Mapping depth-to-bedrock and shallow aggregate resources with airborne electromagnetics

Airborne electromagnetic (EM) results have been applied to estimating depth-to-bedrock and to mapping shallow aggregate resources in selected areas of Northern Ireland. A detailed low-level airborne survey of Northern Ireland was flown in 2005-6 (the Tellus Project) in which magnetic, gamma-radiation and EM data were acquired. Data were acquired with a range of frequencies and inverted into apparent resistivity. Electrical resistivity of rocks is a function mainly of porosity, pore-water salinity, saturation and clay content. ‘Apparent’ resistivity as measured remotely is a complex function of these variables, of the geometry of the measuring system, and of local geology; the thickness and resistivity of the superficial layer are prominent factors. Depth-to-bedrock may only be determined uniquely where other parameters are consistent, where some local borehole control exists and where there is a contrast in apparent resistivity between bedrock and cover.