ABSTRACT
The in situ elastic moduli of rock are measured holographically by a borehole instrument in marble and oil shale. A point force induced displacement field is recorded with double exposure holography. Data consists of a fringe pattern on the viewed hologram. The elastic moduli are determined by modeling of the fringe pattern. Only Young's modulus is fairly constrained by the present data reduction. For an applied force of 520 ± 50 Newtons, a marble was found to have a Young's modulus of 56 ± 8 GPa, near an ultrasonically determined value of 48 ± 6 GPa. In an oil shale, an applied force of 295 ± 10 Newtons indicates a Young's modulus of 25 ± 1GPa.
INTRODUCTION
Accurate knowledge of the in situ elastic moduli is required by most conventional stress measurement techniques (.McGarr and Gay, 1978). The holographic in situ stressmeter, which records a micron scale stress-relief dis- placement field (Bass, Schmitt, and Ahrens, 1986), requires values of Young's modulus and Poisson's ratio for a volume of rock with dimensions 10 cm square by 1 cm deep adjacent to s. borehole wall surface. Other stress measurement techniques: the C.S.I.R. "doorstopper" cell, (Leemah, 1969), the C.S.I.R. triaxial strain cell, (Leemah and Hayes, 1966), and the direct strain-gauge technique, (Swolfs, Handin, and Pratt, 1974) would benefit from small scale elastic moduli magnitudes. With this primary motivation in mind, double exposure holography is applied to the measurement of the in situ elastic moduli. In situ elastic moduli are difficult to measure due to problems in reproducing the original rock properties such as the state of stress, fluid saturation, jointing, and microfractures (Cheng and Johnson, 1981; Jaeger, 1979). Conventional in situ testing methods: compression tests, plate bearing tests, fiat jack tests, dilatometer tests, the "petite seismique" test, and the borehole jack tests (Bieniawski, 1978; Jaeger, 1979), measure the rock elastic moduli on varying length scales, force levels, and strain magnitudes. The rock modulus is dependent upon the sample scale and the density of discontinuities (Bieniawski and Van Heerdeen, 1975). For example, the N-X borehole jack method applies a uni-directional force of up to 703 KNewtons via two 20.3 cm long platens to the walls of a 7.62 cm diameter borehole (Goodman, Van, and Heuze, 1970). Changes in the hydraulic fluid pressure and the borehole wall diameter are measured; typical displacements are on the order of 0.2 into (Meyer and McVey, 1974) resulting in strain magnitudes near 1000 µstrains. The test is affected by the tensional properties of the rock surrounding the borehole (Heuze and Amadei, 1985). In a plate bearing test, compressional forces of up to 11.8 MNewtons are applied to areas on the order of I m2(Jaeger, 1979). Induced displacements are 0.25 to 2.5 mm and are measured with dial gauges; strain levels encountered are also approximately 1000 µStrains. The holographic test, described in this paper, is on a much smaller scale. The test, similar in principle to plate bearing tests, requires application of a normal point force to the borehole wall of 50 Newtons to 5 KNewtons.
