HR: 0800h
AN: T51A-0434    [Abstracts]
TI: Strain Localization and 3-Dimensional Distribution of Strain Gradient Inferred From X-ray CT Imaging
AU: * Louis, L
EM: llouis@notes.cc.sunysb.edu
AF: SUNY at Stony Brook, Department of Geosciences , Stony Brook, NY 11794-2100 United States
AU: Baud, P
EM: pbaud@eost.u-strasbg.fr
AF: Institut de Physique du Globe (CNRS/ULP), 5 rue Rene Descartes, Strasbourg Cedex, 67084 France
AU: Tembe, S
EM: stembe@ic.sunysb.edu
AF: SUNY at Stony Brook, Department of Geosciences , Stony Brook, NY 11794-2100 United States
AU: Wong, T
EM: teng-fong.wong@sunysb.edu
AF: SUNY at Stony Brook, Department of Geosciences , Stony Brook, NY 11794-2100 United States
AB: Strain localization develops from bifurcation and discontinuity in the strain field. In the laboratory a spectrum of localization modes have been observed in relation to dilatant and compactive failure of a porous rock. To investigate the evolution of strain localization it is desirable to map out nondestructively the spatial distribution of the strain field and its gradient in 3 dimensions. In this study micro-CT imaging was used to address this question. Serial sections of X-ray radiographs were acquired on intact and deformed samples of porous sandstones. The X-ray attenuation data reflect the spatial variation of local density, which in a rock may be related to preexisting heterogeneities like sedimentary laminae or to stress-induced porosity changes. While the former can be readily resolved from CT imaging, the latter often results in relatively weak contrasts in X-ray attenuation that require further analysis to resolve. We developed a technique to infer the strain gradient from the X-ray attenuation data, which allows us to map out in fine detail the strain localization features. The average radial gradient of volumetric strain (or porosity change) is inferred from the mean attenuation gradient at a voxel, which in this study is evaluated from the mean of the difference of attenuation values between that at the voxel and its 26 nearest neighbors (weighted by the distances separating them). One of our Diemelstadt sandstone samples was triaxially compressed at 150MPa pressure, and on the exterior cylindrical surface of the failed sample several compaction bands oriented subperpendicular to the maximum principal stress were visible. From the 3D visualization of mean attenuation gradient, we infer that these compaction bands had indeed propagated from the exterior surface into the interior of the sample. These bands can be identified with relatively thin structures with anomalously low strain gradients, implying the development of appreciable homogenization of the porosity distribution within the compaction bands. The data can also be used to characterize the tortuosity and geometric complexity of the strain localization features.
DE: 8010 Fractures and faults
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
DE: 5194 Instruments and techniques
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting