The application of 3‐D kirchhoff migration to VSP data from complex geological settings

Abstract

VSP data were collected in the Larderello geothermal field in Italy in order to attempt to image reflective features below the depth of current geothermal production with the goal of extending and better developing the reservoir. The shallow geologic structures, which are comparatively well known, are strongly 3-D. Since 2-D migrations failed to pretable results and the geometry of the source and receiver locations was not adequate for a true 3-D migration, we performed a modeling study to design a source tion which could be used to collect VSP data suitable for 3-D migration in future experiments at the Larderello site or at other strongly 3-D locations. A hypothetical model of reflecting interfaces was created below the known shallow structures. We applied a 3-D elastic wave Kirchoff migration algorithm to synthetic data computed for several different source arrays and found that a rectangular grid of sources was very robust in terms of collecting data with sufficient redundancy to perform reliable 3-D migrations. This source array allows the development of a fairly good image of the interfaces around and below the region using the observed frequency of the data, 30 Hz. Increasing the central frequency of the synthetic source to 60 Hz allows a much better resolution of interfaces on the order of 200 m apart.