Prediction of fracture orientation from shear wave rotation observed on a near offset VSP

Abstract

Summary Strong, down going shear waves are observed on near offset VSPs in Kuwait. While attempting to construct a time depth curve by tracking the down-going shear wave form, it was noticed that discontinuities existed. It was hypothesized that these sudden changes were caused by shear wave splitting. Shear wave splitting occurs in anisotropic media. A single shear wave will split into fast and slow shear waves which are orthogonal to each other. The directions of motion will depend on the direction of fractures or stress anisotropy. The wave motion is captured by three component accelerometers in the bore hole. It is common practice to rotate the horizontal components based on the horizontal component of the first arrival (P wave). This rotation of coordinates corrects for rotation of the tool in the hole. A rotation based on the down going shear wave gave a more reasonable shear velocities based on the time depth curve. In addition to velocity we were also able to determine information about the changing azimuth of shear wave motion caused by anisotropy. We show that a wealth of information can be obtained from analyzing the wave forms recorded in a simple, near offset VSP.