Seismic reflections that parallel the seafloor at subbottom depths of several hundred meters, so called bottom simulating reflectors (BSR), appear to be associated with the base of the stability field for methane hydrates. As methane hydrates contain more potential fuel energy than is present in conventional oil, gas, and coal deposits, they might be an important future energy resource Kvenvolden (1993). Therefore, a good understanding of the origin, characteristics and material properties of the bottom simulating reflectors is essential. Only limited information on the nature of the hydrate and the BSRs is available from deep-sea drilling. The risk of heating and destabilizing the initial hydrate conditions during the process of drilling is considerably high. Thus, the core samples and well-logs do not necessarily reflect the correct hydrate characteristics and properties. Therefore, most information must be inferred remotely from seismic reflection data Hyndman and Davis (1992); Hyndman and Spence (1992); Miller et al. (1991); Shipley et al. (1979); Singh et al. (1993). The results of these previous investigations, based mainly on AVO responses and synthetic modeling, indicate that BSRs are characterized by large negative reflection coefficients and increasing subbottom depth with increasing water depth. The exact formation of the hydrate, however, is still controversial and different models have been proposed to explain the origin of BSRs Hyndman and Davis (1992); Kvenvolden and Barnard (1983a).
One of the first regions where a bottom simulating reflection was detected and related to the occurrence of methane hydrate is on the eastern United States margin, offshore Florida, on the Blake Outer Ridge. Deep-sea drilling on the Blake Outer Ridge found high methane content which suggested that the reflection was associated with the presence of methane hydrate Shipley et al. (1979). Based only on apparently high velocities in the sediments of the ridge, Bryan 1974 explained the reflection by gas accumulation at the base of the hydrate zone.
In this study, we use the AVO amplitude responses and impedance estimates of marine seismic data recorded at the Blake Outer Ridge to explain the present bottom simulating reflection and to explore the validity of the different origin models. By doing this detailed AVO analysis we can clearly discriminate the effects of the two proposed BSR models. The variation of amplitude versus offset can be an important indicator of free gas at an interface Shuey (1985) and, together with the estimation of material properties at the interface, limits the possible explanations of the physical origin of the BSR considerably.
This paper discusses our work with preprocessing, inversion and interpretation of the methane hydrate seismic data. The data are preprocessed by first applying a spherical divergence correction. Then a source wavelet deconvolution followed by an NMO correction and amplitude calibration are performed. The deconvolution and amplitude correction serve as a receiver compensation for the group array response. To account for the use of a non-linear streamer during the recording, the data are then linearly interpolated at the near offsets. A velocity analysis is performed on the resulting data and they are again processed with an additional static shift to improve the NMO correction. Using the observed AVO curves at the reflectors, the P and S impedance contrasts are estimated and used in a preliminary interpretation of the data.