GPR ? Finding our underground assets

Jul 12, 2007

GIGA was a 2 year, 3 month collaborative project supported by the European Commission’s 5th Framework Research Programme for Energy, Environment and Sustainable Development; it was completed in 2003. A team of Ground Penetrating Radar (GPR) experts, manufacturers and end-users combined to improve the technology used for metallic and non-metallic buried services infrastructure location. Fundamental research and development of existing technology was driven by a comprehensive set of end-user requirements. The technology resulting from GIGA was comprehensively tested under rigorous conditions on a purpose-designed site, and the performance data was analysed and evaluated by the independent team of end-users. Significant performance improvements over the then existing state-of-the art equipment were demonstrated, and areas of investigation for further enhancements were identified.

Introduction
Modern societies are completely dependent upon the buried infrastructure that transports energy, water, waste products and, increasingly, data in electronic form. The systems are so well engineered that, usually, they do their job very efficiently, so much so that ordinary citizens rarely spare them even a passing thought. Yet, without these crucial ‘arteries’, civilisation as we know it could not function.
Despite its immense value, like the absent minded dog with its bone, there is sometimes an imprecise knowledge of its exact location, even though great trouble may have been taken to record its position when it was buried. This is unfortunate, since there will be a continuous need to excavate it for maintenance and repair and, particularly, to avoid it when laying new pipes and cables either to renew, or to in-crease the capacity of, buried systems. Whenever this need arises, the resulting in-convenience and disruption to everyday urban life and business is extremely costly and can, on occasions, be dangerous. It is an operation which should not be under-estimated; it is expensive, and can be prohibitively so when high speed communications links are unintentionally damaged.
There is, therefore, an ever present need for instruments that can locate buried pipes and cables, preferably from the surface, prior to excavation. Increasingly, it is also valuable to retain the knowledge of their positions (and depths) for future reference. If confidence in the knowledge of the location of buried infrastructure could be raised by increased use of no-dig technology, then enormous efficiency improvements would result from the need to dig fewer and smaller holes.
Two main technologies have, so far, emerged as being the most useful for locating buried infrastructure. The first to be developed was a technique that relies upon causing an alternating electric current to flow, either by direct connection or by induction, in buried pipes and cables. The consequent alternating magnetic field surrounding the electrical conductor can be sensed easily by the current it induces in a properly designed coil of wire above the ground surface. Instruments based upon this technique have reached an advanced state of development; they are inexpensive and usually reliable. They are marketed by a number of manufacturers, and most utility companies use them. However, they have a serious disadvantage these days in that they only detect metallic pipes and cables. Of lesser consequence (and not an inherent limitation of the technique) is that none of these systems is capable of recording the position of any object it locates.
GPR is a more sophisticated technology that transmits high frequency, broadband electromagnetic signals into the ground from a suitable antenna. Buried objects reflect this radiation and it is collected as it emerges from the ground by a second antenna. Thus GPR has the ability to locate both metallic and non-metallic objects, because their electrical properties differ from those of the surrounding soil. In addition, because the antenna must be scanned over the ground to collect data in a systematic manner, it is possible to form an image and, as a consequence, GPR is usually used as a mapping device. Among the various state-of-the-art location methods available, GPR alone is capable of accurately detecting and locating both metallic and non-metallic buried objects without a priori knowledge of their position.
Although GPR technology has been in existence for at least 30 years, its price/performance has not been sufficiently advantageous to establish the technique in the day to day operations of organisations that excavate the streets.
Prior to GIGA, development efforts in GPR were oriented towards visualisation improvement, using 3-dimensional plots and GPS data, with little or no attention being paid to addressing the basic, but challenging, radar signal detection problem. Clearly, visualisation developments will not increase system performance but will merely improve the aesthetics of the display. Enhanced graphics software will not solve the basic signal problem or improve the detection performance when the received signal is too weak, as would be the case in wet, muddy ground, or Fig. 1: Street works for utilities and maintenance by horizontal directional drilling when obscured by high levels of energy reflected by unwanted scatterers in either the ground or the radar system itself. This type of target obscuration is termed “clutter”, and is the major limitation of performance in GPR systems.
As a consequence, the GIGA project was established with the main objective of enabling the design of a novel Ground Penetrating Radar capable of providing enhanced performance, especially in terms of location accuracy and detection robustness.
An outcome of the project was an improvement in the performance of GPR as well as identification of those areas of technical development which, if addressed with sufficient determination and resources, would lead to further significant performance gains. The developments include recommendations for a radical change in the microwave signal transmit and receive methods and the development of an ultrawideband antenna system with the capability of adapting its characteristics to the electrical properties of the ground.
There is an additional requirement to minimise traditional excavations, by increasing the use of efficient horizontal directional drilling (HDD) technology (Figure 1). This brings with it a need for information on the immediate surroundings of the drill head.
Further work along the lines indicated above, is now in hand.
The GIGA project
The GIGA (Ground Penetrating Radar Innovative research for highly reliable robustness/ accuracy GAs pipe detection/location) project was a European Commission funded collaborative research study to inform and enable the design and build of a new and reliable Ground Penetrating Radar (GPR). Its duration was 2 years and it was completed in December 2003. The overall objective was to design a GPR with a detection performance significantly improved compared to the then existing generation of equipment, and robust enough to be used with confidence on diverse types of pipes buried in a range of soils types.
The project Consortium comprised of the following organizations
  • Thales Air Defence Systems (France) – Project Coordinator
  • Ingegneria dei Sistemi SpA (Italy)
  • Gaz de France (France)
  • Tracto-Technik Spezialmaschinen (TT Group) (Germany)
  • GERG – The European Gas Research Group (Belgium)
  • OSYS Technology Ltd (UK)
This innovative project approach comprised four main activities:
  1. A meticulous assessment of the performance of a state-of-the-art GPR in surveys conducted under controlled conditions at dedicated test sites
  2. Radar technology improvements, including multi-parameter/variable configuration of the radar
  3. Development of new simulation tools to enable a fresh, theoretical view of the problem to form the basis of an improved equipment design
  4. Development of software processing tools to reduce the need for highly trained operators.
The project’s work plan is shown in Figure 2. An important feature was the key role played by the End-Users who were responsible for both establishing the requirements and an objective analysis of the project results.
The outcome and conclusions of the project were considerably strengthened by the involvement of the End-Users (led by Gaz de France, Tracto-Technik (TT Group) and OSYS Technology) in collecting, formulating and evaluating the requirements specification of the European utilities (gas, water, telecommunications, electricity, etc.). The resulting Requirements Specification was a key project document that guided the research activities.
The definition of the User Requirements was followed by an assessment of the performance of a state-of-the art GPR; several GPR configurations amongst those currently manufactured by IDS were used in the trial area established by Gaz de France in Saint Denis (Paris, France). At the same time, an experimental and flexible multi-parameter/ multi-configuration GPR measurement tool developed by Thales Air Defence – TAD, allowed acquisition of data to provide a good knowledge of the underground environment.
During the project, simulation tools were developed in order to enable a rigorous analysis of ground penetrating radar technology and to acquire useful information on the fundamental limits of detection so as to provide a guide to future equipment design. On this basis, and starting from the measurement of performance achievable with its current technology, IDS has addressed the “bottomup” research and the design of a novel GPR system capable of matching, in the shortterm, most of the end-user requirements. Simultaneously, the “top-down” research led by TAD allowed the definition of the issues that have to be addressed in further research phases, so that the final limitations of the technique can be addressed and, maybe, overcome.
Finally, the End-Users carried out a critical analysis of the new technical solution and compared it against the requirement specification as drawn up earlier in the project.
Within this framework, GERG, the European Gas Research Group, which represents 17 natural gas R&D organisations from 10 European countries, was responsible for the twoway dissemination of data and results of this research within the European gas companies and other utilities, thus ensuring both a solid and comprehensive database of information for the project and a conduit to potential End-Users.
In the following, the main results of the project are described. 
Field trials
To assess the performance achievable by a GPR when used for detecting utilities, an intensive survey was carried out in the trial area established by Gaz de France in Saint Denis, France (Figure 3 and Figure 4).
On the Gaz de France test site, materials covering the pipe were chosen to be as representative as possible of ‘real world’ conditions. In addition, the pipes and cables and their configurations and burial depths were designed both to be representative and to test the major aspects of GPR performance.
A rigorous analysis, by the End-User team, of the huge amount of data collected during the trial also took account of results obtained from other trials to validate the results and concluded that IDS’ state-of-the-art equipment had clearly demonstrated an improvement over other commercially available GPRs by achieving:
  • a detection rate greater than 80 % to 2 metres depth;
  • a false alarm rate less than 10 %;
  • a 40 mm accuracy of location both in the horizontal and vertical plane, and;
  • a resolution better than 300 mm.
These results are, largely, consistent with the main User Requirements, and confirm that GPR is a viable tool for pipe location. The confidence in the reliability of these conclusions is enhanced by the application of new data analysis techniques developed during the course of the project and described below in Section 6.
The analysis has confirmed that two important issues remain to be addressed, in both the short and long term; these are ultimate penetration depth and the capability to detect small diameter plastic pipes (and, perhaps, other small, non-metallic targets) particularly where the ground consists of highly conductive material.
Modelling
During the GIGA project, simulation tools capable of modelling the electrical behaviour of buried targets and their environment were developed.
In particular, two different tools were utilised during the project. The first uses the Transmission Line Matrix (TLM) method for reproducing typical GPR scenarios by working in the time-domain; the second uses an approximation based upon optical propagation laws and ray tracing rather than the more complex approach of solving Maxwell’s equations. This latter method is very economical in processing time, but it cannot be used for analysing complex scenarios.
A comparison between the measured and predicted radargrams for this scenario is shown in Figure 5 . There is good agreement between the shape and strength of the radar reflected radar signals from the pipes.
Further work
In a longer term, the main objective of GPR design is to achieve a clutter-free dynamic range that is as large as possible, specifically to increase the depth range.
A secondary objective is to control the frequency range so that it closely matches the requirements of target detection and antenna characteristics. Unfortunately, this conflicts with practical constraints such as cost, speed of operation and ease of implementation.
Commercial systems that are available now use conventional time domain sampling techniques and relatively inexpensive signal sources based upon avalanche transistor technology. This is tried and trusted technology that is convenient to manufacture. Its main limitations are a restricted dynamic range and, sometimes, a lack of stability with respect to time.
GIGA considered an alternative approach based on stepped frequency sources, and receivers based upon homodyne detection and balanced mixers. Such systems are generally known as “Frequency Modulated Continuous Wave” (FMCW) because they have traditionally used continuously swept frequency microwave sources. Where discrete frequency intervals are used, the technique is known as “Stepped Frequency Continuous Wave” (SFCW). In theory, such systems possess superior dynamic range and stability compared to ‘conventional’ pulse systems and also allow control of the frequency range.
At present, the main disadvantage of this approach is the high cost of the micro-wave sources. It is possible, however, that the particular requirements of GPR may allow a relaxation of the specification of a stepped frequency microwave source that could offer significant cost reductions.
The output of FMCW systems is, by definition, in the frequency domain, thus necessitating a transformation into the time domain in order to extract target and range information. This is most often carried out using Fourier Transformations and the resulting output of this process is equivalent to that of a time domain sampling system with much wider dynamic range and stability.
Thus, in the longer term, it seems reasonable to assert that a Stepped Frequency Continuous Wave GPR will constitute a further step towards addressing and overcoming the remaining limitations of the technique.
Conclusions
Ground Penetrating Radar (GPR) is the only known non-invasive technique that can detect metallic and non-metallic buried objects, but conventional pulse time-domain technology has reached the limit of its development potential. Innovative research is clearly needed to provide an advance in the state-of-the art.
The necessary tasks are to:
  • provide a step change in the depth penetration and spatial resolution of GPR used for surveys carried out from the ground surface. This will be achieved by increasing the frequency and dynamic range of the radar by researching and developing Stepped Frequency Continuous Wave techniques and ultra wide-band antennas with performance independent of ground characteristics.
  • develop a GPR that increases safety when applying trenchless technologies,
  • increase knowledge of the electrical behaviour of the ground, by means of in-situ measurements to enhance understanding of the sub-soil electrical environment, and to provide information for scientifically based antenna design.
These measures will lead to practical solutions that can be implemented cost-effectively to provide a capability to locate buried infrastructure with accuracy and reliability. This will reduce the need for excavations in the highway, thus minimising direct and indirect costs, reducing the incidence of pollution and enhancing safety and quality of life for citizens and motorists.

Contact

Guido Manacorda [INGEGNERIA DEI SISTEMI s.p.a.]

Pisa, Italy

Phone:

+39 050 312411

E-Mail:

G.Manacorda@lds-Spa.it

Internet:

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