During the last three decades significant developments have occurred in the design and application of borehole acoustic measurements. Where during the first decade the main focus has been on slowness analysis and its applications (E.g., porosity, synthetic seismogram, etc.), the last two decades have resulted in more advanced applications: -permeability estimation from Stoneley waves; -intrinsic/stress-induced anisotropy from flexural waves; -and most recently, imaging away from the wellbore. The latter application has great promise in imaging fine structural features away from the wellbore, particularly in hard rock (I.e., less attenuation) and it is complementary to conventional surface seismic. Whereas the seismic method has an imaging resolution of order 10^1 m and a depth of investigation of order 10^3 m, the borehole acoustic method has an imaging resolution of order 10^-1 m and a depth of investigation of order 10^1 m. Furthermore, although the borehole acoustic method has been developed for and applied to mainly wireline configurations, it is considered to have an even greater value in a Logging While Drilling (LWD) setting as a result of its geosteering potential. The cross-dipole acoustic measurement principles, theory and processing steps will be explained in conjunction with two case studies.


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