Asymmetric source acoustic LWD for the improved formation shear velocity estimation
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Chen_2009.pdf
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Author(s)
Chen, Tianrun
Wang, Bing
Zhu, Zhenya
Burns, Daniel R.
Date Issued
2009
Publisher
Massachusetts Institute of Technology. Earth Resources Laboratory
Series/Report no.
Earth Resources Laboratory Industry Consortia Annual Report;2009-03
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.
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