Holographic In-Situ Stress Measurement in Geophysics

Citation

Cohn, Stephen Norfleet (1983) Holographic In-Situ Stress Measurement in Geophysics. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/6vsd-wa54. https://resolver.caltech.edu/CaltechTHESIS:08302019-150635775

Abstract

A new and still experimental method for measuring the absolute in-situ stress field in crustal rocks offers several advantages compared to existing in-situ stress measurement techniques. It employs optical holography to record strain-relief displacements in a borehole environment. We call the prototype instrument the holographic stressmeter. It operates in an uncased borehole where it drills strain-relieving side holes into the borehole wall. An interference holographic recording system records the resulting displacements onto film. The reconstructed interference holograms contain sufficient information in their fringe patterns to determine the three-dimensional vector displacements due to strain relief at every point surrounding the side hole. Assuming isotropic, homogeneous, linearly elastic rock, these displacements define the three stresses acting at the borehole wall at a single point. The three non-vanishing stresses acting at each of three points, distributed azimuthally, on the borehole wall provide sufficient constraint to determine all six components of the desired far-field or virgin-rock stress tensor.

The holographic stressmeter employs an on-board side hole drilling system to produce strain relief. Thus it should be economical to operate and it is not restricted to shallow depths as are overcoring techniques. Furthermore, recording the whole displacement field resulting from drilling the side hole reduces the potential contamination of the measurement by residual stress mechanisms which often affect point strain measurements using foil resistance gauges.

To date a complete stress determination in the field has not been attempted. However, a prototype stressmeter has demonstrated repeatedly that the stability necessary to conduct the measurement using this approach is attainable. Results from field deployment show that the stressmeter can make qualitatively correct measurements at one azimuth in a borehole. Modifications to make measurements at the three azimuths required for a complete determination of the stress tensor components should be easily achieved. We propose additional improvements to permit measuring rock elastic properties in situ to enable accurate, quantitative stress determinations. The theoretically predicted precision of the stress component magnitudes using this measurement approach is estimated at 20%.

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