Advances in Geophysics, Supplement 2 Principles and by Xxx Lee

By Xxx Lee

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Central Recording Site Although the field package usually provides a well-conditioned signal to the telephone system for transmission, the amplitude of the multiplexed seismic signals received at the central recording site may have large variations. These variations pose a serious problem for the analog tape recorders. To achieve adequate signal-to-noise ratio in playbacks of earthquakes from the analog tapes, the multiplexed seismic signals must be recorded within narrow amplitude limits. An automatic gain control (AGC) unit was designed and installed by Jensen (1976b) to accomplish this.

PA0 CENTRAL RECORDING SITE AMPL. LINE COMWTER MULTIPLEXED SEISMIC SIGNAL - DEMULTIPLEXEO SEISMIC SIGNAL ~ RETRANSMIT HELICOROER 4 cm/V TIME CODE IGNALS I SUMMING AMPLIFIER . ONE PER TRACK I DIRECT MODE 14 TRACKS Fig. 7. Major elements of seismic instrumentation of the USGS Central California Mi. croearthquake Network. phone circuit with no special conditioning is used. If the field package output cannot be connected to a drop, it is transmitted via line-of-sight vhf radio to a convenient drop and transmitted from there using the telephone system.

Recent advances in ocean technology, particularly in deep sea drilling, acoustic telemetry, and tech- 42 2 . Instrumentation Systems niques to emplace and recover instrument packages, will facilitate microearthquake observations at sea. In the following subsections we summarize the shallow-water microearthquake network described by Henyey et a / . ( 1979) and then give examples of sonobuoy-hydrophone systems and OBS systems commonly used in deeper water. 1. , 1979). Three of the stations are onshore; five are in shallow water (20-100-m depth) and surround the offshore oil platforms.

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