Shear‐wave logging with dipole sources

Abstract

Laboratory measurements have verified a novel technique for direct shear‐wave logging in hard and soft formations with a dipole source, as recently suggested in theoretical studies. Conventional monopole logging tools are not capable of measuring shear waves directly. In particular, no S waves are recorded in a soft formation with a conventional monopole sonic tool because there are no critically refracted S rays when the S-wave velocity of the rock is less than the acoustic velocity of the borehole fluid. The present studies were conducted in the laboratory with scale models representative of sonic logging conditions in the field. We have used a concrete model to represent hard formations and a plastic model to simulate a soft formation. The dipole source, operating at frequencies lower than those conventionally used in logging, substantially suppressed the P wave and excited a wave train whose first arrival traveled at the S-wave velocity. As a result, one can use a dipole source to log S-wave velocity directly on‐line by picking the first arrival of the full wave train, in a process similar to that used in conventional P-wave logging. Laboratory experiments with a conventional monopole source in a soft formation did not produce S waves. However, the S-wave velocity was accurately estimated by using Biot’s theory, which required measuring the Stoneley‐wave velocity and knowing other borehole parameters.