SEISMIC VELOCITY STRUCTURE AT A GAS HYDRATE REFLECTOR, OFFSHORE WESTERN COLOMBIA, FROM FULL-WAVE-FORM INVERSION

Seismic reflection profiles across many continental margins have image d bottom simulating reflectors (BSRs), which have been interpreted as being formed at the base of a methane hydrate stability field. Such re flectors might arise either from an impedance contrast between high-ve locity, partially hydrated sediments and water-saturated sediments or from a contrast with partially gas-saturated sediments. These alternat ives may be hard to distinguish by conventional amplitude-versus-offse t or waveform modeling approaches. Here we investigate the origin of a high amplitude BSR in the accretionary wedge offshore of western Colo mbia by seismic waveform inversion. The inversion procedure consists o f three steps: firstly, determination of root-mean-square velocities a nd hence estimates of the interval velocities between major reflectors by a global grid search for maximum normalized energy along elliptica l trajectories in the intercept time-slowness domain; secondly, determ ination of accurate interval velocities between these reflectors by a Monte Carlo search for maximum energy; and thirdly, a waveform fit in the frequency-slowness domain, using differential reflectivity seismog rams and a conjugate-gradient optimization algorithm to minimize the s ample-by-sample waveform misfit between data and synthetic. At two loc ations, near a structural high, we find a approximately 30-m thick low -velocity zone beneath the BSR, with the properties of a partially gas -saturated zone, while at a third location, where the BSR amplitude is lower, we find no evidence for anomalously low velocities. The prefer ential development of the BSR in structures that would tend to interce pt fluid flow or migrating gas and the presence of free gas beneath th e BSR indicate a mechanism of BSR formation in which free methane gas migrates upward into the hydrate stability field or is carried there i n advecting pore water.