Elastic waves along a fracture intersection
Abstract
Fractures and fracture networks play a significant role in the subsurface hydraulic connectivity within the Earth. While a significant amount of research has been performed on the seismic response of single fractures and sets of fractures, few studies have examined the effect of fracture intersections on elastic wave propagation. Intersections play a key role in the connectivity of a fracture network that ultimately affects the hydraulic integrity of a rock mass. In this dissertation two new types of coupled waves are examined that propagate along intersections. 1) A coupled wedge wave that propagates along a surface fracture with particle motion highly localized to the intersection of a fracture with a free surface, and 2) fracture intersection waves that propagate along the intersection between two orthogonal fractures. Theoretical formulations were derived to determine the particle motion and velocity of intersection waves. Vibrational modes calculated from the theoretical formulation match those predicted by group theory based on the symmetry of the problem. For the coupled wedge wave, two vibrational modes exist that range in velocity between the wedge wave and Rayleigh wave velocity and exhibit either wagging or breathing motion depending on the Poisson's ratio. For the intersection waves, the observed modes depend on the properties of the fractures forming the intersection. If both fractures have equal stiffness four modes exist, two with wagging and two with breathing motion. If the fractures have unequal stiffness, four modes also exist, but the motion depends on the Poisson's ratio. The velocity of intersection waves depends on the coupling or stiffness of the intersection and frequency of the signal. In general, the different modes travel with speeds between the wedge wave and bulk shear wave velocity. Laboratory experiments were performed on isotropic and anisotropic samples to verify the existence of these waves. For both waves, the observed signals were determined to depend on the applied load, which affects the stiffness of the fractures. These results have significant implications for fracture network characterization using remote techniques in both the laboratory and the field. The coupling parameter used in this discussion, i.e., specific stiffness, is a potential parameter to link the hydraulic properties of the fracture intersections to their seismic response. These results are a first step towards remote characterization to determine the hydraulic connectivity of fracture networks.
Degree
Ph.D.
Advisors
Pyrak-Nolte, Purdue University.
Subject Area
Geophysics|Physics|Acoustics|Geophysical engineering
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.