Efficient Large-scale Underground Utility Mapping Using a New Multi-channel Ground-penetrating Imaging Radar System

Ground-penetrating imaging radar (“GPiR”) is a new technology that combines standard GPR (Ground-Penetrating Radar) with accurate positioning and advanced signal processing to create three­ dimensional (3D) images of the shallow subsurface. These images can reveal soil conditions and buried infrastructure—such as utility lines and conduits—down to depths of about 2 to 3 m (in typical organic soils) with a resolution of centimeters. A commercial GPiR called the CART Imaging System, which was designed for mapping urban infrastructure, has been developed in collaboration between Witten Technologies, Malå Geoscience and Schlumberger. The Electric Power Research Institute (EPRI) sponsored research leading to the development of GPiR. The CART system uses a radar array consisting of 17 shielded antennas (9 transmitters and 8 receivers) that cover a 2 m swath on the ground with 16 bi-static GPR profiles. Each radar element in the array is a wideband bowtie antenna with a peak frequency of 200 MHz and a bandwidth extending from 50 MHz to 400 MHz. The array can collect data while moving at speeds up to about 1 km/h. During operation, a laser theodolite tracks the position of the array at all times. The system collects enough data in a single pass over the ground to form a 3D image beneath its track; multiple side-by-side passes are stitched together—using the positioning information provided by the theodolite and special processing and imaging algorithms—to create a seamless image of the subsurface. GPiR was first tested on a large scale in December 2000, in a pilot project for Consolidated Edison Co. of New York (ConEdison) that mapped a large underground area near the intersection of 149 Street, Southern Boulevard and Prospect Avenue in the south Bronx. Four nights of surveying covered an area of approximately 12,000 m (nearly 3 acres). The positions of surface features, such as curbs, street signs, valve and manhole covers, were also surveyed with the theodolite to provide a local reference grid. Final images were visualized with large-scale maps and electronic movies that scroll through the 3D data volume in depth slices and show the enormous complexity of the subsurface in large cities.