Application of Transient Electromagnetic and Audio-Magnetotelluric Methods for Imaging the Monte Real Aquifer in Portugal

Fifty transient electromagnetic (TEM) and 15 audio-magnetotelluric (AMT) soundings were carried out in the west central part of Portugal over an area of approximately 40km(2) to characterize the geological formations associated with the Monte Real aquifer in order to (1) assess the spatial extent of the aquifer, (2) detect the zones of higher water productivity and (3) evaluate the aquifer salinization risk. The dimensionality and strike direction of the AMT profiles were studied using phase tensor analysis and the AMT data was inverted using a two-dimensional (2D) nonlinear conjugate-gradient algorithm. In addition, the TEM soundings were inverted using a one-dimensional (1D) Levenberg-Marquardt technique in order to obtain 1D layered-earth resistivity models. The TEM results reveal that the aquifer is not uniform in the study area; a more resistive zone is evident in the north and northeast of the investigated area where we expect to have higher productivity. On the other hand, the AMT models suggest that the aquifer basement is probably located at a depth of more than 800m. Based on the results of this investigation, no traces of saltwater intrusion were found for the Monte Real aquifer. Therefore, we assume that the risk of saltwater intrusion is small in the study area.