Asymmetric source acoustic LWD for the improved formation shear velocity estimation

Abstract

Most acoustic LWD tools generate a single pure borehole mode (e.g., dipole or quadrupole) to estimate the formation shear velocity. We propose an approach where many borehole modes are generated and all the modes are used simultaneously to obtain a better shear estimate. In this approach we find the best fit to the dispersion characteristics of a number of modes, rather than one mode. We propose using an asymmetric source, that is a single source on one side of the tool, together with arrays of receivers distributed azimuthally around the tool to allow different modes to be identified and analyzed. We investigate such an approach using synthetic and laboratory data. The lab data uses a scale-model LWD tool with one active sources transducer mounted on the side of the tool. This source geometry generates monopole, dipole, and quadrupole modes simultaneously. Four sets of receiver arrays, each separated by 90 degrees azimuthally, are used to isolate and analyze each of these modes by adding and subtracting the signals received from different arrays. Based on the dispersion analysis and the method of least square fitting, we find that the by simultaneously using both dipole and quadrupole modes, we can reduce the residual error of the best fit shear velocity. It should be noted that higher order modes (e.g., hexapole, etc) will also be generated by an asymmetric source, and these modes could also be utilized with the appropriate azimuthal receiver configuration.