Roger Roberts

Data Analysis Techniques for GPR Used for Assessing Railroad Ballast in High Radio-Frequency Environment

Railroad ballast supports heavy rail loading, prevents track deformation, and provides drainage of water from the track structure. However, over time, ballast is fouled by the breakdown of ballast aggregate and/or the infiltration of fines, which undermine the ballast functions and affect the railroad track structural capacity. Ground penetrating radar (GPR) provides a rapid, effective, and continuous way to assess railroad track substructure condition; especially ballast. However, the GPR system faces some challenges during field surveys including high radio-frequency interference from railroad communication and automation, and strong reflections from rails. In this study, appropriate techniques were used to remove the interference and reduce the strong clutter from rails to obtain clear GPR data of railroad substructure. A time-frequency method, short-time Fourier transform, was then applied to extract ballast fouling condition over depth. A field survey using multiple sets of 2-GHz air-horn antennae was conducted during summer 2007 at the Transportation Technology Center, Inc. in Pueblo, Colo. Compared to ground-truth excavation and ballast gradation analysis results, GPR was found to be an effective technique to assess railroad track ballast substructure condition.

NDE methods for quality assurance of new pavement thickness

Accurate measurement of pavement thickness is an essential aspect of the quality assurance of new pavement construction. Current coring methods are time consuming and provide a very limited representation of the overall pavement structure. The objective of the work described in this paper has been to demonstrate the use of non-destructive evaluation (NDE) methods for rapidly determining the average pavement thickness on a newly constructed section to within 2.5 mm of the true value, without extensive reliance on cores. The effort has considered ground penetrating radar (GPR) and impact echo methods applied to both asphalt and concrete pavement, and has included laboratory and field-testing, with field correlations based on 172 cores. The results show that the 2.5 mm accuracy objective can be met for asphalt pavement, but that accuracy on concrete is limited to 4 mm. The paper describes the techniques that were evaluated, the testing that was conducted, and the results of correlation with core data.

Time-Frequency Approach for Ground Penetrating Radar Data Analysis to Assess Railroad Ballast Condition

Railroad ballast plays an important role in supporting heavy rail loading, preventing the deformation of track, and providing drainage of water from the track structure. However, over time, ballast is fouled by the breakdown of ballast aggregate and/or the infiltration of fines, which undermine ballast functions. This may result in damage to the rail system, such as track settlement. Ground penetrating radar (GPR), a nondestructive method, can be used to rapidly, effectively, and continuously assess railroad track substructure conditions. Ballast under various fouling conditions generates various electromagnetic (EM) scattering patterns. In this study, air-coupled 2 GHz antenna was found to be sensitive to the scattering pattern change. Appropriate data processing was used to remove the effects of ties and rails to obtain clear GPR images of the subsurface layers. Then, the amplitude envelope and time-frequency approaches were implemented to characterize the signal in time and frequency domains simultaneously. Using these techniques, non-fouled ballast thickness can be assessed and trapped water can be detected, along the track.

Development of a Time — Frequency Approach to Quantify Railroad Ballast Fouling Condition Using Ultra-Wide Band Ground-Penetrating Radar Data

This paper discusses the use of ground-penetrating radar (GPR) to assess railroad track substructure conditions. An ultra-wide band (UWB) GPR system, having a centre frequency at or higher than 2 GHz, can be used to detect the scattering pattern and to predict air void volume in railroad ballast. A time–frequency technique was implemented to characterise the signal in time and frequency domains simultaneously. Because electromagnetic energy attenuation is highly frequency dependent, the frequency sub-bands of the reflected UWB GPR signal can be analysed separately to quantify the fouling material and quantify moisture content. Additionally, to validate the GPR system capability, a ground truth field survey was conducted. Using ballast samples collected from the field for validation, this paper shows that a time–frequency analysis may provide a new method to measure the thickness of clean ballast, detect the trapped water and assess the ballast fouling and moisture content along the track.

Characterizing Railroad Ballast Using GPR: Recent Experiences in the United States

Recent work has been conducted in the United States with 2 GHz horn antennas to characterize railroad ballast. There were a number observations made during the course of the project that derive from gaining a more thorough understanding of ballast and the interaction of GPR with the ballast matrix. The major observations from over 238 km of track data at four different geographical locations include: (1) it cannot be assumed that there will be a reflection from the bottom of clean ballast or that there will be a reflection from the ballast-subballast interface; (2) the presence of a strong reflection in the data generally, but not always, infers moderately-fouled to clean ballast above the reflecting boundary; and (3) no observable ballast-subballast interface reflections are generally, but not always, associated with gradational fouling or a fully-fouled ballast section.

Enhanced target imaging in 3D using GPR data from orthogonal profile lines

Continuing improvements in computer technology have made 3-D imaging a standard GPR interpretation technique. The most common data collection methodology for 3-D imaging involves collection of data along parallel profile lines. The data are then often migrated and concatenated into a 3-D file. A 3-D image generated from the file is manipulated to detect linear and finite-size targets. The detection of linear and finite-size targets can be enhanced by creating images generated from data collected along orthogonal profile lines. The fact that the minimum angle formed between the long axis of a linear target and one of the orthogonal profile lines is 45 degrees enhances the detection of a linear target because in at least one profile line direction the reflection from the linear target will form the familiar hyperbola and a series of hyperbolas concatenated from parallel profile lines are readily observed in the 3-D image. Perhaps the most beneficial aspect of using bi-directional data is the ability to perform spatial filtering operators to improve detection of linear targets. Background removal filters applied to parallel profile line data will generally erase reflections from pipes or rebar that trend parallel to the direction of the profile lines. Comparisons of the data visualization capabilities between one-direction and orthogonal profile line data collected on reinforced concrete and on a buried pipe test site clearly show the advantages of imaging using orthogonal profile line data on both small and large scales.

Effect of antenna-surface distance on the radiation of a GPR antenna

An experimental investigation was performed to analyze the influence of the proximity of a dielectric interface on the radiation of a GPR antenna. The experimental setup consisted of two 1.5 GHz resistively loaded transmit-receive antenna pairs. Data were obtained from one transmit-receive antenna pair as it was moved from a height of 29 cm to the dielectric surface at 1 mm increments. Direct transmission data were also obtained at the second antenna pair placed on the other side of the dielectric medium. Data were obtained from three different dielectric media possessing permittivities of 5.2, 10.6, and 80. Comparison of the frequency spectra of the direct-transmission waveforms for different transmitting antenna distances from the dielectric surface revealed a slight shift towards lower frequencies when the antenna enclosure was in contact with the surface. The direct-coupled waveform between transmitting and receiving antennas inside the same enclosure varied significantly over an antenna enclosure-surface distance range of 0 to 10 cm. the changes in the direct-coupling waveform appear to be largely due to the linear superposition of: (1) the direct arrival between the transmitting and receiving antennas, and (2) the surface reflection and associated surface reflection multiples. The waveform collected by the receiving antenna adjacent to the transmitting antenna was remarkably similar to the one-way transmitted waveform recorded at the receiving antenna on the other side of the block.