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Earthquake signal processing and logging with a battery-powered microcomputer

MARINE SCIENCE INSTITUTE UNIVERSITY OF CALIFORNIA, SANTA BARBARA, CALIFORNIA 93106

Abstract

A low power microprocessor controlled event-triggered seismic data logging system has been designed and implemented in the ocean bottom seismic capsule. Three channels of seismic data are digitized, formated, and stored under control of an Intersil IM6100 microprocessor chip. The processor also controls event triggering, acoustic diagnostics, battery voltage monitoring, and self-diagnostic functions. A watchdog system restarts it in the rare case of a failure of proper program sequencing. The entire system consumes less than 0.15 watts in the idle mode. The event triggering algorithm consists of first measuring the ratio of the short-term signal average to the long-term signal average, with amplifier offset removed (STA/LTA ratio). When an STA/LTA trigger occurs, the number of slope reversals in the signal begin to be counted. After a preset time following the first STA/LTA trigger, if the number of subsequent STA/LTA triggers meet preset criteria, a system trigger is declared and data is stored on a 4-track Braemar cassette tape deck with 15-M-bit capacity. Various system diagnostics, unit and experiment identification, and battery voltages are recorded at the beginning of each new event record. Digital filters for the long- and short-term averages, and antialiasing have been efficiently implemented by dividing or multiplying by factors of 2, exclusively. This amounts to a shift right or left of a binary number. The recording system has been tested in numerous field experiments. Circuit cards from the ocean bottom capsule implementation are being assembled and used for land stations and could be applied to a wide variety of data acquisition tasks.

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