Non-Contact in Situ Estimation of Soil Structure Using Acoustic Reflections

The purpose of this study is to investigate the potential for estimation of soil porosity and pore size (key indicators of soil physical health) using the strength of reflection of audio pulses from natural soil surfaces. The motivation for this work is the importance of productive and healthy soils in agriculture and other economic uses of land depending on plant growth. Soil porosity and pore structure are also significant in a wide range of environmental impacts such surface water runoff, and greenhouse gas exchanges. Methods exist for evaluating soil porosity in a laboratory environment or by inserting sensors into the soil in the field. However, no convenient non-contact in situ measurement method exists. This means that existing soil evaluations do not generally sample adequately in either space or time. In the current study we develop a new methodology based on reflection of audio pulses in the frequency range 4 to 16 kHz. Such pulses are readily generated by, and analyzed with, for example, a mobile phone. The key to this very new method is a Taylor series expansion of the functional form of the acoustic reflectivity in a way that includes, embedded as simple linear regression coefficients, in a way which allows three soil parameters (porosity, tortuosity, and average pore radius) to be easily estimated. The requirements to set up these regressions are to generate pulses at a number of audio frequencies and a number of angles of incidence to the ground surface. More than 3 million Monte Carlo simulations are performed using known random noise levels from the proposed sensor system. The simulation results show that both the systematic and random errors in estimation are very small, being typically less than 1 % for all three soil parameters. All assumptions and sources of error related to this method are discussed (to the best of the authors’ knowledge) and it is concluded that a practical new operational tool should be able to be readily manufactured and validated. This tool will be inexpensive, compact, low-power, non-intrusive to either the soil or the surrounding environment and will readily be incorporated into mobile phones as a fast and accurate means of visualizing farm-scale soil health.