Integrated approach in evaluating subsurface corrosivity condition along a proposed gas pipe route at Obasanjo Farm, Obada Oko in Ogun State, Nigeria

 Abstract

In determining a suitable path and depth for immersing gas pipeline for power generation at Obada Oko, Ogun State Nigeria, a geophysical survey was carried out to assess the characteristics of the subsurface underlying formations for soil corrosivity. The corrosivity of the soil was ascertained by using the soil resistivity values, soil moisture content, and soil pH values. Five Wenner electrical resistivity tomography (ERT) traverses were carried out with twenty soil samples and twenty-four Schlumberger vertical electrical sounding (VES). The result reveals an asymmetric resistivity deviancy along the proposed gas pipeline route, which indicates a mixture of sandy clay to clayey sand deposition laterally and vertically. The soil pH value reveals that the study area was slightly acidic to alkaline and high soil moisture content was observed at traverse four due to its proximity to the Ogun River. The electrical resistivity model reveals that a suitable depth of buried gas pipe must be within the depth of 2.5 to 3.5 m at Traverse (1, 2, and 3) and a depth of 3.5 to 5 m at Traverse (4 and 5). The ERT model reconciles with the soil moisture content at all the traverses chaperoned with the soil pH test. It was concluded that corrosion risk to buried metallic structures at the study zone is almost non-existent and the subsurface is highly competent, yet without proper maintenance, external coating, and cathodic protection, the immersion of gas pipeline could eventually fail.