A True-Triaxial Laboratory Seismic Velocity Experiment Under In Situ Stress Conditions: A Comparison With In Situ 3D Stress and Velocity

The purpose of this thesis is to develop a greater understanding of borehole perturbation on the in situ 3D stress field, including observations of the stress field made from a borehole, with a specific focus on seismic velocity surveys. Commonly overlooked concepts of material anisotropy, nonlinear elastic constants, and the effects of σ2 on rock failure are investigated in the laboratory and are subsequently applied to field data. Beginning with an isotropic mine possessing a known 3D velocity structure and in situ stress field of the unexcavated rock mass, an elastic true-triaxial test is used to replicate the in situ stress path of a rock sample from a tunnel designed to maximize excavation damage. Velocity surveys were conducted along the tunnel roof and sidewalls with calibrated sensors and are comparable to laboratory frequencies. However, tensile stresses present in the tunnel sidewall are not replicated during this experiment, therefore acoustoelastic theory is used to describe some of the nonlinearity of the in situ sidewall velocities. In situ P-wave velocities are reproduced experimentally within the associated uncertainties. Another true-triaxial test is performed replicating the stress path of a rock sample located in the tunnel’s roof. Identifying which stress magnitudes and ratios result in significant acoustic emission while focusing on the experimentally elusive σ2. Seismic velocity surveys along all three axes and polarizations are taken at every significant stress path step, giving insight into the influence of crack density and orientation on seismic surveys. These results are then applied to borehole breakouts for interpretation of 3D stress in an anisotropic environment. Using televiewer breakout data, unconfined compression strengths (UCS) and velocities from hydrostatic triaxial experiments on core samples from two boreholes in a deep mine. Additionally, an analytical solution for borehole stress concentrations produced in Python is used to perform a study on the effects of pivotal borehole parameters such as inclination, deviation, and material anisotropy. The results prove the commonly used isotropic assumption is incorrect, and considering material anisotropy, in addition to the role of axial (σ2) stresses, explains breakouts seen along highly anisotropic heterogeneous boreholes.Ph.D