Development of a Time-Frequency Approach to Quantify Railroad Ballast Fouling Condition Using UWB GPR Data

The fouling of ballast in the railroad substructure is detrimental to railroad track effectiveness and its structural capacity. This paper discusses the use of ground penetrating radar (GPR) to rapidly, effectively, and continuously assess railroad track substructure conditions. An ultra-wide band (UWB) GPR system, having a center frequency at or higher than 2GHz, can be used to detect the scattering pattern and to predict air void volume in railroad ballast. To extract valuable information on the ballast fouling condition, various data processing techniques were employed. Effects of rails,and external noise or interference were removed to obtain clear GPR images of subsurface layers. Then, a time-frequency technique was implemented to characterize 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 analyzed separately to quantify the fouling material and quantify moisture content. In addition, to validate the GPR system capability in assessing railroad substructure condition, a ground truth field survey was conducted at the Transportation Technology Center, Inc. (TTCI) in Pueblo, Colorado. Using ballast sample 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.