INVERSION OF REFLECTION TRAVEL-TIMES FOR TRANSVERSE ISOTROPY

In anisotropic media, the short-spread stacking velocity is generally different from the root-mean-square vertical velocity. The influence o f anisotropy makes it impossible to recover the vertical velocity (or the reflector depth) using hyperbolic moveout analysis on short-spread , common-midpoint (CMP) gathers, even if both P- and S-waves are recor ded. Hence, we examine the feasibility of inverting long-spread (nonhy perbolic) reflection moveouts for parameters of transversely isotropic media with a vertical symmetry axis. One possible solution is to reco ver the quartic term of the Taylor series expansion for t(2) - x(2) cu rves for P- and SV-waves, and to use it to determine the anisotropy. H owever, this procedure turns out to be unstable because of the ambigui ty in the joint inversion of intermediate-spread (i.e., spreads of abo ut 1.5 times the reflector depth) P and SV moveouts. The nonuniqueness cannot be overcome by using long spreads (twice as large as the refle ctor depth) if only P-wave data are included. A general analysis of th e P-wave inverse problem proves the existence of a broad set of models with different vertical velocities, all of which provide a satisfacto ry fit to the exact traveltimes. This strong ambiguity is explained by a trade-off between vertical velocity and the parameters of anisotrop y on gathers with a limited angle coverage. The accuracy of the invers ion procedure may be significantly increased by combining both long-sp read P and SV moveouts. The high sensitivity of the long-spread SV mov eout to the reflector depth permits a less ambiguous inversion. In som e cases, the SV moveout alone may be used to recover the vertical S-wa ve velocity, and hence the depth. Success of this inversion depends on the spreadlength and degree of SV-wave velocity anisotropy, as well a s on the constraints on the P-wave vertical velocity.