Pressure Wave vs. Tracer Velocities through Unsaturated Fractured Rock

Rapid fluid pressure changes in unsaturated media due to perturbations at or near the earth-atmosphere interface are commonly attributed to preferential or bypass flow. An alternative mechanism for the rapid propagation of pressure perturbations in unsaturated media is the kinematic response, which results from the nonlinear relationship between unsaturated hydraulic conductivity and water content. The relationship between pressure wave velocities and fluid velocities is described using kinematic wave theory, presented for the Brooks - Corey and van Genuchten -Mualem formulations. The kinematic mechanism predicts fluid pressure pulse velocities to be substantially faster than unsaturated fractured rock tracer velocities for both formulations. A hydraulic form of the advection-diffusion equation based on Richards' Equation is presented that uses the hydraulic diffusivity and kinematic velocity to predict the hydraulic response in unsaturated fractured rock to pressure head perturbations. Pressure pulse velocity monitoring may be an additional tool for estimating unsaturated hydraulic properties in low permeability media.