Summary
3D anisotropic Pre-Stack Depth Migration (PreSDM) was extended to be applicable to borehole seismic data. PreSDM was adjusted to produce angle-dependent Common Image Point (CIP) gathers using both types of imaging conditions: cross-correlation and deconvolution. The effectiveness of this method is demonstrated with 3D borehole seismic data from the Arabian Gulf, Saudi Arabia. The deconvolution imaging condition yields well-balanced migration amplitudes and produces suitable CIP gathers for anisotropic velocity model building and pre-stack elastic inversion. The final migrated results show an enhanced image of the target sand stringers, both in the immediate vicinity and away from the borehole.
Introduction
Seismic surveys can be divided into two main categories: surface seismic surveys and borehole seismic surveys (so-called Vertical Seismic Profile or VSP). The principle is the same for both except that, in surface seismic, the source and receivers are positioned on the surface or close to it; whereas, in VSP, sources are typically located at surface and receivers are located inside the well. Other configurations are possible (Blackburn et al. 2007). The conventional VSP, with an array of geophones within the borehole and the source on the surface, is the most familiar example; but the variety of possible arrangements, borehole orientations, and objectives is much wider than this. This change in view-point opens up a wealth of opportunities that are not available from data collected at the surface.
Borehole seismic data offers improved image resolution compared to that of surface seismic data: fine geologic details are recovered because borehole seismic sensors are deployed below unconsolidated absorbing layers and close to the zones of interest. This acquisition technique brings resolution enhancement to the potentially costly and challenging imaging and interpretation problems in areas of complex geology. For this purpose a 3D anisotropic PreSDM technique was developed that is applicable to both surface and borehole seismic geometries. In the case of 3D VSP data, the method works on common-receiver gathers. PreSDM is extended to produce angle-dependent Common Image Point (CIP) gathers as a function of reflection angle to be used later on for 3D earth model building and pre-stack elastic inversion. The novel PreSDM algorithm is presented using up-going PP and converted wave (PS) seismic wavefields of a 3D VSP data from the Arabian Gulf, Saudi Arabia. The final migrated image result is presented as well as angle-dependent CIP gathers at selected locations.
