Consequences of Oil Spills on the Subsurface Resistivity of Soil in Biseni Community of Yenagoa Local Government Area of Bayelsa State in Niger Delta, Nigeria

Two Electrical Geophysical methods were used at Biseni community in Bayelsa state to determine the effects of Oil spills on the subsurface resistivity in the area. Five Schlumberger Vertical Electrical Soundings (VES) with a maximum Current electrode spread of 300.00m were carried out outside the polluted area while 2D Wenner array of 100.00m coverage was used at the spill location. The interpretation was done using computer programmes, IP2win for VES data and RES2DINV software for 2D data. The resistivity curve types obtained are QKA (VES 1), QA (VES 2), HKA (VES 3) HA (VES 4), and AKA (VES 5), to reveal heterogeneity nature of the subsurface in the area. Six continuous layers were identified from the 2D true resistivity distribution inversion. The first two layers, from VES 1 to VES 5, at depth (0.3m – 3.4m) have resistivity values ranging between 4.46 Ωm and 468.1 Ωm. Correspondingly, at the polluted location, at depth (1.25m – 3.75m), the resistivities recorded are between 11 Ωm and 47.46 Ωm. The next two layers (three and four) situated at depth interval (1.7739 m – 69.85 m) have minimum and maximum resistivity values of 6.38 Ωm and 344 Ωm respectively. Res2inv imaging revealed resistivity between 2.52 Ωm and 11.6 Ωm situated at 3.75m to 8.44m depth. The last three layers (five, six and seven) outside the location have 3.29 Ωm to 31443 Ωm resistivity between 13m and 69m at the substratum. The 2D imaging have 0.133 Ωm to 2. 0 Ωm at depth interval 8.44m to 15.9 m. It shows lower resistivity values at the polluted site and this reduction (0.133 Ωm – 2.52 Ωm) continues downward into the substratum. However, outside the polluted site, the resistivity increases downward in VES 2, 3, VES 4 and VES 5, with recorded values of 31443, 8203, 2051 and 261 Ωm respectively, while high and low resistivity alternate in VES 1. The resistivity values encountered at the Oil spills side range between 0.031 – 900Ωm, and 45.32 to 9347 Ωm at the control location. The depth at about 16.00m, marked the limit of contamination, as indicated by observed reduced resistivity but increased in values beyond this point.