Adsorption Preceding Wetting Front Controls Seismic Velocity

Rock elasticity varies with both humidity and water saturation, yet their combined effects remain poorly understood, although in nature vapor adsorption and liquid infiltration occur simultaneously. Here, we present experimental data of P-wave velocity and volume expansion in a free-standing sandstone subject to progressive wetting. Elastic softening, evidenced by P-wave velocity reduction, precedes the wetting front, followed by stiffening as liquid infiltration reverses this trend. To reconcile these softening/stiffening behaviors, vapor migration ahead of the wetting front is captured by numerical simulation of moisture transport constrained by experimental data. Initial softening is explained by a micromechanical model governed by surface energy reduction at grain contacts and validated by independent vapor adsorption tests. Subsequent stiffening is attributed to water infiltration, consistent with patchy saturation theory. We propose softening and stiffening are transitional processes governed by the advancing wetting front, with implications for seismic imaging of progressive wetting processes in crustal rocks.