Does Dipole Dispersion Crossover Indicate Borehole Stress-induced Anisotropy?

Crossover of flexural wave dispersion recorded in cross-dipole measurements is interpreted as an indicator of stress-induced anisotropy around a borehole. We investigate different factors that influence flexural wave dispersion. Through numerical modeling, we show that for a circular borehole surrounded by an isotropic formation that is subjected to an anisotropic stress field the dipole flexural dispersion crossover is detectable only when the formation is very compliant. This might happen only in the shallow subsurface or in zones having high pore pressure. However, we show that dipole dispersion crossover can also result from the combined effect of formation intrinsic anisotropy and borehole ellipticity. We find that a small change in borehole ellipticity (e.g. ratio of maximum to minimum borehole radii ∼ 1.1) and 1% intrinsic anisotropy can result in a resolvable crossover in flexural dispersion that might be erroneously interpreted as borehole stress-induced anisotropy. A thorough and correct interpretation of flexural dispersion crossover thus has to take into account the effects of both stress-induced and intrinsic anisotropy, and borehole ellipticity.