HTEM data improve 3D modelling of aquifers in Paris Basin, France

In Paris Basin, we evaluate how HTEM data complement the usual borehole, geological and deep seismic data used for modelling aquifer geometries. With these traditional data, depths between ca. 50 to 300m are often relatively ill-constrained, as most boreholes lie within the first tens of meters of the underground and petroleum seismic is blind shallower than ca. 300m. We have fully reprocessed and re-inverted 540km of flight lines of a SkyTEM survey of 2009, acquired on a 40x12km zone with 400m line spacing. The resistivity model is first "calibrated" with respect to ca. 50 boreholes available on the study area. Overall, the correlation between EM resistivity models and the hydrogeological horizons clearly shows that the geological units in which the aquifers are developed almost systematically correspond to relative increase of resistivity, whatever the "background" resistivity environment and the lithology of the aquifer. In 3D Geomodeller software, this allows interpreting 11 aquifer/aquitar layers along the flight lines and then jointly interpolating them in 3D along with the borehole data. The resulting model displays 3D aquifer geometries consistent with the SIGES "reference" regional hydrogeological model and improves it in between the boreholes and on the 50-300m depth range.