- Copyright © 2016, EEGS
Seismic velocity models of the near-surface (<30 m) better explain seismic velocities when all elements of total effective stress are considered, especially in materials with large cohesive and soil suction stress such as clays. Traditional constitutive elastic models that predict velocities in granular materials simplify the effect of total effective stress by equating it to net overburden stress, while excluding interparticle stresses and soil suction stress. A 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 changes and cohesion. At shallow depths, when water saturation increases, theoretically calculated seismic velocities decrease in clay and increase in sand because interparticle stresses suppress the Biot-Gassmann effect. For standard sand and clay properties, net overburden stress becomes more influential than interparticle stresses at depths greater than 10 cm in sand and 100 m in clay. Pore pressure in the new model also incorporates the effect of layer thickness and pore size variation. Traditional calculation of pore pressure assumes a constant pore size medium, but may lead to an under- or overestimation of velocity by up to 20%. In clays, the variation of seismic velocity with water saturation is almost double the range predicted when only net overburden stress is considered to influence stress at the grain contacts. The proposed model predicts seismic velocities that compare well with measured field velocities from the literature.