Contactless ultrasonic system for continuous monitoring of soil moisture

It has long been recognized that soil moisture is a key variable in the terrestrial ecosystem. The dynamics of soil moisture control eco-hydrological dynamics that govern surface and subsurface flow, soil microbes, and vegetation. It is thus imperative to accurately monitor soil moisture to address these intertwined dynamics. However, it is difficult to minimize soil disturbance to preserve the original soil characteristics using conventional soil moisture monitoring methods that require maximum soil contact. To overcome this issue, we developed a non-destructive soil moisture sensor using a contactless ultrasonic system. This system measured leaky Rayleigh waves in the air, which can be measured on the surface. We applied the contactless ultrasonic system to sand, silt, and clay specimens. Their dimensions are 300 mm (length) × 100 mm (width) × 500 mm (height). The frequency of applied ultrasonic sound was 50 kHz. The signals were obtained using National Instruments 6366 data acquisition. We found that there were strong positive relationships between the energy of leaky Rayleigh waves and soil moisture for all three soil cases. These results can be explained by changes in shear resistance depending on soil types and water contents. That is, leaky Rayleigh waves have the potentials to explain soil moisture variations without soil disturbances.