Differences between observed and forward modelled flexural wave dispersion characteristics in cross dipole sonic logging in homogeneous formations are useful for evaluating mud invasion and for differentiating between stress-induced and fabric-related anisotropy. In this paper we investigate the sensitivity of the dispersion effect to borehole size and mud slowness, and show the importance of taking account of the tool’s mechanical properties in the forward model - particularly when the tool diameter is a substantial fraction of the well diameter. We demonstrate that a new generation of small diameter cross dipole tool has a small tool-effect in common hole sizes, and extend the dispersion model results down to hole diameters less than 4 inches; we verify the calculations with real data sets acquired in sub 4-inch wells, as well as larger wells up to a 12 inches diameter. The results are also used to examine the size of dispersion corrections needed to calculate shear slowness in situations where low frequencies from the broadband transmitter are coupled relatively weakly to the formation.

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