A New Sonic Array Tool for Full Waveform Logging

Abstract

Abstract For over 20 years, sonic logging has implied compressional velocity logging using first break detection from two-receiver tools. Today, sonic logging techniques can utilize more of the information contained in the acoustic waveform to provide shear and Stoneley-wave velocity and attenuation measurements. This requires signal-processing techniques for full-waveform analysis. These techniques generally need more spatial samples of the wavefield than are provided by standard two-receiver tools. To fulfill this need, a new multireceiver sonic tool has been designed. In this paper, we describe a new sonic array tool which has an array of eight receivers spaced 6 in. apart, located 8 ft from the nearer of two transmitters. In addition to array capabilities, standard short and long-spaced sonic logs are available. A special section has been incorporated to give a mud-velocity measurement. The downhole electronics provide digitized waveform acquisitions with an effective resolution capability of 11 bits. To extract the additional answers from the full waveform, slowness-time coherence (STC) processing has been developed. Based on semblance techniques, STC identifies coherent arrivals across the array. Examples of waveforms and processed logs from both open and cased wells are presented to illustrate the tool's capabilities.