Over the past decade, a new generation of acoustic logging tools has been developed. These new tools are specifically designed to measure formation shear velocity in all types of rocks-information that is not always available with a conventional acoustic tool. Knowledge of the formation shear velocity can improve well evaluation, particularly by enhancing our understanding of the mechanical properties of formations of interest. Acoustic sources create a pressure wave in the borehole fluid. In a conventional tool, this pressure wave expands uniformly in all directions away from the source. Consequently, movement that is symmetric around the borehole axis is induced in the formation. The acoustic sources in the new tools also generate pressure waves in the borehole fluids, but the waves are not symmetric around the borehole axis. These asymmetric pressure waves result in acoustic propagation along the borehole that differs significantly from the propagation induced by a conventional acoustic source. The properties of the modes produced by these sources can be used to measure formation shear velocity. A conventional acoustic-logging source is a monopole transducer that can be numerically modeled as a single point source. New sources that have been successfully developed into logging tools include dipoles and quadrupoles. Numerical modeling of these sources requires a more complex representation of the source than does the monopole tool. As a class, these new acoustic transducers can be called multipoles.

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