A CUSTOMIZED GPR SYSTEM FOR RAILROAD TRACKS VERIFICATION

Transcription

1 A CUSTOMIZED GPR SYSTEM FOR RAILROAD TRACKS VERIFICATION Guido Manacorda, Davide Morandi, Antonio Sarri IDS Ingegneria dei Sistemi SpA Georadar Division Engineering Dept. Via Livornese 1019, Pisa loc. San Piero a Grado, 56010, Italy g.manacorda@ids-spa.it d.morandi@ids-spa.it a.sarri@ids-spa.it Giuseppe Staccone Ground Control GmbH Dachauer str.37, München, D80335 Germany georadar@saferailsystem.com ABSTRACT Railway is nowadays considered as the most ecological and economical alternative to cars and planes; on the other hand, in order to be really competitive, trains have to go faster and faster on safe tracks that allows an high velocity. From this point of view, speedy track construction is just as important as periodic maintenance and methods of the past (such as coring and visual inspection) are not longer able to provide helpful information especially because too slow and, often, too expensive. Therefore, the use of a Ground Penetrating Radar can be thought as a suitable and economical alternative to the other survey methods. In this paper is presented a GPR equipment especially designed for inspecting and verifying railroad tracks in a reliable and effective way, without interruption of regular traffic. Key words: Ground Penetrating Radar, Railroad track inspection, Non destructive Testing. INTRODUCTION Railroad tracks can suffer from several problems and their analysis requires reliable instruments. As a matter of fact, ballast sacks, contaminations, moist areas and packed layers (each one representing a severe hazard to railway traffic) must be located and monitored continuously. Until now this task has been done by inspecting track s level (if the problems extends to railroad surface) or by performing complex and expensive coring operations. Besides, coring provides punctual information and requires interruption of regular traffic for quite a long time. Obviously, a tool capable to generate a continuous, crosssectional image of the material under the tracks, can solve most of these limitations, especially if the survey can be performed at an high speed. GPR INSPECTION OF RAILWAYS The use of a GPR as railroad track s inspection equipment, is becoming popular especially in Europe, where several railway companies started to utilize GPR data for monitoring condition of ballast and sub-ballast layers [e.g. Hugenschmidt (1998)], but it is usually performed with systems not particularly designed for such a purpose. In fact, several factors due to the particular environment can reduce, sometimes dramatically, GPR performances. First of all, antennae must be used lifted up in order to be not damaged by obstacles during measurements (e.g. switches, crossings, gravel between sleepers), and therefore strong echoes produced by the rails may hidden weak signals coming from railway bed; furthermore, ground-coupled antennae can produce ringing when used not in contact with the surface of the material to be inspected, whereas highfrequency, air-launched antennae often cannot penetrate up to the desired range depth. Then, a multi-channel GPR equipment would be needed in order to survey in a single scan not only the area between tracks, but the whole platform s width; on the other hand, for not affecting the survey s speed, the radar must guarantee high performances in terms of pulse repetition frequency and scan rate as well. Finally, the simultaneous use of antennae at different frequencies allows to extend the range depth (due to the low frequency antennae) without any loss in the resolution characteristics (due to the high frequency sensors).

2 PRELIMINARY TESTS PERFORMED BY IDS In the 1999, IDS Ingegneria Dei Sistemi SpA, carried out some preliminary tests in Italy, along the high-speed railway track between Rome and Orte. The total length covered with the system was of about 16 Km and more than 4 Gbyte of data were collected; the processing sequence which has been found most suitable for all the datasets, includes the following steps: Correction of surface reflection to to time zero Vertical Band pass filtering (FIR) Horizontal Band Pass filtering (FIR) Gain Background removal. I.D.S. Ingegneria Dei Sistemi S.p.A Figure 2: Example of a processed dataset Final results of the preliminary test phase, was encouraging about the performances achievable by the RIS system. In Figure 2 echoes produced by the interfaces between different layers (i.e. 1) ballast/sub-ballast, 2) subballast/base, 3) base/soil, 4) soil/unknown material) are clearly visible. Figure 1: IDS system used for doing preliminary tests The GPR system utilized during these experiments, is shown in Figure 1. A wooden trolley housed the IDS RIS99 multi-channel data collection unit and the antennae array including five 600 MHz, two 200 MHz and one 1600 MHz sensors. In order to reduce the coupling between GPR antennae and the sleepers (which often include metallic rebars), dedicated checks have shown that the most suitable orientation was the one with dipoles main axis parallel to the scan direction. The trolley was pushed by the operator at a walking speed, while a survey wheel has been used both for triggering the acquisition and measuring distances from the starting point. Data were real-time stored into the system s hard disk for the later analysis Figure 3: 1600 MHz data set Figure 3 illustrates how an high-frequency antenna (1600 MHz) may offer valuable information, especially in terms of resolution, when the detailed analysis of a limited extension area is required; echoes due to ballast internal stratification are easily detectable (1,2), whereas interface between ballast and sub-ballast (3) as well as the one between sub-ballast and base (4), are not very strongly marked.

3 FINAL SYSTEM DESIGN Thanks to the cooperation with Ground Control GmbH, a German company experienced in the railways investigation, it has been possible, during the year 2001, to refine the project of a GPR system fully dedicated to that application. IDS took care to adapt hardware components, while Ground Control was responsible for the installation onboard a locomotive and for the design of a small, hand-pushed trolley where to house the system. Data Collection System An IDS RIS 2K Multi-channel Acquisition Unit drives an array of multi-frequency antennae (primarily 200 and 600 MHz) for scanning simultaneously inside and outside the track. Main acquisition parameters are as follows: Range depth: 60 nsec Number of channel: up to 8 Pulse Repetition Frequency: 400 KHz Scan repetition rate: 400 Hz Samples per scan: up to 512 A/D conversion: 16 bit Acquisition speed: up to 80 Km/h Maximum antenna height: 25 cm. Figure 4: Antennae position in front of the locomotive. The survey wheel that triggers the data acquisition is mounted directly on a locomotive s wheel. Data collection is usually performed at about 80 Km/h, so that there is no need for closing the railway tracks during the georadar measurements. Antennae have been hermetically sealed for being used also under extreme weather condition (e.g. hard rain). Data collection software has been modified in order to allow the survey of more than 20 Km during a single scan. For avoiding damages to some critical components of the system, such as the hard disk drive, due to the vibrations caused by train high speed, a special absorbing gel was utilized. System installation onboard the train Antennae are located in front of the locomotive (see Figure 4); a video camera is used for looking after the array during the scan and images are shown in a monitor placed near to the operator seat (see Figure 5). Therefore it is possible, just by activating a motor driven mechanism, to lift or lower the antennae in order to avoid any obstacles during the scan Figure 5: Operator s seat onboard the locomotive Ground Control GmbH has also developed the trolley shown in Figure 6, dedicated to the detailed survey of short sections which have been found critical during the first, high-speed inspection. Depending on the weather conditions, the acquisition unit can be housed within an hermetically closed box (not shown in the picture).

4 8); strong undulations of this layer make the drainage not effective, so that water stagnates under the ballast. Actually, strong reflection on the right side of the picture is due to the high water content within that area. Moist Area Figure 9 shows a fairly regular path of the ballast layer; over the top of the subsoil, strong reflection indicates the presence of moisture. Figure 6: hand-pushed trolley. EXAMPLE OF ACHIEVABLE RESULTS Ground Control GmbH started using the system previously described at the beginning of the year 2001; since then, hundreds of kilometres were inspected in several European countries. Surveys performed with this technique have been found attractive in order to accurately locate damaged track sections and to reduce the number of drillings to a minimum, so that a targeted renewal is possible and maintenance costs can be reduced significantly. Some significant situation are described in the following paragraphs even if these interesting cases should be considered as just a sample of what the system allows to obtain. Ballast Sack And Muddy Area Figure 7 shows the effect due to the presence of a soft soil with an insufficient mechanical strength. Between Km and the base of the ballast is undulated and a strong reflection due to the presence of mud is easily distinguishable (between Km and ), as well as a ballast sack. After the crossing, the base of the ballast shows a linear path due to a renovation. Wrongly-Built Sub-Ballast Layer The protecting layer under the ballast (sub-ballast) was built inaccurately between Km and (see Figure Furthermore are quite evident, in the middle of the picture, the echoes produced by stones placed just above the subsoil; this is the old way for overcoming the limited strength of the ground (e.g. due to the presence of sand or soft soil). CONCLUSION Information collected by the dedicated GPR system are valuable for preventing severe hazard to railway traffic and planning renovation; furthermore the possibility of optimising the location of drill spots and surveying without closing the tracks, makes this method of investigation appreciable. The system described in this paper has been designed for fully matching the requirements of the specific application and was found attractive to several railway companies in Europe. REFERENCES Gobel, C., Hellmann, R., Petzhold, H., Georadar model and in-situ investigations for inspection of railway tracks, Proceedings of Ground Penetrating Radar Conference, Kitchener, Canada, June 12-16, Hugenschmidt, J., Railway track inspection using GPR Some examples from Switzerland, Proceedings of Ground Penetrating Radar Conference, Lawrence, Kansas (USA), May 27-30, Hellmann, R., Göbel, C., Petzold, H., Staccone, G., Anwendung des Georadars - Inspektionsverfahren für das Tragsystem von Eisenbahstrecken, Eisenbahningenieur, (45) 9/1994 Staccone, G., 1999.Georadarverfahren, Eisenbahningenieur, (50) 12/1999

5 Muddy area Level crossing Renewed area Base of Ballast Undulated Base of Ballast Ballast Sack Base of Ballast Figure 7: Example of a ballast sack and a muddy area Ballast Strongly undulating sub-ballast layer Figure 8: Example of a wrongly-built sub-ballast layer High content of humidity at the base of ballast Area with high content of humidity Top of subsoil Stones Figure 9: Example of a moist area.