A Multidisciplinary Analysis of Frequency Domain Metal Detectors for Humanitarian Demining

This thesis details an analysis of metal detectors (low frequency electromagnetic induction devices) with emphasis on Frequency Domain (FD) systems and the operational conditions of interest to humanitarian demining. After an initial look at humanitarian demining and a review of their basic principles we turn our attention to electromagnetic induction modelling and to analytical solutions to some basic FD direct (forward) problems. The second half of the thesis focuses then on the analysis of an extensive amount of experimental data. The possibility of target classification is first discussed on a qualitative basis, then quantitatively. Finally, we discuss shape and size determination via near field imaging

Evaluation of metal detection audio alert signals of AN/PSS-14 by means of real time visualization

The sweep arm of the AN/PSS-14 Landmine Detector attached to an independent visualization device will transform the AN/PSS-14 into a hand-held data acquisition system for conversion of real time MD and GPR audio alert signals into captured visual trace displays on an on-board PDA computer screen so that momentary sounds of millisecond durations can be captured and displayed for visual scrutiny and interpretation by analytical on-board data processing procedures. This is a part of a two-man team that is developing a visual data adapter for the AN/PSS-14 Landmine Detector where the current thesis is concentrating only on MD audio alerts. The visualization device displays the amplitude-time plot and the instantaneous frequency-time plots of the MD audio alert signal for better understanding and for distinction of metallic firing pins from metallic clutter. This is achieved by the identification of the metal type, which would be common in any given landmine field and by observing the symmetric deviations of the amplitude envelope along multiple sweep directions over a buried target. The analysis of this research is constrained to only vertical firing pins. Further, analyzing the frequencies of different types of firing pin (different metal types), it can be concluded that the frequency response of the AN/PSS-14 to metallic targets are specific to metals. This process is only applicable for single, isolated MD audio signals --Abstract, page iii

A critical review on soil ionisation modelling for grounding electrodes

Grounding electrode resistance non-linearly changes under impulse conditions due to soil ionisation phenomenon. Several models have been proposed to model soil ionisation for grounding electrodes applications. However, to date, there is yet an attempt made to compile all these works into a comprehensive review article. Therefore, this paper is written with the objective of summarizing all related works in this field as a one- stop reference. With reference to the literature, this paper is written to summarize the working principles of the soil ionisation models as well as the accuracy and performance analysis of the models. This paper, particularly highlights the deficiencies of the available models in terms of accuracy and performance. This knowledge will contribute to the development of a new accurate and efficient soil ionisation model

Capacitive imaging technique for non-destructive evaluation (NDE)

This thesis describes the development and characterization of a novel NDE methodthe Capacitive Imaging (CI) technique. The CI technique employs a pair of (or multiple) electrodes to form a co-planar capacitor, and uses the fringing quasi-static electric field established across the electrodes to investigate specimens of interest. In general, the CI probe is sensitive to surface and hidden defects in insulating materials, and surface features on conducting materials. The CI technique is advantageous for its non-contact and non-invasive nature, and the capacitive coupling allows the CI technique to work on a wide variety of material properties. The theoretical background to the CI technique has been developed. It is shown that in the frequency range of operation (10 kHz to 1 MHz), the quasi-static approximation is valid and the Maxwell’s Equations describing the general electromagnetic phenomena can be simplified. The practical implementation of the CI system is based on this analysis, and it is shown that the CI technique has features that can complement techniques such as eddy current methods that are already established in NDE. The design principles of the CI probes that are required for an optimum imaging performance have been determined, by considering the key measures of the performance including the depth of penetration, the measurement sensitivity, the imaging resolution and the signal to noise ratio (SNR). It has been shown that the operation frequency is not an influential factor - the performance of the CI probe is determined primarily by the geometry of the probe (e.g. size/shape of the electrodes, separation between electrodes, guard electrodes etc.). Symmetric CI probes with triangular-shaped electrodes were identified as a good general purpose design. Finite Element (FE) models were constructed both in 2D and 3D in COMSOLTM to predict the electric field distributions from CI probes. Effects of thickness of specimen, liftoff distance and relative permittivity value etc were examined using the 2D models. The sensitivity distributions of different CI probes were obtained from the 3D models and were used to characterize the imaging ability of the given CI probes. The fundamental concepts of the CI technique have been experimentally validated in a series of scans where the defects were successfully imaged in insulating (Perspex) and conducting (e.g. Aluminium, Steel and carbon fibre composite) specimens. The detection of corrosion under insulation (CUI) has also been demonstrated. The imaging abilities were assessed by investigating various standard specimens under different situations. The CI technique was then successfully applied to various practical specimens, including glass fibre laminated composites and sandwich structures, laminated carbon fibre composites, corroded steel plate and pipe, and concrete specimens. Further measurements were also conducted using modified CI probes, to demonstrate the wide range of applications of the CI technique

Circuit - based transient model of grounding electrode with consideration of soil ionization and current rate of rise factors

The behaviour of a grounding electrode can be predicted by using either the electrical circuit model or electromagnetic computation. Despite its advantages over the latter, the grounding circuit model fails to accurately predict the behaviour under transient conditions due to the absence of two key factors, namely the soil ionization, and the current rate–of–rise. A new equivalent circuit model of a grounding electrode with dynamic circuit elements (Rd, Cd, and Ld) was developed to consider both soil ionization and current rate–of–rise factors. A generalized formula was derived to calculate the dynamic inductance, Ld, for all standard current wave shapes such as Conseil International des Grands Réseaux Électriques (CIGRE), double–exponential, and IEC 62305–1 (International Electrotechnical Commission). The computed inductance, Ld, dynamically changes with the change in the lightning current parameters, thus improving its accuracy for all current rate–of–rise conditions. The consideration for the soil ionization effect on grounding electrode resistance, Rd, and soil capacitance, Cd, within the equivalent circuit model was achieved by modelling the soil with a network of two layer capacitors (TLC) in which soil particles and air voids are the TLC components. Differential equations were derived to incorporate the soil ionization phenomenon inside the TLC network. The voltage response of the new equivalent circuit model and the dynamic circuit elements were determined by using the above–suggested methods, is more accurate than that of the conventionally determined grounding circuit models. The overall differences between the equivalent circuit model and several experiments are 3.3% for the electrode resistance and 2.8% for the electrode peak voltage. The new equivalent circuit model helps to optimize the overall grounding electrode design, and to provide a better fast transient protection and insulation coordination

EVALUATING FROZEN SOIL PROPERTIES WITH ELECTRICAL RESISTIVITY MEASUREMENT AND ELECTRO-MAGNETIC INDUCTION METHODS

Electromagnetic induction was utilized in the past by the United States Army Corps of Engineers as a method of detecting unexploded ordinance, while it has the potential to act as a novel method of investigating frozen soils in cold regions. In this study, we performed lab-scale 1D electrical resistivity measurements under freeze-thaw circumstances on frost-susceptible soils with varied soil properties. We implemented an empirical model from our experiments into a COMSOL finite element model at both laboratory and field scales to simulate soil electrical resistivity response under both short-term and long-term sub-freezing conditions. Dynamic temperature-dependent soil properties, most notably unfrozen water content, exert significant influences on the observed electrical resistivity. We also characterized the evolution of electrical resistivity during the freeze-thaw cycle with empirical models. Laboratory and field experiments were made to validate the effectiveness of the iFrost Mapper device in detecting typical patterns of metal, liquid, and soil samples of different concentrations and temperatures. The original data were processed by considering both inphase and quadrature responses. Meanwhile, simulation studies with similar parameters to the laboratory tests, including geometry, material properties, and physical conditions, and the samples were made in COMSOL Multiphysics to compare the analytical solutions and experimental data