Simultaneous anisotropy inversion and type identification in the frequency domain for flexural waves in horizontal transverse isotropic media

Abstract

The anisotropy of elastic waves has been widely used to obtain information about the formation structure in geosciences research. The splitting of flexural waves is generally applied to evaluate anisotropy using geophysical inversion methods. However, most of these methods must be combined with other methods, such as dispersion analysis, to distinguish stress-induced and intrinsic anisotropies. The objective function proposed by Tang and Chunduru has been improved by a new objective function that introduces an amplitude ratio of the slow to fast flexural waves in a horizontal transverse isotropic formation. By considering the amplitude difference in flexural wave splitting under stress-induced or intrinsic anisotropy, the new method can perform anisotropy inversion and type identification in the frequency domain. The calculated azimuth of the fast flexural wave as a function of frequency is used to distinguish the anisotropy type. The results from synthetic examples indicate that the intrinsic anisotropy commonly leads to a smooth azimuth curve, whereas the stress-induced anisotropy leads to a sharp step change. Therefore, the distribution of the azimuth in the frequency domain is a better indicator of the anisotropy type than the traditional slowness dispersion curve. Moreover, a new objective function and a new quality indicator for simultaneous anisotropy inversion and type identification have been developed and validated by synthetic and field data sets.