Effects of desiccation on the elastic wave velocities of clay-rocks

Compressional (P) and shear (S) wave velocities were measured in a set of clay-rock samples subjected to a desaturation phase during which the samples were dried at ambient temperature conditions, with a median relative humidity equal to 37%, followed by a heating phase during which the same samples were heated to five different temperature levels, ranging from 65 to 105 °C. The recorded data shows that when the degree of saturation is decreased, both P and S wave velocities increase, thereby lying in the range (0–50%). The increase in S wave velocity following desiccation, by as much as 10%, and the associated increase in dynamic shear modulus, suggests the presence of desiccation-driven hardening, which is commonly observed in clay soils. The existence of such a phenomenon, which is not observed in other sedimentary rocks over such a range of saturation values, proves that the classical models (e.g., the Biot–Gassmann equation) used to study the effect of saturating fluids, fail to correctly assess the influence of variations in water content on seismic velocities measured in clay-rocks.