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Projects: Projects for Investigator
Reference Number	NE/H00257X/1
Title	Reduction of noise on broadband ocean-bottom seismographs through sensor design optimization using numerical and laboratory studies
Status	Completed
Energy Categories	Fossil Fuels: Oil Gas and Coal(Oil and Gas, Other oil and gas) 5%; Not Energy Related 95%;
Research Types	Basic and strategic applied research 100%
Science and Technology Fields	ENGINEERING AND TECHNOLOGY (Electrical and Electronic Engineering) 25%; ENVIRONMENTAL SCIENCES (Earth Systems and Environmental Sciences) 75%;
UKERC Cross Cutting Characterisation	Not Cross-cutting 100%
Principal Investigator	Dr N (Nicholas ) Harman No email address given National Oceanographic Centre, NERC
Award Type	1
Funding Source	NERC
Start Date	01 May 2010
End Date	30 April 2012
Duration	24 months
Total Grant Value	£107,972
Industrial Sectors	No relevance to Underpinning Sectors
Region	South East
Programme
Investigators	Principal Investigator	Dr N (Nicholas ) Harman , National Oceanographic Centre, NERC (99.999%)
	Other Investigator	Dr SR (Stephen ) Turnock , School of Engineering Sciences, University of Southampton (0.001%)
Web Site
Objectives	The primary objective is to develop an effective, low cost alternative to ocean-bottom seismometer burial for noise level reduction. The goal of the proposed research is to develop a small broadband seismometer package that is hydrodynamically stable with a low profile to minimize sensor tilt in ocean-bottom currents. This is a long-standing problem in ocean-bottom seismology, and can effectively half the data returned from the instruments at noisy sites. To be cost effective, the seismometer is deployed from a research vessel and allowed to fall where it may on the ocean floor, where the instrument is coupled to the solid earth and the water column. Typically, the sensor packages are housed in spherical or cylindrical pressure casings, which are not inherently stable in ocean-bottom currents. By reducing the surface area of the sensor package in contact with the water column, and lowering the profile of the sensor into a hydrodynamically stable shape we will be able to reduce instrument tilt and thereby improve the fidelity of the instrument. We will explore both pressure case designs and "pseudo-burial" deployment approaches to determine which is the most effective at reducing noise in horizontal ground motion observations in the presence of ocean currents. The secondary objective is to construct a working prototype for incorporation into the NERC Ocean Bottom Instrumentation Facility's ocean-bottom instrument platform and expand its capabilities to broadband ocean-bottom seismology for use by the wider UK seismology community. The laboratory experiments proposed will verify the functionality of three prototypes and help determine best practices for deployment of these instruments when they go into the field for operational deployments. The technology available to the UK's scientific community lags behind other nations, such at the US, Japan, France and Germany in the development of these types of instruments, but the design innovation proposed herein should give the UK seismological community a competitive advantage in future deployments by providingsuperior data quality and acquisition rates.
Abstract	Seismology is the main tool for understanding the structure and dynamics of the Earth's interior. Yet, due to the extreme environment and remoteness of the deep ocean basins there is a dearth of seismic coverage on the ocean bottom. Within the past decade broadband ocean-bottom seismographs (OBS) have been developed that can be deployed from any oceanographic vessel, record data autonomously for periods of over a year, and be recovered from the seafloor. While the data quality for these instruments for the vertical component of ground motion approaches that of similar temporary deployments on land, the horizontal components, which record shear waves, are plagued by high noise levels caused by tilting due to ocean-bottom currents. The poor data quality of the horizontal components is a serious limitation of broadband ocean-bottom seismology and hinders our ability to study both the dynamics of the Earth's interior and important tectonic processes that occur on the ocean floor, which produce earthquakes, volcanism and tsunamis. Burial of the sensor package effectively reduces noise levels on the OBS horizontal components, but it is prohibitively expensive, requiring an remotely operated vehicle (ROV) and increased ship time for each deployment and recovery. A more cost effective alternative is to reduce the coupling of the sensor package with the water column by reducing the surface area of the sensor package in contact with it. This may now be possible because Nanometrics Seismological Instruments Inc. has produced a compact (90 mm diameter, 128 mm height), low power (150 mW) broadband (0.083-100 Hz) seismometer package that should be ideal for broadband ocean-bottom passive and active source seismology. This smaller sensor is 27% the volume of similar bandwidth models, and would require a substantially smaller pressure housing and gimbaling system. We propose to investigate whether a smaller profile hydrodynamically stable sensor package or instrument "pseudo-burial" can yield significant improvement in horizontal component noise levels. In the firststage of development we will use theoretical understanding coupled with numerical simulations of fluid flow to propose a sensor package shape that helps controls descent attitude, landing, and minimises the flow induced noise when the sensor is in place. In the second phase we will construct prototype sensors and compare and contrast the fidelity of a broadband seismic sensor using analogue fluid experiments. In the analogue experiments, we will also experiment with "pseudo-burial" of sensor packs by simulating a deployment of the sensors in sediment-filled bags to reduce coupling between the pack and the water column. If successful, this research will solve a long-standing issue in ocean-bottom seismology, and will be a huge leap forward in our ability to understand the shear velocity structure of the Earth's interior. These experiments are first steps towards expanding the NERC Ocean-Bottom Instrumentation Facility (OBIF) capabilities into broadband ocean bottom seismology.
Publications	(none)
Final Report	(none)
Added to Database	22/03/11