This paper explores the feasibility of making in situ stress measurements at depth from caliper measurements of deformed wellbores. The work has been carried out in three steps: (1) theoretical development of viscoelastic constitutive equations necessary for calculation of stress directions and magnitudes, (2) application of the equations to field data, and (3) comparison of results to stress measurements made by hydraulic fracturing and overcoring methods. The results of the theoretical analysis are equations that relate the minimum and maximum radii of a deformed wellbore to the maximum and minimum stresses perpendicular to the axis of the wellbore. Calculation of absolute magnitudes of stresses requires the viscoelastic compliance and Poisson's ratio of the rock, but even when this information is not available relative magnitudes of stresses can be determined. The constitutive equations were applied to caliper measurements of a horizontal borehole in unwelded ash-fall tuff near an underground tunnel complex in Rainer Mesa on the Nevada Test Site. The horizontal borehole was aligned in the direction of the maximum horizontal stress. Over a 37 day period the wellbore showed time-dependent deformation, with maximum closure in the maximum stress direction (overburden). The ratio of principal stress calculated from caliper measurements of the deformed wellbore varied from 1.73 to 2.06, depending on the time interval used. The ratios obtained from overcoring and hydraulic fracturing were 1.79 ± .22 and 2.12 ± .12. Limitations of this approach to stress measurement are discussed, but results of the present study suggest that with further development of the viscoelastic consitutive model, stress variations along the length of the wellbore might be readily examined as part of the well logging process.

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