An assessment of the ability to derive regional resistivity maps from geological mapping data

There is a requirement to understand the electrical resistivity structure of the near subsurface (i.e. the upper 10 m). This is the zone into which infrastructure is buried and electrical systems are earthed. Detailed resistivity surveys are carried out for site-specific purposes, but there is a lack of regional data. A synthetic resistivity map has been generated by assigning average intrinsic resistivity values to the superficial and bedrock geology and producing an average resistivity for the top 10 m using the superficial thickness as the weight. To test this approach the synthetic map has been compared with the measured resistivity arising from a high-frequency airborne electromagnetic survey over the Isle of Wight. Many general features of the synthetic and measured maps are in agreement, but some of the resistivity assignments are oversimplified. A revised synthetic map that takes into account the position in the landscape of the geological units and with revised resistivity ranges informed from the airborne survey has been generated that represents a good first approximation of the near-surface resistivity structure. A scheme for generating synthetic maps in the absence of measured airborne data is indicated.