VELOCITY STRUCTURE OF A GAS HYDRATE REFLECTOR AT OCEAN DRILLING PROGRAM SITE-889 FROM A GLOBAL SEISMIC WAVE-FORM INVERSION

Strong subhorizontal reflectors in the upper few hundred meters of oce anic sediments have been imaged by seismic reflection profiling adjace nt to many continental margins. These reflectors are known as ''bottom simulating reflectors'' (BSRs) as they run roughly parallel to the se abed. BSRs generally have been interpreted as representing the base of a methane gas hydrate stability field. A series of boreholes was dril led in October-November 1992 during leg 146 of the Ocean Drilling Prog ram to examine fluid discharge and its relationship to the BSR, in the Cascadia accretionary wedge. independently, we performed a global wav eform inversion of seismic reflection data collected near site 889. We use a multistep strategy for global waveform inversion of seismic ref lection data. The strategy is based on the Bayesian inference theory, in which the a posteriori probability distribution is determined by th e a priori probability distribution and the likelihood function relati ng the data with the model. Three different likelihood functions have been defined relating different parts of the data with different wavel engths of model parameter variation. First, global nonlinear search al gorithms are used to estimate the large-scale features of one-dimensio nal 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 implement ed in the slowness and intercept-time domain. We find that the BSR. co rresponds 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 g as in the low-velocity zone, and a 15-20 m transition zone where the v elocity 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 t hat 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 cl osely with our result. Waveform inversion can recover detailed and wel l-constrained velocity variations from a high-amplitude target such as a BSR; combined with the detailed interpretation of these velocity va riations which the results from site 889 will provide, the inversion a pproach is a powerful tool for analysis of BSRs elsewhere,