Surface‐Wave Dispersion in Partially Saturated Soils: The Role of Capillary Forces.

Improving our understanding of the relation between the water content and the seismic signatures of unconsolidated superficial soils is an important objective in the overall field of hydrogeophysics. Current approaches to constrain the water content in the vadose zone from seismic data are based on computing the ratio between compressional and shear wave velocities Vp/Vs. While this allows for the detection of pronounced changes in saturation, such as the groundwater table, it is essentially insensitive to variations in the saturation‐depth profile. Conversely, evidence shows that surface waves are sensitive to both the location of the water table and the saturation‐depth profile. Classic rock physics models are unable to explain the corresponding observations. We propose to estimate surface‐wave signatures accounting for capillary suction effects. We extend the Hertz‐Mindlin model using Bishop's effective stress definition, thus accounting for stiffness changes associated with capillary stresses acting on the soil's frame. We then compute the elastic properties of the partially saturated medium using the Biot‐Gassmann‐Wood model. Considering a 1D unconsolidated porous medium under steady‐state saturation conditions, as given by Richards' equation, we simulate body‐wave travel times and surface‐wave dispersion characteristics for different water table depths and overlying soil textures. Our results illustrate that surface‐wave phase velocity dispersion curves are remarkably sensitive to capillary effects in partially saturated soils, exhibiting velocity changes of up to 20% in the 10–100 Hz frequency range. These effects, which are particularly important in medium‐to fine‐grained soils, are virtually nonexistent in the corresponding Vp/Vs profiles. Plain Language Summary: Seismic waves are usually employed to study the water content in the shallow subsurface for environmental purposes. Most studies estimate the water content of the soil at different depths relating compressional and shear wave velocities, which are estimated by measuring body‐wave travel times. Even though this method permits to locate the depth at which the soil becomes fully water saturated, it is rather insensitive to changes in the water content of the overlaying portion of the soil, where partial air‐water saturation prevails. Surface waves, however, appear to be significantly sensitive to changes in saturation within this partially saturated region. Classic rock physics models cannot explain this difference in sensitivity between surface‐wave velocity and body‐wave travel times. In this work, we propose to include the effects of capillary forces, which arise when the soil is partially saturated, in the rock physics models. By doing so, we show that capillary action, which acts bringing soil particles further together, can explain why surface‐wave dispersion curves are sensitive to saturation variations in the partially saturated zone while body‐wave travel times remain virtually unperturbed. These results may help to better interpret seismic data for environmental studies. Key Points: Water content variations in the partially saturated zone hardly alter Vp/Vs ratios, but have large effects on surface‐wave signaturesThe effective soil stress is strongly affected by capillary forces, which stiffen the medium at relatively low saturationsCapillary effects permit to explain observed changes in surface‐wave dispersion due to small water content variations in the vadose zone [ABSTRACT FROM AUTHOR]