|This underwater photograph shows the MOISE Broadband Seismometer on the floor of Monterey Bay at a depth of 1,015 meters.|
- Develop and demonstrate ROV installation of a major seismomic monitoring instrument as part of a Seafloor Observatory project. Although this instrument was only be deployed for three months, most of the manipulations were identical to those required for the MOBB seismometer (a permanent deployment).
- Collect and evaluate data from a broadband seismometer deployed on the Pacific plate (western side of the San Andreas system of transform faults) to complement data from the land-based seismographs. Anticipated results are to provide more precise information on local seismic events because the site is on the west side of the major structures.
- Evaluate the merit of sensor burial and contemporaneous collection of tidal data to offset low-frequency marine noise that has historically obscured data from these types of instruments.
- Extract both local and global seismic events to assess the value of a permanent seismic observatory in this type of tectonic setting.
- Correlate the non-earthquake events on the broadband instrument, a 1 Hz seismometer, a single channel hydrophone and a standard 4.5 Hz seismometer to determine whether these represent mass-wasting events in the Monterey Bay region.
MOISE was an international cooperative program involving:
- The Monterey Bay Aquarium Research Institute (MBARI)
- The University of California at Berkeley Seismographic Station (BSS)
- The Institut de Physique du Globe de Paris (IPGP)
- The Division Technique de l'Institut National des Sciences de l'Univers (DT/INSU) in Paris, France
- The Laboratoire de Geophysique Marine of the University de Bretagne Occidentale (UBO), in Brest, France.
The seismometer package were contributed by IPGP and DT/INSU and the recording system, ROV and ship time were contributed by MBARI. The BSS contributed software development and expertise in data encoding, decoding and archival. An independent electromagnetic sensor package, contributed by UBO, was deployed simultaneously.
The primary instrument was a Guralp Seismometer package which contains the sensors on leveling gymbals, a small cpu that drives a self-leveling program, and a rechargeable battery. The aluminum housing includes handles for ROV manipulation and the male side of an 8-pin Nautilus connector.
The selected site for the seismometer was on Smooth Ridge, to the west of the Clam Flats cold seep area. This site was prepared by sinking a PVC caisson and removing the sediment from inside the caisson; this was accomplished ten days prior to the deployment of the seismometer package. Following site preparation, a magnetometer package provided by UBO and CNRS was deployed in the vicinity of the seismometer site. Three days after deployment of the magnetometer, the seismometer package was subsequently lowered to the seafloor by elevator with the sensors and gymbals in a locked position.
After deployment, the ROV Ventana connected the Guralp seismometer to an L-Cheapo datalogger; the datalogger was connected, in turn, to the ROV and to the ship computer, which sent commands to unlock and level the sensors. While the ROV was connected, we observed the data as they were recorded in the flash memory onboard the L-Cheapo to confirm proper operation of the system. The ROV disconnected at that point and left the instrument to collect data. The digitized data were acquired via an RS232 connection onto a 4.2 GB disk.
After allowing a week for additional settling of the sensor package, the ROV returned to reconnect via a second RS232 connection to retrieve samples of noise data and one teleseismic event which occurred on the second day of recording. At the same time, the ROV connected a second Benthos sphere with adequate lithium batteries to power the instrument for a total deployment period of up to 100 days. At the end of the 3 month deployment period, the software automatically instructed the seismometer to shut down and to lock its gymbals. The ROV reconnected with the seismometer and confirmed that the instrument was prepared for recovery. The battery sphere and the logger were disconnected and all the components were recovered in the same manner as they were deployed.