Theory and applications to elastic wave sensors for interpretation of material properties by remote sensing methods
Creators: name: Hill, Jonathan
Publisher: name: Imperial College London
Year: 2015

description: The mathematical treatment of a transient wave theory with complex structural interactions has been investigated in various elastic problems as a tool for detection and interpretation of material properties by remote sensing methods. These problems are of great importance when attempting to gain information of an enclosed material when there is no direct access, with particular significance in worldwide applications including down hole oil exploration and screening of containers. Here we concentrate on the development of a transient analysis of such elastic wave sensor problems, employing both rigorous analytical and numerical methods. The thesis begins with a model problem of a single elastic solid layer under antiplane deformation. With the availability of analytic solutions, the understanding of the transient response is aided considerably. A forcing piston theory is thus formulated by distributing the original line load formulation over part of the free surface of the layer material. From this we evaluate the transient response of the problem numerically using various receiver sensor outputs, with either the layer thickness or the density of the layer material identified. Along with providing an overview of some of the main techniques used in the later chapters, the model problem introduces an averaging method formulation that develops an approximate form of solution, which is utilised throughout the thesis. The second of these problems presented is a two-dimensional analysis of elastic solid layers enclosing a channel comprising of a stationary and compressible viscous fluid. A forcing piston of a given displacement is applied to both of the free elastic surfaces to create a transient disturbance that propagates throughout the layered structure. If the force is applied with some discrete time-signature, the receiver signal processing mechanism is the measurement of the surface displacement either away from the piston, or at the piston location itself, at a later time. A number of ancillary and mathematical tools have been developed here so that various checks on the calculations can be made. The transient response of the problem is then examined numerically in an attempt to detect variation in the material parameters of the viscous fluid channel enclosed within the elastic layers. The two problems considered later in this thesis concentrate on the separate antiplane displacements and plane strain motions of a partially filled cylinder. This annular structure may be arbitrarily filled anywhere between empty and fully filled in a simple two phase system. The sensor is modelled as a long line load which reduces the problem to a two-dimensional analysis at any cross section of the cylindrical pipe, with displacement measurements made at either the source or elsewhere along the external pipe boundary. The piston theory is again introduced to provide further insight into the response mechanism of the cylinder structure. The transient response of the problem is then evaluated numerically to allow detection and quantitative determination of some characteristic material properties, this being the depth of the partial filling material or the density of the content that is enclosed within the cylinder.