Search

Abstract: The paper is aimed at discussing some problems that may arise in the tomographic inversion of data acquired with particular geometries, as vertical seismic profiles (VSP) or implying the use of Ocean Bottom Seismographs (OBS), and presenting the solutions used to solve them. One problem is due to the limited number of sensors (that may be even a single one) and of their sparseness. This affects the information distribution that is concentrated in the vicinity of each sensor. This fact, together with the second problem, i.e. the strong asymmetry of the ray path, may decrease the reliability of the inversion. Two procedures for irregular adaptive gridding of the reflecting interfaces and single ray datuming are proposed as possible solutions to overcome these obstacles, while obtaining a reliable and highly resolved velocity model in depth: the relative procedures are tested through synthetic models and on real data. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]