Geophysical Methods for Environmental Baseline Surveys in Oil and Gas Operations in Ondo State

This study examines the application of geophysical methods in environmental baseline surveys conducted before oil and gas operations in Ondo State, South West Nigeria. Environmental baseline surveys are legally required under the Nigerian Environmental Impact Assessment Act and NOSDRA regulations before hydrocarbon operations commence, providing pre-disturbance characterisation of environmental conditions. Geophysical methods can contribute important baseline data on soil contamination background, subsurface water quality, and soil stability that conventional environmental auditing often neglects. This study reviews 12 environmental baseline survey reports produced for oil and gas operations in the Ondo State onshore oil fields (Ondo marginal field cluster) from 2019 to 2023. Geophysical method application, data quality, and regulatory compliance are assessed against DPR (NUPRC) and NESREA environmental survey requirements. Field validation checks of subsurface baseline characterisation are conducted at three operational sites. Findings reveal that geophysical methods are used in only 4 of 12 reviewed baseline surveys, with the remainder relying solely on soil and water sampling with no subsurface characterisation. Where geophysical surveys are included, resistivity profiling and EM ground conductivity measurements are used to characterise pre-operational contamination background and water table depths. Two sites show geophysical evidence of pre-existing hydrocarbon contamination from earlier unrecorded spills that was inadequately documented in the baseline surveys. The study concludes that geophysical baseline characterisation should be mandatory for all Ondo State oil and gas operational sites and recommends minimum geophysical survey requirements in NUPRC environmental compliance regulations.

Keywords: environmental baseline, geophysical survey, oil and gas, Ondo State, regulatory compliance.