Ground Penetrating Radar (GPR) is a powerful sensing technology widely used for the non-destructive assessment of a variety of structures with different properties including dimensions, electrical properties, and moisture. After an introduction to the underlying concepts, this book guides the reader through the development and use of a GPR system, with an emphasis on the parameters that can be optimized, the theory behind assessment, and a coherent methodology to obtain results from a measured or simulated GPR signal. The authors then embark on a detailed discussion of support tools and numerical modelling techniques that can be applied to improve readings from GPR systems. Ground Penetrating Radar is of interest to engineers, scientists, researchers and professionals working in the fields of ground penetrating radar, non-destructive testing, geoscience and remote sensing, antennas and propagation, microwaves, electromagnetics and imaging. It will also be of use to professionals and academics in the fields of electrical, mechanical, sensing, and civil engineering as well as material science and archaeology concerned with quality control and fault analysis.
Inspec keywords: radar antennas; ground penetrating radar; electromagnetic field theory; numerical analysis; radar imaging; electromagnetic wave propagation; optimisation
Other keywords: problem complexity; ground penetrating radar sensing; ground penetrating radar imaging; radar signal processing; electromagnetic theory; stochastic algorithms; GPR; pattern recognition; numerical modeling; system specification; radar antenna design
Subjects: Radar equipment, systems and applications; Radar theory; General electrical engineering topics; Antennas; Numerical analysis; Optical, image and video signal processing; Optimisation techniques
This chapter has briefly discussed the use of GPR as an NDT and an NDE tool. In contrast to other NDT methods, GPR is based on EM wave propagation, which provides advantages and limitations. Among the advantages, one can cite relatively low-cost of survey, compact size of equipment, excellent trade-off between resolution and depth of detection, and suitability to an overwhelming variety of applications. However, the analysis of data is not straightforward not only because of the complexity of the physical phenomena involved in propagation and scattering of EM waves but also because of the impact of environmental conditions on the EM description of the media and consequently on the data acquired. To reduce this burden, the GPR equipment allows for multiple configurations that have to be carefully explored and selected in the planning stage of surveys. Even with the right choices of these configurations, the ability to predict and detect targets requires considerable expertise as well as a set of specialized post-processing tools. Far from being a closed line of research, the possibilities to improve detection are still open, and more exciting contributions are expected in the future to create more powerful NDT tools.
The principles of ground penetrating radar (GPR) are solidly based on the use of electromagnetic wave radiation to detect buried targets. The article attempts to describe the essentials of electromagnetic wave propagation and transport of power through space and through lossless and lossy materials. By necessity only the fundamental principles can be addressed in the context of this work and within the obvious space limitations. If further depth is required, the reader is encouraged to consult any of the many excellent books and other sources available, including some that are listed as references.
The following sections discuss first the relevant parameters that characterize the radiation from antennas. Then, a set of frequently employed GPR antennas is briefly presented and explained in terms of these parameters. Finally, taking these geometries as a starting point of any antenna design, some recent GPR antenna designs are presented on the basis of computational optimization techniques. It should also be noted here that the same antennas are used for reception of signals, either using a single antenna for both purposes or separate antennas for transmission and reception. It is not necessary to discuss receiving antenna separately because, based on the reciprocity principle, the parameters are the same, that is, the basic electric properties such as impedance, bandwidth, efficiency, and radiation pattern are the same for both functions. The obvious difference is in the power levels but since antennas are typically very efficient, power radiated has little effect on the structure of the antenna.
Radar is an acronym for RAdio Detection And Ranging. Its operating principle is analogous to the broader pulse-echo (or pitch-catch) technique, except that pulsed electromagnetic (EM) waves (short radio waves or microwaves) are used instead of stress waves. Radar was developed during World War II to provide early warning of approaching enemy aircraft. From there, it evolved into indispensable tools in air-traffic control, shipping, weather prediction, and a multitude of other applications that range from probing distant planets to law enforcement. The application of radar is based on the use of sensors (antennas) that transmit and receive EM waves. First, an EM wave is generated in the instrument's circuitry and propagated to the antenna through a transmission line. This wave is then launched by an antenna in some medium (in general free space but in some cases may be coupled directly into a medium such as soil). The wave propagates within the medium (with attenuation) until it reaches a target with different electrical characteristics than the surrounding medium.
Numerical modeling is a powerful support tool in the quest for improvements in the design and performance of ground penetrating radar (GPR) systems and beyond. To be effective, the representation must consider all relevant factors that influence realistic, practical problems. Archeology, hydrology, civil engineering, forensic, geo-physics, and humanitarian demining are some of the complex problems addressed with GPR. Each of these applications presents different issues that need to be addressed effectively, including antenna performance, data interpretation, and false alarm rates. There is a range of numerical methods that can be brought to bear in attempts to address each of these issues. None of these methods, of course, provides solutions to all problems under all possible conditions, but numerical tools can alleviate many of the tasks involved in the use of GPR, including such issues as generation of training data, verification of results, and, indeed, in the design of systems. The article discusses the relevant issues and features that should be addressed for the proper choice of a numerical method for GPR modeling and simulation. Given that the GPR problem is multidisciplinary in nature, the numerical methods must adapt to its needs. Therefore, some methods are more useful than others in the context of GPR.
There is an indefinite number of different scenarios when it comes to nondestructive testing (NDT) using ground penetrating radar (GPR). Each scenario has its own properties with respect to system specification, material properties, and environment conditions. In the preceding chapters, it was made amply clear that electromagnetic (EM) wave propagation results in complex responses even from simple configurations of materials and targets in any given scenario. In GPR, the responses from the testing environment are used to identify and quantify targets and other features that may be of interest. The raw data must be processed in a way that provides useful, dependable information that can be acted upon. Pattern recognition is a very important step in the overall purpose of extracting useful information from the received signals.