Geophysical Characterization of Oil Spill Sites in Parts of Niger Delta Using Electrical Resistivity Tomography, Southern Nigeria

Abstract

Geophysical survey using 2D resistivity tomography was acquired to characterize two hydrocarbon impacted sites of Ikarama and Kalaba. In the impacted sites six (6) 2D resistivity imaging profiles were carried out to understand the resistivity distribution as basis for interpreting the depth and lateral extent of contamination. Additionally, two (2) 2D resistivity imaging profiles were also carried out in the un-impacted sites to serve as control.  Surface geophysical survey was acquired using ABEM SAS 1000 terammeter set with electrode constant separation of 6 step array system with a maximum electrode separation of five metres in a traverse of  100 meters.  The 2D resistivity imaging data was interpreted using Res2Div. The results indicated the distribution of electrical resistivity within the subsurface. The resistivity sections were displayed in shades of rainbow colours. The values for the models range from 1.05   the resistivity models show a general increase of resistivity with depth relative to the   control points.  The 2D resistivity models correlate well with the borehole logs as top soil, clay, sandy clay and sand. Also, some of the 2D models indicate increases in resistivity with depth. They also show interaction of water interplaying with the contaminant plume.  The resistivity range of 0.005  to 11.3  which occurred from the surface is interpreted as top soil. Immediately underlying this layer is a region of high resistivity with range of 9.4  to 78.1  which indicate petroleum contamination that may have seeped through the clay horizon to the sandy layer. This may be due to the fact that the invading crude oil may have exceeded the retention capacity of the porous medium hence allowing crude oil to spread to other zones of high hydraulic conductivity materials. In addition, it is also possible that   the clay bodies were not localized.    Superimposition of the  main profile generated to monitor petroleum contaminant and  the control profiles established a remarkably difference showing  the main profiles  increases of resistivity  with depth, while the resistivity of the control profiles decreases with depth and possibly with materials highly resistive at the top. The study established that biodegradation increases as the depth decreases. The characterization of high resistivity with decrease in depth of the impacted sites were due to oil spill contamination whereas, decrease in resistivity with respect to depth is agreeable to non-petroleum site having materials that were restive at the top. The study pointed out hydrocarbon contamination of the both sites between shallow depths of 3m to 7m which may infiltrate to change the composition of near surface aquifers in the locality. Therefore, more geophysical studies is recommended in this area.

Keywords: Electrical Resistivity Tomography, Oil spill, Geophysical Characterization

DOI: 10.7176/JEES/11-2-08

Publication date: February 28^th 2021