Wireless Retrieval of High-Rate Ocean Bottom Seismograph Data and Time Synchronization Using the WHOI Optical Modem and REMUS AUV

摘要

Sustained earthquake monitoring of offshore areas of high seismic risk such as the continental shelf and slope above the Cascadia, Alaska/Aleutians, and Puerto Rico subduction zones would not only improve our understanding of the internal structure and rupture properties of these faults but also potentially allow advanced warning of increased short-term seismic risk. Studies of these areas have demonstrated that there are time periods on the scale of days to weeks when large subduction earthquakes are more likely to occur. To fully understand this basic fault behavior and possibly utilize it to reduce risk to society, we need access to high-quality data from offshore directly above areas where the great earthquakes rupture. Retrieving high quality seismic and geodetic data from the seafloor pushes the limits of currently affordable telemetry systems. Fiber optic cabled real-time seismic stations, such as those deployed offshore Japan, the west coast of Canada, and the northwest coast of the U.S., are superb platforms for earthquake monitoring but are expensive to deploy. Alternatively, autonomous Ocean Bottom Seismometers (OBS) could be used for cost-effective, long-term, earthquake monitoring if: (1) the OBS could operate for multiple years without servicing, and (2) accurately-timed data could be retrieved in real-time or on-demand without the expense and effort of sending a research vessel to recover and redeploy the OBS. A number of recent technological advances developed at WHOI, including a factor of >10,000 increase in underwater telemetry speeds, can make long-term deployments with routine data access a reality. A collaborative effort among three well-established instrumentation groups at WHOI - the Optical Modem, REMUS Autonomous Underwater Vehicle (AUV), and Ocean Bottom Seismograph Labs - proposes to solve two severe limitations of OBS deployments, namely infrequent and expensive data retrieval by OBS recovery using a research vessel, and non-optimum timing brought about by large and non- linear clock drifts that increase with deployment duration. We have developed the capability for complete data retrieval from autonomous OBS using a high-speed optical telemetry modem capable of sustained transfer rates of 10 Mbits/second from a variety of platforms, including fully autonomous underwater vehicles. Precise and accurate timing is a requirement not only for measuring earthquake locations and spatio-temporal migration rates, but also for measuring temporal changes in rock velocity, which are indicative of stress changes that can in turn be indicators of near-term rupture. In addition to retrieving data, the offset of the OBS clocks relative to GPS-referenced time can be accurately measured and logged via the optical link. Both of these capabilities will make multiple-year OBS deployments worthwhile. We present the results of our initial proof-of-concept, short-duration deployment offshore Woods Hole, Massachusetts. A timing transfer algorithm and protocol have been developed to enable communication of timing information alongside the telemetry data. A REMUS AUV, equipped with the optical modem and a freshly disciplined timing source, traveled to an OBS deployed close to shore on two missions and successfully retrieved telemetry data. The offset of the OBS clock was measured relative to the GPS- synchronized time signals provided over the optical link to an accuracy of 10 ppm. This study will be followed by a two-year deployment of two modem-equipped OBS in a seismically active area at a location to be determined.