Magnetotelluric Study in the Guarani Deep Thermal Aquifer System, NE Argentina

ALICIA FAVETTO, CRISTINA POMPOSIELLO, CRISTINA DAPEÑA, ANA CURCIO

Beijing , China

Workshop; IAGA WG 1.2 on Electromagnetic Induction in the Earth; 2008

The Guaraní Aquifer System (GAS) is one of the most important transboundary groundwater reservoirs covering an area of ca. 1.200.000 km2. It forms part of the Paraná and Chacoparanense sedimentary basin and it is shared by Brazil, Paraguay, Uruguay and Argentina. The geological units related to this system are the Triassic-Jurassic eolian and fluvial sandstones of the Piramboia and Botucatu Formations, and the Serra Geral Basalts which present clastic intercalations. This effusive Cretaceous complex covers the sandstones and provides a high-confinement degree. The research activities carried out in each country by scientists coming from different disciplines and institutions allowed them to define the main characteristics of this deep thermal aquifer system. The geological, hydrogeological, hydraulic, hydrochemical and thermal characteristics of the GAS vary throughout the whole basin and are controlled by variations in the depositional environment, in the structural evolution and in the residence time of water within the formations, which generate heterogeneities and point out the presence of compartments and, consequently condition the recharge mechanisms, storage, circulation and water quality. The object of our previous vertical electrical soundings (VES) was to estimate the geometry of the geological units, the depth of the top of the crystalline basement and to evaluate the existence of thermal waters in the southeastern part of the Entre Ríos province, Argentina. Considering that the main factor determining the presence of the sediments which contain the GAS, in this area, is the structural setting which is directly related to the depth of the crystalline basement top, new MT sites were collected during 2007-2008 along an N-S profile. Geoelectrical models coming from this profile and also from a previous deployment along an E-W profile allowed us to find the depth of the sediments - crystalline basement interface. Although, at the western part of Entre Ríos province, the depth of the crystalline basement obtained from the E-W MT model is around 4000 m and it should be difficult determined the prolongation of the Serra Geral Basalts because of fractures and clastic intercalations with high conductivity due to the presence of very saline waters. The results have been correlated with the existing thermal wells and significant variation in the basement depth is shown.