Ultrasonic velocity tomography method for damage evaluation in rocks

Abstract

The naturally occurring fractures and discontinuities have a significant influence on the mechanical and hydraulic properties of rocks and various engineering problems such as reservoir development, hydraulic fracturing, slope stability, underground construction, and earthquake prediction require an accurate characterization of such natural discontinuities and their progression. Therefore, it is very important to understand several damage processes associated with the loading of rock materials, including the closure of existing micro-cracks and the initiation of cracks and their propagation. The Ultrasonic Velocity Tomography (UVT) technique is one of the full-field measurement technique that can be used, independently or in combination with other techniques, to study the changes in mechanical properties in the material, by mapping the ultrasonic wave velocity in the entire specimen. UVT is capable of identifying the heterogeneous changes (e.g. strain localization) occurring inside a material due to external loading. The main objective of this thesis was to develop and implement UVT techniques for characterizing the damage in prismatic brittle rocks subjected to uniaxial loads. By comparing the generated velocity field tomograms at different stages of loading, this thesis aimed to identify the formation of damage zones (e.g. strain localization zones) in a laboratory specimen, which are often a precursor to crack initiation, propagation and ultimately material failure. To achieve the research objectives, the following tasks were undertaken: (a) Evaluation of the applicability/suitability of the UVT technique in identifying the velocity field in prismatic brittle rock specimens; (b) Validation of the UVT technique: first, by using synthetic data and second, by using material with a known response; (c) Sensitivity analysis to determine the influential parameters for velocity tomograms; (d) Determination and comparison of the velocity field in prismatic brittle rock specimens with the flaws at different stages of loading to infer the changes occurring inside the specimen; and (e) Determination of the full-field strain profile, using the Digital Image Correlation (DIC) technique for better interpretation of the relationship between the mechanical changes and the geophysical response. Piezoelectric ultrasonic sensors were used to generate and receive elastic waves across the specimen and a fast LabView-based data acquisition system was used to record the waveforms that provided a great spatial resolution. The three major steps involved in the UVT procedure included Arrival time picking, Double Beamforming (DBF) for low Signal-to-noise ratio waveforms, and the inversion. Arrival times of the received compressional waves were used as the input to the MATLAB implemented tomographic inversion method and sectional velocity field tomograms were generated, during different stages of the uniaxial loading. By analyzing and comparing the changes in generated velocity tomograms, the initiation and growth of cracking and damage in the specimen were monitored. The ultrasonic velocity tomography technique was able to detect progressive damage in the specimen as low-velocity zones in the generated tomograms.