Reflectivity analysis from the low symmetric anisotropic media

• Author / Creator

  Malehmir, Mohammadreza

• The earth’s crust and mantle is known to be anisotropic to the propagation of seismic waves. Despite this knowledge, the analysis and processing of seismic data still primarily assumes isotropy, an assumption that, in the context of active source seismic imaging, is an oversimplification that can result in flawed interpretations. Most work on seismic anisotropy has focused on improving seismic imaging by more properly accounting wave propagation paths. The incidence and azimuthal angle variations in reflectivity from anisotropic formations, however, remain poorly understood. Most analyses using structurally-constrained approximations for transversely isotropic half-spaces that are not always representative of real geological formations in the earth’s crust. The work in this thesis seeks to contribute further by both analytic and physical modelling of the reflectivity from the contact between anisotropic half-spaces. This is primarily accomplished by carrying out laboratory measurements of the acoustic reflectivity from variously tilted blocks of a weakly-orthotropic composite phenolic grade CE. This material has characteristics reminiscent of fractured and layered formations in the earth. Following the lead of earlier researchers, we repeated measurements of reflectivity with respect to the angle of incidence at four azimuths from each of the four blocks studied. As expected, the reflectivity varied with both incidence and azimuth, but it could not be explained using plane-wave, Zoeppritz-type, solutions. Interpreting the results required, first, development of the appropriate understanding of the expected plane-wave reflectivity and, second, tools to account for the propagation and reflectivity of finite beams in the geometry of the laboratory. The first issue was overcome by solving the general problem for the reflections originating from the welded-contact between two half-spaces of any symmetry and arbitrary orientation with respect to one another. This solution was then developed into an open-source and readily available algorithm entitled Anisotropic Reflectivity and Transmissivity calculator (ARTc). The second issue was addressed by the development of a second algorithm that, following earlier work of others, models the propagation and reflection of a spatially and temporally ‘bounded’ ultrasonic pulse within a water column overlying a flat interface. The program propagates the launched bounded pulse through the water to the interface and modulates in the 2-D Fourier domain the amplitude and phase of each of the pulse’s component plane waves, before returning the pulse to the point of observation. This algorithm successfully reproduced the complicated features of the post-critical angle reflectivity from isotropic test samples in the laboratory. More importantly, however, we were able to reproduce the reflectivities observed from the anisotropic block’s in the laboratory; this strong match between observation and modelling points validates the theory within ARTc. Further, the peak of the observed reflectivity curves does not coincide with that for the plane-wave solution which occurs at the critical angle. There are two implications of this work with respect to field investigations of azimuthal and incidence angle dependent seismic reflectivity. First, the azimuthal variations were not strong and this may suggest caution when attempting to deconvolve such variations out of more complicated and noisy field seismic data. Second, since all real seismic sources may deviate strongly from the plane wave assumptions normally employed, in field studies of seismic reflectivity workers may need to take account of this departure more fully.

• Subjects / Keywords

  • Elasticity
  • Anisotropy
  • Seismology

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3DN4089K

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.