| Summary: | 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.
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