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In September 2009, Associate Professor Preston Wilson and ME graduate student Chad Greene spent two weeks onboard a U.S. Coast Guard ship studying methane hydrate, an organic material found on and beneath the ocean floor. They embarked from Barrow Alaska, in the Beaufort Sea, at the top of the world. Wilson and Greene are studying methane hydrate as a potential source of energy and are also studying its potential role as a greenhouse gas. The primary purpose of the expedition, Methane in the Arctic Shelf 2009 (MITAS 2009), was to establish benchmark measurements of the rate of methane flux from the sediment, into the water column and ultimately into the atmosphere. The scientists want to know if the rate of methane entering the atmosphere in polar regions is increasing with global warming.
The Purpose of the Expedition
The expedition cruise was focused on the arctic shelf, specifically the continental shelf off the North Slope of Alaska in the Beaufort Sea. Because this area has historically been covered with ice for most of the year, there has been little study of methane hydrate in the area, but the ice is currently receding due to climate change, and this area is ice-free for longer parts of the year than in the past, with the trend projected to increase in the future. The data collected will be used as a benchmark for comparison to future data in hopes of understanding the behavior of methane hydrate in polar regions subject to climate change.
Methane Hydrate—it looks like ice, but it burns
Methane Hydrate burning, U.S. Naval Research.
Methane hydrate, also called methane clathrate or methane ice, is a naturally occurring substance that derives from the decay of organic material, like the natural gas that is commonly used for heating and energy. This material is formed and exists at the temperatures and pressures found at the bottom of the ocean, on or near the continental shelves, at certain locations in the world. It is found both on the ocean bottom, and within ocean bottom sediments. The methane hydrate molecule is composed of a methane molecule surrounded by a cage of water molecules, and is a solid, like ice. It is often mixed with sediment particles and looks like dirty ice, but if lit, this ice will burn. Upon a change in pressure or temperature, methane hydrate can change phase and the methane emerges in gaseous form. This methane gas rises up out of the sediment, due to buoyancy, and can rise through the water column and into the atmosphere. This results in what is known as a methane seep, a flair, or a plume. These rising methane bubbles are also found at numerous locations throughout the world. Methane hydrates have been studied for some time, both for their potential as a source of future energy, and their potential as a greenhouse gas.
The Research Cruise, Methane in the Arctic Shelf 2009
U.S. Coast Guard Cutter Polar Sea used for expedition.
Consequently, a research cruise, Methane in the Arctic Shelf 2009 (MITAS 2009), was undertaken in September 2009. The primary purpose of MITAS 2009 was to establish benchmark measurements of the rate of methane flux from the sediment, into the water column and ultimately into the atmosphere. Scientists wish to monitor the rate of methane entering the atmosphere in polar regions subject to climate change.
The expedition occurred onboard the U.S. Coast Guard Cutter Polar Sea and began in Barrow, Alaska, with logistical support provided by the Barrow Arctic Science Consortium. The continental shelf region off the coast of Alaska from Barrow to Prudhoe Bay was investigated. An international team of geologists, geochemists, biogeochemists and University of Texas at Austin Mechanical Engineering acousticians (Preston Wilson and Chad Greene), made up the science party, with funding provided by the US Department of Energy, the National Science Foundation and the US Navy Office of Naval Research. Additional funding was provided from the home countries of the international scientists.
Core sample of sediment about 12 inches high.
Sediment core samples, water column samples and atmospheric samples were taken along several tracks throughout the region. The concentration of either free methane gas, or the concentration of methane dissolved in the sediment pore water was measured for all of these samples. In addition, the microorganisms that create the methane were studied. These organisms seem to be similar wherever methane hydrate is found. One wonders how did these organisms get so thoroughly distributed throughout the world? These investigations were conducted during the cruise, and the samples collected will be brought back to the laboratories of the various participants for further study. Initial results indicated the presence of free gas bubbles of methane in some of the core samples, dissolved methane in the sediment pore water of other samples, in the ocean water itself and in the atmosphere. The analysis will continue and the full impact of the measurements will not be known immediately, but their ultimate worth will be for comparison with future measurements. The team’s goal is to return to this site in the future, but funding for future expeditions has yet to be secured.
Acoustic Testing to Determine Location of Methane Hydrate Deposits
The University of Texas Mechanical Engineering team is interested in using acoustic remote sensing to help determine the location and extent of methane hydrate deposits in the future. To do this, the acoustic properties of methane hydrate need further study. For example, seismic surveys are currently used extensively to prospect for oil and gas. In order to more fully exploit such data (to detect and quantify methane hydrate), the propagation velocity and attenuation of sound waves in the material must be better understood. Wilson and Greene are particularly interested in the gas phase of methane hydrate. During the methane's migration from deep inside the sediments up into the water, an intermediate condition is a sediment/water/methane gas bubble mixture. This multiphase material has very different acoustic properties than the solid form, and in addition displays significant sound speed dispersion (it has a sound speed that is dependent on the frequency of the acoustic excitation). For example, laboratory work conducted in Wilson's group using artificial sediments at atmospheric pressure, [1] indicated that the sound speed can be around 100 to 200 m/s below 1 kHz, but is around 1500 m/s at 400 kHz.
Scientists must take initial data from core samples as soon as possible after retrieval, hence it is done outside on the ship's deck. The core processing continues indoors for several additional hours.
Traditionally, once a sediment core is brought on deck, a core logger (a device that measures a number of physical properties of the core) measures the acoustic sound speed and attenuation (the loss of acoustic amplitude, or the loudness of a sound, due to various physical processes). These measurements are done at ultrasonic frequencies, around 200 to 400 kHz.
Ultrasound definition
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Although this limit varies from person to person, it is approximately 20 kilohertz (20,000 hertz) in healthy, young adults and thus, 20 kHz serves as a useful lower limit in describing ultrasound.
Seismic surveys and marine streamers, Seismic Surveys, excerpted from Wikipedia
Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. Seismic surveys in water-covered regions are conducted using vessels capable of towing one or more seismic cables known as "streamers." Modern 3D surveys use multiple streamers deployed in parallel to record data suitable for the three-dimensional interpretation of the structures beneath the sea bed.
Chad Greene taking measurements of a core sample on board the Polar Sea.
The frequencies used in seismic surveys are much lower than this, around 3 kHz or less. Hence there is a discrepancy between the acoustic behavior observed with a core logger (the data typically used for seismic survey analysis), and the actual acoustic behavior associated with the low frequency excitation used in seismic surveys. Further, their work, and other work in the past, indicates that the acoustic velocity in gas-bearing sediments is an excellent indicator of the gas volume. Their goal is to more fully understand the acoustics of methane hydrate-bearing sediments, and to determine the correct acoustic properties for use in the analysis of existing and future seismic survey data.
This will lead to a more accurate knowledge of the presence and quantity of methane hydrate in the world, for both climate change and energy applications. Initially, their work was done at their on-campus laboratory, using artificial sediments and a pressure vessel to simulate the pressure found at depth in the ocean. MITAS 2009 was their first opportunity to work with real methane hydrate. Although during this cruise, a number of operational difficulties, including the weather, prevented them from finding any methane hydrate in solid form, they did find methane-gas-bearing sediments and performed acoustic measurements on them. Their initial results with the natural gas-bearing sediments corroborate their earlier laboratory results with artificial sediments. Wilson and Greene found a much lower sound speed at the seismic frequencies than at the ultrasonic frequencies. Data analysis has just begun and relies upon data from other researchers that has yet to be completely processed, so confirmation of their initial results will have to wait. For now, they are conducting additional laboratory research using artificial sediments, measuring the sound speed as a function of gas content, gas molecular weight and pressure.
About Wilson and Greene
Chad Greene began the master's program at The University of Texas at Austin in the summer of 2007 after receiving his BS in in Mechanical Engineering from Virgina Commonwealth University. The research he conducted on the arctic expedition is the basis for his thesis work. Chad will graduate in May 2010 and plans to travel Europe by bicycle before starting work doing environmental research.
Wilson is an Associate Professor in the ME Department and arrived at The University of Texas at Austin in the fall semester of 2003, after receiving the Ph.D. degree in Mechanical Engineering from Boston University. He has studied the acoustics of multiphase materials since that time for medical, environmental and naval sonar applications and is jointly appointed at the University's Applied Research Laboratories.
References
[1] P.S. Wilson, A.H. Reed, W.T. Wood, and R.A. Roy, "The low-frequency sound speed of fluid-like gas-bearing sediments," J. Acoust. Soc. Am. 123, pp. EL99–EL104 (2008).
