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感测的方法将有助于更好地决定适当的土壤管理做法。缺乏适当的方法来直接测量与作物生长和土壤环境功能（如对高强度降雨的吸收）有关的土壤物理特性，是制定可持续土壤管理方法的障碍。土壤可视为部分饱和多孔介质。充气多孔介质的声学特性在各种背景下得到了广泛的研究。这些属性的模型包含与框架弹性和孔隙结构相关的参数。最广泛使用的模型，毕奥理论预测，这种介质支持两种耦合的压缩波，有时称为I型和II型波，和剪切波。I型波和剪切波主要通过固体基质传播，并涉及颗粒之间的相互作用。它们等同于由直接机械激励引起的P波和S波，例如在地震折射勘测期间。II型波主要通过充满流体的孔隙传播，并通过粘性摩擦和热交换衰减。它在声激励期间占主导地位，即来自非饱和土壤表面上方的声源，因为声音进入土壤的主要路径是通过连接到表面的孔隙。近年来研究表明，土中的纵波速度与土中的内应力高度相关。这表明，P波速度确定远程从非侵入性声地震探测可用于测量土壤中的机械应力，因此其电阻根伸长。此外，在实验室和室外仪器坑中的测量表明，土壤中的声速和衰减与土壤密度、含水量、基质势和孔隙度有关。申请人（Attenborough和Taherzadeh）已经开发了一种模型（PFFLAGS）来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源，该模型考虑了土壤的机械和结构性质。通过将该模型应用于使用探头麦克风的声学测量和使用地震检波器的地震测量的组合，已经发现可以获得与独立测量值合理一致的几个土壤参数的值。当然，使用埋在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，我们建议调查的结合使用麦克风测量反射从土壤表面的声音从一个点源（扬声器）和扫描激光多普勒振动计（LDV）测量地震表面对这种声穿透的响应。我们建议发展推导渗透率所需的理论和实践知识（土壤的一种物理性质，在很大程度上取决于大孔隙的数量和连通性）、含水量和土壤内应力，并绘制这些量与深度的关系图。所提出的技术将作为一个原型，为随后的工程开发系统的自动数据采集和处理领域。

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.