This tool has improved the accuracy of transit time measurements in formations which have been affected by the drilling process. The longer the spacing between transmitter and receiver, the greater the separation of the various waves from each other. Furthermore, the greater the spacing the larger the attenuation of the wave train. Spacings for this tool are for example 8, 10, or 12 ft.

Present acquisition and processing technology (Schlumberger) allows computation of velocities and energies of the shear and compressional waves. It has been found that the ratio between shear and compressional wave energies provides a useful fracture indicator. Figure 3 6 shows an example of Delta-T shear processing. The first track presents amplitude and the energy ratio (RSCE). The second track shows compressional and shear wave energies (CEW1 and SEW1). The last track shows the computed ?t compressional and ?t shear (DTC and DTSM), and the long spacing sonic ?t (DTC).

Figure 3 6: Long Spacing Sonic Delta T (DTL), together with the computed Delta T Compressional (DTC) and Delta T Shear (DTSM). Compressional and Shear Wave Energies (CEW1 and SEW1), and the ratio of these energies (RSCE) are also displayed (after Schlumberger)

Note that when both waves are strongly attenuated, ?t _c and ?t _s are not reliable as cycle skipping occurs. The ratio of energies and the other curves suggest the presence of fractures in the shaded areas. It must be stressed, however, that other...