Resistivity characterization of the Krafla and Hengill geothermal fields through 3D MT inverse modeling

Abstract

Krafla and Hengill volcanic complexes, located 300 km apart, are both known as hightemperature geothermal systems located within neo-volcanic zones of Iceland. This paper demonstrates the utilization of three-dimensional (3D) magnetotelluric (MT) inversions from three different inverse modeling algorithms, which leads to characterizing the electrical resistivity structure of geothermal reservoirs with a much greater level of confidence in accuracy and resolution than if a single algorithm was employed in the data interpretation. These are the first 3D MT inversions of a Krafla MT dataset. The inverted model of electrical resistivity is a classic example of a high-temperature hydrothermal system, with a highly resistive near-surface layer, identified as unaltered porous basalt, overlying a low resistivity cap corresponding to the smectite-zeolite zone. This layer is in turn underlain by a more resistive zone, identified as the epidote-chlorite zone, also called the resistive core, which is often associated with production of geothermal fluids. The electrical structure in the upper 1-2 km does not correlate with lithology but with alteration mineralogy. At the location of the IDDP-1 well, which encountered magma at 2.1 km depth, the resistivity image shows high resistivity, most likely due to the epidote-chlorite geology and/or the presence of deeper supercritical fluids. Two km northwest of the well, however, an intrusive low-resistivity feature is imaged rising from depth, and a plausible interpretation is that of a magma intrusion. One possible explanation for the magma encounter at the IDDP-1 well is the existence of pathways or fissures connected to the magma chamber and intersected by the well. The MT response to these magma pathways is not discernible in the data, perhaps because this magma volume is below the threshold of resolvability. The electrical resistivity structure of the Hengill geothermal area also reveals characteristic features of a high temperature geothermal system with two low-resistivity layers. The nature of the uppermost low-3 resistivity layer and the increasing resistivity below it is attributed to hydrothermal mineral alteration, while the nature of the deep low-resistivity layer, centered over the northeast, is not yet well understood. The geothermal system in the northeast area appears to be shallower than the system manifested in the southwest. 3D MT inversions of Krafla and Hengill data sets show that knowledge of the subsurface electrical resistivity contributes substantially to a better understanding of complex geothermal systems.

Resistivity characterisation of Hakone volcano, Central Japan, by three-dimensional magnetotelluric inversion

“…This temperature estimation supports our interpretation, because the smectite-rich zone or the transition zone from smectite to illite/chlorite generally occurs in the 100-250 °C temperature range and possesses both a low resistivity and a low permeability (e.g. Ussher et al 2000;Gasperikova et al 2015). Comparison of this resistivity structure with the distribution of earthquake swarms in the region (Yukutake et al 2017) highlights the good agreement between the upper limit of seismicity beneath the central cones and the bottom of the low-resistivity bell-shaped conductor (Fig.…”

“…Since resistivity is sensitive to temperature (including thermal alteration) and the presence of fluids, it is commonly applied to infer the structural setting in volcanic and geothermal environments (e.g. Nurhasan et al 2006;Kanda et al 2008;Komori et al 2013;Yamaya et al 2013;Gasperikova et al 2015;Seki et al 2015Seki et al , 2016Usui et al 2017). In our study area, we obtained magnetotelluric (MT) data in the dense site array (Ogawa et al 2011;Yoshimura et al 2012Yoshimura et al , 2013 to infer the structure of Hakone volcano, with a focus on shallow caldera Fig.…”

Contribution of the Paragenetic Sequence of Clay Minerals to Re-Examination of the Alteration Zoning in the Krafla Geothermal System

“…In recent decades, electrical and electromagnetic (EM) methods have been developed as high-temperature geothermal reservoir prospecting tools (e.g., [87][88][89][90][91][92]). These EM data, along with borehole data [90], served to create a general model of geothermal reservoirs by correlating temperature and resistivity measurements with hydrothermal alteration.…”

“…In recent decades, electrical and electromagnetic (EM) methods have been developed as high-temperature geothermal reservoir prospecting tools (e.g., [87][88][89][90][91][92]). These EM data, along with borehole data [90], served to create a general model of geothermal reservoirs by correlating temperature and resistivity measurements with hydrothermal alteration. Most high-temperature geothermal reservoirs within basaltic rocks, as is the case for the Icelandic context, present a similar 3D resistivity distribution, with the occurrences of a low resistivity anomaly at shallow depths (for temperatures ranging from 110 to 200 • C) underlain by a more resistive core (dominated by chlorite and epidote minerals and for which temperatures exceed 250 • C).…”

Deep geothermal resource assessment of the St. Lawrence Lowlands sedimentary basin (Québec) based on 3D regional geological modelling

“…Relatively small-scale 3D geothermal models, on the order of ≤100 km in diameters, are described in the literature with non-exhaustive examples from around the world including Denmark, Germany, Iceland, Italy, New Zealand, The Netherlands, the United States of America and Taiwan (e.g. Guglielmetti et al, 2013;;Chang et al, 2014;Gasperikova et al, 2015;Ratouis et al, 2016;Siler et al, 2016;Przybycin et al, 2017;Békési et al, 2020;Fuchs, 2020). On the other hand, regional or large-scale 3D models, on the order of >100 km diameters, are just starting to be used for the assessment of geothermal resources.…”