Strong subhorizontal reflectors in the upper few hundred meters of oceanic sediments have been imaged by seismic reflection profiling adjacent to many continental margins. These reflectors are known as ''bottom simulating reflectors'' (BSRs) as they run roughly parallel to the seabed. BSRs generally have been interpreted as represented the base of a methane gas hydrate stability field. A series of boreholes was drilled in October--November 1992 during leg 146 of the Ocean Drilling Program to examine fluid discharge and its relationship to the BSR in the Cascadia accretionary wedge. Independently, we performed a global waveform inversion of seismic reflection data collected near site 889. We use a multistep strategy for global waveform inversion of seismic reflection data. The strategy is based on the Bayesian inference theory, in which the a posteriori probability distribution is determined by the a priori probability distribution and the likelihood function relating the data with the model. Three different likelihood functions have been defined relating different parts of the data with different wavelengths of model parameter variation.

First, global nonlinear search algorithms are used to estimate the large-scale features of one-dimensional velocity variations; then a nonlinear iterative search is used to retrieve the velocity structure in fine detail. At each stage, bounds on the model parameters are estimated. The strategy has been implemented in the slowness and intercept-time domain. We find that the BSR corresponds to a 15--20 m zone where the velocity drops from about 1.8 km/s to a minimum of 1.4 km/s, suggesting the presence of free methane gas in the low-velocity zone, and a a 15--20 m transition zone where the velocity gradually increases to a velocity of the surrounding sediments. Thus the total thickness of the low-velocity zone is about 30--40 m. A small or negligible velocity increase just above the BSR suggests that the hydrate fills less than 10% of the pore space in the sediments. Our result was confirmed by the drilling. The velocity derived using a vertical seismic profile and sonic log from the drill hole agree closely with our result. Waveform inversion can recover detailed and well-constrained velocity variations from a high-amplitude target such as a BSR; combined with the detailed interpretation of these velocity variations which the results from site 889 will provide, the inversion approach is a powerful tool for analysis of BSRs elsewhere. ¿American Geophysical Union 1994