2.5D FULL-WAVE-FIELD VISCOACOUSTIC INVERSION

Full-wavefield inversion for distributions of acoustic velocity, densi ty and Q on a vertical slice through a 2.5D model is implemented for c ommon-source gathers in a cross-hole geometry. The wavefield extrapola tion used is 3D, so all geometrical spreading, scattering, reflection, and transmission effects are correctly and automatically compensated for. In order to keep the number of unknowns tractable, application wa s limited to 2.5D models of known geometry; the latter assumes a prior step, such as tomography, to fix the layer geometries. With the model geometry fixed, reliable solutions are obtained using synthetic data from only two independent source locations. Solutions from data with n oisy and missing traces are comparable to those from noise-free data, but with higher residuals. When the source locations are spatially wid ely separated, common-source gathers may be summed and treated as a si ngle wavefield to yield the same model estimates as when the individua l source wavefields are treated separately, at substantially reduced c ost. Inversions for full 3D parameter distributions can be handled wit h the same software, requiring only solution for more unknowns.