Asymmetric source acoustic LWD for the improved formation shear velocity estimation

Abstract

Most acoustic logging while drilling (LWD) tools generate a single pure borehole mode (e.g., dipole or quadrupole) to estimate the formation shear velocity. We propose an approach where multipole modes are generated simultaneously and used to obtain a better shear estimation. This approach uses an asymmetric source with arrays of receivers distributed azimuthally around the tool to generate and identify signals from different modes. We investigate such an approach using both synthetic and laboratory data. The laboratory data are collected from a scale-model LWD tool with one active source transducer mounted on the side of the tool. Four sets of receiver arrays, each separated by 90 degrees azimuthally, are used to isolate monopole, dipole and quadrupole modes by coherently adding and subtracting received arrivals. A method is then apply to perform dispersion analysis on these arrivals. With least square fitting, formation shear velocities are estimated from both dipole and quadrupole modes' arrivals. We find that, by averaging the estimates obtained independently from dipole and quadrupole modes, we can reduce the uncertainty and improve the confidence of the estimation for the formation shear velocity.