Multidisciplinary Observatory Development in Mon- terey Bay

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Multidisciplinary Observatory Development in Mon-

terey Bay

Debra Stakes, Paul McGill, Ken Johnson, Hans Jannasch, C.Geoff Wheat.

debra@mbari.org, mcgill@mbari.org, johnson@mbari.org, jaha@mbari.org, wheat@mbari.org

Monterey Bay Aquarium Research Institute, 7700 Sandholt Road, Moss Landing CA 95039-

9644, USA

Barbara Romanowicz, Doug Neuhauser, Robert Uhrhammer

barbara@seismo.berkeley.edu, doug@seismo.berkeley.edu, bob@seismo.berkeley.edu

Seismological Laboratory, U.C. Berkeley, Berkeley CA USA

Monterey Bay Aquarium Research Institute has

several ongoing parallel development efforts that

have a common goal of providing long-term multi-

sensor measurements of ground motion and fluid

flow on the seafloor. Instrument packages are be-

ing designed to operate in a low-power, autonomous

mode, but a planned logger/controller will provide

ethernet connection to available cables to either a

shore laboratory or a telemetered mooring.

Seismometer development was initiated at

MBARI in 1996 with the development of a short-

period corehole seismometer, designed and fabri-

cated by the JPL Microdevice laboratory. This sen-

sor package uses three standard 4.5 hz geophones

with an extended response to about 0.8 hz,. The

sensor package is placed into small-diameter bore-

holes drilled into basement outcrops to provide su-

perior coupling and greatly reduced noise. A new

low-power datalogger, the GeoSense LP1, was de-

veloped in 1997, and can provide more than 24 GB

on less than 0.3 watt of power, easily supporting 12

month deployments.

Methodologies to deploy broadband seismic sen-

sor packages has been undertaken in collaboration

with UC Berkeley. In the summer of 1997, a suite

of geophysical instrument packages was deployed

on the ocean floor for 3 months using MBARI’s

ROV Ventana, under the aegis of MOISE (Mon-

terey Bay Ocean Bottom International Seismic Ex-

periment), an international cooperative pilot exper-

iment between MBARI, DT/INSU,IPGP and UBO

(France) and UC Berkeley. The deployment site was

in Monterey Bay, 40km offshore and 10km west of

the San Gregorio fault at a water depth of 1015m.

The instruments deployed included; a 3 component

Guralp CMG-3T broadband seismometer system,

mounted on leveling gymbals, with a self-recentering

program designed by DT/INSU. The sensor pack-

age was half buried in the sediments. The broad-

band was connected on the seafloor by the ROV

to an L-CHEAPO recording system (designed by

Scripps/IGPP and modified by MBARI) and to an

external battery pack. A CTD/Paroscientific pres-

sure gauge package and an S4 current meter were

placed nearby to measure local bottom currents.

A stand-alone electromagnetic sensor package, con-

tributed by UBO was also deployed by the ROV and

recovered via elevator. In addition, two MBARI

well-coupled corehole seismometers were deployed

at adjacent sites along with a variety of conven-

tional short-period seismometers and hydrophones

deployed as stand-alone, contemporaneous measure-

ments. On three separate dives a connection was

made from the ship to the L-CHEAPO via the ROV

RS232 with command and data links through the

datalogger to the seismometer package. Through

this link we were able to a) successfully level and ini-

tiate the data collection and recentering routines, b)

extract samples of both historical and real-time data

to the state-of-health of the system and c) re-level

the seismometer sensors. This experiment was, in

particular, a successful feasibility test for ocean floor

deployment of broadband sensors using an ROV.

As a follow-up to the MOISE experiment, in a

cooperative project between MBARI and UC Berke-

ley, with partial support from NSF, we are currently

planning the permanent deployment of a broad-

band, three component seismometer package, in

Monterey Bay. The deployment will comprise a Gu-

ralp CMG-1T sensor package, which will be com-

pletely buried under the seafloor, a modified LP1

data logger, current meter and pressure gauges, and

will be initially configured for stand-alone operation

for at least 1 year. The deployment is planned for

September 2001. The data will be available through

the NCEDC (Northern California Earthquake Data

Center) at UC Berkeley, as part of the Berkeley Dig-

ital Seismic Network. Eventually, data telemetry to

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UC Berkeley will be achieved by hooking up the

seafloor packages to the buoy system and/or ocean

floor cable developed in the framework of the MOOS

program at MBARI.

Five MBARI corehole seismometers were de-

ployed in Monterey Bay during 9 months in 1998

as part of the MBARI Margin Seismology Project.

The sensors were placed into holes in granite out-

crops drilled by the ROV diamond coring system.

On sedimented seafloor, the instruments were de-

ployed within low-provide cement ”portable” bore-

holes. The data from these extended deployments

have constrained the distribution of seismicity on

the off-shore segments of the San Gregorio fault,

identifying the northern San Gregorio as a zone of

anomalously high seismicity (Begnaud and Stakes,

2000) with dominantly thrust focal mechanisms. An

improved local crustal velocity model permitted the

relocation of all moderate events dating back to 1926

(Begnaud et al., 2000). Based on these results, the

southern San Gregorio is apparently aseismic. Dur-

ing 2001, three of these corehole instruments will be

deployed to both supplement data from the land-

based array and to provide developmental sites for

a planned logger development to the LP2. This ad-

vanced datalogger will use a new cpu for faster com-

putations and insitu data analyses. The LP2 will

also provide an ethernet connection for faster data

transfers. We envision the LP2 as the core of a mini-

observatory to operate in an autonomous mode or

as a multiple sensor node on a larger cabled experi-

ment.

During the next two years our goal will be

to interface multiple sensors onto the LP2 log-

ger/controller for studies of fluid flow in volcanically

and tectonically active sections of the mid-ocean

ridge. This effort will combine the achievements of

the seismology efforts with new chemical sensors de-

veloped at MBARI and MOOS, the MBARI Ocean

Observatory System, a telemetered mooring capa-

ble of providing near real-time communication to

instruments on the seafloor. As part of this latter

effort, the ROV will be used to install thin cables

between instruments and a central node.

Chemical sensor systems with long-term (order

1 year) endurance and stability now exist for sev-

eral chemicals of relevance to hydrothermal systems

including sulfide, bromide and iron. These systems

can be deployed in an observatory network to couple

seismic, heat flow and chemical observations in an

integrated study of the processes that transport and

transform mantle energy into hydrothermal ecosys-

tems. The Fe-Osmoanalyzer has successfully de-

ployed on Axial Seamount, monitoring Fe-variations

in vent fluids. The new sensors will be supplemented

by conventional physical sensors to measure temper-

ature and particulates adapted to the extended peri-

ods of observation. The Osmosampler collects con-

tinuous small volumes of fluid into capillary tubes.

These samples will be used to supplement the data

provided by the in-situ analyzers. Other, more com-

plex, fluid and microbial samplers have yet to be

adapted to seafloor deployments.

References cited:

Begnaud, M.L. and D.S. Stakes, Constraining

continental margin seismicity by extending on-shore

seismograph stations to critical off-shore sites, Bull.

Seism. Soc. Am., 90, 414-424, 2000.

Begnaud, M. L., K. C. McNally, D. S. Stakes

and V. A. Gallardo, A crustal velocity model for

locating earthquakes in Monterey Bay, California,

Bull. Seism. Soc. Am., in press,