An expedition to define the elastic properties of the shallow subseafloor at Woolsey Mound, site of the Gulf of Mexico Hydrate Research Consortium’s Seafloor Observatory, was completed April 2-9, 2011. The experiment was designed by MMRI scientists in collaboration with geophysicists from the University of Texas Bureau of Economic Geology and Scripps Institution of Oceanography.
The purpose of the experiment is to discover how seafloor features such as pockmarks, craters mini-basins, and cones relate to shallow subsurface-gas hydrates and to predict responses to activities such as fluid migration and salt movements. Differences in the propagation of different wave-types can be used to produce high-resolution images of the shallow subsurface (up to ~400 meters). From these images, seismic attributes used to characterize conventional oil and gas reservoirs can then be extracted and used to describe the internal architecture of the hydrate system.
Woods Hole Oceanographic Institution’s Ocean Bottom Seismometer group provided the instruments and the technical expertise to execute the experiment. The 4-component seismometers (receivers) were deployed on a 2.25 kilometer-long transect that crossed faults and other seafloor features of interest. Placement of the instruments was designed to accommodate experiments using both active (gas-Injection and water-gun) and passive (wave, ship, drilling) energy sources that produce signals used to investigate subseafloor properties. The navigation and active surveying were accomplished by MMRI.
Sediments containing gas hydrates behave differently from sediments that do not contain gas hydrates when crossed by elastic waves. The objective of the Ocean Bottom Seismometer experiment is to produce and record such waves and determine the form of the hydrates in the sediments: large layers, small amounts, isolated pockets or layers, or interconnected. We will also use the OBS data to map small vertical faults, fractures, or disruptions that can affect fluid movements, and identify important reflectors that may be present. Seismic images will be extended from the seafloor to below the base of the hydrate stability zone. Evaluation of this dataset will provide an important component of characterization of the Seafloor Observatory site.