Coda Wave Interferometry analysis for brittle rock deformation during uniaxial compression

Coda Wave Interferometry analysis is studied for rocks subjected to brittle deformation which is induced by an Uniaxial Compressive Strength testing machine. Ultrasonic pulses, 1 MHz and 2.25MHz, are sent through rock samples and recorded throughout the loading up-to failure. A novel method is proposed, the rolling cross correlation, which can aid in the identification of the brittle fracture process. Unlike in previous CWI research a different method of referencing is used. The focus is not on velocity change of the medium with a fixed reference wavelet, but a variable reference point is used with a lag in respect to the newly measured wavelet. With this rate-of change method it is possible to quantify an incremental change in the scatter distribution as a result of micro-fracturing. Laboratory testing results showed similar curve behavior for the following rock types: Bentheimer Sandstone, Fontainebleau Sandstone, Indiana limestone and the Benin Granite. A correlation was found between the rolling cross correlation curves and the geomechanical phases of initial crack closure, purely elastic region and inelastic deformation. The detection of the latter was found to be a precursory indicator for yielding and eventually rock failure. The trend of the rolling cross correlation curve became more erratic when using a higher frequency as pulse source. In some cases individual events of faster displacement rate due to fracturing were detected in the change of coda waves. CWI analysis identified as an interesting tool for monitoring in-situ rock, such as fault stability in earthquake prediction, though future research is needed to enhance the correlation of change in coda to the actual change in the medium.

Subject

Coda Wave Interferometry
Brittle rock deformation
Uniaxial Compression

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Student theses

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master thesis

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(c) Boerrigter, T.J.

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Thesis2462017ThijsBoerrigter.pdf

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