Merging Regional Time and Frequency Domain Electromagnetic Data, Experiences from the Tellus Survey, Ireland

The Tellus Project is an airborne geophysical and geochemical mapping programme managed by the Geological Survey of Ireland. Blocks of airborne data are collected annually and then merged together with the aim of completing national coverage across Ireland. Both Frequency Domain (FDEM) and Time Domain (TDEM) electromagnetic data have been collected. One of the main aims of the survey programme is to produce seamless integrated data. Inverting the FDEM data together with conductivity depth transforms of the TDEM data have allowed successful merger of apparent conductivities at specified depths. Before inversion, the FDEM data were smoothed to minimise the impact of negative in-phase and quadrature values resulting from high magnetic susceptibility areas. In areas of lower quality data the level of confidence in the apparent conductivities derived from inverted FDEM may be limited. Therefore another merging methodology based on equivalent time / frequency relationships between the TDEM and FDEM data was investigated. Although not perfectly equivalent the result of merging these approximate grids provided good spatial resolution and maintained the character of the data. All collected data, corresponding technical reports and map results are made freely available from the project website www.tellus.ie.