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

全球气候预测模型需要关于海底沉积物作为游离气体和天然气水合物所含温室气体(甲烷、甲烷和二氧化碳)数量的准确信息。地球物理调查在世界各地的大陆边缘发现了海底甲烷气体和甲烷气体水合物的广泛分布，而随着未来全面的碳捕获和储存设施的上线，监测海底二氧化碳储藏库的二氧化碳渗漏将变得越来越重要。然而，使用地球物理遥感方法对就地天然气的数量进行量化仍然是一个挑战。在这项以技术为主导的提案中，我们打算提供必要的知识变革，使我们能够将海底地球物理测量与气体含量联系起来，从而向海洋界提供必要的测量专门知识。取得进展的主要障碍是我们对气体和天然气水合物形态(即大小和形状)对测量的地球物理沉积物性质的影响的知识状况较差。众所周知，沉积物中的气泡具有复杂的形状和大小分布，受沉积物类型的影响很大。泥质沉积物表现为裂隙状气泡，砂质沉积物表现为球形气泡。如果这些沉积物出现在大陆斜坡足够深的水中，那么甲烷气体水合物可能会形成等效的裂隙状或浸染状水合物形态。只有专心致志、控制良好的实验室实验才有希望揭开天然气和水合物形态、沉积物类型和观测到的地球物理性质之间的复杂相互作用。不幸的是，目前还没有这样的实验能力，所以我们必须开发自己的实验室测量系统。我们的解决方案是建造世界上第一个用于含气和天然气水合物沉积物研究的声波脉冲管。它将能够在模拟海底压力和温度下测量含有天然甲烷(或二氧化碳)气泡或水合物的长达1米的大型沉积物岩心样品的整体声学和电学性质。对甲烷和水合物含量已知的合成泥浆进行的实验也将有助于我们理解这些物理性质的相互关系。我们还将研究相关的理论模型，并将其与实验室实验结果进行验证。这些经过验证的模型是我们根据原位气体和水合物含量来解释海底地球物理测量所需的。我们将与寻求量化与北极和海底下二氧化碳储存地点的甲烷水合物有关的海底温室气体的其他科学家，以及与海底地球物理技术的潜在行业和政府终端用户进行互动。

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.