GPR grid survey was conducted on a busy intersection in New York City. The survey successfully located and differentiated between a PVC natural gas main and an underlying cast iron water line.

GPR can be a powerful tool for mapping the location and depth of metallic, non-metallic, plastic, concrete and asbestos-concrete utilities. Mapping with GPR to systematically cover an area can quickly reveal the distribution and character of multiple buried utilities. GPR surveys on regular grids are now recommended in Subsurface Utility Engineering (SUE) projects.

Plastic and cast iron pipes are not readily located by traditional methods. This site in New York City was one where the location of plastic gas mains and cast iron water lines were in question. Construction design and planning called for completion of Level B SUE mapping of potential conflicts with existing utilities.

GPR was selected as the method of investigation since it could detect both metallic and non-metallic underground utilities. To make a complete study, the locating team used a grid survey approach to create a full subsurface view of the critical area.

A Noggin® 250 MHz GPR in the SmartCart® configuration was used. The 250 MHz system provided the optimal trade-off between depth of exploration and target resolution. The SmartCart provided quick and easy maneuvering of the GPR. (Alternatively an LMX200™ GPR system could have been used to get the same result.)

Subsurface Reflections by Peter Annan