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OS1B-8:A LABORATORY PULSE TUBE STUDY OF THE EFFECT OF METHANE HYDRATE ON THE ACOUSTIC AND ELECTRICAL PROPERTIES OF SEAFLOOR SEDIMENTS
发布时间:2014-07-28
Angus I. BEST 1, Laurence J. NORTH 1, Jeremy SOTHCOTT 1, Clive MCCANN 1, Timothy L. LEIGHTON 2, Paul R. WHITE 2 & Hakan DOGAN 2
1. National Oceanography Centre, University of Southampton , UNITED KINGDOM; 2. Institute for Sound and Vibration Research, Engineering and the Environment, University of Southampton, UNITED KINGDOM
Geophysical remote quantification of seafloor methane hydrates is needed for studies of the impact of global warming on greenhouse gas release into the water column and atmosphere, for geohazard prediction and for economic resource assessment. Both methane hydrate, and especially any associated free methane gas, are known to give rise to acoustic velocities and attenuations that are highly dependent on measurement frequency. However, the exact relationships between these acoustic properties and hydrate content are poorly understood, but are known to be influenced by hydrate and gas bubble morphology, which are influenced in turn by sediment type. Gas “bubbles”, and resultant hydrate morphologies when the sediment is within the gas hydrate stability zone, take on sheet-like, lenticular or nodular forms in fine grained, cohesive sediments (clays and muds), but tend to be finely dispersed and spherical within the pore spaces of coarse grained sediments (sands and gravels). Previous laboratory studies on methane hydrate-bearing sand showed how seismic frequency (< 500 Hz) velocity and attenuation vary with hydrate content, results useful to marine seismic data interpretation. The aim of this study is to extend those observations to the higher, acoustic frequency range of 1 – 10 kHz, pertinent to high resolution seismic surveys of the seabed and also to sonic borehole logging. Hence, we are developing a novel acoustic pulse tube capable of accepting large sediment core samples (7 cm diameter, < 1 m long) and simulating seafloor gas hydrate formation conditions (temperatures down to 4 ˚C, pore pressures in excess of 20 MPa). Crucially, the size of the samples will enable us to measure bulk behavior of centimeter-scale heterogeneities in sediment samples representative of in situ gas and hydrate morphologies. A novel electrical resistivity tomography system will aid the characterization of sample heterogeneity and give complementary electrical resistivity information, needed for improving joint acoustic-electrical interpretation schemes. The experimental results on sands and muds will be used to develop acoustical theory on gas bubble resonance and its application to seafloor hydrate quantification.
1. National Oceanography Centre, University of Southampton , UNITED KINGDOM; 2. Institute for Sound and Vibration Research, Engineering and the Environment, University of Southampton, UNITED KINGDOM
Geophysical remote quantification of seafloor methane hydrates is needed for studies of the impact of global warming on greenhouse gas release into the water column and atmosphere, for geohazard prediction and for economic resource assessment. Both methane hydrate, and especially any associated free methane gas, are known to give rise to acoustic velocities and attenuations that are highly dependent on measurement frequency. However, the exact relationships between these acoustic properties and hydrate content are poorly understood, but are known to be influenced by hydrate and gas bubble morphology, which are influenced in turn by sediment type. Gas “bubbles”, and resultant hydrate morphologies when the sediment is within the gas hydrate stability zone, take on sheet-like, lenticular or nodular forms in fine grained, cohesive sediments (clays and muds), but tend to be finely dispersed and spherical within the pore spaces of coarse grained sediments (sands and gravels). Previous laboratory studies on methane hydrate-bearing sand showed how seismic frequency (< 500 Hz) velocity and attenuation vary with hydrate content, results useful to marine seismic data interpretation. The aim of this study is to extend those observations to the higher, acoustic frequency range of 1 – 10 kHz, pertinent to high resolution seismic surveys of the seabed and also to sonic borehole logging. Hence, we are developing a novel acoustic pulse tube capable of accepting large sediment core samples (7 cm diameter, < 1 m long) and simulating seafloor gas hydrate formation conditions (temperatures down to 4 ˚C, pore pressures in excess of 20 MPa). Crucially, the size of the samples will enable us to measure bulk behavior of centimeter-scale heterogeneities in sediment samples representative of in situ gas and hydrate morphologies. A novel electrical resistivity tomography system will aid the characterization of sample heterogeneity and give complementary electrical resistivity information, needed for improving joint acoustic-electrical interpretation schemes. The experimental results on sands and muds will be used to develop acoustical theory on gas bubble resonance and its application to seafloor hydrate quantification.
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