuous recording for up to 12.5 days. CMOS electronics are used throughout and, together with a low-power tape drive motor, the unit requires less than 100 mW of power. This permits long recording times from 9 D-cell alkaline batteries. Syntactic foam flotation, good to 6400 m, permits a low-profile design 1.0 m long and 0.7 m high. It weights 114 kg in air and descent and rise rates are 0.64 m/sec. For retrieval, an expendable base plate is released by a cable cutter on command from a clock comparator circuit. Data playback is at 10 to 100 times recorded speed.