Working in a difficult environment: GPR sensing on the railroads

Selecting antenna systems for ground penetrating radar along railroad tracks is a challenge. The antennas must provide sufficient depth of investigation (roughly 2 m), and yet have sufficient resolution to detect and map changes in ballast gravel and the ponding of centimeter thicknesses of water. They must operate in high cross winds during high speed rail travel, with at least 20 centimetres of ground clearance. They must also tolerate the high EMI/RFI environment from railroad communications and automation, and the high clutter environment from metallic objects, and yet provide an adequate field of view to see sub-meter scale variations in subsurface properties along and across the track and between the ties, as well as the condition of the ties themselves. The antenna mounting must be flexible to fit on a variety of rail vehicles and be easily reconfigurable to accommodate frequent track changes. Frequency, antenna type, mounting, position, orientation, and polarization of the antennas are important. Field experiments have proven the utility and effectiveness of one configuration of antennas utilizing 1 GHz air launched horns with one orientation and polarization between the rails and another outside the rails beyond the end of ties. Antenna and radar system calibrations are required to turn the data into true cross sectional images and to model for water content in the subsurface, and such calibrations are challenging.