where and are Thomsen anisotropy parameters Thomsen (1986) and is the tilting angle of the media. Theoretically, equation 1 can be solved analytically, but there is no explicit analytical expression for its solution. The solid line in Figure shows how the dispersion relation looks, given the anisotropy parameters , and the tilting angle .Note that Sz is not a symmetric function of Sx. And Sz has two branches when Sx>0.8. One of them represents the up-going waves and the other one represents the down going waves. Therefore, in a TTI medium waves may overturn even though it is homogeneous.
Conventional implicit finite-difference methods are designed by truncating the Taylor series of the dispersion relation. The dispersion relation for TTI media is so complex that it is difficult to derive an analytical Taylor series used for an implicit finite-difference scheme.
Generally, the Padé approximation suggests that if the function , then Sz(Sx) can be approximated by a rational function Rn,m(Sx):
are polynomials of degree n and m, respectively. The coefficients ai and bi can be obtained either analytically by Taylor-series analysis or numerically by least-squares fitting.
Sz is an even function of Sx for isotropic and VTI media. In contrast, Sz is not an symmetric function of Sx for TTI media. It's well known that an general function can be decomposed into an even function and an odd function. We approximate the even part of the dispersion with the even rational functions, such as Sx2, Sx4 and approximate the odd part with odd rational functions, such as Sx,Sx3.
Considering the stability of the finite-difference scheme, I approximate the dispersion relation of TTI media with rational functions as follows:
For a laterally varying medium, the anisotropy parameters vary laterally. As a consequence, the coefficients for the finite-difference scheme vary laterally. It is too expensive to estimate these coefficients for each discrete grid during the wavefield extrapolation. After estimating the minimum and maximum value of the anisotropy parameters and the tilting angle, I compute the coefficients for the anisotropy parameters and the tilting angle in these ranges and store them in a table before the migration. During the wavefield extrapolation, given the anisotropy parameters , , and the tilting angle , I search the coefficients in the table and put them into the finite-difference scheme. Given the coefficients found from the table, the finite difference algorithm in TTI media is the same as the isotropic media. The table of coefficients is small, and the computation cost for table-searching is trivial compared to that of solving the finite-difference equation. Therefore, the cost of the optimized implicit finite-difference for TTI media is similar to that of the conventional finite-difference methods for isotropic media.