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| Titel |
A Computational Method for 3D Anisotropic Travel-time Tomography of Rocks in the Laboratory |
| VerfasserIn |
Mehdi Ghofranitabari, R. Paul Young |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250072111
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| Zusammenfassung |
| True triaxial loading in the laboratory applies three principal stresses on a cubic rock
specimen. Elliptical anisotropy and distributed heterogeneities are introduced in the rock due
to closure and opening of the pre-existing cracks and creation and growth of the new aligned
cracks. The rock sample is tested in a Geophysical Imaging Cell that is armed with an
Acoustic Emission monitoring system which can perform transducer to transducer
velocity surveys to image velocity structure of the sample during the experiment.
Ultrasonic travel-time tomography as a non-destructive method outfits a map of wave
propagation velocity in the sample in order to detect the uniformly distributed or localised
heterogeneities and provide the spatial variation and temporal evolution of induced damages
in rocks at various stages of loading. The rock sample is partitioned into cubic
grid cells as model space. Ray-based tomography method measuring body wave
travel time along ray paths between pairs of emitting and receiving transducers is
used to calculate isotropic ray-path segment matrix elements (Gij) which contain
segment lengths of the ith ray in the jth cell in three dimensions. Synthetic P wave
travel times are computed between pairs of transducers in a hypothetical isotropic
heterogeneous cubic sample as data space along with an error due to precision of
measurement. 3D strain of the squeezed rock and the consequent geometrical deformation is
also included in computations for further accuracy. Singular Value Decomposition
method is used for the inversion from data space to model space. In the next step, the
anisotropic ray-path segment matrix and the corresponded data space are computed for
hypothetical anisotropic heterogeneous samples based on the elliptical anisotropic
model of velocity which is obtained from the real laboratory experimental data.
The method is examined for several different synthetic heterogeneous models. An
“Inaccuracy factor” is utilized to inquire the accuracy of inversion results and to
obtain an optimal number of singular values for inversion as well as the minimum
heterogeneity percentage that can be recovered. Furthermore, the computational codes are
developed as a ray-based tomography tools to implement any set of transducer
distribution and any 3D model space resolution of cubic samples in a true triaxial test. |
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