Elsevier

NDT & E International

Volume 43, Issue 1, January 2010, Pages 20-28
NDT & E International

Optimization of antenna configuration in multiple-frequency ground penetrating radar system for railroad substructure assessment

https://doi.org/10.1016/j.ndteint.2009.08.006Get rights and content

Abstract

This paper discusses the use of ground penetrating radar (GPR) to rapidly, effectively, and continuously assess railroad track substructure conditions, especially ballast. To overcome the limited electromagnetic waves penetration for high-frequency antennae and the low resolution of low-frequency antennae, this study uses a multiple-frequency GPR system to assess railroad substructure conditions. High-frequency antennae were used to detect the scattering pattern, which is related to air void volume in railroad ballast, and low-frequency antennae are used to assess deeper substructure conditions. Considering the scattering energy attenuation is highly frequency and material dependent, a time–frequency method based on tracking the frequency spectrum and energy change over depth can be used to extract ballast fouling conditions. From GPR field collected data, ground-truth observation, and ballast gradation analysis, the multiple-frequency GPR system demonstrates a promising capability to assess railroad track substructure condition.

Introduction

To ensure the safety and timely delivery of freight and passengers, the railroad network system is continuously maintained. This maintenance is a challenging task. It is difficult and expensive to accurately assess the condition of the track system. In the United States, there are over 373,000 km of railroad track that must be periodically evaluated and possibly repaired. To optimize the limited funds available to maintain the track system, an accurate assessment of track conditions is necessary.

The main components of the track structure are grouped into two main categories: superstructure and substructure. The superstructure consists of rail, the fastening system, and ties (sleepers). The substructure consists of ballast, subballast, and subgrade. The railroad track effectiveness and its structural capacity depend on the characteristics and the general condition of all layers under the ties, especially the ballast.

The main functions of ballast are the following: resist vertical, lateral, and longitudinal forces applied to ties to retain track in its required position; reduce pressure from the tie-bearing area to acceptable stress levels for the underlying material; provide resiliency and energy absorption for the track; absorb airborne noise; facilitate maintenance surfacing and lining operations; provide water drainage; and prevent frost formation.

However, ballast fouling may jeopardize the aforementioned expectations of the ballast layer. Selig [1] divided sources of ballast fouling into five categories: (1) ballast breakdown, (2) infiltration from ballast surface, (3) tie wear, (4) infiltration from underlying granular layers, and (5) subgrade infiltration. The most common source of fouling is ballast breakdown; it contributes up to 76% of the fouling, which means most fouling particles are first generated in the area under ties.

The present methods used to assess ballast condition are visual survey, selective drilling and digging at intervals along the track, and ground penetrating radar (GPR) survey. The visual survey is inaccurate and may not reveal subsurface conditions. The drilling method is time consuming and cannot supply continuous information about the track subsurface. Considering that significant variation may occur along tracks within short distances, a continuous, rapid measurement technique is needed. Ground penetrating radar, a nondestructive evaluation technique, is designed to gather information from the subsurface.

According to Olhoeft [2], selecting antenna frequency is very important for GPR survey of track substructure condition. The antennae used need to provide enough penetration depth to reach the bottom of the ballast layer. At the same time, high resolution is needed to detect air void volume change, which indicates ballast fouling condition. According to Gallagher et al. [3], antennae with frequency greater than 900 MHz are required for high resolution and those with frequency less than 900 MHz can provide deep penetration. In order to identify the appropriate antennae for railroad substructure assessment, several studies have been conducted on this topic in recent years.

Sussman et al. [4] used a GPR system with 100 MHz, 400 MHz, and 1 GHz antennae for track substructure survey. The 100 and 400 MHz antenna data were successfully used to identify layer interfaces using automatic layer tracking algorithms. An increase in moisture and localized soil change can be detected from the 400 MHz collected data. In the study of Olhoeft et al. [5] and Hyslip et al. [6], the GPR equipment with multiple sets of 1 GHz air-coupled antennae was used. The GPR data provides a good indication of subsurface layer configuration. To improve the estimation of the ballast layer depth, detectable geosynthetics were used by Carpenter et al. [7]. Keogh et al. [8] used the RAIL RADAR™ system to measure the signal travel time and material dielectric constant of each detected subsurface layer, as well as the tie condition. A multi-offset 1.2 GHz ground-coupled antenna was used. The results show that the GPR pulse propagation velocity significantly decreased (10–30%) from clean to fouled ballast. Accordingly, RAIL RADAR™'s was used to define a ballast fouling threshold.

In a study by Clark et al. [9], data collected by antennae at various frequencies, including 500 MHz, 900 MHz and 1.5 GHz, was used to analyze the frequency spectrum change from clean and fouled ballast. Fast Fourier transform (FFT) was used to analyze the data and showed good potential to assess ballast condition. Silvast et al. [10] used the frequency sum method on the data collected from two 400 MHz antennae. Research results demonstrated that frequency analysis can provide information on ballast fouling condition and subgrade soil type. Roberts et al. [11] used scattering information of void space in ballast to predict fouling condition. The study concluded that clean ballast could potentially be distinguished from fouled ballast by the intensity of the void scattering using a 2 GHz-antenna data.

Narayanan et al. correlated GRR measurements using 400 MHz to ballast modulus [12]. Their preliminary results indicated that a relationship between reflected energy and the track modulus existed. According to Narayanan et al., 400 MHz is the optimal frequency for assessing the condition of subballast and subgrade. However, it has to be noted here that electromagnetic waves reflection is related to the dielectric and volumetric characteristics of the material and may not be related to material modulus, unless a unique material is considered. Furthermore, reflected energy may depend on several factors including moisture content. In addition, the low-frequency antenna may not provide detailed information about the ballast condition; especially at critical areas, such as under sleepers. The use of multi-frequency GPR system may, however, provide more reliable subsurface information as shown by Loizos et al. [13]. In summary, there is a trade-off between GPR depth of penetration and data resolution. The higher-frequency 2 GHz antennae provide greater resolution to extract ballast condition but less penetration depth. Lower-frequency antennae, on the other hand, provide deeper penetration but reduced resolution. In this paper, a combination of high-frequency 2 GHz antennae and low-frequency 500 MHz antennae are used to obtain detailed information on ballast condition and layer interfaces.

Section snippets

Theoretical background on ground penetrating radar

Ground penetrating radar is based on sending electromagnetic (EM) waves into the ground using a transmitting antenna. A receiver antenna is used to collect the reflected signal from the interfaces between the materials and scattering from inhomogeneities having different dielectric properties within the materials. If each layer can be assumed homogeneous and lossless, information about layer thicknesses and material properties, such as hot-mix asphalt (HMA) density and base moisture content,

Conclusion

A multiple-frequency GPR system, having two 2 GHz and one 500 MHz horn antennae, is used to assess railroad substructure condition. The 2 GHz antennae radiate wavelengths that are sufficiently short enough to detect the presence of air voids in clean ballast. The scattering pattern changes when ballast fouling condition changes. Given that scattering energy attenuation is highly frequency and material dependent, a time–frequency technique, based on short-time Fourier transform (STFT), can be used

Acknowledgements

The authors gratefully acknowledge the assistance of the staff at the Transportation Technology Center, Inc. during data collection and cross-trenching. The help provided by Douglas Jones, Erol Tutumluer, Chris Barkan, and Jeff Boyle is greatly appreciated. This research was partially funded by the Federal Railroad Administration Project DTFR53-05-D-00200 and the University of Illinois at Urbana-Champaign (UIUC)-Association of American Railroad (AAR) Technology Scanning Research. The contents

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