Audiomagnetotelluric study at the hotsprings of the northern segment of the La Candelaria Range: preliminary results

BARCELONA, HERNAN; UNGARELLI, CARLO; FAVETTO, ALICIA; POMPOSIELLO, M. CRISTINA; PERI, VERÓNICA GISEL

Milan

Congreso; 86° Congresso Nazionale della Società Geologica Italiana, Arcavacata di Rende; 2012

WesternGeco GeoSolutions ? Integrated EM Center of Excellence, Milan, Italy

INTRODUCTION Geothermal fields usually develop complex systems in depth constrained by the geological framework, the deep fluid circulation associated to the fractures control and the geometry of the system. Commonly, superficial manifestations are related to these systems in intricate ways and not reveal the geometry and the magnitude of the phenomenon. The northern segment of the La Candelaria Range (LCR) is an example of these complex relations and shows several kinds of geothermal manifestations, including hyperthermal hot springs. Seggiaro et al. (1997) proposed a geothermal water circulation model based on infiltration of meteoric water in the upper highland areas where permeable rocks are exposed. Several discontinuities allow the conduction of water in depth where its temperature rises and emerges at lower topographic levels at the northern apex range. This theoretical system of water circulation appears to be plausible. However, no detailed data is available and very little is known about the deep configuration of the geothermal system at the hot springs zone. The aim of our work was to define the resistivity distribution of the northern segment of LCR. Particularly, the work is focuses on the hot springs zones in order to determine the geometry and extension of the geothermal system. For this purpose, a detailed distortion and dimensional analyses of the signal was carried out. Several bidimensional inversions were performed according with different profiles and a series of conductive anomalies were defined. GEOLOGICAL SETTING LCR is located between the Cordillera Oriental and Santa Barbara system morphotectonic units, northwestern Argentina. Regionally, is part of a N-S fault and thrust system evolved in response to the stress transference from Western Cordillera to foreland during the final stages of Andean deformation (Ramos, 1999). Also, LCR represent a thick-skinned deformation front which a Miocene-Pliocene main event (Mon, 2001). The stratigraphic column shows the basement deformation and describes a tectonic events sequence associated to the evolution of the cretaceous rift system until the thermal subsidence stage interrupted by Andean orogeny deposits (Iaffa et al., 2011). Locally, two anticlines are the major structures of the zone. These anticlines and the NW-SE, E-W inherited fractures are supposed to be the structural framework to all the geothermal system. AMT METHOD The audiomagnetotelluric method is based on the measure of high frequencies variations of the horizontal components of the magnetic and electric fields. A total of 30 AMT sites were sampled using a Geometrics STRATAGEM equipment during October of 2011. The frequency range used during the study was 10 Hz to 1000 Hz, but depending on the obtained curve features the frequency range was increased up to 7000 Hz in order to obtain a good detail in the first tens of meters deep. The geometric arrange of the sites was roughly an orthogonal mesh with a preferential E-W orientation, according to the geological features of the area. The sites were placed with an equidistance of 1Km when was possible but on the edge of the range the sampling distance was shorter to describe better the hot springs. The AMT signal was pre-processed and processed according to robust analysis using IMAGE software. Also, dimensional and distortion analysis was carried out. PRELIMINARY RESULTS Five bidimensional inversions were done according to four E-W and one roughly N-S profiles in order to characterize the resistivity distribution in depth. The inversion algorithm was developed by Rodi & Mackie (2001) and the grid included the topography. The adjustment of the resulting models were good and with a homogeneous distribution of the misfit among sites. A series of shallow conductive anomalies were identified at the four E-W profiles. The thickness of the anomalies decreases from north to south. Also, the shallow anomalies are surrounding the range, probably showing the topography control over the conductive zones. The N-S profile, which partially crosses the hot springs area is characterized by a shallow resistive medium (>20 Ωm) and a more conductive (~ 8 Ωm) geoelectric structure below 100 m depth. Also, a superficial conductive anomaly that continues in depth is highlighted in the model and is placed below the hot springs area. CONCLUSIONS We applied the AMT over the understudied hot springs field at the northern segment of LCR. The resistivity distribution was determined and shallow and deep conductive anomalies were identified. The distribution of the anomalies is in agreement with the hydrogeological features of the zone. Then, the results suggest that the shallow conductive anomalies should be carefully interpreted in a geological, hydrological and topographical framework. In addition, the deep conductive anomaly on the summit of LCR supports the involvement of potential geothermal source of the fluids. Additional geophysics studies are needed to investigate the geothermal system at more depth and determine its source and complete geometry. ACKOWLEDGEMENTS We thank to Geotermia Andina for providing financial support to improve the logistics. Also, we kindly thank E. Llambias for his invaluable help in the field.