Geoelectrical characterization of Socompa lagoon area in the Andean Central Volcanic Zone from 3-D audiomagnetotelluric inversion

Highlights

• The Socompa Geothermal Prospect is located in the Andean Central Volcanic Zone.

• An audiomagnetotelluric survey was carried out in the Socompa Geothermal Prospect.

• A high conductive layer at 400–500 m and at least 200 m thick was determined by the 3D model.

• The main scenarios for the high conductivity layer are: a saline aquifer; a Mio-Piocene? clay cap.

Abstract

The Socompa stratovolcano is located in the Andean Central Volcanic Zone, where active volcanism turns it into an area of interest for geothermal resources. In the surroundings of the volcano, the Socompa Geothermal Prospect was carried out looking for thermal manifestations related to an active geothermal system. The results of these preliminary studies are not conclusive.

Here we present a geoelectrical model from a 3-D inversion of audiomagnetotelluric data acquired in an area of 16 km². It covers the topographic low where the Socompa lagoon and the thermal springs are located, in order to determine the reservoir of circulating geothermal waters. The most distinctive feature determined by the 3-D model is a high conductive layer (less than 10 Ωm) at a depth of about 400–500 m and at least 200 m thick. Due to the environment and the depth to which the conductive layer is found, there is more than one possibility to explain its presence. It could be due to a deep saline aquifer that would be hosting the circulating waters. But since the low topography could coincide with a cryptic caldera of Miocene - Pliocene(?) age, the high conductivity layer could also be explained by the presence of a clay cap developed during the period of activity. From the inversion of the audiomagnetotelluric data it was possible to carry out a geoelectrical characterization of the upper units present in the study area, as well as to delimit, in a first approximation, the depth to which the hydrothermal reservoir would be found.

Introduction

The Socompa is one of the largest stratovolcanoes of the Central Volcanic Zone (CVZ) of the Andes (see Fig. 1). The region is characterized by an active volcanic arc related to the subduction of the Nazca plate below the South American plate, in a high plateau environment, from semi-arid to hyper-arid weather conditions (Stern, 2004). Thermo-mechanical studies of the Andes state indicate that the region where the volcano is located shows the highest heat-flow and the shallowest depths of the Curie isotherm (Ibarra and Prezzi, 2019). The active volcanism makes the region attractive for the exploration of geothermal systems (e.g., Bissig et al., 2001; Tassi et al., 2009; McCoy-West et al., 2011; Lahsen et al., 2015; Munoz-Saez et al., 2018; Chiodi et al., 2019; Filipovich et al., 2020).

Previous studies in the Socompa area suggest favorable preliminary geological and hydrogeological features for a blind geothermal system characterized by heat source, related to the current magmatic system that feeds the Socompa volcano, reservoirs and seals within a possible calderic structure system, and water availability from different feeding zones and flow paths (Galliski et al., 1987; Seggiaro and Apaza, 2018; Lelli, 2018). The most recent studies were carried out within the framework of the Socompa Goethermal Prospect (SGP) which is located in the topographic low with mean altitude of 3500–3800 m.a.s.l. located between the southeastern flank of the volcano and the western boundary of the Caipe range, encompassing the Socompa lagoon. The main manifestations are the thermal springs in the Socompa lagoon and in Quebrada del Agua locality and pliocene rocks that are altered by the presence of circulating fluids on the Quebrada del Agua fault (see Fig. 1a) (Seggiaro and Apaza, 2018).

Electromagnetic methods are widely used for exploration and characterization of environments associated with geothermal systems (Arnason et al., 2000; Harinarayana et al., 2006; Farquharson and Craven, 2009; Amatyakul et al., 2015; Blake et al., 2016; Barcelona et al., 2019). In particular, the audiomagnetotelluric technique has been used to explore hydrogeological resources in geothermal environments due to its ability to detect water-bearing rocks in the subsurface (Blake et al., 2016; Siniscalchi et al., 2019). Several factors strongly influence the conductivity of the rock such as temperature, porosity, permeability, poral content and the presence of clay minerals (Spichak and Manzella, 2009). The conductivity of the fluid in the pore space is a factor that influences the resistivity of the rock. In turn, fluid conductivity generally increases with salt concentration, taking extreme values of 0.01 Ωm for hypersaline fluids with 25 wt% salt concentration (Bedrosian, 2007). On the other hand, in rocks with clay minerals, the mechanism that causes the high conductivity is the electrical double layer, that is formed at the interface of the clay mineral surface and the water. This additional conductivity contribution is called surface conductivity (Ussher et al., 2000). Besides, the effect of surface conductivity is generally more important when porosity or permeability is low (Spichak and Manzella, 2009). In turn, temperature is the main factor influencing mineralogical alteration.

This paper presents an AMT model of the first few hundred meters of the region between the Socompa lagoon and Quebrada del Agua. The main objectives are the geoelectrical characterization of the geological formations present in the topographic low, as well as the structures that facilitate the circulation of hydrothermal waters in order to determine the reservoir of such waters at depth.