Abstract: A drill module for use in a holographic borehole instrument which is operable to determine the stress in the wall of a borehole by recording successively on the same holographic recording medium two holograms of a selected surface region of the wall and, in the interval between the two recordings, drilling a stress relief hole in the wall in stress relieving relation to the selected surface region to produce on the recording medium a double exposure hologram containing holographic information representing the stress. The drill module embodies a drill which is normally retracted from the field of view of the holographic system to permit unobstructed holographic recording of the selected wall surface region and which is extended into the field to drill the stress relief hole in the interval between the two holographic recordings.

Abstract: An instrument for determining the stress in the wall of a borehole by recording successively on the same holographic recording medium two holograms of a selected surface region of the wall, and, in the interval between the two recordings, forming a stress relief in the wall in stress relieving relation to the surface region to produce on the recording medium a double exposure hologram. This hologram contains holographic information which represents the stress in the wall in the vicinity of the surface region and may be converted to data defining the magnitude and direction of the stress.