Abstract

Shale plays are naturally heterogeneous and frequently anisotropic due to tectonic stresses and fracturing. Determining stress or fracture intensity and its orientation from the seismic method is a desirable outcome for the drilling engineer, bu t a challenging one for geophysicists, particularly at significant depths in the subsurface. Wide Azimuth (WAZ) data is routinely used to study the amplitude variation with angle and azimuth (AVAZ) and velocity variation with angle -azimuth (HTI velocity). Vertical fractures in the presence of VTI are the simplest real life geological scenario. An orthorhombic anisotropic model is required (Leon Thomson, 1986) to describe the kinematics of wave propagation in this media. Orthorhombic models require 9 interval parameters to describe them fully. Inversion procedures to derive them are very unstable for field data and difficult to implement. Time imaging, which uses straight-ray approximations and relies upon surface azimuth measurements for imaging and analysis, only yields effective anisotropic parameters. This approach is not very suitable as the anisotropy could be as small as 1%, and may be unrecoverable because of averaging in the time migration process