Propagation of Leaky Rayleigh Waves across a Fracture along a Fluid-Solid Interface
Abstract:Rayleigh waves and leaky Rayleigh waves are sometime used to interpret crustal properties of fault zones or the seafloor. Interpreting these surface wave modes requires an understanding of the effect of mechanical discontinuities such as faults, joints and fractures on leaky Rayleigh wave propagation. In this work, laboratory wavefront imaging experiments were performed to capture the behavior of the leaky Rayleigh waves propagated across fractures at a liquid-solid interface.
Cubic samples of aluminum submerged in water were used in this study. The fractured sample was composed of two identical aluminum blocks (dimension: 102 mm x 102 mm x 61 mm), while the intact sample was measured around 102mm on edge. A contact piezoelectric shear-wave transducer (1MHz) was used as the source. A spherically-focused water-coupled compressional-wave transducer (1MHz) was used to receiver the radiated component of the leaky Rayleigh wave. Measurements were made for a range of normal stresses (0 – 6 MPa). For each loading condition, the receiver scanned a 60 mm x 20 mm region in 1 mm increments to map out the arriving wavefront as a function of time.
The measured waveforms at different depths from the intact reference sample were the same under different loading conditions. For the fractured sample, the following phenomenon were observed: (1) Prior to crossing the fracture, the velocity of the leaky Rayleigh wave did not change with stress; (2) After crossing the fracture, the velocity was lower than that measured above the fracture and the velocity increased as the stress increased from 0 to 6 MPa; (3) the amplitude of the leaky Rayleigh wave transmitted across the fracture increased with stress; (4) High frequency components experienced more attenuation than the low frequency components; (5) At high stress, the velocity of the leaky-Rayleigh wave approached that observed above the fracture. Correct interpretation of leaky-Rayleigh waves in fractured/faulted regions must account for time delays and attenuation caused by mechanical discontinuities.
Acknowledgments: This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Geosciences Research Program under Award Number (DE-FG02-09ER16022) and by the Geo-mathematical Imaging Group at Purdue University.