Seismic-Imaging Research Cruise Investigates Deepwater Gas Hydrate Deposits in the Gulf of Mexico
Interest is mounting in the possibility that gas hydrate—a naturally occurring ice-like substance that contains vast quantities of methane—might be a viable source of natural gas. A research cruise by the U.S. Geological Survey (USGS) Gas Hydrates Project in the northern Gulf of Mexico in spring 2013 shed new light on that possibility. The 15-day cruise was conducted by USGS scientists and technicians with partial financial support from the U.S. Department of Energy (DOE) and the U.S. Bureau of Ocean Energy Management (BOEM). The scientific surveys focused on sites in the deepwater Gulf where 2009 drilling had revealed thick sections of gas-hydrate-rich reservoir rocks (see related Sound Waves story "Significant Gas Resource Discovered in U.S. Gulf of Mexico"). The 2013 cruise imaged sediments deep beneath the seafloor in and around two of the drilled areas, Green Canyon and Walker Ridge (see map). The primary objectives were to constrain the saturation of gas hydrate (the percentage of available pore space in the sediment that is filled by gas hydrate) and to produce high-quality imagery of faults, sediment layers, and gas-related features in the sediments above the gas hydrate deposits. The new high-resolution imaging will enable the most detailed interpretation to date of the distribution, saturation, and geologic setting of gas hydrate and associated free gas at these sites.
Gas hydrate is a naturally occurring ice-like compound that forms in sediment when water and certain gases combine at the pressure and temperature conditions common at ocean depths greater than approximately 500 meters (1,600 feet) and in areas of continuous permafrost. Methane, popularly known as natural gas, is the gas most often trapped in gas hydrate deposits. Gas hydrate concentrates large amounts of methane into compact crystals and globally sequesters far more methane than exists in proven natural-gas reserves. There is substantial interest in gas hydrate as a potential energy resource—Japan produced methane from deepwater gas hydrate deposits for the first time in March 2013 (see related Sound Waves story, "Groundbreaking Gas Hydrate Research"). The USGS Gas Hydrates Project studies energy-resource issues, as well as the interaction of gas hydrates and the climate system (for example, see "Gas Hydrates and Climate WarmingWhy a Methane Catastrophe Is Unlikely") and the potential contribution of gas hydrates to seafloor destabilization. Ongoing studies characterize and quantify gas hydrate deposits beneath many of the world’s continental margins and in permafrost areas.
To image deep gas hydrate-rich sediments during the 2013 cruise, scientists used a high-resolution multichannel seismic (MCS) reflection system and ocean bottom seismometers (OBS). Both systems make use of acoustic (sound) energy. For the MCS reflection surveys, seismic sources were towed behind the vessel to produce low-energy sound waves that traveled to and beneath the seafloor, reflecting off surfaces with contrasting physical properties, particularly different densities. The returning sound energy was received by 72 hydrophones (underwater microphones) arrayed in a 450-meter (1,500 foot) digital streamer that is owned by the USGS. The data were recorded on shipboard computers and then processed to yield a cross-section, or side view, of sediment layers beneath the seafloor (see example above).
An OBS is an instrument that is deployed on the seafloor, where it records the seafloor motion produced either by earthquakes or by artificial sound sources. The 25 OBS used during the 2013 cruise are partially managed by the USGS and were supplied by the Woods Hole Oceanographic Institution (WHOI). These OBS recorded signals and tiny seafloor vibrations caused by the same sound sources as used by the MCS system. Each OBS employed a hydrophone like those in the MCS system to record energy waves transmitted through the water and a geophone to record energy waves transmitted through the seafloor. The geophone recorded vertical seafloor motion and two directions (at right angles to one another) of horizontal seafloor motion. Thus each OBS collected what are referred to as “four-component” data—one component from the hydrophone and three components (three directions) from the geophone. Because much of the energy recorded by the OBS traveled paths at a wide angle from vertical, the OBS data are also called “wide-angle.”
Sound energy travels through the water as compressional waves (P-waves, like compression waves in a slinky; see http://web.ics.purdue.edu/~braile/edumod/waves/Pwave.htm), but in sediment layers beneath the seafloor, part of the energy is converted into shear waves (S-waves, like waves in a rope; see http://web.ics.purdue.edu/~braile/edumod/waves/Swave.htm). Shear waves cannot travel through fluid and so are not recorded by hydrophones. The OBS geophones, however, can record shear waves as well as compressional waves, thus providing information not available from the MCS system. This information yields clues about the composition of sediment layers, including whether or not the pores contain gas hydrate and free gas.
The 2013 seismic cruise continues a long tradition of interagency cooperation and coordination among the USGS, DOE, and BOEM to advance national research and development objectives related to determining the viability of gas hydrates as a potential energy resource. These groups collaborated with a Chevron-led industry consortium to conduct the 2009 gas hydrate-drilling program in the Gulf of Mexico (see related Sound Waves story, "Significant Gas Resource Discovered in U.S. Gulf of Mexico"). The USGS Gas Hydrates Project, the DOE National Methane Hydrates R&D Program, and BOEM jointly funded the 2013 seismic program, which was conducted by the USGS during around-the-clock operations from April 18 to May 3 aboard the research vessel (R/V) Pelican.
The 35-meter (115 foot) R/V Pelican is a University-National Oceanic Laboratory System (UNOLS) vessel operated by the Louisiana Universities Marine Consortium (LUMCON) and home-ported in Cocodrie, Louisiana. Seth Haines, of the USGS Energy Resources Program (ERP), Denver, Colorado, and Patrick Hart, of the Coastal and Marine Geology Program (CMGP), Santa Cruz, California, were co-chief scientists on the cruise. Additional science crew included Thomas O’Brien, Wayne Baldwin, and Eric Moore (CMGP, Woods Hole, Massachusetts) and Ray Sliter, Jenny White, Pete Dal Ferro, and Rob Wyland (CMGP, Santa Cruz). Tim Kane and Peter Lemmond (WHOI) ran the OBS shipboard operations.
The seismic data-acquisition equipment was installed on the Pelican over a 3-day period beginning April 16. After a 20-hour transit during the night of April 18 to the first study site in the Green Canyon area, work began with deployment of a pattern of 21 OBS in water approximately 2,000 meters (6,500 feet) deep. Once these OBS were in place, MCS data acquisition commenced. The 72-channel digital hydrophone streamer was deployed along with three depth-controller “birds” to maintain the 450-meter (1,500 foot) streamer at a constant 3-meter (10 foot) depth below the sea surface. A pair of generator-injector (GI) air guns was used as a low-energy sound source to provide the signal recorded by the digital streamer and the OBS. Lamont-Doherty Earth Observatory loaned these GI air guns to the USGS for use on this project. Air-gun operations were conducted under guidelines in an Incidental Harassment Authorization (IHA) issued to the USGS by the National Marine Fisheries Service. Two contracted Protected Species Visual Observers were onboard during the cruise to ensure compliance with the IHA.
A closely-spaced grid of 65 two-dimensional (2D) seismic-reflection profiles and longer OBS-appropriate seismic lines totaling 397 kilometers (nearly 250 miles) were recorded at the Green Canyon site, centered on the 2009 gas hydrate drilling locations. The OBS were recovered after completion of the MCS acquisition, and the Pelican transited to the Walker Ridge site, where the OBS and MCS operations were repeated. Twenty-five OBS were deployed, and 450 kilometers (about 280 miles) of 2D data along 43 profiles were acquired at Walker Ridge. Data acquisition was completed on May 2, and the Pelican was back at LUMCON in Cocodrie on May 3. (Additional information about the cruise, including maps of the ship’s track, is posted at http://walrus.wr.usgs.gov/infobank/p/p0113la/html/p-01-13-la.meta.html.)
Preliminary processing of the MCS data was completed at sea soon after the acquisition of each line. This onboard processing provided continuous quality control and enabled underway adjustments to be made to the pre-planned program. The preliminary MCS seismic images show dramatic improvements in vertical resolution in comparison with available private-sector data at the two study sites. Increased resolution is critical for the interpretation of these gas hydrate deposits, which the 2009 logging-while-drilling data (data collected during drilling by instruments positioned behind the drill bit) have shown to be considerably thinner than the resolution possible with the previously available seismic-reflection data. The seismic-reflection image near the top of this page shows an example of the new MCS data from the Walker Ridge site.
Final processing of the MCS and OBS data is currently underway. OBS data-processing efforts are focusing on:
On the basis of the quality and volume of seismic data acquired, the USGS 2013 Gulf of Mexico gas hydrate research cruise was highly successful. The data processing and interpretation phases are just beginning, but the outlook is promising for a much more detailed understanding of gas hydrates in the Gulf of Mexico.
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