A hidden toll of the collapse of the World Trade Center on September 11 caused extensive damage to the underground utility network that supports lower Manhattan. Sponsored by a grant from the Swedish government, a project using radar tomography to map below streets near ground zero was established underway to help with reconstruction of the network. In less than two months, a continuous 3D radar image down to a depth of about 6 feet was created beneath 320,000 square feet of lower Manhattan ith the CART Imaging System, a new type of ground penetrating imaging radar that uses an array of antennas to dramatically increase the density and efficiency of underground coverage.

The figure shows horizontal slices through the radar image at depths of 24 and 42 inches below street level near the southwest corner of the Trade Center quadrangle (see photos at top). Numerous underground features, including gas, electrical, sewer and water services, are clearly visible in these two high-resolution image slices, which cover roughly 10,000 square feet with a pixel size of about 3 inches. (Slices are created at 1-inch depth increments in the full image.) The images are being used to help locate underground facilities not clearly shown on maps, to provide "ground truth" to design engineers planning new installations, and to reduce the number of test pits needed to validate designs.

The project was a collaboration of City of New York Department of Design and Construction, ConEdison and the Urban Utility Center of Polytechnic University. NASA Jet Propulsion Laboratory (CalTech) has helped with the analysis of the underground images.