Abstract

Physical properties of soils in the vadose zone, and especially their water content, are characterized by strong spatial and temporal variations mostly driven by weather and anthropogenic activities. To understand this variability and help water resource management, seismic methods have been recently suggested as a complement to electrical and electromagnetic techniques. The simultaneous in situ estimation of pressure (P) and shear (S) wave velocities (VP and VS, respectively) and their ratio (VP/VS) offers novel perspectives for the monitoring of space and time variations of vadose zone mechanical properties. However, the seismic response in partially saturated and unconsolidated soils remains complex and deserves to be studied both theoretically and experimentally. In this study, we tested the sensitivity of seismic measurements (i.e., P-wave travel times and surface-wave phase velocities) to water saturation variations using controlled physical models at the laboratory scale. Ultrasonic techniques were used to reproduce a small-scale seismic acquisition setup at the surface of glass bead layers with varying water levels. Travel times and phase velocity measurements obtained at the dry state were validated with both theoretical models and numerical simulations and serve as reference datasets. The increasing water level clearly affected the recorded wave field in both its phase and amplitude. In these cases, the collected data cannot yet be inverted in the absence of a comprehensive theoretical model for such partially saturated granular media. The differences in travel time and phase velocity observed between the dry and wet models show patterns that interestingly match the observed water level and depth of the capillary fringe, thus offering attractive perspectives for studying soil water content variations.

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