Method of using a circularly-polarized source to characterize seismic anisotropy

摘要

A method is disclosed for detecting the presence or absence of anisotropy in a subterranean geological formation and for determining its orientation. In a first preferred embodiment, the method utilizes a seismic source which generates circularly polarized shear waves and directs them to a target formation. These waves undergo wave splitting on encountering anisotropic conditions and the resulting components are linearly polarized and are reflected to the surface or to a bore hole where they are detected by receivers. The reflected arrivals are then analyzed to detect the presence and orientation of the linearly polarized arrivals as is known in the prior art. In an alternative preferred embodiment, the use of a circularly polarized source is simulated by generating two orthogonal waves linearly polarized 90 DEG out of phase with respect to each other. Each wave undergoes splitting upon encountering anisotropic conditions in the formation and the resulting components are reflected back to the surface and detected by receivers. In a preferred receiver embodiment, a first receiver is used to detect the reflected arrivals of the first components of each wave. A second receiver oriented at 90 DEG with respect to the first receiver is then used to detect the reflected arrivals of the second components of each wave. A first receiver component of a circular source is then simulated by summing the reflected arrival of the first receiver component of the first source wave with the Hilbert transform of the first receiver component of the second source wave. A second simulated receiver component of a circular source is then generated by summing the second receiver component of the first source wave with the Hilbert transform of the second receiver component of the second source wave. The simulated components are than analyzed to detect the presence and orientation of the linearly polarized components as is known in the prior art.