Non-invasive acoustic-seismic sensing of soils

Lead Research Organisation: Rothamsted Research
Department Name: Sustainable Soils and Grassland Systems

Abstract

A method for non-invasive sensing of soil structure and the mechanical strength of soil would permit better decisions about appropriate soil management practices. The lack of suitable methods to measure soil physical characteristics directly that are relevant to crop growth and soil environmental function (e.g. absorption of high intensity rainfall) are barriers to the development of approaches for sustainable soil management. Soils may be regarded as partially-saturated porous media. The acoustical properties of air-filled porous media have been studied widely in various contexts. Models for these properties incorporate parameters related to the frame elasticity and the pore structure. The most widely-used model, Biot theory predicts that such media support two kinds of coupled compressional waves, sometimes called Type I and II waves, and a shear wave. The Type I and shear waves travel mainly through the solid matrix and involve interactions between particles. They are equivalent to the P- and S- waves induced by direct mechanical excitation, for example during a seismic refraction survey. The Type II wave travels mainly through the fluid-filled pores being attenuated by viscous friction and thermal exchanges. It is dominant during acoustic excitation i.e. from sound sources above an unsaturated soil surface since the primary path for sound into the soil is through the pores connected to the surface. Recently it has been demonstrated that the P-wave velocity in soil is highly correlated with the internal stress in a soil. This suggests that P-wave velocities determined remotely from non-invasive acoustic-seismic probing can be used to measure mechanical stress in soil and hence its resistance to root elongation. Furthermore measurements in the laboratory and in instrumented pits outdoors have shown that the velocity and attenuation of sound in soil is related to soil density, water content, matric potential and porosity. The applicants (Attenborough and Taherzadeh) have developed a model (PFFLAGS) to predict the interaction of sound with layered soils, from sources above or within the soil that takes into account both soil mechanical and structural properties. By applyng this model to a combination of acoustic measurements using probe microphones and seismic measurements using geophones it has been found to be possible to obtain values of several soil parameters in reasonable agreement with independently measured values. Of course techniques using buried microphones and geophones are invasive. There remains a need to develop non-contact non-invasive acoustical techniques and to extend them to encompass the determination of moisture content. In this project we propose to investigate the conjunctive use of microphone measurements of reflection from the soil surface of sound from a point source (loudspeaker) and scanning Laser-Doppler Vibrometer (LDV) measurements of the seismic surface response to such insonification.We propose to develop the theory and practical knowledge needed to deduce permeability (a physical property of soils that depends strongly on the number and connectivity of macropores), moisture content and the internal stress in soil and to map these quantities as a function of depth. The proposed technique will serve as a prototype for subsequent engineering development of systems for automated data acquisition and processing in the field.

Publications

10 25 50
 
Description The stress environment around plants is determined by soil composition, moisture content and cultivation practices. In its turn the stress environment determines root growth potential and thereby cultivation yields. Knowledge of how plants respond to the stress environment is important for
• Food security
• Sustainability
• Irrigation and water conservation
Work carried out by The Open University and Rothamsted for EP/H040617/1 Non-invasive acoustic-seismic sensing of soil strength and structure, has established (a) the feasibility of remote determination of wave speeds and layering in soils [Shin et al 2013] and (b) empirical relationships between these speeds, penetrometer resistance and matric potential (a measure of moisture content) [Gao et al 2013].
Exploitation Route A non-invasive method for sensing/imaging of soil structure and the mechanical strength of soil would permit better decisions about appropriate soil management practices. This project as established that this approach is feasible. We are currently seeking funding to take this work forward in a proposed submission to BBSRC with a proposal to panel B which was considered in the February 2017 panel meeting.
Sectors Agriculture, Food and Drink

URL http://acoustics.open.ac.uk/soils
 
Description We have used the output of this project to develop a new project proposal to integrate EMI technology with acoustic-seismic measurements. This project proposal was submitted to BBSRC and we await the decision of the panel. Unfortunately this was not funded.
Sector Agriculture, Food and Drink
Impact Types Economic

 
Description BBSRC China Partnering Award
Amount £30,000 (GBP)
Funding ID BB/P025595/1 
Organisation Biotechnology and Biological Sciences Research Council (BBSRC) 
Sector Public
Country United Kingdom
Start 04/2017 
End 03/2020
 
Description An optimal calibration function and process for a dielectric tensiometer sensor of the matric potential of soil water 
Organisation Delta T Devices Ltd
Country United Kingdom 
Sector Private 
PI Contribution Richard Whalley and his team have a strong track record of developing novel sensing techniques to measure soil conditions for plant growth. These sensors and sensing approaches have been developed with funding from EPSRC, BBSRC and the EU, but they have never been taken through to the point of commercialization. This has limited their wider use at Rothamsted because of the low number of prototype sensors available for research. The hurdle to commercialization is sensor calibration and this applied in particular to the dielectric tensiometer sensors of water potential (Whalley et al. SSSAJ 75:1652-1657). While the Rothamsted team does have many of the "soil physics" skills needed to develop a rapid calibration method for dielectric tensiometers, to do so in the absence of commercialization strategy, involving the use of pre-commercial sensor designs, would not effective or fruitful. For this reason we are proposing to develop a sensor calibration protocol in parallel with the development of pre-commercial prototype sensor at Delta-T Devices.
Collaborator Contribution Delta-T expects to start commercialising sensors that arise from this collaboration within 12 to 18 months of the end of this project. The successful completion of this project would provide Delta-T with its first low cost and accurate dielectric tensiometer for the measurement of matric potential of soil water. The demand for such a sensor is thought to be very high. We already have a royalty agreement, which is not related to any patent, in place with Delta-T for the technology that is at the core of the dielectric tensiometer technology that they are looking to commercialise.
Impact This collaboration is about to start. It exists because of the existing projects identified (BBS/E/C/00004861 and BBS/E/C/00005204)
Start Year 2015