Apparatus for low frequency torsional shear wave logging

A system for shear wave logging of formations adjacent a borehole utilizing a controlled frequency torsional wave, comprising:

• elongated housing means suspended for controlled vertical positioning within said borehole;
• source means carried by said housing and positionable against the wall of said borehole for generating a selected frequency torsional shear wave;
• first and second variable reluctance torsional detector means carried by said housing at a first and second known spacing from said source means and being in contact with the wall of said borehole, each detector means having an armature means of ring shape having plural permanent magnet segments of equal arcuate length intersticed in equal spacing by plural segments of high permeability metal, rotor means of circular shape disposed for close positioning within said armature means and having a non-magnetic central portion secured to a concentric high permeability outer portion that is formed to provide plural cores, and having plural coils equi-spaced therearound as formed on said respective cores for disposition in flux associative positioning to said plural permanent magnets, and means for rigidly securing said armature means to said housing means, and torsion bar means for axially securing said rotor means to said housing means, and means connecting opposite ends of each of said plural coils to provide the detector output;
• means for processing output signals of said first and second detector means to determine signal phase difference which for known spacing difference is indicative of velocity in the formation.