The continuous, worldwide availability of very low-frequency signals from the Omega navigation system makes them useful for application to timing of portable seismographs, particularly in remote parts of the world where other radio sources may be difficult or impossible to receive. In our seismograph system, Omega phase data, which are sampled relative to a temperature-compensated quartz crystal oscillator, are digitized and recorded along with seismic data, and used to correct chronometer errors in postexperiment processing. An analysis procedure that statistically optimizes the correction process has been developed. Once time is known to ±5 sec, corrections are determined using a combination of two processes: (1) synchronizing the received Omega pattern to the propagation-corrected transmitted Omega pattern, and (2) measuring the drift rate from short-term (10-sec) changes in Omega phase. The Omega pattern synchronization yields the absolute time reference, and the cumulative drift rate gives the relative time correction. Standard errors of less than 1 msec are obtained under most circumstances.

Absolute timing accuracies as high as 4 μsec are obtainable, however, through continuous phase tracking of Omega signals and application of very low-frequency, phase-velocity models. This suggests Omega's suitability for earth-imaging experiments using dense arrays of portable seismographs. Although it is probable that Omega will eventually be replaced by satellite-based systems such as the Global Positioning System, it will most certainly be years before such a system can compete in terms of cost, reliability, and worldwide applicability.

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