Holographic in situ stress measurements

A new instrument for measuring the in situ level of stress in boreholes has been developed. The instrument operates on the principle of locally relieving the stresses acting on a rock mass by drilling a small hole into the borehole surface and recording the resultant displacement field by holographic interferometry. Because the recording technique is optical, the entire displacement field due to stress relief is obtained. A description of the stressmeter, theory of the interferometric technique, data reduction methodology, and results of laboratory stress relief calibration tests are presented. In addition, we present results from a field deployment of the instrument in an underground shale mine in Garfield County, Colorado using a test borehole within a support pillar. Sufficient data were obtained to constrain five of six stress components at a shallow level of the test borehole, thereby demonstrating the viability of the holographic technique. The holographic stress-relief data yield an approximate EW maximum horizontal stress direction. By comparison with previous hydrofracture measurements of Bredehoeft et al., our results indicate substantial stress-relief near the pillar face, thus masking any relicts of the far-field tectonic stress.