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A seismic telemetry system of large dynamic range

GEOPHYSICAL LABORATORY UNIVERSITY OF SOUTHERN CALIFORNIA, LOS ANGELES, CALIFORNIA 90089-0741

Abstract

An optimum telemetry system (OTS) has been developed that can be plugged directly into the front-end electronics of a field seismic station to enable large dynamic range (120 dB) data transmission, 60 dB resolution, and three-component seismic recording. This new system is designed to be accommodated within a conventional single ±125-Hz transmission channel. With CMOS microprocessors and a minimum of 64 kB memory, the OTS buffer stores 3.5 min of data for one channel or 1 min of data for three channels at 128 sps. The buffer storage can easily be expanded by an order of magnitude without significant increase in cost, but the present buffer size is sufficient for regional seismic networks of coverage less than 500 km in size; i.e., enough preevent memory is allowed for event trigger and gain-ranging operations. There is no sacrifice on timing accuracy, and to apply corrections to the travel-time delays is simple as the delays are exactly known. To guarantee the 60 dB resolution in a ±125-Hz FM transmission band, a new digital voltage-controlled oscillator has been designed for the OTS which keys the FM center frequency accurate to 0.1 per cent and practically eliminates the drift of the voltage-controlled oscillator center frequency. The OTS offers a practical solution to the severe signal-clipping problem that has plagued many high-performance telemetered seismic networks.

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