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Vol. 49, n.2, June 2008
pp. 255-264

P- and S-wave migration of while drilling data

G. Bernasconi

Received: March 28, 2007; accepted: November 21, 2007

Abstract
I investigate some aspects of multicomponent processing in seismic while drilling (SWD) acquisitions. I start from the consideration that a working roller cone is a good source of both P- and S-waves. Then, by examining the radiation pattern of the bit, I evaluate the subsurface illumination in simple models and for different geophone directivity. Results show that S-waves can increase the lateral extent of the imaged area, while P-waves achieve a higher illumination in the vicinity of the borehole. By using the appropriate velocity model, it is possible to obtain a migrated gather for P-waves and one for S-waves separately. In addition, migration gathers can be observed in the aperture angle domain. Results on SWD synthetic data confirm this analysis. I also produce P- and S-wave migrated gathers from field data: their comparison reveals the presence of a coherent signal for the two wave modes. The information thus collected about both compressional and shear properties of the subsurface, especially during the drilling process, can be crucial to predicting overpressured zones and to reducing the ambiguity between variations in pore pressure and variations in lithology.

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