Ground Penetrating Radar (2nd Edition)
The book will help users to assess the potential of the technique and currently avaliable technology and to apply them effectively.
Inspec keywords: ground penetrating radar; landmine detection; archaeology
Other keywords: Mathcad worksheets; forensic investigations; archaeology; ground penetrating radar; man-made structures; landmine detection
The general structure of this volume is based on an earlier paper which was published in a special edition of the IEE Proceedings Part F by Daniels et al. (1988), which served as an introduction to GPR techniques. This still serves well as a primer and the introduction is still relevant and is quoted below. GPR in the hands of an expert provides a safe and noninvasive method of conducting speculative searches without the need for unnecessary disruption and excavation. GPR has significantly improved the efficiency of the exploratory work that is fundamental to the construction and civil engineering industries, the police and forensic sectors, security/intelligence forces and archaeological surveys.
This chapter considers the principal factors affecting the design of a GPR in order to illustrate those factors which need to be considered. The aim is to illustrate the technical options available to the operator or designer. This is not a rigorous treatment of radar system analysis but does enable an order of magnitude estimate of the various loss components to be assessed. Many radar systems generate a fast rise time impulsive voltage, so the signal level is best considered from the point of view of voltages across particular nodes in the network. A consideration of this approach is given at the end of this section, and an expression suitable for evaluation using MathCAD™ is included, together with a series of modelled results for particular values.
Models of the GPR situation range from a simple single frequency evaluation of path losses to complete 3D time domain descriptions of the GPR and its environment. This chapter introduces some of the approaches and provides a starting point for further exploration of the literature.
A significant number of researchers have extensively investigated the dielectric properties of earth materials. They have shown experimentally that for most materials which constitute the shallow sub-surface of the earth, which in this case is taken to be a zone of depths of 100 m or less, the attenuation of electromagnetic radiation rises with frequency and that at a given frequency wet materials exhibit a higher loss than dry ones. From this generalisation a number of predictions can be made relating to the performance of a surface-penetrating radar system. Before this can be done it is necessary to understand those characteristics of materials which affect both the velocity of propagation and attenuation.
Surface-penetrating radar presents the system designer with significant restrictions on the types of antennas that can be used. The propagation path consists in general of a lossy, inhomogeneous dielectric, which, in addition to being occasionally anisotropic, exhibits a frequency dependent attenuation and hence acts as a lowpass filter. The upper frequency of operation of the system, and hence the antenna, is therefore limited by the properties of the material. The need to obtain a high value of range resolution requires the antenna to exhibit ultra-wide bandwidth, and in the case of impulsive radar systems, linear phase response. The requirement for wide band width and the limitations in upper frequency are mutually conflicting and hence a design compromise is adopted whereby antennas are designed to operate over some portion of the frequency range 10 MHz to 5 GHz depending on the resolution and range specified. The requirement for portability for the operator means that it is nor mal to use electrically small antennas, which consequently results generally in a low gain and associated broad polar radiation patterns. The classes of antennas that can be used are therefore limited, and the following factors have to be considered in the selection of a suitable design; large fractional bandwidth, low time sidelobes and in the case of separate transmit and receive antennas, low crosscoupling levels. The interaction of the reactive field of the antenna with the dielectric material and its effect on antenna radiation pattern characteristics must also be considered.
Each of the various modulation techniques used for ground penetrating radar systems has its relative merits, and in this chapter we consider the general system architecture and system specifications associated with each. The most frequently used system design is that of the impulse radar, and the majority of commercially available radar systems use short pulses or impulses which generally come in the category of amplitude modulation (AM). The next most frequently used modulation technique is frequency modulation (FM) followed by synthesised pulse (SPM), holographic (HM) and finally coded and noise modulation (NM). In this chapter we consider the most commonly used techniques in turn and discuss the key parameters, which need to be considered in the design process.
This chapter has introduced basic signal processing techniques and has, by means of contributions, provided examples of some of the methods and principles of GPR processing being currently developed. The selection of suitable signal processing methods must start from a clear appre ciation of the modulation technique and the likely form of the received wavelet. The main initial objective is to select suitable processing to optimise the wavelet output in terms of each individual A-scan sample time series, and deconvolution techniques have been described. If the subsequent objective is to generate an image it is reason able to consider some type of 3D 'spiking' filter or migration of the data. If, however, the objective is to classify the wavelet, i.e. by Prony processing, then 'spiking' filters are not appropriate. Consideration can also be given to the removal of multiple reflec tions. Once the A-scan data are optimised, processing methods based on B-scan data sets can be considered. Again, if the objective is image 'spiking' or focusing, a num ber of migration or synthetic aperture methods are available, each of which is more or less tolerant to variations in propagation conditions. Examples of the application of migration techniques have been given. Where this method is not preferred, image pattern recognition techniques based on standard image processing methods of template matching can be used. Transforms such as Hough or neural network techniques can also be considered.
This chapter presents selected archaeological applications using ground penetrating radar.
This chapter describes some of the civil engineering applications of GPR and considers roads and pavements, concrete structures, bridges and tunnels. GPR has become an established and routine method of inspection of civil engineering structures. Further information can be found by contacting the appropriate national authorities, some of whose websites are referenced.
This chapter considers the application of ground penetrating radar (GPR) for forensic investigation. It presents the principle of GPR forensic search and provides some case histories where GPR has been successfully used to assist investigation.
This chapter has considered some of the geophysical applications of GPR . This is one of the main founding applications for GPR and this chapter, in conjunction with Chapter 4 on the properties of materials, should provide a suitable reference source for the reader. There is now a wealth of information on GPR techniques for the probing of rocks, soils, snow and ice. Given the considerable amount of work that has been carried out on conventional GPR probing of rocks and soils, the contributors to the second edition have concentrated on two specific applications, namely frozen materials and borehole radar. Most geophysical probing is carried out at the lower end of the frequency range for GPR, and this in turn brings different hardware configurations into existence.
GPR is beginning to be fielded as a sensor for mine detection where its ability against the minimum metal mine often surpasses the ubiquitous metal detector. The US HSTAMIDS hand-held detector is now being evaluated in Afghanistan. Throughout the world, airborne, vehicle mounted and hand-held systems have been extensively researched, developed and trialled. The process has taken over two decades from the early systems devised for Vietnam and the Falkland Islands and has often been fragmented and intermittent. In most soils, GPR can detect mines at greater depths than the metal detector, but in clay or salt-laden soils it does not perform as well. However, in some mineralised soils where the metal detector struggles, GPR has a performance advantage.
The detection of buried utilities has been a standard application of GPR for several decades. However, the performance of GPR is limited by the physics of propagation and in many countries there are significantly large regions where the attenuation of the ground limits the application. Unlike the simpler low radio frequency pipe detection technology, GPR is more expensive and requires, at least for single channel systems, a systematic search pattern. This makes the cost per unit area more expensive. One way of looking at the predicted performance could be on a statistical basis with an estimate of the probability of detection per percentage of the country. This might suggest that in a country with significant regions of clay then 60% of buried utilities could be expected to be detected in 60% of the country in question. The economic case for both investment and survey can then be more accurately considered. As many GPR systems use only a single polarisation, then survey in orthogonal directions is necessary to ensure that all utilities are detected. Multi-element systems are being successfully used in those places where access is possible. However, in the city streets of many European countries the pedestrian walkways, parked cars and street furniture severely limit access. The use of GPR is one method among a number that can be used for utility detection, and the prospective user should take much of the marketing of GPR with a pinch of salt. The type of ground should be established before survey operations, with the aim of estimating the achievable performance from the outset.
This chapter describes some of the work that has been carried out on radar systems for remote sensing below the surface of the earth and the planets. Radar systems can be mounted on aircraft or on satellites. The types of radar used are radically different from typical GPR systems, and the aim of this chapter is to provide an insight into the remarkable results and sophistication of radar systems. Most of the work is based on synthetic aperture radar (SAR) processing and there is a wealth of material available on this topic.
There is an ever increasing range of commercially available equipment for surface penetrating radar applications. There are now a number of manufacturers and suppliers of equipment and a list is provided. This chapter provides a brief introduction to selecting equipment.
The relatively low numbers of GPR in use throughout the world suggests that GPR is not a major source of electromagnetic radiation. As most GPR systems radiate into the ground the likelihood of interference is very low. For these reasons the licensing authorities appear to have been persuaded that GPR poses little threat. However, both the FCC and ETSI will be supervising the use of GPR more strictly in the future, and manufacturers, developers and users should ensure that they comply with all of the statutory requirements. The vast majority of GPR systems radiate at levels well below the internationally agreed limits for radiological hazard. Care should be taken by users of GPR that mobile phones and other sources of possible interference do not contaminate the data being gathered, but fairly simple filtering can be used to post-process the data and remove the effects of interference.
