Fracture Interface Waves in an Anisotropic Medium
Abstract
The detection of fractures in an anisotropic medium is complicated by discreet modes that are guided or confined by fractures and that travel with velocities close (~92%) to the shear wave velocity. For instance, fractures can mask the presence of textural anisotropy in a rock, and can increase the apparent shear wave velocity anisotropy. In this study, we examine how fracture interface waves affect the interpretation of shear wave velocities for two orthogonal polarizations propagating parallel to the layers. Samples with textural anisotropy measuring 100 x 100 x 100 mm were fabricated from garolite, an epoxy - cloth laminate, with layer thickness on the order of 0.5 mm. Three fracture samples were created with: (1) a fracture oriented parallel to layering, (2) a fracture oriented perpendicular to layering, and (3) two intersecting orthogonal fractures. An intact sample without fractures was used a standard. A seismic array, consisting of source and receiver arrays, was used to perform full waveform measurements. Each array contained two compressional and five shear wave piezoelectric contact transducers with a central frequency of 1 MHz. Shear wave transducers were polarized both perpendicular and parallel to the layering as well as to the fracture. Measurements were made for a range of stresses (0.4 - 4MPa). When the shear wave was polarized parallel to a fracture, the shear wave traveled at the bulk shear velocity respective to the layering. However, when the shear wave was polarized perpendicular to a fracture, the measured velocity ranged between the Rayleigh wave velocity at low stress and the bulk shear wave at high stress. The shear wave velocities perpendicular and parallel to the layering (propagation direction parallel to the layers) were ~1500 m/s and ~1600 m/s, respectively, in the intact sample. However, in the fractured samples, the observed shear wave anisotropy depended on the stress and fracture orientation relative to the layering. When the fracture was oriented perpendicularly to the layers, at low stress, the sample appeared almost isotropic. When the fracture was oriented parallel to the layers, the observed shear wave velocity anisotropy increased at low stresses as the shear wave (measured perpendicular to the fracture) traveled with a velocity at or near the Rayleigh velocity. Rock masses often have competing textural and structural properties that affect interpretation of the presence of fractures. For instance, shear wave energy can couple completely into fracture guided-modes. The existence of structural guided modes, such as fracture interface waves, depends on the frequency of the signal and the specific stiffness of the fracture, which is stress sensitive. Therefore, interpretation of the presence of fractures in isotropic or anisotropic media can be unambiguously interpreted if measurements are made as a function of stress, which eliminates many fracture-generated discreet modes. Acknowledgment: The authors wish to acknowledge support of this work by the Geosciences Research Program, Office of Basic Energy Sciences US Department of Energy (DE-FG02-09ER16022).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.H21B1087P
- Keywords:
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- 0935 EXPLORATION GEOPHYSICS / Seismic methods;
- 1859 HYDROLOGY / Rocks: physical properties;
- 5104 PHYSICAL PROPERTIES OF ROCKS / Fracture and flow;
- 5144 PHYSICAL PROPERTIES OF ROCKS / Wave attenuation