MOBB: An Ocean Floor Broadband Station

The Monterey Ocean Bottom Broadband observatory (MOBB) is a collaborative project between the Monterey Bay Aquarium Research Institute (MBARI) and the BSL. Supported by funds from the Packard Foundation to MBARI, NSF/OCE funds and UC Berkeley funds to BSL, its goal has been to install and operate a long-term seafloor broadband station as a first step towards extending the on-shore broadband seismic network in northern California, to the seaside of the North-America/Pacific plate boundary, providing better azimuthal coverage for regional earthquake and structure studies. It also serves the important goal of evaluating background noise in near-shore buried ocean floor seismic systems, such as may be installed as part of temporary deployments of ``leap-frogging" arrays (e.g. Ocean Mantle Dynamics Workshop, September 2002).

BSL staff put significant effort in the development of procedures to minimize instrumental noise caused by air circulation inside the seismometer package casing (see 2001-2002 and 2002-2003 BSL Annual Reports). These procedures were later applied to the preparation of 3 similar packages destined for installation on the Juan de Fuca plate in the framework of University of Washington's Keck project.

This project follows the 1997 MOISE experiment, in which a three component broadband system was deployed for a period of 3 months, 40 km off shore in Monterey Bay, with the help of MBARI's ``Point Lobos" ship and ROV ``Ventana" (Figure 3.7). MOISE was a cooperative program sponsored by MBARI, UC Berkeley and the INSU, Paris, France (Stakes et al., 1998; Romanowicz et al., 1999; Stutzmann et al., 2001). During the MOISE experiment, valuable experience was gained on the technological aspects of such deployments, which contributed to the success of the present MOBB installation.

The successful MOBB deployment took place April 9-11, 2002 and the station is currently recording data autonomously (e.g. Romanowicz et al., 2003). It comprises a 3 component very broadband CMG-1T seismometer system, a diffential pressure gauge (DPG, Cox et al., 1984) and a current meter. Data from the DPG are acquired with a sampling rate of 1 sps, and are crucial for the development and implementation of a posteriori noise deconvolution procedures to help counteract the large contribution of infragravity wave noise in the period range 20-200 sec. Figure 3.6 shows preliminary results of removing the infragravity noise from the seismic channel by using a simple linear regression in the time domain (Zuern and Widmer, 1995). We are also developing a procedure in frequency domain that will routinely remove coherent noise derived from pressure measurements, following the approach described in Crawford and Webb (2000). Removal of infragravity noise from the MOBB seismic channel will make station MOBB an important addition to the BDSN land stations for use in studies that utilize high quality long-period seismic data as well as provide directions for data processing from the future ocean bottom broadband seismic stations.

Fifteen ``dives" involving the MBARI ship ``Point Lobos" and ROV ``Ventana" have so far taken place to exchange dataloggers and battery packages, the last one on June 16, 2005. In February 2004, the N/S component seismometer failed. It was temporarily replaced, from 05/19/04 to 07/09/04 by one of the Keck seismometer packages which was conveniently available at that time. The original seismometer was sent back to Guralp Inc. for repair and successfully reinstalled on 07/09/04.

The data collection from the broadband seismic system is fairly complete. However, there have been recurring DPG sensor as well as DPG data storage problems in the first two years of the MOBB operation. Well recorded DPG data are available since 03/18/2004.

Figure 3.6: Left: Comparison of the power spectral density (PSD) calculated for a 5.5 hour period without earthquakes for the vertical acceleration (blue) and DPG signal (red). The DPG signal has not been corrected for the instrument response. Increased noise level for periods between 30 and 200 seconds that is due to infragravity waves is observed in both datasets. Right: Vertical acceleration PSD before (blue) and after (green) correction to remove the noise due to infragravity waves from the seismic channel.
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Figure 3.7: Location of the MOBB station in Monterey Bay, California, against seafloor and land topography. The projected path of the MARS cable is indicated by the solid line.
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With input from BSL staff, MBARI engineers are currently working on hardware and software developments needed to connect the MOBB sensors to the MARS (Monterey Accelerated Research System; http://www.mbari.org/mars/) cable (Figure 3.7). This will provide access to real-time, continuous seismic data from MOBB to be merged with the rest of the northern California real-time seismic system.

Berkeley Seismological Laboratory
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