Traditionally, subsurface pipelines such as sewerlines and storm water drains have been inspected using manual access. This involves a person entering the pipeline system and carrying out a manual inspection of pipe walls. However, this method reveals only surface defects in the pipe walls and gives no information on defects in the surrounding soils. This method also has inherent dangers and health risks for the person carrying out the inspection.
Remote Controlled Closed Circuit Television (CCTV) has also been employed in inspecting subsurface pipelines. This method involves the use of a small camera which is mounted on the end of along flexible cable. The camera is placed into the pipeline through an access opening and is then remotely controlled from the surface. This method removes the dangers involved in a person entering the pipeline, however, this method again reveals only surface defects in the pipe walls and gives no information on defects in the surrounding soils.
In order to inspect the bedding of a subsurface pipeline and to detect defects in the surrounding soils, more elaborate techniques have been employed such as ground probing radar (GPR) and seismic methods. The GPR systems consist of a transmitting antenna emitting electromagnetic radiation (le: generated by an oscillator), a receiving antenna and an energy detecting device, or receiver. A portion of the transmitted signal is intercepted by a reflecting object, such as the wall of the pipeline, and is reradiated in all directions. The energy reradiated in the back direction is collected by the receiving antenna and delivered to a receiver, where it is processed to detect the presence of the pipeline. The time taken for the radar signal to travel through the pipeline and back is measured. Defects in the soil surrounding the pipeline, which can result in deformation or collapse of the pipeline, are detected by using the time measurement and known soil characteristics, and comparing this information to site drawings.
Seismic methods, including techniques such as tomography, measure the velocity and refraction of seismic waves in a manner similar to the electromagnetic radiation measurements of GPR. However, seismic methods are based on long wavelengths with a resulting reduction in resolution. Further, both the GPR and seismic methods require complex equipment and processing which results in low productivity and unacceptably high costs.