Acoustic wave propagation in a fluid‐filled borehole is affected by the type of rock which surrounds the hole. More specifically, the slowness dispersion of the various body‐wave and borehole modes depends to some extent on the properties of the rock. We have developed a technique for estimating the dispersion relations from data acquired by full‐waveform digital sonic array well‐logging tools. The technique is an extension of earlier work and is based on a variation of the well‐known Prony method of exponential modeling to estimate the spatial wavenumbers at each temporal frequency. This variation, known as the forward‐backward method of linear prediction, models the spatial propagation by purely real‐valued wavenumbers. The Prony exponential model is derived from the physics of borehole acoustics under the assumption that the formation does not vary in the axial or azimuthal dimensions across the aperture of the receiver array, but can vary arbitrarily in the radial dimension. The exponential model fits the arrivals of body waves (i.e., head waves) well, because the body waves are dominated by a pole rather than a branch point. Examples of this processing applied to synthetic waveforms, laboratory scale‐model data, and field data illustrate the power of the technique and verify its ability to recover dispersion relations from sonic array data. The interpretation of the estimated dispersion in terms of rock properties is not discussed.

Classical sonic logging employs the acquisition and analysis of data with a simple monopole source. For this type of source, physics limits shear speed determination to speeds faster than the acoustic velocity of the borehole fluid. A dipole source excites the borehole flexural mode, providing a means to determine shear speed without this limitation. Propagation models and waveforms from computer Propagation models and waveforms from computer simulations of hard and soft formations with both monopole and dipole sources are presented. These simulations are then compared to laboratory scale model data. Real-world borehole data acquired with both monopole and dipole sources, along with the instrumentation employed to acquire the data, are described. In particular, a combined monopole and dipole logging instrument with a high fidelity monopole and dipole receiver array is discussed. A processing algorithm for extracting compressional, shear and Stoneley speeds from monopole and dipole full waveform data is presented. Log samples are presented for hard, soft and presented. Log samples are presented for hard, soft and extremely soft formations from both monopole and dipole sources. Application of log data to evaluation of rock mechanical properties, such as Poisson's ratio, fracture evaluation, and correlation to compressional and shear seismic data, is also discussed. Introduction Since the 1950's, borehole sonic logs have been made with sonic logging tools which incorporate transmitters and receivers which exhibit monopole radiation characteristics. These instruments were first used to detect sonic first arrivals for determination of formation compressional velocities. As sonic logging technology evolved, observing and recording the full wavetrain permitted detection of the later arriving waves associated with shear propagation. From these data, compressional and shear propagation velocities were determined and applied to estimate formation properties, such as rock lithology, fluid content, stiffness, compressibility and other mechanical properties, as well as assisting in the interpretation of surface and borehole seismic data. However, for monopole-based technology, borehole physics limits shear determination to velocities faster than the physics limits shear determination to velocities faster than the acoustic velocity of the borehole fluid. Although these "hard rock" formations are quite common, many important reservoirs fall into the category of "soft rock" or a mixture of both. In these soft, slow, poorly consolidated formations, the shear waves are primarily directed away from the bore hole and direct measurement of the formation shear velocity is not possible. possible. Dipole transmitters and receivers, on the other hand, remove this fluid velocity barrier. In particular, dipole sources produce a flexural mode in the borehole from which the formation shear velocity may be determined regardless of fluid velocity. In the following sections, the borehole physics of monopole and dipole logging tools will be discussed with presentation of computer simulations and comparison with presentation of computer simulations and comparison with laboratory scale model data. Following that discussion, the technology incorporated in a commercial logging instrument combining both monopole and dipole transmitters and receivers will be presented. Waveform processing techniques, example logs from both the hard and soft rock environments and data application examples conclude the discussion. P. 267

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 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. Introduction After more than 20 years of compressional-wave logging, the field of acoustic well logging is moving toward a more complete analysis of the full sonic waveform. This trend is being driven by the use of other wave components to probe rock properties and mechanical characteristics. Shear-wave logging, for example, can be effectively used in lithology and fluid identification, porosity determination, rock elastic and inelastic properties measurement, and in shear seismics. Stoneley waves can be used to determine shear-wave propagation characteristics in soft, unconsolidated propagation characteristics in soft, unconsolidated formations where converted shear waves are often absent from the borehole. Achieving this requires both digitizing the full acoustic waveform and applying modern signal-processing techniques. This paper describes a tool and processing technique that addresses both requirements. Digitizing capabilities are placed downhole so that more than one received wave form can be simultaneously digitized free of cable-induced distortion. Instead of the standard two receivers, a linear array of several receivers is used to provide more spatial samples of the propagating provide more spatial samples of the propagating wavefield. This allows a much better picture of the composite wave and its propagation characteristics. These additional data make possible the use of more sophisticated array-processing techniques that give unambiguous, more highly resolved estimates of the various wave component slownesses. In the sections that follow, the tool hardware and logging capabilities are described along with the signal processing. Examples of logs made in open and cased wells illustrate the tool's utility.

A new technique for producing S-wave logs from borehole acoustic waves has been developed. The procedure is based on direct phase calculations for timewindowed waveforms obtained with an acoustic logging tool. A window is positioned over the S-wave portion of the signals, and window moveout is chosen to give zerophase differences in a band of frequencies across the array of receivers.A major issue in application of this method is the error introduced by windowing and by interfering signals such as casing arrivals, residual P-waves, and modes propagating in the borehole. Examples using synthetic data are presented illustrating these errors and the means of reducing them. A capture effect, characteristic of phase methods, may be exploited to reduce the effect of interference.Examples are presented showing logs made in different lithologies using a conventional two-receiver longspacing tool. A pulsed transmitter was used with energy in a frequency range 10 to 20 kHz. In hard formations there is little difficulty in obtaining good shear logs. In softer formations, with reduced shear amplitudes, the problems caused by interfering waves become more severe. Careful choice of frequency bands used in the analysis can reduce interference problems and may improve logs in soft formations.

This paper addresses the problem of estimating the velocity of a propagating wave in the presence of other waves traveling at different velocities. Signals from a linear array of receivers are first windowed to attenuate interfering signals. Velocities are calculated from spatial frequency estimates obtained for each temporal frequency using a variation of Prony's method. This procedure is applied to the sonic well logging problem where shear wave velocity is estimated from data which contain interfering cornpressional and guided fluid waves.