Efficient Large‐Scale Utility Mapping with Radar and Induction Arrays

A project sponsored by the United States Department of Transportation (U.S. DOT) has developed a new mobile geophysical system combining an array of broadband electromagnetic induction (EMI) sensors with an array of ultra-wideband ground-penetrating radar (GPR) antennas. This “dual-array system” was designed for mapping underground utility networks efficiently over large areas, but can also be useful in environmental surveying for applications such as leak detection and hazardous waste monitoring. The project was part of the Pipeline Safety Research and Development Program of the U.S. DOT Research and Special Programs Administration. Several utility companies, including Consolidated Edison Company of New York and Regional Water Authority of South Central Connecticut, participated in the project. The EMI array consists of 16 vector magnetometers (induction coils) with a flat frequency response from about 1 to 100 kHz. Signals from each sensor are recorded and digitized as time series, with a sampling rate of 1 MHz. The EMI sensors are arranged in two linear arrays, each consisting of 8 sensors with a spacing of 30 cm; the arrays are offset vertically by about 50 cm. The system works with two types of transmitters: “clamp-on” transmitters which can inject current at discrete frequencies onto individual pipes (by galvanic or toroidal clamps) and a 3-axis induction coil which rides “on-board” with the transmitters and operates over the same frequency range as the sensors. The GPR array, which is based on the commercial CART Imaging System (Birken et al., 2002), consists of 17 antenna elements in an arrangement that creates 16 independent radar channels (transmitter-receiver pairs). The GPR array system has two antenna sets: one set has a central frequency of about 200 MHz and a channel spacing of about 14 cm; the other, a central frequency of 400 MHz and a spacing of 8 cm. The positioning system is designed to allow surveying in arbitrary patterns. Each array is mounted on a trailer whose position is monitored by a laser surveying instrument as the array moves over the survey area. Special algorithms merge data from different passes of each array to create a regular data grid. The radar data are imaged into a 3D volume using standard synthetic-aperture seismic imaging techniques adapted for GPR. The EMI data are inverted using a parametric model that assumes currents in the subsurface flow mainly along a network of (possibly interconnected) pipes. Two large surveys have been conducted with the dual-array system. One survey in the spring of 2004 successfully mapped a complex network of subsurface water, electrical, gas and telecommunication lines in Connecticut for a local water utility company. This survey covered over 2000 sq m with radar traces on a 10 cm grid and EMI data on a 30 cm grid. A second survey done in the summer of 2004 mapped electrical lines emerging from an electrical substation in New York. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the RSPA or the U.S. Government.