Low-Frequency Nonlinear Elasticity: a Powerful Tool for Probing Fractures
Lawrence Berkeley National Laboratory, USA
Harry Lisabeth is a Research Scientist in the Energy Geosciences Division at Lawrence Berkeley National Laboratory (LBNL). He received AB’s in Geological Sciences and Literary Arts from Brown University in 2010 and a Ph.D. in Geology from the University of Maryland, College Park in 2016. His graduate work focused on the effects of chemistry on rock deformation. From 2016 to 2018 he was a Postdoctoral Research Fellow at Stanford University working with the Stanford Center for Carbon Storage (SCCS) and the Stress and Crustal Mechanics Lab (SCML). Harry’s research focuses on the interaction between chemistry and materials under stress and utilizing novel experimental tools to characterize material behavior under conditions relevant to the Earth. Specific research applications range from Geological Storage (GS) and geothermal energy to planetary geophysics.
Introduction of the Lecture
Wave propagation in rocks is typically treated as a linear elastic phenomenon; however, as strain increases, nonlinear stress-strain behavior can result. Experimental evidence for nonlinear elastic behavior in geomaterials has existed for years, but the ability to use this property as a diagnostic tool has only begun development in the last several decades. The basis of this tool is that the degree of nonlinearity in fractured material is much greater than that in intact material, resulting in signals that are highly sensitive to the state of the fracture (stress, chemistry, fluids). Elastic nonlinearity can cause a propagating wave to distort, resulting in generation of harmonics, multiplication of waves of different frequencies and, under resonant conditions, shifts in resonance frequency peaks. One way to exploit these behaviors is to propagate waves of differing frequencies through a material and observe nonlinear wave mixing phenomena, a technique referred to in the literature as nonlinear wave modulation spectroscopy (NWMS). When two waves propagate colinearly through a nonlinear material, nonlinear wave mixing manifests as wave distortion, harmonic generation and the creation of sum and difference frequencies (sidebands). I’ll discuss the results of a NWMS study of fractured rocks at a range of conditions to highlight the strain (10^-6 – 10^-5), frequency (0.001 – 100 Hz), normal stress (0.1 – 10 MPa) and fluid dependence of nonlinear parameters in rocks. Analysis of various dependencies can provide insight into the mechanisms of nonlinearity and has the potential to provide methods of monitoring damage processes in the subsurface.