Semi-elastic plane wave depth migration

A method of utilizing mode converted S waves at target horizons without resorting to full elastic migration is presented. The only requirement for the applicability of the methods is the need for long offsets, which are already being recorded in present day exploration. The imaging ability of the mode-converted waves is demonstrated with numerical data, scaled model data from the Physical Modeling System at the University of Houston and also a real data example.; Encountering high velocity layers is a common phenomenon in seismic exploration. These layers act as acoustic barriers, blocking the penetration of the P waves. Fortunately, such layers also support mode conversion when there is an efficient coupling between the S waves inside the high velocity layer and P waves outside. Imaging beneath such high velocity layers using mode converted S waves in the plane wave domain is the goal of this study.; A migration velocity analysis and model building method is presented. Kirchhoff migration is used to efficiently migrate constant P (ray parameter) sections for a range of trial velocities starting from the surface. A decimated pre-stack depth migration from which the undulations of the analysis horizon are picked completes one iteration of the repetitive analysis. The method is extended to include the velocity analysis of the mode converted S waves within the high velocity layer. The velocity of the events whose depth coincides with the depth from the P wave model is selected as the S wave velocity of the interval. After establishing the P and S wave velocity models, migrations are performed in the plane wave domain with each model. The resulting images may be interpreted separately and/or combined into a single section.; An envelope may be utilized in combining the P and mode converted sections. Such sections are a measure of intensity, which is similar to photographic images. Separate interpretations of the sections, aided by the P and S wave velocity information and the phase and amplitude variation within the common image point gathers, may be utilized in identifying the lithology of the target horizons.