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Summary

Soil structure, moisture content and strength have profound effects on plant growth. Traditional methods for monitoring soil condition are invasive and therefore may affect the samples of interest. We have demonstrated the potential of a non-invasive measurement technique for the in situ monitoring of soil physical properties in the field. When soils are regarded as porous and elastic media, sub-surface wave propagation can be indicative of the soil status. Such propagation can be initiated by airborne sound through acoustic-to-seismic (A–S) coupling. Measurements of near-surface sound pressure and acoustically induced soil particle motion can be exploited to estimate the pore-related and elastic properties of soils. We have conducted laboratory measurements on dry and wet sand and field measurements on an arable soil growing wheat using a compression driver, microphones and a laser Doppler vibrometer. The excitation levels were chosen so as to reduce the influence of soil non-linearity while still yielding sufficient signal-to-noise ratios. Measured data were compared with model predictions based on wave propagation in layered homogeneous isotropic poro-elastic media described by linear Biot-Stoll theory. Soil properties were estimated through an optimization process minimizing the differences between the measurements and predictions. Latin hypercube sampling was adopted to ensure uniform seeding for optimization throughout the multi-dimensional search space. The fitted soil characteristics are air permeability, porosity, P-/S-wave speeds (related to bulk and rigidity moduli) and a loss factor. Layer depth was also estimated for multi-layered samples. The current work has demonstrated that soil can be characterized non-invasively by using A–S coupling. It is also shown that field soils can be represented adequately by multiple homogeneous layers.