Gas hydrate bottom-simulating reflector analysis on the Colombia-Ecuador convergent margin from multichannel seismic reflection data by viscoacoustic asymptotic waveform inversion

Many studies have been performed to analyse the presence of gas hydrate through the observation of bottom- simulating reflectors (BSR). Much of the available information has come from seismic reflection data calibrated by limited in situ (drilling, borehole) data. Numerous studies involving forward modeling and inversion of multichannel seismic reflection and OBS refraction data have been carried out to constrain seismic velocity enhancement due to hydrate above the BSR and the presence of low velocity associated with free gas in the sediment beneath it. However, few studies involving forward modeling and inversion of multichannel seismic reflection data have been carried out to constrain seismic velocity and attenuation Q factor along the gas hydrate bottom-simulating reflector (BSR) and to analyse presence of free gas in the sediment beneath it. In particular, the volume of gas stored in the gas hydrate reservoir is currently poorly known. To analyse the physical properties (velocity and attenuation Q factor) along the bottom-simulating reflector we present an application of the conventional preserved amplitude prestack depth migration/inversion (PSDM) coupled with a specific post-processing of the tomographic model. The specific post-processing sequence allows us to: (1) mitigate effects of limited source bandwith and source-receiver aperture range from the tomographic images; (2) obtain the absolute values of the seismic attributes; (3) obtain the correct geometry of seismic reflectors reaching the theoretical seismic resolution of the source wavelet. During the post- processing sequence the model space exploration is made automatically via a random search of velocity and Q models, and optimal models are determined using the very fast simulated annealing algorithm. We present an application to multichannel seismic reflection data to obtain 2-dimensional viscoacoustic quantitative imaging of the BSR on a seismic profile (SIST-40) cutting across the Colombia-Ecuador subduction zone. Preliminary results suggest that the BSR corresponds to a 30 meters-thick layer. The BSR presents a dominant reverse polarity. Along the BSR we can observe regions with velocity between 1650-1200 m/s and with Q factor between 500-300, indicating the presence of hydrates. Locally, we can observe velocities around 1000 m/s and Q less than 90, indicating the presence of gassy sediments