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By Charles G. Sammis, Thomas L. Henyey

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SEISMIC INSTRUMENTATION 21 effectively extend the period of the pendulum. Since the sensitivity of a pendulum to acceleration at low frequency is proportional to the square of the pendulum period, finding a stable long-period vertical pendulum was the most important problem in instrumental seismology for many years. This problem was not solved until LaCoste and Romberg (1942) invented the zero-initial-length spring in 1935. LaCoste’s spring can theoretically achieve an infinite period,* but in actual applications, long-term stability can be achieved only at a period up to about 30 sec.

As the word length increases from 16 to 32 bits, both the dynamic range and the resolution exceed any practical needs of seismic data recording. Even with a 16-bit computer, one can easily realize a 120-dB dynamic range by a simple assignment of 12-bit mantissa and 4 gain bits. This may be compared to the maximum 45-dB dynamic range of analog tape recorders and the maximum 60-dB dynamic range of the present seismic telemetry links. This newly acquired large dynamic range of digital recording devices has caused a sweeping revolution that affects the fundamental design of the sensors, the signal conditioning and telemetry electronics, as well as the seismic network operation.

In timing seismic signal arrivals, especially for the purpose of location of epicenters, an accuracy of 20msec or better is commonly required. The electronic clock is essentially a time code generator that outputs both serial time codes (IRIG-C and IRIG-E) and a 30-bit parallel time code providing time of day in units of Julian day, hour, minute, and second. Serial time codes are fed into analog seismic recorders such as the drum and the analog tape system ;whereas the parallel time code is delivered to a digital recorder such as a computer.

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