In‐situ holographic elastic moduli measurements from boreholes

We have developed a unique technique employing optical holography to measure the static Young’s modulus (E) from a borehole. In the experiment, a known point force induces micron scale displacements on the borehole wall which are recorded by a double‐exposure hologram. Raw data consist of dark fringes superimposed on the three‐dimensional image whose pattern is modeled to find E directly. In the laboratory, the holographic technique determined E on rock and metal samples to an uncertainty better than 10 percent. For example, double exposure holograms of a saw‐cut sample of dolomitic marlstone gave an E of 16.8 ± 2.8 GPa in agreement with 17.2 ± 2.0 GPa predicted by published density‐modulus relationships. Field tests of a holographic tool in a horizontal mine pillar borehole gave in‐situ Es which range from 26.9 to 36.0 GPa. Although these data could be interpreted as localized elastic heterogeneity within the rock mass, elastic anisotropy of the rock is a possible explanation for this variation.