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

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.

Improved seafloor geophysical surveys are increasingly in demand to help answer pressing societal questions such as: 1) how will the oceans affect global climate change in future? and 2) how can we sustainably exploit global ocean resources? While the academic sector and inter-governmental agencies are mainly focussed on answering question 1), question 2) is largely in the hands of the private sector. UK companies are major players in the highly competitive global seafloor survey market driven currently by the oil and gas (energy) sector. The leading edge geophysical survey know-how developed here would enable them to sustain and increase their market share, creating jobs and wealth for the UK as a whole. Most importantly, it will enable them to deliver the seafloor data needed, at the scale needed, to manage the ocean economy in an environmentally responsible way. In particular, the proposed research will be of great value to the offshore community working in the following areas:
1) Geophysical and geotechnical survey. Improved geophysical methods are needed to gain faster (ie more economic) and more accurate information on sediment load-bearing capacity and slope stability for the safe siting of seafloor structures, such as wind farms, tidal power generators, oil platforms, pipelines and cable routes. Better knowledge of gas content would also benefit the avoidance of shallow gas pockets as a drilling hazard (e.g . could allow drilling in gassy seafloor areas with only minor gas content that previously would be avoided altogether because present methods can only say if gas is present or not). The project results would also be of interest to geotechnical companies offering specialised autoclave sediment sampling and physical properties measurements.
2) Hydrates exploitation. Better geophysical methods are needed for the commercial assessment of hydrate deposits for natural gas fuel in future (reservoir exploration, characterisation and monitoring during production). Improved high resolution geophysical methods would be particularly useful for locating and assessing relatively small hydrate reservoirs (compared to conventional deep natural gas reservoirs). Hydrate deposits may be exploited in future in combination with CO2 storage (eg German SUGAR project) by replacing methane with CO2 hydrate. Such complex industrial scale seabed operations would require tight control of seabed gas and hydrate contents through geophysical survey.
3) Carbon capture and storage (CCS). There will be an increasing demand for monitoring of seafloor CO2 leakage (and CO2 distribution within storage reservoirs themselves) as large marine CO2 storage projects come online around the world (e.g. Sleipner, Snoehvit, China, India, etc). CCS is currently the only pragmatic engineering solution to arrest the rise in anthropogenic CO2 emissions from burning of fossil fuels. The oil and gas sector will need the new geophysical survey know-how provided by this project to adapt to this emerging new market, estimated to be worth billions of pounds in future.
4) Naval defence. Improved knowledge of the acoustic and electrical properties of seafloor sediments, particularly gas-bearing shelf and continental slope sediments, will be of direct benefit to the protection from mines of military, civilian and relief sea transport and landing operations, as well as submarine warfare. Also, knowledge of possible energy sources on the seabed (eg from methane gas) would be useful for long-term seafloor monitoring stations, including autonomous underwater vehicle (AUV) docking stations (the same technology is relevant to future autonomous oceanographic surveys). Geophysical survey methods are needed for rapid environmental assessment of large areas of seabed prior to military operations. The UK military and strategic allies would benefit from this know-how, thus contributing to national security.