In-situ holographic elastic moduli measurements from boreholes

Abstract

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.