High-Resolution Seismic Characterization of Shallow Sedimentary Structure for Freshwater Resources and Karstic Voids

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This thesis encompasses a comprehensive exploration of seismic source evaluation, high-resolution characterization of near-surface coastal sediments, and the development of novel methodologies for subsurface void detection. The research integrates various geophysical measurements and numerical modeling to address these multifaceted objectives. First, the field performance of two impulsive borehole seismic sources, Scorpion and Ballard, is rigorously examined at a Gulf of Mexico coastal site. Both sources are evaluated in crosswell and reverse vertical seismic profile (RVSP) configurations. The study reveals distinct P-wave dominant frequencies for each source and geometry, along with direct S-wave arrivals for the Ballard source. Signal-to-noise ratios (SNR) are determined, and source radiation patterns and signature wavelets are analyzed, providing valuable insights into the behavior of these seismic sources. Subsequent investigations focus on high-resolution characterization of near-surface coastal sediments in the Gulf of Mexico. Seismic tomography is employed to analyze the region between two boreholes, yielding crucial information about P-wave and S-wave velocities. The results align with geological data, confirming the presence of fresh water-saturated sand zones and validating the geological characteristics of the area. The study also introduces a Q factor calculation, further enhancing our understanding of the sediment properties. Moreover, the research delves into the propagation of Gaussian Wave Packets (GWPs) within complex media. Leveraging insights from this study, two innovative methodologies for subsurface void detection are introduced. These methodologies employ data-driven algorithms utilizing GWPs. By emphasizing void scattering events while eliminating direct waves and unwanted reflections, these approaches allow for the precise localization of subsurface voids, particularly karst formations. Numerical synthesis of directional wave packets from existing shot gathers enhances the practicality of these methods

Borehole seismic source, Near-surface sediment characterization, Karst detection
Portions of this document appear in: Souri, Zohreh, Robert R. Stewart, and Yingcai Zheng. "Field evaluation of two impulsive downhole seismic sources in crosswell and reverse VSP geometries and high-resolution characterization of near-surface Texas Gulf Coast sediments." Journal of Environmental and Engineering Geophysics 28, no. 1 (2023): 28-44.