Dissertation/Thesis Abstract

Extracting physical parameters from marine seismic data: New methods in seismic oceanography and velocity inversion
by Fortin, Will F.J., Ph.D., University of Wyoming, 2015, 145; 3722992
Abstract (Summary)

The utility and meaning of a geophysical dataset is dependent on good interpretation informed by high-quality data, processing, and attribute examination via technical methodologies. Active source marine seismic reflection data contains a great deal of information in the location, phase, and amplitude of both pre- and post-stack seismic reflections. Using pre- and post-stack data, this work has extracted useful information from marine reflection seismic data in novel ways in both the oceanic water column and the sub-seafloor geology.

In chapter 1 we develop a new method for estimating oceanic turbulence from a seismic image. This method is tested on synthetic seismic data to show the method’s ability to accurately recover both distribution and levels of turbulent diffusivity. Then we apply the method to real data offshore Costa Rica where we observe lee waves. Our results find elevated diffusivities near the seafloor as well as above the lee waves five times greater than surrounding waters and 50 times greater than open ocean diffusivities.

Chapter 2 investigates subsurface geology in the Cascadia Subduction Zone and outlines a workflow for using pre-stack waveform inversion to produce highly detailed velocity models and seismic images. Using a newly developed inversion code, we achieve better imaging results as compared to the product of a standard, user-intensive method for building a velocity model. Our results image the subduction interface ~30 km farther landward than previous work and better images faults and sedimentary structures above the oceanic plate as well as in the accretionary prism. The resultant velocity model is highly detailed, inverted every 6.25 m with ~20 m vertical resolution, and will be used to examine the role of fluids in the subduction system. These results help us to better understand the natural hazards risks associated with the Cascadia Subduction Zone.

Chapter 3 returns to seismic oceanography and examines the dynamics of nonlinear internal wave pulses in the South China Sea. Coupling observations from the seismic images with turbulent patterns, we find no evidence for hydraulic jumps in the Luzon passage. Our data suggests geometric resonance may be the underlying physics behind large amplitude nonlinear internal wave pulses seen in the region. We find increased levels of turbulent diffusivity in deep water below 1000 m, associated with internal tide pulses, and near the steep slopes of both the Heng-Chun and Lan-Yu ridges.

Indexing (document details)
Advisor: Holbrook, W. Steven
Commitee: Dueker, Ken G., Mallick, Subhashis, Schmitt, Raymond, Snider, Jefferson R.
School: University of Wyoming
Department: Geology & Geophysics
School Location: United States -- Wyoming
Source: DAI-B 77/02(E), Dissertation Abstracts International
Source Type: DISSERTATION
Subjects: Geophysics, Marine Geology
Keywords: Seismic imaging, Seismic inversion, Seismic oceanography, Turbulence
Publication Number: 3722992
ISBN: 9781339054759
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