Characterization of the shallow structure of El Tatio geothermal field in the Central Andes, Chile using transient electromagnetics

Highlights

• Identification of fluid pathways <200 m depth in El Tatio geothermal field.

• Electrical resistivity image in El Tatio using Transient Electromagnetic Method.

• Detection of shallow structural differences between El Tatio Middle and Upper basin.

Abstract

This study presents the first high resolution geophysical survey conducted in The El Tatio geothermal field, northern of Chile, focused on the detection of shallow subsurface structures and identification of ascending fluid pathways. TEM data was collected along 5 profiles crossing the two main geothermal basins (Upper and Middle Basin) to obtain an electrical resistivity model up to 200 m depth. The models show important structures that allowed us to improve the conceptual model of the field connecting these geophysical observations with the geology and the geochemistry of the area. We found a shallow (<60 m) high conductivity layer in all profiles. This layer was interpreted as a shallow aquifer of thermal water, which is probably the water supplier of surface manifestations. In the Upper Basin a main permeable zone allows the ascent of fluids from deep aquifers to the shallower one, and a structure that probably act as impermeable geological barrier that forces the fluids to ascend has been detected. In the Middle Basin fluid ascent zones are less clear than in the Upper Basin but it is possible to observe areas of lower resistivity that could be associated with higher permeability.

Introduction

The El Tatio geothermal field is located in the Chilean Altiplano at an elevation of 4200 m above sea level (Fig. 1). It is the largest geyser field in the southern hemisphere and the third largest in the world (approximately 10% of the geysers in the world), covering more than 30 km² and containing more than two hundred active geothermal manifestations such as: geysers, perpetual spouters, fumaroles, hot spring pools and mud-pools (e.g., Glennon and Pfaff, 2003; Munoz-Saez et al. 2018).

Systematic exploration of El Tatio began around 100 years ago with geological studies of the geothermal potential in the area (Tocchi 1923). However, today the internal structure and physical processes controlling fluid flow at El Tatio are still poorly understood. In the 1960's and 1970's 13 deep drillings between a depth of 571 m and 1816 m were conducted and the first local geological maps were obtained (Lahsen and Trujillo 1976). Other studies in the 1970's included geochemical analysis (Cusicanqui 1975; Giggenbach 1978) and geophysical imaging using Vertical Electrical Soundings (Lahsen and Trujillo 1976). In the last two decades, new studies were conducted to understand the large-scale dynamics of the geothermal area (e.g., Cortecci et al. 2005; Glennon and Pfaff, 2003; Lucchi et al. 2009; Mourguesr, 2017; Munoz-Saez et al., 2015, Munoz-Saez et al., 2018; Tassi et al. 2010; Cumming et al. 2002; Ardid et al. 2019; Figueroa 2019). Most of these previous studies were on a regional scale, providing insights into the subsurface water flow, possible geothermal heat source, and electrical resistivities at kilometer scale. However, they did not address the local subsurface structure of the El Tatio geothermal field.

The conceptual models of the geothermal field have been mainly based on geochemical data and information from the wells (e.g., Giggenbach 1978; Munoz-Saez et al. 2018). Geothermal wells (Fig. 1) identified two permeable zones at different depths forming geothermal aquifers confined by relatively impermeable rock formations (e.g., Cusicanqui 1975; Giggenbach 1978; Lahsen and Trujillo 1976). According to the geothermometry, the temperature of the deepest reservoir is ~230 °C, and the heat flow of the system is greater than 150 MW (Munoz-Saez et al. 2018). The isotopic signature of the thermal waters suggested that snowmelt from the mountains, located >15 km to the east of El Tatio at an elevation of >5000 m, recharge the aquifers through a deep fault system (Giggenbach 1978; Munoz-Saez et al. 2018). The surface water geochemistry and isotopic composition indicated that regional meteoric water interacts with the surrounding rock in the geothermal reservoir, before ascending to the surface as boiling water (e.g., Cusicanqui 1975; Giggenbach 1978; Cortecci et al. 2005; Munoz-Saez et al. 2018). The lack of tritium in the surface thermal waters indicates residence times of >60 years for the water in the reservoir (Cortecci et al. 2005; Munoz-Saez et al. 2018). Local meteoric water accumulated in a shallow aquifer (Munoz-Saez et al. 2018) reached the surface and created the existing wetlands in the area (Fig. 1). During the ascent, the thermal waters were mixed in different proportions with this local meteoric water (Giggenbach 1978). Those previous studies provided good explanations about the origin and evolution of the ascending fluids, however the subsurface structures that control the pathways of the fluids have not been well defined.

There have been limited geophysical imaging studies at El Tatio, and most that have been done focused on regional scales. Some magnetotelluric soundings were collected near the main volcanic centers in the area (Cumming et al. 2002; Figueroa 2019), and these data produced electrical resistivity images which verified the existence of deep aquifers as well as confining impermeable layers. The overall objective of this study is to improve the existing conceptual model using high resolution geophysical data from the main basins and identify the subsurface fluid pathways. It is the first geophysical survey which focuses on shallow depths (< 200 m) on a local scale around the geothermal manifestations and sinter hydrothermal deposits, which typically have a high electrical resistivity (Munoz-Saez et al. 2016).

The loop source Transient Electromagnetic method (TEM) is widely used for imaging shallow subsurface structures related to groundwater and salinization problems (e.g., Fitterman and Stewart 1986; Ruthsatz et al. 2018), sedimentary basin studies (Danielsen et al. 2003; Yogeshwar and Tezkan 2017), general geomorphological studies, and environmental and engineering studies (cf. Tezkan 1999; Goldman et al. 1994). TEM is also a reliable method for determining the subsurface structure and the composition in volcanic and geothermal areas (e.g., Martínez-Moreno et al. 2016; Arnason et al., 2010 ;Cumming and Mackie 2010; Kajiwara et al. 2000; Goto and Johmori 2011; Jousset et al. 2011; Descloitres et al. 1997; Dickey 2018; Bouligand et al. 2019; Gresse et al. 2017; Lévy et al. 2019).We collected dense TEM data along five profiles in order to derive the electrical resistivities of the subsurface beneath El Tatio geothermal field. The models are derived from applying 1D inversion techniques. Since the resistivity structure varies significantly along the profile, we also performed a 2D modeling analysis to verify the structures and the interpretation.