The ocean-bottom MOBB station is currently comprised of a seismometer package, a current meter, and a recording and battery package.
The seismic package contains a low-power (2.2W), three-component CMG-1T broadband seismometer system built by Guralp, Inc., with a three-component 24-bit digitizer, a leveling system, and a precision clock. The seismometer package is mounted in a cylindrical titanium pressure vessel 54cm in height and 41cm in diameter, custom built by the MBARI team and outfitted for underwater connection.
Because of the extreme sensitivity of the seismometer, air movement within the pressure vessel must be minimized. In order to achieve this, the top of the pressure vessel was thermally isolated with two inches of insulating foam and reflective Mylar. The sides were then insulated with multiple layers of reflective Mylar space blanket, and the vessel was filled with argon gas.
The current meter is a Falmouth Scientific 2D-ACM acoustic current meter, sampled at 1Hz. The current meter is held by a small standalone fixture and measures the magnitude and direction of the currents about 1 meter above the seafloor.
The recording system is a GEOSense LP1 data logger with custom software designed to acquire and log digital data from the Guralp system and digital data from the current meter over RS-232 serial interfaces. The data are stored on a 3GB, 2.5in disk drive. All the electronics, including the seismometer and the current meter, are powered by a single 10kWh lithium battery.
Deployment:
All installations were done using the MBARI ship Point Lobos and the ROV Ventana. Photographs of the deployment are linked throughout the following text.
Prior to the instrumentation deployment, the MBARI team manufactured and deployed a 1181-kg galvanized steel trawl-resistant bottom mount [photo 1] to house the recording and power systems, and installed a 53cm diameter by 61cm deep cylindrical PVC caisson [photo 2 3 4] which would eventually house the seismometer pressure vessel.
The bottom mount for the recording system was placed about 11m away from the caisson to allow the future exchange of the recording and battery package without disturbing the seismometer.
The actual deployment occurred over three days (9-11 April, 2002).
On the first dive, the seismometer package was lowered into the PVC caisson [photo 5], and its connection cable [photo 6] brought to the site of the recording unit. On the second dive, the recording package was emplaced [photo 7] in its trawl-resistant mount, and connected to the seismometer package. Tiny (0.8mm) glass beads were poured into the caisson [photo 8 9] until the seismometer was completely covered, to further isolate it from water circulation. The seismometer package is now buried at least 10 cm under the seafloor level [photo 10]. On the third dive, the ROV connected to the seismometer through the recording system, leveled and recentered it and verified that the seismometer was operational. The current meter was also installed and connected to the recording system [photo 11 12 13].
Three-component seismic data are being recorded continuously at a sampling rate of 20Hz, as well as current-meter data at a sampling rate of 1Hz.
Initial Results:
On April 22nd, the ROV returned to the MOBB site to check the functioning of the seismometer and recording system. Some slight settling of the seismometer pressure vessel had occurred, and so the seismometer was commanded to recenter electronically. Over 3MB of data were then downloaded from the system over a period of about two and a half hours, including the recordings of two earthquakes that occurred in California, and two teleseismic events that occurred in Guerrero, Mexico and in Northern Chile. These data are currently being analyzed.
The site will be revisited every three months to replace the data recording and battery module. Eventually, the data recording package will be plugged into the MARS cable, enabling continuous real-time data acquisition on land.