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

Jie Shen, Department of. Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803: jshen6@lsu.edu, Juan M. Lorenzo, Department of. Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803: gllore@lsu.edu, Frank Tsai, Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, LA 70803: ftsai@lsu.edu.

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 near-surface 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) (V∝⁶√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 sand-tank 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.

Key words: seismic velocity, inversion, water saturation, soil properties