Multiple‐shot processing of array sonic waveforms

Abstract

Signal processing techniques are needed to estimate the slownesses (inverses of velocities) of compressional, shear, and Stoneley waves from array sonic waveforms recorded by array sonic tools. Existing processing techniques process entire array waveforms, assuming that each wave component has linear moveout (a constant velocity) across the array. The results show poor vertical resolution if the formation spanned by the array aperture is not homogeneous. Multiple‐shot processing is a technique to improve the vertical resolution of slowness logs. The algorithm incorporates redundant information from the multiplicity of overlapping tool locations. Specifically, the method includes four steps: (1) subarrays are selected from full arrays associated with shots from successive sources; (2) subarray waveforms are processed using the semblance statistic; (3) the semblance statistic is projected onto the slowness axis; and (4) the projected semblances of all selected subarrays are combined and the slownesses of the wave components are estimated from the combined statistic. The technique improves the vertical resolution of slowness logs because the subarrays are shorter than the full array. It also increases the accuracy of estimates of slowness because more data are used in the processing. This procedure is applicable to common‐shot subarrays as well as to common‐receiver subarrays. Combining common‐shot and common‐receiver results provides two additional advantages, namely, borehole compensation and measurement enhancement.