Shear-Wave Sonic Imaging of Complex Structure

Summary Shear sonic data generated by dipole sonic sources were used to image near-borehole structure more than 25 m from the borehole. Time-domain analysis of the sonic data in single-receiver iso-offset depth-array displays and multishot full-array displays were used for initial insights and quality control. Subsequent reverse time migration imaging showed bedding images at the expected relative dip for the main structure penetrated by the borehole of ~50 degrees and, in addition, showed an image of a structure at a higher-relative dip of around 75 degrees. This interpretation was greatly aided by pre-imaging quality-control analysis of full-array displays that identified reflected arrivals from a higher-angle structure more than 15 m from the borehole. In addition, successful imaging was demonstrated in the cased-hole section above 3917 m with coherent images seen to 25 m from the borehole. Imaging was accomplished using two orthogonal source drives, with results from the source drive oriented closest to the SH mode orientation of 90 degrees from the high side of the borehole giving the sharpest images.