Application of GPR in assessing the stability of oil pipeline

Oil pipelines are normally buried at a shallow depth for protection against environmental hazard and pilferage. Because of their shallow depth of burial, they have the least known stability threat unless they pass over some hazardous region. In the present case with the crude oil pipeline in India, 42 km stretch (out of 300 km) passes through a coal mining belt where series of coal mines are still active. Due to shallow occurrence of good quality coal seam, pilferage due to illegal mining is quite common. Many of the old coal mines in this area were as shallow as at 20 m depth, most of them are now inundated and inaccessible. Incidences of mine fire in the exposed pits or subsidence over an old mining region has threatened the pipeline stability in the past by way of abnormal sag resulting in the opening of joints and leakage of oil. After a few such occurrences, it was desired to assess the pipeline stability by mapping the subsurface conditions around the pipeline through appropriate geophysical methods. In the present study, geophysical survey along selected using GPR was done using stepped frequency radar and the results were verified with 2D resistivity imaging survey. Survey results identified two vulnerable locations where the pipeline stability was under potential threat. Though shifting of the pipeline is the best alternative, proper ground reinforcement is recommended till alternate alignment is finalised.