An Ocean Bottom Seismograph Using Digital Telemetry and Floating-Point Conversion

An ocean bottom seismograph (OBS) using digital telemetry to convey eight channels of information to a shore station through 10 km of single-conductor armored cable was installed July 27, 1976 at a depth of 600 m off the northeastern coast of Oahu, Hawaii. Power for the OBS is supplied by the shore station through the same cable. A floating-point scheme requires only 12 bits for the digitization of each channel within a 108-dB dynamic range. The eight channels are each sampled at 70/g rate and include: 1) the outputs from three orthogonal 1-Hz geophones; 2) a crystal hydrophone output; 3) a rectified output from the same hydrophone in the 500-1500-Hz band; 4) a temperature and water current sensing thermistor; and 5) two system test channels. The geophones are mounted in self-leveling gimbals with brakes which are released three times each day. A calibration signal is also applied to each geophone calibration coil three times a day. The OBS has recorded on the average two events a day, 70 percent of which are of apparent local origin, and 20 percent of which are apparently earthquakes originating near the island of Hawaii. The digital telemetry scheme enables direct computer connection and digital recording. Using these digital records, ocean noise spectra have been computed and correlated with temperature and current records.