Proceedings of the National Seminar & Exhibitionon Non-Destructive Evaluation

NDE 2011, December 8-10, 2011

BALLAST VELOCITY ANALYSIS

Ballast fouling appears as a decrease of the volume of air voidsinside the ballast. The reduced proportion of air within theballast leads to an increase of the relative permittivity. Typicalvalues for the relative permittivity of clean ballast are in therange of 3-3.5 and for fouled ballast in the range of 4-38 [15].The increased relative permittivity causes a decrease of theGPR propagation velocity v and can thus be measured usingthe common midpoint (CMP) method [1,2]. In a CMPmeasurement receiver and transmitter are moved in oppositedirections from a center point with the same speed. The traveltime t of the reflection from layered structures will increasewith increased offset x between receiver and transmitter by:

(1)

This formula is also valid for the movement of only oneantenna, if a layer parallel to the surface causes the reflections.This antenna configuration with the movement of only oneantenna is designated as common reflection surface (CRS).

With this formula the difference tNMO= t- t0 (NMO: NormalMove Out) between zero offset t0 and the travel time t can becalculated and used to compensate the increase of travel time(see Figure 1). If the velocity v is chosen properly, the correctedtravel time t* is constant for each CMP trace.

GPR ANTENNA ARRAY FOR THE INSPECTION OF RAILWAY BALLAST

Th. KindBAM Federal Institute for Materials Research and Testing,

Division 8.2, 12205 Berlin, Germany

ABSTRACT

A GPR antenna array was investigated for automated measurement of wave propagation velocity as a measure of theballast quality. This goes beyond the regular application of conventional radar systems offering only qualitative structuralinformation. The new approach provides a fast NDT method for the classification of ballast fouling. A main advantageof the multi-offset radar is that regular ballast digging for velocity evaluation can be avoided and travel time can bedirectly transformed into an equivalent depth by using the automatically evaluated velocity. Fast monitoring of ballastfouling condition can reduce maintenance cost significantly by supporting the works necessary for ballast cleaningand furthermore this method reduces interruption time for a better railway service.

Keywords: Railway, ballast, track bed investigation, velocity analysis

PACS: 89.20.Bb

INTRODUCTION

Railway ballast fouling reduces the safety of the wheel-tracksystem, due to the increase of stiffness and shear strength ofthe degenerated rail track bed. Fast monitoring of ballastfouling condition can reduce maintenance cost significantlyby supporting the works necessary for ballast cleaning [11].The application of GPR for rail track investigation has beenproven to be an useful tool for detecting layer interfaces likethe ballast/subgrade interface [3-14]. It is performed bymonitoring the travel time of the reflection from the ballastsubgrade interface along the track. The depth of the ballastlayer can be calibrated by calculating the propagation velocityof the radar signal in the ballast. Therefore depth measurementsat selected points along the track have to be correlated withcorresponding travel times. This kind of depth calibration isexpensive because it requires digging or drilling of the ballast.The calculated depth between two calibration points dependseither on a change of propagation velocity or on an actualchange of the thickness of the ballast layer. This leads to anuncertainty because with local calibration it is not possible todistinguish between a change in the vertical position of thereflection interface or a change of the propagation velocity.Measurements with a multi-offset antenna arrangement cansolve this problem in a similar way to the velocity analysisapplied to seismic data for a common midpoint measurement[1].

282 Kind : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation

Stacking the corrected CMP traces results in an increase ofthe reflection amplitude at t=t0. In the case of improperselection of velocity the reflection amplitude is spread outalong the trace. A sequence of stacked traces with differentapplied velocities will result in a velocity spectrum along thetime axis. The velocity spectrum can be calculated by using asub-set of CMP traces, too. An example of a velocity spectrumand a comparison between a velocity spectrum calculated bydifferent numbers of CMP traces will be given below.

RAILWAY MULTI-OFFSET ANTENNA ARRAY

A multi-offset array for ballast investigation was built. Arraydesign is one of a linear type. One transmitter and up to fivereceivers are set up in a line (see Figure 3 and 5b).

To verify the feasibility of a multi-offset array in advance, anantenna array has been simulated by using two 500MHzantennas (GSSI: SIR20) one after the other in each antennaposition of the array. Tests were carried out with the lineararray aligned along and perpendicular to the track.

BALLAST TEST SPECIMEN

For testing the concept of ballast velocity analysis, a 4m x 4mtest specimen has been constructed (see Figure 4). The height

of the ballast is about 80cm. Four reinforced concrete sleepersare placed in a distance of about 70cm, followed by a regionwithout sleepers of about 140cm.

MEASUREMENTS

A trolley with a position wheel, which was led along the topsurface of one trail, was used for the measurements. For allmeasurements two 500MHz antennas were used. Thetransmitting antenna was placed in a fixed position with anelevation of 19cm above the ballast. The receiving antennawas mounted on the trolley in the same elevation as thetransmitting antenna (see Figure 5a). Ten parallel scans wererecorded using the trolley. The measurement traces weretriggered by the wheel every 5mm. In one completemeasurement cycle an area of 1.2m x 2m was scanned. Parallelpolarisation between transmitting and receiving antenna wereused. The polarisations were directed along and across thetrack.

RESULTS

For the velocity analysis a scan of the receiving antenna hasbeen used, which is in line with the transmitting antenna andalong the track. Figure 6 shows the scan.

Fig. 2 : Diagram of an CMP/CRS measurement; t0 denotesthe minimum travel time of the layer reflection; reddots indicates the recorded reflection of five differentantenna offsets

Fig. 1 : CRS antenna configuration (TX: transmitter, RX:receiver)

Fig. 3 : CMP/CRS by an linear antenna array (a); Prototype of the linear antenna array (b)

NDE 2011, December 8-10, 2011 283

Fig. 4 : Ballast test specimen

Fig. 5 : Multi-Offset measurement with one fixed transmitterand moved transmitter on the ballast test specimen (a);Multi-Offset measurement with a linear antenna arrayon a ballast railway track (b)

Fig. 6 : Radargram with fixed transmitter and moved receiver

The data were processed only with dc removal and withoutapplication of time gain. Reflections from the ballast surfacean d the ballast bottom side are clearly visible. Figure 6 showsonly a minor influence of the sleeper on the radargram. Onlythe ballast bottom side reflection is slightly affected by thesleepers. From the radargram seen in Figure 6 and a givenpropagation velocity the normal move out time for each traceis calculated by the offset between transmitter and receivingantenna. In a second step the time position of each trace iscorrected by the corresponding normal move out time. Thereflection shown in Figure 6 appears tilted. If the appropriatevelocity is chosen, the reflections will be displayed ashorizontal lines in the radargram. The sum of all correctedtraces will increase the summed amplitude of the reflectionsto a maximum at the zero offset position. To find the mostappropriated propagation velocity, the normal move out hasbeen corrected and all traces in a velocity range of 5x107 -3.5x108 m/s have been summed up. Velocities greater than thespeed of light are used only for visualizing the velocityspectrum. They have no practical meaning. The velocityspectrum of the summed traces is shown in Figure 7a.

Two regions of high amplitudes of the summed reflectionsare formed around the velocities of 3x108m/s in the upper rightcorner and around 2x108m/s in the centre of the Figure 7a.The upper right region represents the ballast surface reflectionand the centre region can be referred to the ballast bottomside reflection. This velocity analysis measurement shows thatthe multi-offset measurement of the propagation velocityinside the ballast can be used as an evaluation tool of the ballastdegradation: the shifting of the areas of high amplitudes ofthe summed reflections within the travel time vs. propagationvelocity diagram (Figure 7a) is an indication for the state ofthe ballast.

284 Kind : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation

Fig. 7 : Velocities spectrum (a); (b) – (d) calculated from a limited number of 8,6 and 4 traces

For a multi-offset antenna array the number of traces and thelateral position of the traces are limited by the scale of a singleantenna. Choosing an antenna in the frequency range of about500-1000MHz will limit the number of antennas to 4 –10elements per meter inside the array. For fast acquisition a multi-offset antenna array should collect all traces for one velocityanalysis at one array position at the same time. The velocityanalysis has been carried out with the data shown in figure 4but with a sub set of traces in order to test the impact of reducedtrace numbers on the quality of the velocity analysis. 4, 6 and8 traces with equal distance between the traces have beenselected. The velocity analysis was done as described abovefor the results presented in Figure 7a. Figure 7b-7d shows theradargrams together with spectra of the propagation velocity.

CONCLUSION

Conventional propagation velocity analysis can be applied forthe investigation of ballast track beds. For a fast acquisitionon a railway inspection train, a multi-offset array can be used,which is acquiring the necessary traces for velocity analysisin one step and with minor impact to the quality of the velocityspectrum. In a ongoing investigation it will be investigated, ifa decrease of propagation velocity caused by fouling of theballast will lead to a significant shift of the high amplitudes inthe velocity spectrum towards lower velocities on the left sideof the spectrum.

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8 Sussmann, T.R., Selig, E. T., Hyslip, J. P., Railway trackcondition indicators from ground penetrating radar,NDT&E International, Elsevier, Vol. 36, pp. 157-167,(2003)

9 Gallagher, G.P., Leiper, Q., Williamson, R., Clark, M.R.,Forde, M.C., The application of time domain groundpenetrating radar to evaluate railway track ballast,NDT&E International, Elsevier, Vol. 32, pp. 463-468,(1999)

10 Hugenschmidt, J., Ballast Inspection Using GPR, Proc2nd Int’l Railway Conference, London, UK (1999)

11 Selig, E.T. and Waters, J. M., Track Geotechnology andSubstructure Management, Telford, England (1994)

12 Kathage, A., Nissen, J., White, G., Bell N., Fast Inspectionof Railway Ballast by Means of Impulse GPR Equippedwith Horn Antennas, Railway Engineering-2005,London, UK (2005)

13 Smekal, A., Berggren, E., Hrubec, K., Track-SubstructureInestigations Using GPR and Track Loading Vehicle,Railway Engineering-2003, London, UK (2003)

14 Saarenketo, T., Silvast, M.,Noukka, J., Using GPR onRailways to Identify Frost Susceptible Areas, RailwayEngineering-2003, London, UK (2003)

15 Clark, M.R., McCann, D.M., Forde, M.C., GPR as a Toolfor the Characterisation of Ballast, Railway Engineering-2003, London, UK (2003)

ACKNOWLEDGEMENTS

Part of this work was part of the project SAFERAIL, whichwas founded by the European Union in the 7th Frameworkprogram.

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