Monterey Bay Aquarium Research Institute
MOBB Seismometer
Instruments and installation process

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 (see Figure 1) to house the recording and power systems.

A 53-cm diameter by 61-cm deep cylindrical PVC caisson was installed in the seafloor to house the seismometer pressure vessel (see Figure 2).

The bottom mount for the recording system was placed about 11 meters away from the caisson to allow the future exchange of the recording and battery package without disturbing the seismometer.


Figure 1 - Trawl-resistant bottom mount

Figure 2 - PVC Caisson

The seismometer was deployed during several ROV dives over a three-day period (9-11 April, 2002).

During the first dive, the seismometer package was lowered into the PVC caisson (See Figure 3). Next, the data and power cable from the seismometer was carried to the recording unit (See Figure 4).


Figure 3 - Lowering seismometer into caisson.

Figure 4 - ROV leading cable from seismometer

During the second dive, the recording package was emplaced in its trawl-resistant mount (See Figure 5), and connected to the seismometer package. Tiny (0.8mm) glass beads were poured into the caisson (see Figures 6-8) until the seismometer was completely covered, to isolate the seismometer from vibration due to bottom currents. When the installation was complete, the seismometer package was buried at least 10 cm below the level of the seafloor.


Figure 5 - Lowering recording package into bottom mount.


Figure 6 - Caisson with seismometer and bead-filling tube.


Figure 7 - Filling caisson with glass beads.

Figure 8 - Seismometer housing in caisson, covered with beads

During the third ROV dive, the ROV connected to the seismometer to the recording system, leveled and recentered the seismometer, and verified that the seismometer was working properly. A current meter was also installed nearby and connected to the recording system (See Figures 9-11).


Figure 9 - Carrying the data/power cable from the current meter to the bottom mount.


Figure 10 - Approaching the bottom mount with cable from current meter.


Figure 11 - Plugging the cable from the current meter into data-recording unit (cable from seismometer is on right).
 

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.

Since installation, three-component seismic data have being recorded continuously at a sampling rate of 20Hz, as well as current-meter data at a sampling rate of 1Hz. The site has been revisited every three months to replace the data-recording and battery modules. Eventually, the seismometer data will be routed through the MARS cable, enabling continuous real-time data acquisition on land.