Velocity and attenuation 3D tomography for gas-hydrates studies: the NW offshore Svalbard case

This work presents results of the tomographic analysis of 3D-3C seismic data acquired within the Hydratech EU Project off shore W-Svalbard, to detect and quantify gas-hydrate and free-gas amount within the sediments. The data have been acquired using an array of 20 Ocean Bottom Seismographs (OBS) 400 m spaced, shooting 36 seismic lines, typically 10 km long, implying offsets of 5-6 km for the OBS instruments, and spanning a relevant azimuth interval. The use of OBSs allowed the recording of both P as well as of PS converted waves. As known, P wave velocity (Vp) is particularly sensitive to changes in even small amount of gas in the pore spaces, while S wave velocity (Vs) is not, and therefore from the analysis of their ratio and of its variations with depth important information about fluid content in the sediments may be retrieved. The travel-times and frequency content of the 3D seismic data (P and S) have been object of the tomographic inversion, so to retrieve information not only on Vp and Vs velocities, but also on the relative quality factors (Qp and Qs) and therefore on their attenuation. The analysis of the so obtained multi-parameter 3D cube, therefore, enables to study the spatial distribution of the gas-hydrates and free-gas bearing sediments, the former indicated by high Vp and high Qp, the latter by low Vp and low Qp. In agreement with the theory, S waves show a constant velocity increase with depth. What is interesting, is the correlation of the shape of the velocity and attenuation anomalies, i.e. of the gas-hydrate and free-gas bearing lenses, with the fault pattern present in the survey area, to confirm that the faults control the fluid propagation within the sediments.