Locating the Unlocatables: GPR

In the past, the standard practice of locating non-metallic utilities (i.e. PVC, other plastics, terracotta, and concrete) had to be done by gaining access to the utility through a manhole and feeding a metal rod down the pipe. Only then could the utility be tracked by traditional designating equipment. This labor-intensive process has been surpassed by the improved technology of Ground Penetrating Radar (GPR).

How GPR Works

Traditional electronic utility locating equipment relies on metal within cables, metal pipes, or having unobstructed access to the utility, allowing for the introduction of metal tracers into the target utilities for locating. GPR, however, does not require any metal within the utility or direct access to the utility for it to be located. Therefore, all non-metallic utilities can typically be detected using GPR.

GPR transmits radar waves into the ground to detect changes in soil reflectivity and density. The radio signals are transmitted at a specific frequency into the ground and are then received, interpreted, and displayed to create a complete map of soil conditions, and objects lying beneath the surface.

Uses for GPR

Besides locating and mapping underground utilities, GPR can also be used to:

• Detect voids and underground tanks
• Locate buried manholes, vaults, and valves
• Map septic drain fields
• Map cemeteries and grave sites

Locating with GPR in the Mid-Atlantic Region

GPR’s effectiveness is based on the soil type and condition. Optimal soil conditions produce the most accurate mapping. The best results occur in loose soils where utilities can be detected as far as 15 feet below the surface. Optimal soil conditions can be found in the Mid-Atlantic region mostly near major waterways and coastal areas, and in the Eastern Panhandle of West Virginia. In these areas, soils consist mainly of loose sand or shale, instead of tightly compacted clay found more throughout western parts of Virginia. Although soils composed mainly of clay are not considered optimal conditions for GPR, it can still be effective when used in conjunction with traditional methods to locate utilities.