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Ocean Floor Broadband Station in Monterey Bay

Subsections

Introduction

This 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 U.C. Berkeley funds to BSL, its goal is to install and operate a permanent 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.

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 10.1). MOISE was a cooperative program sponsored by MBARI, UC Berkeley and the INSU, Paris, France (Stakes et al., 1998; Romanowicz et al., 1998; 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. Eventually, it will be linked to the planned (and recently funded) MARS (Monterey Accelerated Research System; http://www.mbari.org/mars/) cable and provide real-time, continuous seismic data to be merged with the rest of the northern California real-time seismic system. The data are archived at the NCEDC, as part of the Berkeley Digital Seismic Network (BDSN).

Figure 10.1: Location of the MOBB and MOIS stations in Monterey Bay, California, against safloor and land topography. Fault lines are from the California Geological Survey database. MOBB is located at 1000 m below sea-level.
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Instrumentation

The ocean-bottom MOBB station currently comprises a three-component seismometer package, a current-meter, and a recording and battery package. A differential pressure gauge (DPG) with autonomous recording (e.g. Cox et al., 1984) will be deployed in the vicinity of the seismometer package during the next data recovery dive, in September 2002.

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 on a cylindrical titanium pressure vessel 54 cm in height and 41 cm 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, after extensive testing at BSL (Chapter 11), 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. It 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 seismic data are sampled at 20 Hz and current-meter data at 1 Hz, and stored on a 3 GB, 2.5 in 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. Prior to the instrumentation deployment, the MBARI team manufactured and deployed a 1181 kg galvanized steel trawl-resistant bottom mount to house the recording and power systems, and installed a 53 cm diameter by 61 cm deep cylindrical PVC caisson to 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. Prior to deployment, the seismometer package was tested extensively at BSL, then brought to MBARI where its internal clock drift was calibrated in the cold room against GPS time.

The actual deployment (04/09/02-04/11/02) occurred over 3 days. On the first dive, the seismometer package was lowered into the PVC caisson (Figure 10.2), and its connection cable brought to the site of the recording unit. On the second dive, the recording package was emplaced in its trawl-resistant mount, and connected to the seismometer package. Tiny (0.8 mm) glass beads were poured into the caisson 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. On the third dive, the ROV buried the cable between the seismometer and recording packages, then connected to the seismometer through the recording system, levelled and recentered the seismometer and verified that it was operational. The current-meter was also installed and connected to the recording system.

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 3 MB of data were then downloaded from the recording system over a period of about two and a half hours, including the recordings of two regional earthquakes in California and two teleseismic events that occurred in Guerrero, Mexico and in Northern Chile.

Figure 10.2: Installation of the seismometer package inside the PVC caisson. This was later completely covered by glass beads
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The site was revisited two months later, on June 27th, to check the functioning of the system and replace the data recording and battery module, in the first of a series of such dives planned for the next 3 years. The following functions were performed:


1. Disconnected the current meter and seismometer from old data logger frame

2. Removed old data logger from from the trawl-resistant mount.

3. Installed new data logger frame in the trawl-resistant mount.

4. Connected the current meter to the new data logger frame.

5. Connected the ROV to the new data logger frame, and verified that the data logger was alive.

6. Connected the seismometer to the new data logger frame, and watched it reboot.

7. Centered the seismometer.

8. Turned on auto-centering on the seismometer.

9. Verified that the Guralp was seeing the GPS clock signals (NMEA time messages and pulse per second), and recorded the clock offset. During this dive, the Guralp clock was not resynchronized to GPS time.

10. Brought the old data logger frame with data logger and batteries back to the ship.


During two months of recording, many regional and teleseismic events were recorded. These data have just started being analyzed. The plan is to revisit the site every three months to replace the data recording and battery module. Between each dive, improvements to the data acquisition software can be made. During each visit, the seismometers can be recentered, and the clock resynchronized to GPS time. Eventually, the data recording package will be plugged into the MARS cable, enabling continuous real-time data acquisition on land.

Examples of data

First, we show in Figure 10.3, the horizontal component tilt signal obtained from the seismometer mass position channels (MME, MMN). These data indicate that the seismometer package has been experiencing an exponentially decaying tilt in a south-southwesterly direction, which is also the down slope direction (e.g. Figure 10.1). The large step on day 112 (04/22/02) was caused by recentering, when the instrument was checked 12 days after installation. The small step on day 134 (05/14/02) is coincident with the occurrence of a $M_{w}$ 4.96 earthquake which occurred 5 km N59W of MOBB on the San Andreas fault near the town of Gilroy. The instruments were recentered again on day 178 (06/27/02), and, with the gradual settling of the package, we expect that the subsequent tilt decay will take at least two months to reach saturation (which it did between days 161-178 on the MMN component), in time for the next planned dive and recentering operation. On the MMZ component, the semidiurnal gravitational tide is visible, riding on the tilt signal. This signal has been detrended with a 2 day running average and the scale was expanded to emphasize the fortnightly beating of the semidiurnal gravitational tides. As with the horizontal components, the largest signals are associated with rapid changes in the second derivative of the tilt, caused by recentering or by significant ground shaking (day 134).

Figure 10.3: Mass position data for the time period 04/11/02-06/27/02. The large steps are associated with: 1) installation (day 100); 2) recentering (day 112), and a smaller step with a local $M_{w}$ 4.95 earthquake on day 134. The tide signal is clearly visible on the vertical component.
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Figures 10.4 and 10.5 show a component by component comparison of the recording of the 04/26/02 $M_{w}$ 7.1 teleseism in the Mariana Islands at MOBB and 3 nearby stations of the BDSN. In Figure 10.5, only the P-wave portion of the seismograms is displayed. The comparison shows consistency between the recordings of MOBB and nearby stations. On the horizontal components, there appears to be some signal-generated noise following the S waves and the Love wave, which is likely associated with ringing in the shallow mud layers. It is less apparent in the P waves in the pass-band shown, however at higher frequency the P wave shows a 3 min long coda. Such observations should be helpful in understanding the triggering of submarine landslides in strong motion events, and may be relevant for ocean floor structures such as oil platforms and pipelines. On the other hand, this type of noise may be unavoidable in a shallow buried installation. Our plan is to evaluate it further and investigate ways to suppress it by modelling and post-processing.

Figure 10.4: Comparison of vertical, N and E component records of the 04/26/02 $M_{w}$ 7.1 Mariana earthquake (depth = 85.7 km, distance = $85.2^o$, azimuth = $283^o$ from MOBB, at MOBB and 3 stations of the BDSN. The records have been band-pass filtered between 10-100 sec.
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Figure 10.5: Same as Figure 10.4, zoomed in on the P wave portion of the seismograms
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To further demonstrate the consistency of the MOBB data, we also show the results of a single station moment tensor inversion using only MOBB, and the comparison of the corresponding synthetic predictions with the actual data at the four other BDSN stations, for a $M_{w}$ 3.63 regional event which occurred on 04/23/02 on the San Andreas fault at a distance of 53.4 km from MOBB. The single station solution results in nearly identical focal mechanisms, but a slightly larger CLVD component and scalar moment, which is not unlike other single station inversions (Figure 10.6).

Figure 10.6: Results of moment tensor inversions for the M 3.63 regional event shown in Figure 10. Top: inversion using 4 stations of the BDSN and MOBB (BDM, BKS, and CMB are bandpassed between 0.02 and 0.05Hz; MHC and MOBB, between 0.05 and 0.10Hz). Bottom: results of inversion using only MOBB, showing the good fits of the single station solution to the other BDSN data. Courtesy of D. Dreger.
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We plan to systematically analyze MOBB data acquired over the next year to assess the data quality and possible improvements, through post-processing and/or installation adjustments. We plan to evaluate the long term time evolution of background noise, as the system continues to settle and stabilize, and the shorter term noise fluctuations in relation to tides and currents as recorder by the current-meter (e.g. Figure 10.7) as well as the DPG (after the installation of the DPG in September 02). Since the auxiliary data are sampled at sufficiently high rates (1 sps) compared to what was available for the MOISE experiment, we will be able to investigate ways to reduce the background noise correlated with the pressure and current data at periods longer than 10 sec.

Figure 10.7: Distribution of currently available current velocity data as a function of azimuth. The contour label units are fractions of the average density distribution of the current. The two dominant maxima (centered at $60^o$ and $240^o$, i.e. orthogonal to the continental shelf) are associated with the semi-diurnal tidal currents. The third directional peak is roughly parallel to the coastline and appears to be related to the dominant ocean circulation.
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Acknowledgements

MOBB is a collaboration between the BSL and MBARI, involving Barbara Romanowicz, Bob Uhrhammer, and Doug Neuhauser from the BSL and Debra Stakes and Paul McGill from MBARI. The MBARI team also includes Steve Etchemendy (Director of Marine Operations), Jon Erickson, John Ferreira, Tony Ramirez and Craig Dawe.

The MOBB effort at the BSL is suppored by funds from NSF/OCE and UC Berkeley.

References

Cox, C., T. Deaton and S. Webb, A deep-sea differential pressure gauge, J. Atm. Ocean. Tech., 1, 237-245, 1984.

Romanowicz, B., D. Stakes, J. P. Montagner, P. Tarits, R. Uhrhammer. M. Begnaud, E. Stutzmann, M. Pasyanos, J.F. Karczewski, S. Etchemendy, MOISE: A pilot experiment towards long term Sea-floor geophysical observatories, Earth Planets Space, 50, 927-937, 1999.

Stakes, D., B. Romanowicz, J.P. Montagner, P. Tarits, J.F. Karczewski, S. Etchemendy, D. Neuhauser, P. McGill, J-C. Koenig, J.Savary, M. Begnaud and M. Pasyanos, MOISE: Monterey Bay Ocean Bottom International Seismic Experiment, EOS Trans., A.G.U., 79, 301-309, 1998.

Stutzmann, E., J.P. Montagner et al., MOISE: a prototype multiparameter ocean-bottom station, Bull. Seism. Soc. Am., 81, 885-902, 2001.



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