Elastic Reverse Time Migration Method for Single-Well Imaging and Elimination of Azimuthal Ambiguity With a Combined Receiver System

Single-well imaging (SWI) is a borehole measurement technique that aims at detecting geological structures a few to tens of meters away from the borehole. In SWI, both the acoustic source and the receiver are located in a fluid-filled borehole, making it a great challenge to image complex structures such as faults and cavities. In addition, the current SWI is unable to identify the azimuth of the reflector uniquely. In this article, a new imaging framework based on elastic reverse time migration (RTM) is proposed for SWI with a combined monopole and dipole receiver system, which can eliminate the azimuthal ambiguity. A decoupled elastic RTM based on the Helmholtz decomposition is extended into SWI for the first time. Wave-mode decomposition and the effect of the fluid-filled borehole are considered in the method. Furthermore, combining with the asymptotic solutions, the theoretical analysis shows that the fluid pressure and the horizontal displacement signals recorded by a combined monopole and dipole receiver system are sensitive to the reflector azimuth. By taking advantage of the sensitivity of the acoustic signals, the fluid pressure and the horizontal displacement records are migrated using the proposed elastic RTM, respectively. Finally, the normalized images are stacked to get the final images, from which the orientation of the reflector can be identified uniquely. Numerical examples demonstrate that the proposed imaging framework can image complex structures such as steep dipping interfaces and cavities outside the fluid-filled borehole with high imaging quality and clear physical meaning, and can solve the azimuthal ambiguity of SWI.