Publications

Today, 3D seismic data contains by default a diversity of azimuths for rays travelling from shot to receiver. By design, modern datasets are being acquired with richer and wider azimuths. Such acquisitions have been found to be highly desirable for fractured reservoir or stress analysis, as fractures or stress can produce measurable differences in velocity as a function of azimuth. This velocity dependency on azimuth is referred to as anisotropy, and in the presence of vertically aligned fractures or stress direction is referred to as anisotropy of the HTI type (or horizontal axis of symmetry). Because these velocity differences (moveouts) are detectable with the seismic method, the goal is to capture those differences by evaluating multiazimuth or even full azimuth angle gathers. In other words, rich azimuth data, whether by planning or simply due to the acquisition’s inherent geometry, captures these anisotropic effects and provides a means to determine fracture orientation and density or stress direction and intensity. Since anisotropic velocities are the basic seismic signatures of fractured reservoir characterization, accurate anisotropic velocity analysis is a critical step when processing rich azimuth data. Anisotropic velocity differences (moveouts) can be used directly in inversion processes to determine the properties of fractured or stressed reservoirs, or they may be used as a preconditioning step for amplitude versus angle versus azimuth (AVAZ) analysis. AVAZ requires ‘flattened’ gathers. The preferred approach creates full azimuth angle gathers in depth, using a rich decomposition procedure designed to preserve all subsurface azimuths.