Structure of the Guaraní aquifer in the surroundings of the Uruguay river from magnetotelluric soundings

Abstract

We use wide band magnetotelluric soundings to infer the geological and structural surrounding of the Guarani Aquifer System (SAG) in an area of major exploitation. The studied region comprises both sides of the Uruguay river, between latitudes 30º 30’ and 31º 52’ South and longitudes 57º 00’ and 58º 22’ West. A total of 31 AMT and MT soundings were used for the geological interpretation, which were calibrated with 14 deep wells with lithologic information. Prior to a quantitative interpretation of the data, a distortion analysis of the impedance was carried out to define the dimensionality of the predominant geological structures in the zone. The results show that at high frequencies, larger than 1 Hz, the earth behaves as a stratified ground or one-dimensional (1D) earth, excepts at some of the sites where the observed distortion is associated to local geological structures. From the one-dimensional inversion of the impedance determinant of the data set we estimate the distribution of the main lithological units that conform the sedimentary filling in the zone of the Chaco-Paranense basin, including the SAG. The calibration of the obtained models and the assignation of the resistivity ranges of the main geological units were done using seven resistivity logs from deep wells available in the area. The results show that the Precambrian basement becomes deeper towards the west, reaching depths larger than 4 km in the Argentinian side of the studied area. The results also show the existence of a structure near and parallel to Uruguay river, that seems to correspond with a major regional fault, suggesting vertical displacements larger that 2 km. According to this interpretation the Uruguay river is a superficial expression of the deep regional fault having roughly the same azimuth. At frequencies below 1 Hz the ground turns out to be bi-dimensional with a regional strike angle of around 0º, which is consistent with the NS trend of the Uruguay river. Using this information a 2D inversion was performed along four profiles perpendicular to the river to contrast the results obtained using the more simple 1D approach. The 2D models obtained yield consistent results showing similar features and structure for the SAG obtained from the 1D inversion of the invariant of the impedance.