The interest in methane hydrates as a potential future energy resource has been constantly increasing during recent years. Assessments show that worldwide there might be twice as much hydrocarbon available from hydrate resources than from all known recoverable and non-recoverable coal, oil and gas deposits Kvenvolden (1993). Recent drilling at the Blake Outer Ridge offshore Florida and Georgia has shown that there might be as much as 4.7 x 1016 g CH4, or 35 GT of carbon available there. This quantity could meet the 1996 United States natural gas consumption needs for the next 105 years Dickens et al. (1997).
In order to evaluate the future impact of methane hydrates realistically, it is essential that we understand its elastic and mechanical properties. Information available from surface seismic and drilling has to be linked with rock physics to estimate the amount of hydrate present, its distribution in the pore space, permeability and potential recoverability Andreassen et al. (1995); Dvorkin and Nur (1993); Ecker et al. (1996); Ecker and Lumley (1994a); Lee et al. (1992). Most studies were restricted to using only the P-wave velocity information to infer those properties, assuming a known in-situ hydrate Poisson's ratio based on laboratory studies. However, when trying to assess hydrate structures, S-wave velocity behavior might be an important piece of information that can influence the estimated hydrate properties significantly Ecker et al. (1996). Therefore, it is essential to perform an integrated characterization using as much information as possible.
In this paper I present the characterization of a hydrate reserve at the Blake Outer Ridge offshore Florida and Georgia using both compressional and shear information contained in the data. Part of the data have been described in previous reports Ecker and Lumley (1994b); Ecker (1995). Data preprocessing includes spherical divergence correction, source wavelet deconvolution and amplitude calibration. The seafloor multiple was used to normalize the zero-offset amplitudes, thus resulting in an zero offset, reflection strength map of the section. A detailed NMO stacking velocity analysis was performed to infer the P-wave velocity structure. Subsequently, I applied AVO analysis on the data, followed by a true amplitude migration/inversion. The results show strong variations in the zero-offset reflection coefficient of the BSR. The strongest BSR amplitudes seem to coincide with the presence of a low velocity zone. Weak BSR reflections occur over an area of diminished velocity contrasts and discontinuous hydrate appearance. The impedance inversion results suggest very strong P-impedance contrasts at the BSR and the seafloor, dominating the entire seismic section. The S-impedance contrast shows a strong contrast at the BSR compared to the seafloor. The data show a significant spread of BSR AVO responses laterally. This might be partly due to interference with underlaying structure, patchy hydrate or gas saturation or fracturing that affects the hydrate locally.