Dispersion Analysis - An Essential Consideration for All Sonic Tools

Many studies have been published showing dispersion characteristics for borehole sonic data, particularly for monopole, dipole, and quadrupole modes for both wireline and LWD tools. However, with the exception of LWD dipole tools, the dispersion corrections are often considered "negligible" and not, in practice, applied systematically to the data. Instead, the measured flexural velocity is computed with time domain methods and often presented as the shear velocity. Whereas it is true that the dispersion correction for wireline flexural and LWD quadrupole modes are often minimal, (less than 2%) there are many cases where the dispersion amount is significant and must be accounted for to provide an accurate shear velocity. <br>Modelling of dispersion curves and excitation functions are presented for wireline and LWD borehole modes. Data examples showing the application of dispersion curves in a variety of formations environments are discussed. In addition, the advantages of frequency domain processing for analysing dispersive modes are illustrated.<br>