Attenuation tomography: an application to gas-hydrate and free-gas detection

We estimate the quality factor (Q) from seismic reflections by using a tomographic inversion algorithm based on the frequency-shift method. The algorithm is verified with a synthetic case and is applied to offshore data, acquired at western Svalbard, to detect the presence of bottom-simulating reflectors (BSR) and gas hydrates. An array of 20 ocean-bottom seismographs has been used. The combined use of traveltime and attenuation tomography provides a 3D velocity–Q cube, which can be used to map the spatial distribution of the gas-hydrate concentration and free-gas saturation. In general, high P-wave velocity and quality factor indicate the presence of solid hydrates and low P-wave velocity and quality factor correspond to free-gas bearing sediments. The Q-values vary between 200 and 25, with higher values (150–200) above the BSR and lower values below the BSR (25–40). These results seem to confirm that hydrates cement the grains, and attenuation decreases with increasing hydrate concentration.