Publication Date

5-2013

Type of Culminating Activity

Dissertation

Degree Title

Doctor of Philosophy in Geophysics

Department

Geosciences

Supervisory Committee Chair

Kasper van Wijk

Abstract

Wave propagation in scattering media is a complicated topic, but scattered elastic waves carry important information about the internal structure of the medium. It is a current topic of research and for the foreseeable future. Advances in theory and applications described in this manuscript benefit from new ways to collect more densely sampled, multicomponent, true-amplitude data.

Thus far, most fracture characterization experiments in the laboratory involve contacting transducers as elastic wave sources and receivers. Similarly, rock properties such as anisotropy and attenuation are also measured with contacting techniques. These type of measurements are well-suited for time-of-flight measurements, but for scattering experiments issues arise. These include coupling issues between transducers and sample, ringing of the mechanical transducer, time-consuming steps to repeat the measurements with different source/receiver locations, and the relatively large sensor size. As a result, contacting techniques are less than ideal to the study of heterogeneous and anisotropic media.

In this work, we show that contacting devices can successfully be replaced by remote laser ultrasonic sources and receivers. Using fully non-contact measurement techniques, we are able to avoid the aforementioned drawbacks, acquire high-quality laboratory data with dense source and/or receiver locations, and with computer-controlled acquisition that is fully automated and takes on the order of hours to complete.

First, we describe the experimental setup used throughout this work to acquire laboratory data on small-scale samples. We show that using a novel laser inter- ferometer design allows us to measure two components of the elastic displacement field. Combined with a laser source, this results in a fully non-contacting system that makes automated scanning acquisition possible with a source/receiver footprint small compared to the wavelength.

Second, we study a single fracture, whose size is comparable to the elastic wave- length, in an otherwise homogeneous medium. In a first step, we apply the linear slip model to a single finite planar fracture under the Born approximation. We derive new expressions for the scattering amplitude in the frequency domain and illustrate this theoretical work with a laboratory experiment. We measure the scattering amplitudes and estimate the compliance of a single fracture generated in a clear plastic sample, which shows good agreement between the theoretical and experimental results. We also show that the laser-based experimental setup allows us to directly excite elastic waves at a fracture inside a solid sample. We measure the associated displacement field, and use tip diffractions to estimate the size of the fracture.

Finally, we investigate the properties of an anisotropic medium with vertical trans- verse isotropic (VTI) symmetry. We can accurately measure the P-wave arrival along a dense range of angles, but also the S-wave arrival, for selected directions. We there- fore estimate the elastic constants and Thomsen parameters of the medium, as well as the attenuation anisotropy. This series of results demonstrate the potential of laser- based ultrasonics for laboratory measurements. In particular, we are able to rapidly acquire high-quality, densely sampled data in situations where contacting transducers would introduce issues related to their size, and ringing. These findings pave the way for wider use of laser ultrasonics in rock physics applications.

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