Non‐invasive estimation of the depth profile of soil strength with acoustic‐to‐seismic coupling measurement in the presence of crops

A traditional penetrometer measurement of soil strength in terms of penetration resistance, a useful indicator of a soil's mechanical property, is invasive and manually laborious. An alternative non‐invasive technique uses an acoustic source, two microphones and a laser Doppler vibrometer to obtain acoustic‐to‐seismic (A–S) transfer functions. Transfer functions between the acoustically‐induced soil vibration and the acoustic signals have been compared, through numerical optimizations, with predictions of a wave‐propagation model for layered poro‐elastic media. The numerically optimized shear modulus of soil was converted to the soil strength equivalent of penetration resistance by applying an empirical relation for the type of soil in question. The A–S and penetrometer measurements were made in experimental fields planted with winter wheat. The depth profiles of A–S‐deduced soil strength agree reasonably with those of penetrometer readings in shallow layers of five arable plots. We have demonstrated that the strength of soil in the presence of growing crops can be measured by this non‐invasive approach. Highlights Measurement of soil strength with a penetrometer is invasive and laborious. Acoustic‐to‐seismic (A–S) coupling has been applied to soil planted with crops. Numerically optimized shear modulus from A–S data can be converted to penetration resistance. Non‐invasive A–S method can be used as an alternative to a penetrometer.