3D Imaging and Characterization of Fractures in Rock

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

Geometric and interfacial properties of the fractures and faults in rock are the subject of critical importance to many facets of our society including mining, seismology, earthquake engineering, environmental protection, hydrogeology, and utilization of geothermal energy. One particular parameter embodying the fracture's interfacial condition is the so-called specific stiffness, quantifying for instance its rigidity under shearing or compression. Beyond its immediate relevance to the stability analyses in rock masses (e.g. during mining operations), the fracture specific stiffness has been found to: i) bear an intimate connection to the fracture's hydraulic properties (governing for example the performance of enhanced geothermal systems), ii) serve as a precursor of shear failure along rock discontinuities, and iii) help understand the mechanism of shallow earthquakes. In general, however, the fracture's response to given activation is equally driven by its geometry, which is inherently not limited to the planar condition. Thus a holistic characterization of subterranean fractures, that unveils both their geometric and mechanical characteristics, is a paramount. To help meet the challenge, this research aims to establish a comprehensive analytical, computational, and experimental platform for the geometric reconstruction and mechanical characterization of arbitrarily-shaped discontinuities in rock by way of seismic waves. The focus is on developing and validating a robust framework for the waveform tomography of fractures that is capable of resolving their three-dimensional geometry and spatial distribution of specific stiffness without iterations. Typically, approaches to the waveform tomography entail recursions owing to a highly nonlinear relationship between the fracture characteristics and seismic observations. Recently, however, the research in applied mathematics has produced a suite of non-iterative approaches to the waveform tomography such as the method of Topological Sensitivity. By building on such advancements, this research will cater for the imaging and characterization of curved fractures while allowing for significant flexibility in terms of the sensing arrangement. This is made possible by an innovative 3-step approach where the seismic waveforms are used to sequentially - and non-iteratively - reconstruct: i) fracture geometry, ii) fracture opening displacement profile, and iii) heterogeneous specific stiffness. The developments will be verified in a laboratory setting, making use of the recently acquired Scanning Laser Doppler Vibrometer that is capable of remotely monitoring triaxial waveforms on the surface of rock specimens with exceptional resolution and accuracy.