P. R. McGill

Identifying and removing noise from the Monterey ocean bottom broadband seismic station (MOBB) data

When compared to quiet land stations, the very broadband Monterey ocean bottom seismic station (MOBB) shows increased long-period background as well as signal-generated noise. Both sources of noise are unavoidable in shallow ocean bottom installations, and postprocessing is required to remove them from seismic observations. The long-period background noise observed for periods longer than 20 s is mainly due to infragravity waves and ocean currents. The shorter-period signal-generated noise, on the other hand, is due to reverberations of the seismic waves in the shallow sedimentary layers as well as in the water layer. We first present the steps that were taken prior to and during the instrument deployment to minimize instrument generated noise as well as to avoid noise due to water flow around the instrument. We then present results from two postprocessing methods that can be used to remove the long-period background noise, which both utilize the ocean bottom pressure signal locally recorded on a differential pressure gauge (DPG). One consists of subtracting the locally recorded ocean bottom pressure from the vertical seismic acceleration signal. In this case the frequency-independent scale factor is linearly estimated from the data. The other one makes use of the transfer function between the vertical seismic and pressure signal to predict the vertical component deformation signal. The predicted signal is then removed from the vertical seismic data in either frequency or time domain. We also present two methods that can be used to remove the signal-generated noise. One employs the empirical transfer function constructed from MOBB data and nearby land station data that do not show the signal-generated noise. The other one uses a synthetic transfer function computed by modeling the response of shallow layers at the MOBB location. Using either of the two transfer functions, most of the signal-generated noise can be removed from the MOBB data by deconvolution.

The MOBB experiment: A prototype permanent off‐shore ocean bottom broadband station

Technical accomplishments of the past 10 years in the design and deployment of sea floor broadband seismic systems are now making it possible to start addressing the issue of the limited coverage of the Earth that can be achieved through land-based installations, at the regional or global scale. In particular, the September 2002 Ocean Mantle Dynamics (OMD) workshop in Snowbird, Utah [OMD Workshop Committee, 2003] proposed the development of two “leap-frogging arrays” of about 30 broadband sea floor instruments to fill geophysically important target holes in ocean coverage for deployment periods of 1 to 2 years. The rationale for an off-shore (“Webfoot”) component of the SArray/Earth-scope “Bigfoot” array was also highlighted at this meeting, pointing out that the study of the North American continent should not stop at the ocean margin. The ocean floor environment is challenging for broadband seismology for several reasons. Broadband seismometers cannot be simply “dropped off” a ship with the expectation that they will produce useable data, particularly on the horizontal components. Several pilot experiments, [e.g., Montagner et al., 1994; OSN1, 1998; Suyehiro et al., 2002] have addressed the issue of optimal installation of ocean bottom stations, and in particular, have carried out comparisons between borehole, sea floor, and buried sea floor installations.

A seismic swarm and regional hydrothermal and hydrologic perturbations: The northern Endeavour segment, February 2005

The February 2005 swarm at the overlapping spreading center (OSC) on the northern end of the Endeavour segment is the first swarm on the Juan de Fuca Ridge recorded on a local seafloor seismic network. The swarm included several larger earthquakes and caused triggered seismicity and a hydrothermal response in the Endeavour vent fields as well as regional-scale hydrologic pressure perturbations. The spatial and temporal pattern of over 6000 earthquakes recorded during this seismic sequence is complex. Small-magnitude events dominate, and seismicity rates wax and wane, indicating a magmatic process. The main swarm initiates at the northern end of the Endeavour ridge. However, most of the moment release, including six strike-slip events, occurs in the southwest Endeavour Valley, where the swarm epicenters generally migrate south. The main swarm is contemporaneous with a hydrologic pressure response at four sealed seafloor boreholes, ∼25–105 km away. We infer that the seismic sequence is driven by a largely aseismic magma intrusion at the northern Endeavour axis. Resulting stress changes trigger slip on tectonic faults and possibly dike propagation at the opposing limb of the Endeavour OSC in the southwest Endeavour Valley, consistent with the eventual decapitation of the Endeavour by the West Valley segment. Furthermore, 2.5 days after the start of the main swarm, seismicity is triggered beneath the Endeavour vent fields, and temperature increases at a diffuse vent in the Mothra field. We infer that this delayed response is due to a hydrologic pressure pulse that diffuses away from the main magma intrusion.

Seismic experiment paves way for long-term seafloor observatories

The Monterey Bay Ocean Bottom International Seismic Experiment (MOISE) has successfully deployed a suite of geophysical and oceanographic instrument packages on the ocean floor using the Monterey Bay Aquarium Research Institutes (MBARI) Ventana, a tethered Remotely Operated Vehicle (ROV).The goal of this international cooperative experiment is to advance the global Seafloor Observatory effort by developing a prototype suite of instruments and installing them on the western side of the San Andreas fault system offshore of central California. The centerpiece of the instrument suite was a digital broadband seismometer package partially buried within the sediment-covered floor of Monterey Bay, 40 km offshore and 10 km west of the San Gregorio fault at a depth of 1015 m (Figure 1).

MARS Benthic Rover: In-situ rapid proto-testing on the Monterey Accelerated Research System

The Benthic Rover is an autonomous, bottom-crawling vehicle being developed at the Monterey Bay Aquarium Research Institute (MBARI) to conduct long-term deep-ocean ecological research. In 2009 MBARI researchers deployed the Rover on the Monterey Accelerated Research System (MARS) cabled observatory for component and operational testing. MARS is located near-shore in the Monterey Bay near Monterey, CA, at a depth of approximately 900 meters, providing the power reliability and network accessibility similar to an on-shore laboratory. By enabling immediate feedback and the ability to quickly re-program control software and re-configure mission scripts, MARS facilitates a kind of “in-situ rapid proto-testing”. MBARI researchers were able to run numerous experimental procedures and analyze results in a relatively short timeframe, converging on desired operational profiles quickly and at very low cost. This paper will cover recent development work on the Benthic Rover with emphasis on the deployment and testing on MARS.

Subsea instrument deployments: methodology and techniques using a work class remotely operated vehicle (ROV)

The conventional deployment of sensors and data-logging equipment by free fall from the oceans surface is fraught with many technical problems, including unpredictable instrument placement, lack of control over site selection, unreliable instrument orientation, lack of communications with the instrument during deployment, and short measurement duration due to limited battery power. Through several experiments, MBARI has attempted to reduce or eliminate the impact of a number of these problems by utilizing the communicative and manipulative capabilities of the ROV Ventana. The development of corehole seismometers required methods to maintain the connection to a data logger during the deployment, and then allow in situ connection of the logger to an instrument to verify data collection and integrity in real time. The Monterey Bay Ocean Bottom International Seismic Experiment (MOISE) allowed for specific site selection and preparation prior to instrument deployment, real-time data access, control and verification of instrument and data integrity, and the ability to revisit the site to access both the instrument and data logger as well as supply additional power. The Autonomous Video Camera System (AVCS) allows selection of sites or subject matter for long-term video observation with the ability to verify the operation and target subject prior to disconnection of the ROV from the camera system. The One- and Two-Meter Hydraulic Tube Coring experiments allowed for precise placement and controlled execution of sediment coring operations. The preceding are some development efforts that demonstrate how technical and methodological problems were approached. By the very nature of the solutions, new questions have arisen and more advanced methods of achieving technical goals are being pursued.

The Deployment of a Long-Term Seafloor Seismic Network on the Juan de Fuca Ridge

From 2003-2006, a novel seismic network comprising seven short-period corehole seismometers and a broadband Guralp CMG-1T OBS was deployed using remotely operated vehicles (ROVs) in a subseafloor configuration on the Endeavour Segment of the Juan de Fuca mid-ocean ridge as part of a multi-disciplinary prototype NEPTUNE experiment to investigate the linkages between seismic deformation, hydrothermal fluxes, and microbial productivity. The networks recorded high quality data that illustrate the advantages of using an ROV to deploy seismometers in well-coupled configurations away from the effects of ocean currents. The data is presently being analyzed to understand the linkages between seismic deformation and hydrothermal circulation, and the nature of seismic swarms that are associated with ridge spreading events. A subset of the seismometers will be incorporated into the NEPTUNE Canada cabled observatory.

Initial Deployments of the Rover, an Autonomous Bottom-Transecting Instrument Platform for Long-Term Measurements in Deep Benthic Environments

Rover is a bottom-crawling, autonomous vehicle capable of making continuous time-series measurements at abyssal depths up to 6000 m for periods exceeding six months. The Rover control system and instrumentation suite are being designed at the Monterey Bay Aquarium Research Institute (MBARI), building on the earlier rover work of Smith and associates at the Scripps Institution of Oceanography. The vehicle weighs 68 kg in water and crawls on two wide tracks with a combined surface contact area of about one square meter; this provides good traction while minimizing the disturbance to benthic sediments. A typical mission scenario is to take measurements for a few days at each site before picking up the instruments and moving forward ~10 m to a new site. Up to fifty sites may be visited in a single mission. Engineering field tests have been performed with the Rover in the Monterey Bay in California (890 m depth), and at Station M, 220 km west of the central California coast (4200 m depth). Rover operations have been observed with the ROVs Ventana and Tiburon, and with the manned submersible DSV Alvin. Knowledge gained from these engineering deployments has resulted in numerous modifications and improvements to The Rover.