A mechanical-dielectric-high frequency acoustic sensor fusion for soil physical characterization

Abstract

The aim of this study was to develop a new soil multi-sensor fusion by combining mechanical, dielectric and acoustic responses in layered soils for estimation of soil water content, degree of compactness and texture. The load cell, dielectric and acoustic sensors were integrated on a 25 mm diameter, 45-deg conical probe. Functioning of the dielectric sensor is based on the fringe-field between the two adjacent ring electrodes. Passive acoustic responses (emissions) at high frequency (10–350 kHz) were measured using an acoustic emission (AE) piezo sensor during probe penetration into the layered soil. The AE sensor was installed inside the cone (close to the cone tip) to receive passive AEs originating at the cone-soil interface and between soil particles surrounding the moving probe. The multi-sensor probe was evaluated using laboratory vertical penetration tests in various layered soil columns consisting of three soil types: clay, loam and sand. Each soil texture was prepared at three soil water contents (0.4, 0.6 and 0.8 × lower plastic limit) and remolded at three bulk densities (1.25, 1.4 and 1.55 Mg m−3) in the column. The acoustic frequency distribution spectra clearly distinguished the sandy soil texture (irrespective of bulk density and water content) from the clay and loam soil textures, with a relatively poor discrimination between clay and loam soil. The results indicate that the dielectric sensor output is highly correlated to soil volumetric water content (R2 = 0.78, mean absolute error (MAE) = 0.025 m3 m−3) with a minor effect of bulk density. Data fusion of mechanical, dielectric and acoustic sensors significantly improved the measurement of soil water content (R2 = 0.93, MAE = 0.014 m3 m−3). Degree of soil compactness was well predicted from data fusion of the three sensors (R2 = 0.80, MAE = 3.32%). Therefore, the multi-sensor probe developed in this study is an economical and viable instrument that shows promising potential for in-situ use in soil physical characterization. Laboratory and field evaluations to further examine the sensor for practical applications, such as on-the-go soil physical sensing in precision agriculture, are needed.