Ground Penetrating Radar

The first peer-reviewed scientific journal dedicated to GPR

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ISSN 2533-3100

Ground Penetrating Radar 2018, Volume 1, Issue 2, GPR-1-2-1, https://doi.org/10.26376/GPR2018007

 

GPR system performance compliance according to COST Action TU1208 guidelines

Lara Pajewski, Milan Vrtunski, Željko Bugarinović, Aleksandar Ristić, Miro Govedarica, Audrey van der Wielen, Colette Grégoire, Carl Van Geem, Xavier Dérobert, Vladislav Borecky, Salih Serkan Artagan, Simona Fontul, Vânia Marecos, and Sébastien Lambot

 

Full text: PDF [4.9 MB, open access]

 

Abstract: Ground Penetrating Radar (GPR) systems shall be periodically calibrated and their performance verified, in accordance with the recommendations and specifications of the manufacturer. Nevertheless, most GPR owners in Europe employ their instrumentation for years without ever having it checked by the

manufacturer, unless major flaws or problems become evident, according to the results of a survey carried out in the context of COST (European Cooperation in Science and Technology) Action TU1208 “Civil engineering applications of Ground Penetrating Radar.” The D6087–08 standard, emitted by the American Society for Testing Materials (ASTM International), describes four procedures for the calibration of GPR systems equipped with air-coupled antennas. After a critical analysis of those procedures, four improved tests were proposed by a team of Members of the COST Action TU1208, which can be carried out to evaluate the signal-to-noise ratio, short-term stability, linearity in the time axis, and long-term stability of the GPR signal. This paper includes a full description of the proposed tests and presents the results obtained by scientists from Belgium, Czech Republic, Portugal, and Serbia, who executed the tests on their GPR systems. Overall, five pulsed control units and nine antennas were tested (five horn and four ground-coupled antennas, with central frequencies from 400 MHz to 1.8 GHz). While the performed measurements are not representative enough to establish absolute thresholds for the tests, they provide a valuable indication about values that one could obtain when testing GPR equipment, if the equipment is working reasonably well. Moreover, by periodically repeating the tests on the same equipment, it is possible to detect any significant shift from previously obtained values, which may imply that the GPR unit or antenna under test is not working in a normal or satisfactory manner. We also believe that executing the tests described in this paper is a useful exercise to gain awareness about the behaviour of a GPR system, its accuracy and limits, and how to best utilize it.

 

Keywords: Ground Penetrating Radar (GPR); antennas; calibration; system performance compliance; signal-to-noise ratio; signal stability; signal linearity in the time axis.

 

Introduction

Early Ground Penetrating Radar (GPR) technology was relatively primitive, data presentation was complex and interpretation of results was a difficult task [1]. Over time, the GPR technology has improved in terms of sensitivity, functional form, ease of use and information presentation. Systems have become lighter, more portable and self-contained; efficient data processing algorithms have been developed, the interaction of electromagnetic waves with soil and targets is better understood, and there is a stronger awareness of GPR limitations [2]-[5]. As a consequence, the GPR technique is nowadays increasingly used in a wide range of applications and is considered as a safe and versatile method, which is capable to provide accurate and reliable information in a fast and efficient way [6]-[11]. GPR surveys are successfully conducted in various environments, under conditions that may sometimes change on a daily basis in the context of long surveys. Thanks to the continuing technology and methodology improvements, it is expected that GPR will further advance in the coming years.

High precision and reliability in GPR measurements obviously require systems with very high linearity and stability, generating very low levels of disturbancies.

As is well known, the measurement accuracy is the closeness of agreement between the measured quantity value and the true quantity value of a measurand (e.g., the amplitude of the electric field as a function of time, in the GPR case); the sensitivity is a relation between the indication of an instrument and the corresponding change in a quantity being measured. Ideally, the accuracy and sensitivity of a GPR should be constant over its full operating range; in practice, most measurements involve some changes in accuracy and sensitivity and this type of imperfection is referred as non-linearity of the equipment (which is often emphasized at the extremes of the expected operating range). Being aware of the linearity properties of a GPR and understanding their impact on the measured values significantly aids data interpretation and contributes to the effectiveness of a survey; if the equipment demonstrates non-linearity, it may be not properly calibrated in some portions of the operating range, or else some components may be worn, or the signal-to-noise ratio (SNR) may be too low.

Stability is the key to predictability: if the measuring process is changing over time, the ability to use the gathered data for the evaluation of electromagnetic and geometrical properties of media and targets is diminished, and so is the capacity to use GPR results in making decisions. Selectivity is defined as the instrument’s insensitivity to changes in factors other than the actual measurand, for instance to environmental factors (humidity, pressure, temperature); an instrument with better selectivity guarantees a higher stability. There are many further factors that may introduce instability in a GPR system, such as internal and external electromagnetic noise, alterations of feeding voltage, antenna shielding problems, mechanical vibrations, variations of antenna matching due to permittivity and conductivity changes in the surveyed media, and more; additionally, as in all electronic devices, the GPR stability can worsen over time due to deterioration or ageing of system components.

Noise is the unwanted electromagnetic energy that interferes with the ability of the receiver to detect the useful signal. Noise is always present in the environment and is also generated within the GPR system. If the level of disturbancies generated by the radar is low, the detection probability of small signals is enhanced [12]. The use of appropriate signal processing procedures can improve the SNR in GPR investigations [13]-[15].

To verify the performance compliance of GPR equipment, suitable stability, linearity and SNR tests should be carried out on a regular basis and in a controlled environment, by following procedures that should be standardized. However, few recognized international standards exist in the area of GPR [16] and, to the best of our knowledge, the calibration topic is covered only within one of them, namely in the ASTM D6087 - 08(2015)e1 “Standard Test Method for Evaluating Asphalt-Covered Concrete Bridge Decks Using Ground Penetrating Radar” emitted by the American Society for Testing Materials (ASTM International) [17]; therein, four procedures for testing GPR systems equipped with air-coupled antennas are described. Moreover, reliable GPR manufacturers shall suggest calibration and verification procedures for the equipment they produce.

Besides the poor availability of standards in the field, the importance of periodically testing and calibrating GPR instrumentation is often underestimated in Europe, according to a survey conducted during the Third General Meeting of COST Action TU1208 “Civil engineering applications of Ground Penetrating Radar.” This event was held in London, United Kingdom, on 4-6 March 2015, and was attended by 90 participants from 29 countries, from academia and industry: in addition to the only GPR manufacturer participating in the meeting, just a researcher from France, a team of researchers from Belgium, and another researcher from Belgium claimed to have experience on testing the stability, linearity and SNR levels of GPR systems. In particular, the researcher from France stated that in the scientific network of the French Ministry of ecological and solidary transition (MTES), composed by the Institut Français des Sciences et Technologies des Transports, de l’Aménagement et des Réseaux (IFSTTAR, Nantes, France) and the Centre d’études et d’expertise pour les risques, la mobilité, l’environnement et l’aménagement (CEREMA, France), procedures similar to those described in [17] had been executed various times throughout the years, to test the equipment owned by the institute. The research team from Université catholique de Louvain (UCL, Louvain-la-Neuve, Belgium) reported about their studies on the topic, which were published in [18], [19] and are resumed in the following paragraphs. The researcher from the Belgian Road Research Centre (BRRC, Brussels, Belgium) communicated that she executed the procedures of [17] during her PhD thesis (see Appendix 5 of [20]); in particular, she tested a commercial 2.3 GHz ground-coupled antenna, which did not fulfill the thresholds set by the ASTM standard for the long term stability and signal to noise ratio, namely because of the short-term noise in the acquired signal. Additionally, a Member from Spain reported about research activities performed in the University of Vigo by her colleagues [21], which are resumed in the following of this section, too.

In [18], the stability over time and repeatability of a frequency-domain and a time-domain GPR system were investigated. The frequency-domain GPR was a combination of a vector network analyser and an 800–5200 MHz horn antenna. The time-domain GPR was a commercial control unit with a 900 MHz bow-tie antenna. Both GPR systems were calibrated several times by performing measurements with the antennas at different heights over a perfect electric conductor (PEC) in the laboratory, as well as over a water layer. Further measurements were performed over a thin water layer and a relatively thick sandy soil layer, as validating media. The frequency-domain GPR turned out to be relatively stable, while the time-domain GPR presented a significant drift, which according to the authors can be accounted for using corrections based on the air direct-coupling waves. Inversions for the thin water layer and the sandy soil layer provided reliable results and showed a high degree of repeatability for both radar systems. Results presented in [18] also show that water- and PEC-based calibrations provide very similar results for the GPR calibration functions, with useful practical implications in case the calibration of a low-frequency antenna is necessary and when a sufficiently large metal plane is not available. Furthermore, the error on the calibration due to inaccurate antenna heights over PEC (or water) yields significant uncertainties on the inversion results for the horn antenna and smaller uncertainties for the bow-tie antenna.

In [19], the time drift of a time-domain GPR with a 900 MHz antenna was quantified over a certain time period (28 hours, non consecutive but in identical situation) and the maximum observed time drift was 0.0978 ns. As a second step of the study, the maximum time and amplitude drift were characterized in the frequency domain, via the calculation of a frequency-dependent ratio, to be multiplied by the original spectrum of the signal in order to illustrate the effects of the drift. Third step of the study was the quantification of the sensitivity of soil characterization (by full-wave inversion) in response to a drift: the overestimation of the dielectric permittivity reached 50% for low dielectric permittivity values, whereas the maximum underestimation was 25% for high permittivity values, following a gradient. The error on the estimation of the electric conductivity turned out to be much higher, reaching an extreme of 105.4% for the lowest original values, with an average of 102.5%. These results show that the inaccuracy of recorded GPR data caused by drift phenomena, or more in general by the system instability, can be disastrous for an inverse problem solution.

In [21], several tests were carried out in order to evaluate the short-term and long-term amplitude and arrival-time stability of a time-domain commercial GPR working with three different ground-coupled antennas having central frequencies of 500, 800, and 1000 MHz. The tests were taken and further developed from [22], where procedures for the calibration of GPR equipment were presented; such procedures were in turn taken from a Texas Department of Transportation report [23]. Actually, in [23] eight procedures to test the performance of GPR systems were proposed. Four of them are based on the evaluation of the GPR reflection from a large metal plate and allow measuring the noise-to-signal ratio, the short-term signal stability, the amplitude of the so-called ‘end reflection’ directly preceding the metal plate reflection (caused by impedance mismatch at the end of the antenna, according to [23]), and the variations in the time-calibration factor; another procedure makes use of a non-reinforced concrete slab placed on top of a metal plate and aims at measuring the signal penetration in concrete; one more procedure, with the antenna pointed directly up into the air, is to measure an “end reflection waveform” (superimposed on every waveform collected by the system, according to [23]). Finally, two procedures allow compensating the bouncing effects of air-coupled antennas mounted on vehicles and evaluating the influence of vehicle speed on GPR amplitudes.

Coming back to [21], inspiration concerning the warm-up time before executing the tests was taken from [24] and the obtained data were used to determine some parameters proposed in [25], as well. The results of [21] show that, after a warm-up time of about 10 minutes, the GPR system under test had high arrival time stability, ensuring correct positioning of the recorded reflections in time. On the other hand, the amplitude stability was not satisfactory; for practical purposes, amplitude instability may cause significant errors in the estimation of the electromagnetic properties of media (for example, when applying the procedures customarily used in road pavement investigations for the estimation of the electromagnetic properties of road layers, where the amplitude of the signal reflected by a particular layer is compared with the amplitude of the signal reflected by a metal plate). As suggested in [21], when accurate amplitude values are needed, it is safer to repeat static measurements several times and take an average of the received signals, in order to minimize the amplitude instability effects. All stability tests of [21] were carried out in air and repeated in distilled water; in the latter medium the amplitude stability was significantly improved, which suggests that the examined antennas work better when placed in contact with an absorbent medium having an impedance different than the air (as is expected for ground-coupled antennas).

Following the Third General Meeting, two Members of COST Action TU1208 from Italy and United Kingdom analysed the ASTM SNR test proposed in [17]. They considered a reduced Taylor's expansion up to the second order of the expressions of SNR bias and variance; and, they derived a formula for tuning the SNR threshold according to a fixed target value of the GPR signal stability [26]. Moreover, they executed the SNR test of [17] on a time-domain commercial GPR equipped with three different horn antennas produced by the same manufacturer, having central frequencies of 1 GHz, 2 GHz, and again 2 GHz, to investigate the effects of the antenna frequency on the SNR [26]. While the authors of the present paper appreciate the valuable efforts done in [26], it is the opinion of the authors that the SNR test of [17] is inherently not correct, for reasons explained in Sub-section 2.1 of the present paper.

Before introducing and describing the content of the present paper, a few more studies available in the GPR literature are worth being mentioned.

Various scientific-technical reports from the United States can be found on the web, where the tests proposed in [17], [21] and [23] are suggested; the report [27] is especially interesting and includes, in Appendix B, the description of almost all tests of [21] and [23], plus a procedure for evaluating the metal plate reflection symmetry. In [28] it was recommended to calibrate GPR systems at least once per year, based on the results of the tests described in [27].

In [29], the authors assessed the accuracy of GPR evaluations of propagation velocity and two-way travel time. By using time picks from a common midpoint radargram recorded by a GPR equipped with a 200 MHz antenna, confidence limits of the order of 0.01 m/ns were found for velocity estimates; the confidence limits for two-way travel time estimates were of the order of 1 ns. In [30], the velocity inaccuracy level found in [29] was translated to an uncertainty of 12% in the estimated moisture content of a typical soil.

In the recently published paper [31], a new method for the stability evaluation of GPR systems was proposed, based on statistics. Four sets of experiments were carried out in an anechoic chamber and on a sandbox, to compare the stability performances of a commercial impulse GPR system with a 900 MHz ground-coupled antenna and a stepped-frequency GPR system covering the 50 MHz – 4 GHz range, based on a vector network analyser equipped with a pair of homemade bow-tie antennas. The influence of the warm-up time, environmental noise and antenna vibration on the GPR signal instability was investigated. In agreement with [18], it was found that the GPR signal recorded by the stepped-frequency GPR system was more stable than the impulse GPR system (at a cost of a longer sweep time, hence a slower survey speed). A warm-up time of several minutes turned out to be enough for the impulse system, whereas the stepped-frequency system needed no warm-up time (but only because it was warmed up before the measurement, to perform a standard calibration of the vector network analyser and coaxial cable). Environmental noise was found to have a negligible influence on the stability performance of the impulse system, probably because – in normal conditions – the environmental noise is much weaker than the instantaneous electromagnetic power radiated by a GPR. Mechanical vibrations, instead, were found to have a severe impact on the GPR stability (in agreement with [27]): the instability index was increased by more than one order of magnitude in a vibrating condition, compared to a static condition; it is therefore very important to undertake shock-proof measures when using a GPR mounted on a vehicle. Finally, the instability index evaluated by considering the direct wave, only, turned out to be similar to the instability index evaluated by considering the reflection from a metal plate; therefore, by using the index proposed in [31], a simple measurement of the direct signal seems to be enough for the evaluation of the instability of a GPR system (no need of using a metal plate). However, it is the opinion of the authors of the present paper that the implementation of the test proposed in [31] may be too difficult for an average GPR user lacking of a scientific background; to make that text executable by everyone, the calculation of the instability index should be incorporated in the GPR system software, which should also assist the user in performing the test.

In this context, the international group of TU1208 Members authoring this paper focused on the standardised procedures described in [17] and critically analysed them. After a series of exploratory experiments carried out at the BRRC, with the aim of better understanding the merits and limits of the ASTM tests, in-depth discussions took place and four improved tests were defined as an output of the 2017 Working Group Meeting “Guidelines for the use of GPR in civil engineering” of COST Action TU1208, held at the COST Association premises in Brussels, Belgium, on 9-13 January 2017. In this paper, the four improved tests are presented (Section 2). They can be used to test GPR systems equipped with both air-coupled and ground-coupled antennas; and, they allow the quantitative experimental evaluation of the SNR (Sub-Section 2.1), signal stability over time (Sub-Section 2.2), signal linearity in the time axis (Sub-Section 2.3), and signal long-term stability (Sub-Section 2.4). During 2017, the four improved test were executed by research teams from Belgium (BRRC), Czech Republic (University of Pardubice, Pardubice), Portugal (National Laboratory of Civil Engineering, LNEC, Lisbon), and Serbia (Faculty of Technical Sciences of the University of Novi Sad, Novi Sad), to verify the performances of five commercial impulse GPR control units and nine commercial antennas with central frequencies ranging from 400 MHz to 1.8 GHz (five horn and four ground-coupled antennas); all the obtained results are reported and commented herein. The tests and experimental results were presented at the Final Conference of COST Action TU1208, held in Warsaw, Poland, on 25-27 September 2017 [32]; the results of measurements carried out in Serbia were also presented at the 2018 European Geosciences Union General Assembly (EGU GA), held in Vienna, Austria, on 8-13 April 2018, in the framework of the session “COST Actions in Geosciences: breakthrough ideas, research activities and results” [33].

The improved tests are being integrated in the guidelines for the use of GPR in civil engineering proposed by the COST Action TU1208. Given the afore-mentioned scarce availability of standards in the area of GPR and having observed the existence of inhomogeneous recommendations and use practices in different countries, COST Action TU1208 worked hard on leveraging the gaps and yielded three guidelines for the use of GPR in some civil engineering tasks, plus a volume of recommendations for a safe geophysical prospecting [34]. The main focus of the three guidelines is on GPR road inspection, detection and localization of utilities in urban areas, and assessment of concrete structures (concrete bridges, tunnels and floors). As in all civil engineering applications of GPR it is very important to be aware of the stability, linearity and repeatability of the employed equipment, it was decided to include the four improved GPR performance compliance tests in the guidelines. Such guidelines are currently being refined and finalized, before being published in open access on the website of the Action (www.gpradar.eu).

We hope that the GPR performance compliance tests described in this paper will be executed by other research teams, private end-users and manufacturers, in the near future, on a wide variety of control units and antennas, on both brand new and older equipment; by sharing information about the obtained results, the GPR community can establish reasonable thresholds for the tests, which will help to distinguish between equipment working properly and flawed equipment (so that, in case of flawed equipment, the manufacturer can be contacted to check and possibly repair or calibrate the equipment). Our plans for future work also include investigating how the results of the proposed performance compliance tests translate into accuracy levels of measured physical and geometrical quantities, in various applications of the GPR technique.

 

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Unrestricted use, distribution, and reproduction in any medium of this article is permitted, provided the original article is properly cited. Please cite this article as follows: L. Pajewski, M. Vrtunski, Ž. Bugarinović, A. Ristić, M. Govedarica, A. van der Wielen, C. Grégoire, C. Van Geem, X. Dérobert, V. Borecky, S. Serkan Artagan, S. Fontul, V. Marecos, and S. Lambot, "GPR system performance compliance according to COST Action TU1208 guidelines," Ground Penetrating Radar, Volume 1, Issue 2, July 2018, pp. 2-36, doi.org/10.26376/GPR2018007.

For information concerning COST Action TU1208 and TU1208 GPR Association, please take contact with the Chair of the Action and President of the Association, Prof. Lara Pajewski. From 4 April 2013 to 3 October 2017, this website was supported by COST, European Cooperation in Science and Technology - COST is supported by the EU RTD Framework Programme Horizon2020. TU1208 Members are deeply grateful to COST for funding and supporting COST Action TU1208. As of 4 October 2017, this website is supported by TU1208 GPR Association, a non-profit association stemming from COST Action TU1208.