VELOCITY STRUCTURE OF A BOTTOM SIMULATING REFLECTOR OFFSHORE PERU - RESULTS FROM FULL-WAVE-FORM INVERSION

Much of our knowledge of the worldwide distribution of submarine gas h ydrates comes from seismic observations of Bottom Simulating Reflector s (BSRs). Full waveform inversion has proven to be a reliable techniqu e for studying the fine structure of BSRs using the compressional wave velocity, We applied a non-linear full waveform inversion technique t o a BSR at a location offshore Peru. We first determined the large-sca le features of seismic velocity variations using a statistical inversi on technique to maximise coherent energy along travel-time curves. The se velocities were used for a starting velocity model for the full wav eform inversion, which yielded a detailed velocity/depth model in the vicinity of the BSR. We found that the data are best fit by a model in which the BSR consists of a thin, low-velocity layer. The compression al wave velocity drops from 2.15 km/s down to an average of 1.70 km/s in an 18 m thick interval, with a minimum velocity of 1.62 km/s in a 6 m interval, The resulting compressional wave velocity was used to est imate gas content in the sediments. Our results suggest that the low v elocity layer is a 6-18 m thick zone containing a few percent of free gas in the pore space. The presence of the BSR coincides with a region of vertical uplift. Therefore, we suggest that gas at this BSR is for med by a dissociation of hydrates at the base of the hydrate stability zone due to uplift and subsequently a decrease in pressure.