Recognizing Apparent P-wave Azimuthal Velocity Anisotropy Caused by Isotropic Lateral Velocity Variations

Azimuthal variation in traveltimes is becoming increasingly used as a tool for reservoir characterization and fracture detection. One issue in interpreting the results of azimuthal velocity analyses is distinguishing intrinsic azimuthal anisotropy from apparent anisotropy caused by lateral velocity variations. In an endeavor to differentiate between the two cases, I model discrete overburden lateral velocity variations in an isotropic medium. This paper illustrates the impact of those lateral variations on azimuthal velocity analyses through several models and a field data example. In general I conclude that the spatial patterns observed in the apparent velocity anisotropy are very distinctive with obvious correlations between ellipse parameters (fast and slow velocities and azimuth of the fast velocity). These patterns can also be expected to correlate between RMS and interval attributes, something that is unlikely to occur within truly azimuthally anisotropic media.