Constitutive elastic models to predict shallow (<30m) seismic velocities: Natural soils beneath an earthen levee: Marrero, Louisiana, U.S.A.

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Current constitutive elastic models of granular materials are able to predict shallow (< 30 m) seismic velocities in sands, but can be improved to predict seismic velocities in clay-rich soils where additional interparticle stresses exist, caused by capillarity and cohesion. We calculate the elastic moduli of granular matrices in nearsurface environments with an updated definition of total effective stress which also incorporates granular cohesion and capillary pressures. Commonly, Hertz-Mindlin (HM) theory is used to calculate the elastic moduli of granular materials by extending Biot-Gassmann theory to include pressure effects induced by water saturation changes. Hertz-Mindlin theory predicts that seismic velocity (V) will increase as a power function of stress (σ) (Vα6√σ). HM theory can readily adapt to include the additional effects of interparticle stresses. Currently the proposed extended model calculates seismic velocities that compare well with sandtank lab experiments (depths < 1m). However, in mixed organic-rich lower-delta sediments, measured velocities require additional consideration of clay interparticle stresses. We use field velocity measurements from a case study in soils beneath coastal flood-protection levees, south of New Orleans, U.S.A For shallow depths (<100 m), interparticle stresses can be larger than net overburden stress in clay-rich soils. © 2013 SEG.

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SEG Technical Program Expanded Abstracts

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