Computer model study of the effect of resistive topsoil on interpretation results of subsurface geoelectric layers' parameters

Abstract

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.