Continual post-construction checks on dam embankment are inevitable to forestall the possible catastrophes associated with its structural failure. In view of this, electrical resistivity (ER) surveys involving vertical electrical sounding (VES) and dipole-dipole profiling were conducted along the Oba dam embankment as a part of routine deformation monitoring for any possible seepage(s) around the dam embankment. Thirty-one vertical electrical soundings and thirty-five dipole-dipole horizontal profiling data were carried out on a 350 m long (west-east trending) traverse established along dam embankment. The iterated VES curves were mainly KH-type suggesting that Oba dam embankment is underlain by four geoelectric layers; topsoil (caprock), fill (granite materials), core (plastic-clay/wet clay) and basement bedrock with resistivity values range of between 53-249 Ωm, 205-1084 Ωm, 15-124 Ωm and 281-1375 Ωm, respectively. The dipole-dipole 2-D resistivity structure along the dam embankment showed no evidence of structural distress/weakness within the underlying sequences. Therefore, by integrating results obtained from geoelectric section and 2D resistivity structure based on the layer resistivity/ lithological composition of the core materials (15-124 Ωm/plastic clay/clay/sandy clay), Oba earth-fill dam embankment integrity was good to very good indicating no seepage zone(s).

This paper investigates the effect of resistive topsoil (upper layer) on interpreted subsurface layers' geoelectric parameters. Five hypothetical computer geoelectric models were employed for this study. Model A is a three-layered model with constant thicknesses and varied topsoil resistivity of 100 ohm-m to 2500 ohm-m. Model B is a threelayered model with constant resistivities and varied topsoil thicknesses from 2 to 12 m. Model C is a four-layered model with constant thicknesses and varied topsoil resistivities from 250 to 2500 ohm-m. Model D is a four-layered model with constant thicknesses and varied second layer resistivities from 1000 to 10000 ohm-m. Model E is a four-layered model with constant resistivities and varied second layer thicknesses from 2.0 to 20.0 m.A total of forty theoretical Vertical Electrical Sounding (VES) curves of the A, H, HA, and KHtype were generated and interpreted using the conventional partial curve matching method and computer assisted 1-D iterative forward modeling with the RESIST 1.0 (Vander-Velpen, 1988) software. Statistical analysis of percentage deviations between model and final interpretation parameters was carried out.Results obtained from Model A show insignificant overestimated and underestimated second layer geoelectric parameters. Results obtained from Model B show significant overestimated second layer geoelectric parameters -10.7% to 187.5% and 13% to 204% for percentage resistivity and thickness deviation respectively. Results obtained from Model C show highly overestimated second layer thicknesses and third layer resistivities -124% to 238% second layer percentage thickness deviation and 96% to 334.7% third layer percentage resistivity deviation. Results obtained from Model D showed highly overestimated third layer geoelectric parameters -66.30% to 300.4% and 68% to 316% for percentage resistivity and thickness deviation respectively. Results obtained from Model E show extremely overestimated third layer geoelectric parameters -183% to 800.7% and 182% to 876% for percentage resistivity and thickness deviation respectively.The study concludes that the VES technique can yield abnormally high layer parameters for subsurface layers overlain by a resistive topsoil or near surface layer.