NDT.net • Sep 2005 • Vol. 10 No.9

Fast Inspection of Railway Ballast By Means of Impulse GPR Equipped with Horn Antennas

Dr. A. Kathage*, GSSI, North Salem, N.H., USA
J. Niessen, GBM Wiebe, Achim, Germany
G. White, N. Bell, Allied Associates Geophysical Ltd., Dunstable, U.K.

*Corresponding Author Contact:
Email: KathageA@geophysical.com

Published in Proceedings of RAILWAY ENGINEERING-2005, The Eighth International Conference
“Maintenance & Renewal of Permanent Way; Power & Signalling; Structures & Earthworks”, Internet: www.railwayengineering.com


The inspection of railways ballast by use of ground penetrating radar devices has been performed for several years now. A continuous and non destructive profiling of the ballast and subsoil offers obviously significant advantages compared with the traditional way of coring and sampling. The latter method is appropriate for layered structures which can be described by statistical methods but for example local structural defects can easily be overlooked because statistical drilling patterns can not be used for addressing this kind of problem.Last but not least it is well known that the application of non destructive inspection methods like GPR is reducing the inspection costs compared with the traditional approach. One serious handicap for the application of GPR on railways is survey speed. Exploiting the full economical potential of GPR would allow users to fit the surveys in between regular train schedules.This was not possible until newly developed equipment became available. The risk of hitting switches or similar obstacles close to the ground was the main reason for slow data collection speed with bow-tie antennas. Bow-ties need to be operated within a quarter of a GPR-signal wavelength.Practically this forced the GPR operators to mount their antennas not higher than 10 cm above the ballast. This low height allowed a maximum data collection velocity of only 30 km/hour.

The operation of horn antennas avoids this problem because they can be mounted about half a metre above the ballast. The development of a new 400 MHz horn antenna for railways ballast and subsoil inspection was additionally triggered by the availability of new GPR control units like the GSSI SIR-20. These units allow data collection rates of several hundred scans per second with a time resolution of 5 picoseconds for 512 or 1024 samples per scan. Using the 400 MHz horns with 50 nanoseconds time range offers survey velocities of more than 100 km/hour with 20 scans per metre. This scan separation has been identified to be an important parameter for good data quality. Less scans per meter would mean less information between the sleepers.

The use of the newly developed hardware and software for collecting high speed GPR data in combination with other sensors like RTK-PS,Doppler radar, video as well as the streamlined data processing and data interpretation routines will be presented in this paper. Examples of typical survey data and the final survey results will be shown for demonstrating the high performance of this new technology.

KEYWORDS: GPR, horn antennas, ballast inspection, computer analysis,


The traditional method for investigating the quality of the ballast uses sampling and analysing of drill cores.The sampling interval depends on local parameters.Usually drill cores are collected every 50, 100 or 200 meter. Local defects can be missed by this approach because they are not distributed along the railway route according to statistical laws. They can be located just far enough from a drill hole to be missed.

Nowadays big machines are employed for carrying out the required maintenance work. Unexpected standstill times caused by unforeseen obstacles in the ballast like muddy spots, bedrock, compact layers can lead to enormous rise of costs. A solid economical planning can not be performed based on these technical boundary conditions.

The application of ground penetrating radar (GPR) solves this information problem.Four GPR profiles are collected along the railways route. The planner obtains a non destructive section of the ballast structure from surface down to about 4 meters depth. Based on this continuous data the sampling of cores is guided exactly to the spot where defects have been detected by the GPR scan.

Standard bow-tie antennas have been used for this application in the past. These antennas need to be operated closely to the ground surface allowing a maximum survey speed of about 30 km per hour.Due to the increasing traffic on the railways network – especially in east-- west direction – this speed limit posed a serious handicap on the application of this method. Most of the GPR surveys were forced to be performed during night time when the traffic situation allowed the slow moving survey. That procedure turned out to be risky – antennas could get damaged on near surface obstacles ((like switch boxes) – and too expensive due to many long waiting periods (regular traffic had to pass by) during the data collection. Horn antennas needed to be used in this situation.This antenna type can be operated half a meter above the ground surface. Therefore the speed limitation would only be depending on the data collection rates of the GPR control unit. 1 GHz horns or even higher frequencies were commercially available. But the detection range of 4 meters required a lower frequency to be applied. Therefore customized 400 MHz horns were developed.


The following table illustrates the typical key parameters of a railway line GPR survey project.

Table 1: Main activities for GPR railway line survey.

a) Data collection:
Four profiles are collected per railway line. 400 MHz data are taken from the field side,rails centre profile, rails side. One profile is collected with 1 GHz horn antenna for detailed analysis of the possibly compacted layer and ballast quality.
The following equipment is used for the data collection:

Table 2: Equipment used for railways GPR surveys.

Two GSSI SIR-20 control units are used for collecting the GPR data (see figure 1). These systems allow scan rates of more than 800 scans per second (single channel, dual channel operation is multiplexed) which means that in combination with horn antennas a survey speed of 120 km/h can be achieved while collecting 20 scans per meter.A Differential GPS (see figure 2) in combination with a DMI (distance measuring device) and a Doppler-radar provide accurate distance-/position information. A PC is used for managing and saving of position as well as video data while an external Firewire hard-disk serves for backup of all survey data.

Fig 1: Two GSSI SIR-20 systems are mounted in the train for high speed GPR data collection.

Fig 2: Differential GPS system with receiver for reference data (ASCOS)

The antennas are mounted in front of the train. The mounting system allows hydraulic changing of the antenna height.

Fig 3
: Mounting of the different sensors in front of the train of the Rhätische Bahn, Switzerland.

Horizontal resolution
The horizontal resolution of the GPR measurement is a function of:
1)the width of the antenna radiation pattern/Fresnel zone, 2) the capability of the radar system,to collect a certain number of scans per metre at a specific scanning speed.In the case of a GPR system,the number of scans per meter can be used as a measure of the capability for imaging objects in the radargrams starting from a specific minimum size. The images of relevant objects or structures have to be clearly and distinctly visible for the human eye. For the investigation of railway line buildings,reflection amplitudes are significant which are caused by structures or objects starting at a dimension of several metres. But the reflections of cables and conduits are just as significant. In order to be able to meet these requirements, at least 20 scans/m is required for each profile. This results in an optically distinct scale of 1:250. For the raw data scan, this is an absolute minimum requirement. The measurements have to be distance triggered.

In practice, the following applies: One metre of measuring distance along the rails means: space between sleepers (approx.40 cm) +sleeper (approx.20 cm) +space between sleepers (approx.40 cm). In total this distance corresponds to approx. 1 metre. At twenty scans per metre, a total of 16 scans can be collected in the gaps between sleepers and four scans each through the sleepers. Four pulses per sleeper are sufficient for being able to image these sleepers clearly visible in the radargrams. In most cases the sleepers can then be counted individually! Sleepers are part of the building structure and therefore they need to be visibly imaged in the data. Ringing effects in the data due to sleeper reinforcement are caused by an inappropriate polarisation of the antennas. The figure 4 shows an example of the four GPR sections which are typically collected.

Fig 4
: Raw data as well as filtered data. The typical 4 profiles are shown which are usually collected along railways. In this case the radargrams were collected with 400 MHz bow-tie antennas and a 1000 MHz horn antenna.

b) Data processing and interpretation:
Special software was developed in order to optimize the post- processing as well as the interpretation of the different survey data.

The program GeoExplorer allows the efficient handling and interpretation of profile data from railways or road sections. Standard data processing is not included because this part is already well covered by other available software like GSSI RADAN.

Each profile can be described with standardized abbreviations. All detected features and signatures can be saved in a special railways data base.The position accuracy of these data base elements is +/-1m.That means also that the tracked layer interfaces are stored in that data base.
Also other data than GPR results can be stored in the data base:

  • Geometrical rail data
  • Drill core graphics
  • Data sheets, specifications and graphical data of buildings (bridges, tunnels, foundation walls,…)
  • Data as well as graphics from building grounds.
  • Digital video data
  • Customized information

Fig 5: Interpretation with dedicated GPR interpretation/visualization software optimized for rail- ways which (GeoExplorer)generates information for the data base.
Fig 6: Examples of three parallel profiles (raw data) recorded on a railways line in Switzerland with 400 MHz horn antennas.The GeoExplorer list boxes besides the radargrams show the optional details for half-automatic evaluation and interpretation.

Fig 7: Interpretation elements in GeoExplorer.

The GeoInspector is a viewer program for the customers of the GPR survey company. It is part of GeoEx-plorer and the functions of this program are:

  • Display of radargrams (up to 8 channels)
  • Individual gain settings
  • Display of symbols (buildings, drill core graphics, etc.)
  • Distance reference points (railways milestones, GPS, …)

This standalone program reads all information which was generated by GeoExplorer. Then it creates automatically a profile view with simultaneous displaying of data base information. This includes the display of the digital video film. Furthermore this program allows the calibration of layer interface depth by using drill core information. A vertical scale is created. Non calibrated profiles are highlighted. The user can also display the slope of the gravel base perpendicular to the rail axis. This display can work with information from 3 profile lines for a reconnaissance survey. The information from 9 profiles is used in case of a detailed quality control survey. This 2d-display can be created in a certain distance interval like every 5 km.

Fig 8: GeoAnalyzer display of database sheet with filtered signatures for subsoil and superstructure.

Fig 9: 2d-presentation of all results by GeoAnalyzer. This 2d-visualization contains all relevant railways parameters, incl.GPS and video.

c) Data Examples:
The following two figures show examples of GPR data demonstrating how structural defects of the embankment show up in the GPR sections.

Fig 10:
The photo shows a contaminated layer within the ballast.The yellow arrow in the radargram corresponds with the line in the photo.

Fig 11: Data taken with 1000 MHz horn antennas for quality control. Ballast layer and sand layer are not regular. The positions where the abnormal thickness changes start to appear along the profile are marked by yellow arrows.


PR has proven to be a successful as well as beneficial method for the non destructive investigation of railways embankments and building grounds. Meanwhile thousands of kilometres of railways lines in several European countries like Germany, Austria, Switzerland, Hungary, Norway, etc. have been investigated with GPR for the benefit of safety as well as the optimized allocation of financial resources in times of tight budgets. The success of this method was based on the combination of dedicated hardware and software. Fast GPR control units in combination with air coupled horn antennas of 400 MHz and 1000 MHz allow high speed data collection,the specialised software allows fast data processing and interpretation. Due to the high quality of the survey results and not at least due to the time efficiency of this method PR already plays an important role for the geotechnical inspection of railways with high potential for the future.


Publications in two different German railways journals:

1) Der Eisenbahningenieur

  • 6/2000 Flächendeckende Untersuchung der Bettungs-und Untergrundverhältnisse von Schienenverkehrswegen Stefan Haszio,Michael Funke
  • 6/2000 GeoRail – Unterwegs auf sicherem Grund,,Jürgen Niessen
  • 10/2002 GBM GeoRail Xpress – Modulares Multisensorsystem erfasst Schienenwege in einer Datenbank ,Jürgen Niessen
  • 9/2004 GeoRail Xpress als Systemvoraussetzung für eine qualitätsgerechte Projektvorbereitung, Jürgen Niessen (printed paper of the Oberbaufachtagung des VDEI in Darmstadt)
2) Güterbahnen
  • 2/2004 GeoRail Xpress:Schienenprüfung mit Hochgeschwindigkeit, Jürgen Niessen, Mario Giertz

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