Geophysical quantification of seafloor greenhouse gas: the effect of gas bubble and hydrate morphology on sediment geophysical properties.

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Global climate prediction models need accurate information on the amount of greenhouse gases (methane CH4 and carbon dioxide CO2) hosted by seafloor sediments as free gas and gas hydrates. Extensive distributions of seafloor methane gas and methane gas hydrate have been detected by geophysical surveys on continental margins around the world, while monitoring of carbon dioxide seepage from sub-seafloor CO2 reservoirs will become increasingly important as full scale carbon capture and storage facilities come online in future. However, quantification of the amount of in situ gas using geophysical remote sensing methods remains a challenge. In this technology-led proposal, we intend to provide the required step change in knowledge that will allow us to relate seafloor geophysical measurements to gas content and thus provide the marine community with the necessary survey know-how.The main barrier to progress is our poor state of knowledge of the effect of gas and gas hydrate morphology (i.e., size and shape) on the measured geophysical sediment properties acoustic velocity and attenuation, and electrical resistivity. Gas bubbles in sediments are known to show complex shapes and size distributions that are strongly influenced by sediment type. Muddy sediments show crack-like gas bubbles while sandy sediments show spheroidal gas bubbles. If these sediments occur in deep enough water on the continental slope, then methane gas hydrate may form producing equivalent crack-like or disseminated hydrate morphologies. Only dedicated, well controlled laboratory experiments can hope to unravel the complex interaction between gas and hydrate morphology, sediment type and the observed geophysical properties. Unfortunately, no such experimental capability exists at present, so we will have to develop our own laboratory measurement system.Our solution is to build the world's first acoustic pulse tube for gas- and gas hydrate-bearing sediment studies. It will enable the bulk acoustic and electrical properties of large sediment core samples, up to 1 m long, containing natural methane (or carbon dioxide) gas bubbles or hydrate, to be measured under simulated seafloor pressures and temperatures. Experiments on synthetic muds with known amounts of methane and hydrate will also assist our understanding of these physical property inter-relationships. We will also study relevant theoretical models that will be tested against the laboratory experimental results. These validated models are what we need to interpret seafloor geophysical measurements in terms of in situ gas and hydrate content. We will interact with other scientists seeking to quantify seafloor greenhouse gas associated with methane hydrates in the Arctic and sub-seafloor carbon dioxide storage sites, and with potential industry and government end-users of seafloor geophysical technologies.