CROSSWELL TOMOGRAPHIC ESTIMATION OF ELASTIC-CONSTANTS IN HETEROGENEOUS TRANSVERSELY ISOTROPIC MEDIA

The procedure to estimate elastic constants of a transversely isotropic medium from limited-aperture traveltimes has two steps. First, P- and SV-wave traveltimes are fitted with elliptical velocity functions around one of the axes of symmetry. Second, the parameters that describe the elliptical velocity functions are transformed analytically into elastic constants. When the medium is heterogeneous, the process of fitting the traveltimes with elliptical velocity functions is performed tomographically, and the trans-formation to elastic constants is performed locally at each position in space. Crosswell synthetic and field data examples show that the procedure is accurate as long as the data aperture is constrained as follows: it should not be too large otherwise the elliptical approximation may not be adequate, and it should not be too small because the tomographic estimation of elliptical velocities fails, even if the medium is actually isotropic.