Introduction: Over the last decade, large deposits of methane hydrates have been identified along the world continental margins. Frozen mixtures of hydrocarbon gas (mostly methane) and water occur over large areas of the ocean floor and vastly exceed other carbon-energy reservoirs. With a maximum content of 164 m³ of methane and 0.8 m³ of water at standard temperature and pressure per cubic meter of hydrate and an estimated range of 26 to 139 X 1015 m³ globally, this is a significant new energy source. The content of methane in hydrates is variable and is controlled by geothermal gradients and biological methane production. International research has begun, with a primary goal of obtaining the methane in these hydrates as an energy source.

This requires a broad range of scientific efforts to address the methane hydrate presence, develop mining strategies, and predict the impact on the environment and platform stability. The Naval Research Laboratory (NRL) has developed strong research topics regarding methane hydrates over the last 30 years. NRL has unique field and laboratory expertise that couples physical, chemical, and biological parameters to address methane hydrate distribution, formation, and stability. Recent, current, and planned field work is active on the Texas-Louisiana Shelf in the Gulf of Mexico, Nankai Trough off the eastern coast of Japan, Blake Ridge in the northwestern Atlantic Ocean, the Cascadia Margin in the northeastern Pacific Ocean, and the Haakon-Mosby Mud Volcano (MV) in the Norwegian-Greenland Sea (Fig. 7).

Research Approach: Key program efforts at NRL includes integration of: geoacoustical surveys to predict hydrate locations; methane sensors to trace hydrate rich regions in the ocean floor; field and laboratory analysis of hydrate structure and content; and stable and radio carbon isotope analysis to assist in the interpretation of methane sources and understanding of the hydrate content and stability. The following sections describe Code 6000's efforts in this project.

Testing and Development of a Methane Sensor: Methane sensing is applied to identify potential hydrate-rich regions in the sediments and to study the flow of methane from these regions into the water column. A methane sensor, METS, from ASD Sensortechnik Gmgh (Germany) became commercially available at the start of the Methane Hydrate Advanced Research Initiative (ARI). The METS sensor specifications list an operational depth range from 0 to 2000 m, temperature range of 0 to 40 C, and a methane concentration range of 50 µmol/l to 10 µmol/l. The methane sensor is a semiconductor (metal oxide) that works on the principle of hydrocarbon adsorption. The data collected during the summer 2000 cruise to the Gulf of Mexico was obtained from one sensor, D21. The METS sensor was placed on the forward platform of the submersible in view of the operator. This allowed the operator and observer to properly annotate the sampling events since multiple experiments were ongoing on each dive. Figure 8 shows data collected while working with hydrate mounds or with pieces of hydrate. Methane concentrations rise from a background level of ~0.1 µmols/l to a high of ~8.8 µmols/l. The first peak (~2.1 µmols/l) at 5,788 seconds was obtained when working around a loose piece of hydrate. While working a hydrate mound at 6,634 seconds, a concentration of ~2.9 µmols/l was obtained. The highest concentration (~9 µmols/l) occurred while cores were being taken in a hydrate mound. The peak located at 10,523 seconds occurred while working in a mussel bed located to the left of the hydrate mound.

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