Apparatus for low frequency torsional shear wave logging

A borehole logging device for obtaining shear wave velocities in formations adjacent the borehole, a device which utilizes a closely controlled, relatively low frequency torsional shear wave. The system includes an elongated housing for suspension within the borehole, said housing including a shear wave source for generating shear waves directly within the borehole wall, and including two spaced, variable reluctance torsional detectors which are also placed in direct contact with the borehole wall. The torsional detectors are, of course, at known spacing from the source such that cross-correlative procsssing of the detector output signals will enable a very accurate phase difference indicative of the shear wave velocity in the formation.