Non-invasive acoustic-seismic sensing of soils

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.

Because they can give information about soil strength, acoustic-seismic methods offer a unique way to infer forces between particles as a function of water content and soil structure. So there is an important scientific case to convey. Moreover there is important economic case for the proposed research which we plan to stress when disseminating the findings to non-academic audiences. In the UK the cost of lost production resulting from droughts on wheat crops alone is estimated to be between 112 and 224M. This proposal includes collaboration with Delta-T devices and with Syngenta through their funded research group at Manchester. The project will also benefit from close links with plant scientists at Rothamsted and from collaboration with US researchers (Sabatier - see Letter of Support)). As well as publicising the results from this project at international research meetings, we will promote the results through the popular press and at public meetings. A regularly-updated website detailing the main project components and outputs will be initiated and maintained by Rothamsted. The website will have a FAQ section designed to help public understanding and will include a frequently revised blog and podcast updates. We will seek to publicize the project in the farming press and in the various media (as for project BB/D010683/1) including the BBC with whom the OU has an excellent relationship and track record. The main findings of the project will be communicated also to industry (e.g. the Association of Independent Crop Consultants) through regular meetings held at Rothamsted. We will disseminate our findings to academic audiences through peer reviewed publications in high impact scientific journals. Peer-reviewed publications arising from this grant will be registered on the Open University's open access institutional repository - Open Research Online (ORO) at http://oro.open.ac.uk. The OU PI will propose Special Issues of Acta Acustica combined with Acustica and Applied Acoustics devoted to the (novel) topic of this research. To ensure dissemination to potential end users and to soil science instrumentation developers, we plan to advertise and hold an international workshop at Rothamsted six months before the end of the project (supported with 5K by Delta-T, see support letter, Jenkins and by matched funding from this project). We propose that EPSRC match the 5K funding for the international workshop (see above) promised by Delta-T to ensure substantial invited international participation at a prestigious venue.