Abstract: A borehole tool including an interferometer, a light source, a chamber containing a sliding weight having a first optical prism, a second optical prism located within the chamber, a tilt measuring device, and a timing device operatively associated with the interferometer. The light source, the interferometer, and the first and second optical prisms are configured to cause light emitted by the light source to form a first beam and a second beam that interfere with each other. The interferometer measures distances traveled by the sliding weight in the upward and downward direction by counting the fringes caused by the interference between the first beam and the second beam. The tilt measuring device measures the angle of the chamber relative to vertical. The influence of friction on the sliding weight's motion is eliminated by comparing the distances traveled by it in its upward and downward path over an equal time interval.

Abstract: The present invention provides a method that is useful in determining the permeability of rock within a subterranean formation surrounding a borehole. The method involves the establishment of a compressional wave in the borehole rock of interest and the subsequent detection of a slow compressional wave that is directly related to the permeability of the rock. The slow compressional wave typically has a velocity of approximately 900 m/s. In contrast, the compressional waves used to make borehole measurements in the past have, velocities of approximately 2000-3000 m/s.