ABSTRACT

Conventional acoustic logs are based on a small fraction of the potential information available in the acoustic wavetrain propagating down the borehole. The objective of full wave acoustic logging is to record, process, and interpret the entire acoustic wavetrain. Better understanding of wave propagation around the borehole, new generation logging tools, and application of digital processing to the borehole acoustic signal are providing the ability to extract more of the information. The acoustic wavetrain associated with the borehole is composed of compressional, shear, normal mode and Stoneley waves. Characteristics of each of these phases are discussed briefly. These characteristics have guided the design of full wave logging systems: tools, data acquisition and processing New generation full wave sonic logging tools have more receivers, greater transmitter-to-receiver spacings, and lower frequency response than their earlier counterparts. Waveforms are generally recorded and processed digitally. Non-symmetrical sources and receivers are being used to preferentially generate shear waves. Seismic-type data processing is being used to extract velocity and amplitude information from the full wave acoustic signals. Phase velocities of compressional, shear and Stoneley waves are being determined almost entirely by some type of correlation technique. These techniques include cross-correlation, semblance and linear and non-linear delay and sum. Full wave acoustic logging has advantages and disadvantages. The use of relatively sophisticated digital processing and the fact that full wave tools generate about ten times more data than do other logging tools places considerable demand on computing facilities, particularly at the wellsite. The large dynamic range of full wave signals requires gain ranging techniques if digitization is performed in the logging tool. Array-type processing requires new borehole compensation techniques. Some of the most important applications of full wave sonic information include estimation of formation permeability, vertical extension of hydraulically induced fractures, porosity in cased holes and the generation of compressional and shear synthetic seismograms.

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