Non-invasive acoustic-seismic sensing of soils

一种对土壤结构和土壤机械强度进行非侵入性传感的方法将使人们能够更好地决定适当的土壤管理做法。缺乏直接测量与作物生长和土壤环境功能(如吸收高强度降雨)有关的土壤物理特性的适当方法，是制定可持续土壤管理办法的障碍。土壤可视为部分饱和的多孔介质。充气多孔介质的声学特性在各种环境中得到了广泛的研究。这些特性的模型包含了与框架弹性和孔隙结构相关的参数。最广泛使用的模型，Biot理论预测，这种介质支持两种耦合的纵波，有时被称为I型波和II型波，以及一种横波。I型波和横波主要通过固体基质传播，涉及颗粒之间的相互作用。它们相当于直接机械激发的P波和S波，例如在地震折射测量中。第二类波主要通过被粘性摩擦和热交换衰减的充满流体的孔隙传播。在声激励期间，即来自非饱和土壤表面上方的声源，它占主导地位，因为声音进入土壤的主要路径是通过连接到土壤表面的孔隙。最近的研究表明，土中的P波速度与土中的内应力高度相关。这表明，从非侵入性声波-地震探测中遥测的P波速度可以用来测量土壤中的机械应力，从而测量其对根部伸长的阻力。此外，在实验室和室外仪表井中的测量表明，土壤中声波的速度和衰减与土壤密度、含水率、基质电势和孔隙度有关。申请人(Attenborough和Taherzadeh)开发了一个模型(PFFLAGS)，以预测声音与分层土壤的相互作用，该模型来自土壤上方或土壤内部的声源，该模型同时考虑了土壤的力学和结构特性。将该模型应用于使用探头传声器的声学测量和使用地震检波器的地震测量的组合中，可以得到与独立测量值合理一致的几个土壤参数的值。当然，使用埋入地下的麦克风和地震检波器的技术是侵入性的。仍然需要开发非接触式、非侵入性的声学技术，并将其扩展到包括水分含量的测定。在这个项目中，我们建议联合使用麦克风测量从土壤表面反射的来自点声源(扬声器)的声音和扫描激光多普勒振动仪(LDV)测量地震表面对这种声辐射的响应。我们建议发展所需的理论和实践知识来推断渗透率(土壤的物理性质，强烈依赖于大孔隙的数量和连通性)、水分含量和土壤中的内应力，并绘制这些量作为深度的函数。拟议的技术将作为外地自动化数据采集和处理系统的后续工程开发的原型。

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.