Accurate timing of seismic events is critical our ability to analyze them. Seismometers that are installed on land often use the precise timing available in GPS satellite transmissions to provide the time synch for data loggers. Unfortunately, in the subsea domain, GPS radio signals do not propagate through the water.
Historically, OBS's have relied on precision crystal osillators that are either miroprocessor corrected for frequency drift as a function of temperature changes or incorporate integrated ovens to maintain the oscillator at a stable temperature. Even with these forms of temperature compenstation, the long term (and potentially non-linear) drift of crystal based sources can become significant over the duration of a typical year long OBS deployment, and can be a significant source of error in the data recorded. While more precise time sources exist (rubidium, cesium atomic sources) their high power requirements preclude their long term deployment in remotely deployed ocean bottom seismographs.
Recently, there have been significant advances in the development of small, low cost, and low power atomic based clocks. The Symmetricom Chip Scale Atomic Clock, or CSAC has been integrated on WHOI broadband OBS, and is planned to be integrated in LDEO instruments. Initial performance evaluation of the CSAC indicates an improvement in long term drift of approximately 2 orders of magnitude over similar size and power crystal oscillators.
Several presentations have been made at the IEEE Oceans meetings by A. T. Gardner and J. A. Collins of Woods Hole Oceanographic Institution on the long term timing quality of CSAC clocks:
For more information about the CSAC, visit the CSAC product page.