Computer Assisted Radar Tomography (CART)

Since its introduction in 2001, the CART (RT for short) system has created 3D imagery over 14 million square feet for a wide variety of public and private sector clients, proving itself as a viable, cost-effective technology for an equally wide variety of subsurface concerns. Although it was originally created to address the needless cost and damage inherent to utility-related excavation, RT is also applicable to certain environmental, military, and forensic scenarios…anywhere lack of information about the subsurface creates additional risk and cost.

RT employs a mobile GPR array, which can be towed by a vehicle or pushed in front of a modified commercial lawnmower (Figure 1) at speeds up to about 1 km/h (30 cm/s).

Figure 1: The radar array in the CART Imaging System can be mounted in a trailer that towed by a vehicle (left) or in a special deck mounted in front of a commercial riding lawnmower (right).

The standard CART system uses a fixed array of 9 transmitters and 8 receivers (Figure 2). Each radar element in the array is a standard ultra-wideband GPR that broadcasts an impulse with a frequency spectrum from about 50 to 400 MHz (A system with higher-frequency elements, having a spectrum between 100 and 650 MHz, has also been tested.). The array is controlled by special electronics that fires the transmitter elements and controls the receivers in sequence to create 16 standard bi-static GPR channels covering a 2 m swath on the ground (Figure 2 right). In this standard "bi-static" mode of operation, each transmitter fires twice in sequence, with each firing being recorded by an adjacent receiver. A multi-static mode, in which each transmitter fires once in sequence and is recorded by all the receivers, is also possible.


Figure 2: (Left) Photo inside the CART trailer housing the 17 radar antennas. (Right) Schematic top view of CART antenna array, which distributes 16 standard GPR channels over a 2 m swath considering a bi-static fixed offset data acquisition mode.

The CART systems rely on precise geometry control provided by a self-tracking laser theodolite. RTK GPS can also be used to control precise radar surveying, however, there are still issues with GPS in city environments . As the CART array moves along the ground, a laser theodolite locks on and follows a prism mounted next to the array. The CART system records the geometry data independently from the radar data and merges the two data streams using information provided by an internal trigger wheel that controls firing of the radar antennas.

As part of standard CART surveys, the laser theodolite is also used to map surface features-such as curbs, manholes, valve covers, fire hydrants, and light posts-to provide a reference map for the final 3D radar images.

The CART's 3D images clearly show the approximate size, shape and depth of buried pipes and other underground structures, such as trench walls or concrete footings. CART images also contain information about the material composition of buried structures (metal vs. plastic) and soil conditions. Special image processing software is used to extract and highlight utility lines and conduits.