Marine survey for gas hydrate

Abstract

Abstract Oceanic methane hydrate and associated gases occur in continental slope marine sediments in very large quantities. Current interest is being driven by the economic potential of these methane deposits. But their affect on the global carbon cycle and climate change, seafloor stability and safety, plus the influence on the biological food chain have only begun to be resolved. Survey, observation, measurement, and sampling methods must be developed specifically for hydrate system deposits as all occur in the upper 1.5 km of the seafloor in relatively similar marine sediments. Some instrumentation can already characterize these deposits; application of geophysical equipment should be directed toward the range of physical properties common to hydrate system deposits at the target depths. I Introduction Concentrations of gas hydrate that extend over large areas are now being recognized widely in continental slope sediments virtually on a global scale1. Both single and multichannel seismic reflection surveys have revealed hydrate2,3 through the presence of the strong impedance contrast reflector at the base of the hydrate - the bottom simulating reflector or BSR. This is normally seen where free gas is present beneath the hydrate2, 3. Large scale surveys using relatively high resolution side-scan sonar acoustic imagery of the seafloor have revealed pockmarks, mud diapirs and mud volcanoes that are often related to methane gas flux4, 5. These surveys have also mapped large scale slope disturbances and sediment mass wasting often attributed to gas hydrate instability6, 7. Larger scale failures appear to correlate with regions of higher gas hydrate occurrence, at least on the European margins8. However this relationship can be tenuous in some localities, such as in the Mediterranean Sea, where gas hydrates are not known to occur widely9. This illustrates the need for better measurement of gas hydrate distribution in both area and depth. To date these wide-area survey techniques have been serendipitous in their revealing of the presence of oceanic hydrate system diagenetic features in that they were tuned to explore deeper geophysical - geological features10. The acoustic systems and processing were usually designed to reveal hydrocarbon exploration targets or crustal features of scientific interest. Methane gas flux and oceanic hydrates however, are confined to the uppermost 0.5 to 2 km of marine sediments, with hydrate methane resource issues focusing at between 200 and 800 m depth in the seafloor. The impact of hydrate on seafloor stability, safety and the environment may be confined to the top few hundred meters of sediment. A commercially successful strategy for methane production does not yet exist though Japan has stated their resources are on the brink of being considered as reserves. Therefore it is pragmatic to consider effective survey logistics for characterizing the various hydrate issues. This is relevant irrespective of whether gas hydrate is of commercial interest in its own right, is an environmental and engineering hazard, or an acoustic layer of scientific interest within the seabed. This paper considers the two important modes of gas hydrate occurrences and defines the current technology that can be used to effectively measure, or at least map, its distribution on the continental margins of the world's oceans. We propose an outline for survey and monitoring requirements for seafloors in which gas hydrate occurs.