Most acoustic logging while drilling (LWD) tools generatea single pure borehole mode (e.g., dipole or quadrupole) toestimate the formation shear velocity. We propose an approachwhere multipole modes are generated simultaneouslyand used to obtain a better shear estimation. This approachuses an asymmetric source with arrays of receivers distributedazimuthally around the tool to generate and identify signalsfrom different modes. We investigate such an approach usingboth synthetic and laboratory data. The laboratory data arecollected from a scale-model LWD tool with one active sourcetransducer mounted on the side of the tool. Four sets of receiverarrays, each separated by 90 degrees azimuthally, areused to isolate monopole, dipole and quadrupole modes by coherentlyadding and subtracting received arrivals. A methodis then apply to perform dispersion analysis on these arrivals.With least square fitting, formation shear velocities are estimatedfrom both dipole and quadrupole modes' arrivals. Wefind that, by averaging the estimates obtained independentlyfrom dipole and quadrupole modes, we can reduce the uncertaintyand improve the confidence of the estimation for the formationshear velocity.
展开▼
