| EPSRC Reference: |
EP/H018573/1 |
| Title: |
Developing methods for high-precision measurement of shear-wave splitting |
| Principal Investigator: |
Haacke, Dr RR |
| Other Investigators: |
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| Department: |
Earth Sciences |
| Organisation: |
Royal Holloway, Univ of London |
| Scheme: |
First Grant - Revised 2009 |
| Starts: |
24 April 2010 |
Ends: |
23 April 2011 |
Value (£): |
53,808
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| Panel History: |
| Panel Date | Panel Name | Outcome |
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26 Nov 2009
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Process Environment and Sustainability Panel
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Announced
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Summary on Grant Application Form |
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The nature of the seabed is increasingly important to modern science, technology, and industry as exploration and exploitation of the oceans moves further offshore and into deeper water than ever before. Humankind needs to build on, dig through, observe, and depend upon the seabed, the uppermost few hundred metres of which are at the interface between the geological processes that occur below the seabed and the oceanographic, atmospheric, and environmental processes that occur above it. It is this near-surface part of the seabed that must be understood and relied upon when seafloor infrastructure is built, or cables and pipes are laid across oceans and seas.Recent work has shown that near-seabed sediments are elastically anisotropic. Seismic measurements of this anisotropy will provide a useful tool for observing small changes in the physical properties of these sediments and in the processes that affect them. Such properties include the state of stress and strain, the pore-fluid pressure, the presence and alignment of fractures, and the overall sediment permeability and competence. Quantifying changes in the seismic record over space or time would enable us to monitor processes such as slope stability and failure, subsidence, or stress build-up and release during the earthquake cycle, which can damage expensive seabed installations and pose a hazard to people and their environments.Shear-wave splitting is a particularly useful seismic expression of elastic anisotropy. However, we face two problems in analysing split shear waves in sediments near the seabed. The first problem is to generate and record shear waves at sea, which is usually done with a pressure-wave source at the sea-surface. The downgoing pressure waves mode-convert to upgoing shear waves beneath the seabed and are recorded on seafloor receivers. Unfortunately, however, the amount of shear-wave splitting is small for near-surface reflectors, typically a few milliseconds or less. This signal is about the same size as the effects of structural heterogeneity and anisotropy and heterogeneity of the pressure-wave sound-speed profile in the ocean and seabed. The second problem is thus one of signal separation and precision of measurement, which becomes increasingly important and problematic as signal frequency or the ratio of pressure-to-shear pathlengths increases. Current methods work adequately for reservoir rocks many kilometers beneath the seabed, but do not properly account for pressure-wave and reflector effects, so have limited precision and do not work well for the near-surface.The proposed research will develop a powerful new method for isolating the shear-wave splitting from interfering pressure-wave and reflector effects. A feasibility study indicates that the new methodology is unusually robust and will lead to unparalleled levels of precision in measurement. Funding is sought to develop this new method and apply it to data from three sites around the north Atlantic. The high levels of precision achieved by this method are of interest to: (1) geotechnical engineers planning to build on, develop, or otherwise perturb the seabed, since improved site surveying and monitoring techniques will increase site safety and longevity, and reduce negative impacts on the surrounding environment; (2) geoscientists trying to understand the causes and effects of physical processes occurring in the seabed; (3) geophysicists and reservoir engineers seeking to increase the precision and reliability with which elastic anisotropy is mapped in their reservoirs as a proxy for fracture orientation and intensity, thus maximizing production efficiency. The research is particularly timely given the current efforts in Canada, the US, and Europe to build large seafloor observatory networks for scientific research, and given our ongoing efforts to decrease the costs and consequences of energy supply and consumption.
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Key Findings |
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This information can now be found on Gateway to Research (GtR) http://gtr.rcuk.ac.uk
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Potential use in non-academic contexts |
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This information can now be found on Gateway to Research (GtR) http://gtr.rcuk.ac.uk
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Impacts |
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| Description |
This information can now be found on Gateway to Research (GtR) http://gtr.rcuk.ac.uk |
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Sectors submitted by the Researcher |
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This information can now be found on Gateway to Research (GtR) http://gtr.rcuk.ac.uk
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