Doctor of Philosophy (PhD)
Geology and Geophysics
Seismic investigation in the near-surface is complicated by highly attenuating media, large interparticle stresses, and variable water saturation, so new tools and methodology are necessary to understand the relationships between velocity, attenuation, and physical properties of the propagating media. A new shear wave source is developed for investigation of gas-charged, organic-rich sediments because compressional waves are highly attenuated and currently available sources are inadequate. The new source compares favorably to a traditional hammer impact source, producing a signal with a broader-band of frequencies (30-100Hz cf. 30-60Hz) and signal-to-noise ratios (SNR) equivalent to ~3 stacked hammer blows to the hammer impact source. Ideal source signals must be broadband in frequency, have a high SNR, be consistent, and have precise start times; all traits of the new shear source. A new constitutive model predicting seismic velocity is developed because current models do not include interparticle stresses which are especially important in materials with large cohesive and capillary pressures such as clays. The new proposed methodology calculates elastic moduli of granular matrices in near-surface environments by incorporating an updated definition of total effective stress into Hertz-Mindlin theory and calculates the elastic moduli of granular materials by extending Biot-Gassmann theory to include pressure effects induced by water saturation. As water saturation increases in shallow sediments, theoretically calculated seismic velocities decrease in clay and increase in sand because of the respective interparticle stresses in these media. The proposed model calculates seismic velocities that compare well with measured field velocities from the literature. A field-transferrable lab experiment shows the simultaneous dependence of quality factor (Q) on water saturation and stress in unconsolidated sand. Local Q values (Qint) increase the most with depth (dQ/dz=43 m-1) and stress (dQ/dS=0.0025/Pa) in dry sand and the least in partially saturated sand (dQ/dz=10m-1 and dQ/dS=0.0013/Pa) where attenuation created by local fluid flow reaches a maximum. Expectations for Qint values with depth can be extrapolated from dQ/dS and are bounded by Qint of the dry (QD) and partially saturated (QPS) media (e.g.,QD>Qint>QPS). Qint deviations outside this range can be explained by a divergence in effective stress, attenuation mechanism, or lithology.
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Crane, James Michael, "Effects of stress and water saturation on seismic velocity and attenuation in near surface sediments" (2013). LSU Doctoral Dissertations. 3714.