Shear‐wave logging with quadrupole sources

Abstract

This paper provides laboratory verification of a novel technique for direct, on‐line, shear (S)-wave velocity logging in hard and soft formations using a quadrupole source as recently suggested by theory. Conventional monopole logging tools are not capable of directly measuring the shear‐wave velocity in soft formations. Previous theoretical and laboratory experimental work has already shown that a dipole source can be used for direct shear‐wave velocity logging in hard and soft formations. I demonstrate in this paper that quadrupole sources can also achieve this objective. Furthermore, the quadrupole source can produce higher resolution of S-wave velocity, since it can be operated at higher frequencies. A power comparison of monopole, dipole, and quadrupole indicates that a field quadrupole logging tool may be expected to produce strong enough signals for practical operation. The present studies were conducted on laboratory scale models representative of sonic logging conditions in the field. I used a limestone model to represent hard formations and a plastic model to simulate a soft formation such as Gulf Coast soft shale or near‐surface materials.