Magnetotelluric Investigation of Geoelectrical Dimensionality and Study of the Central Betic Crustal Structure, A

[eng] The magnetotelluric (MT) method is a geophysical method that allows characterizing the electric properties of the Earth materials. Within the frame of this method, this thesis is divided in two parts. The first part presents a series of methodological contributions to the dimensionality analysis of the MT data, emphasizing on the effects produced by errors. Thus, the WAL and the Bahr dimensionality methods, both based on the use of rotational invariants, have been studied. In the first, the method has been extended in order to consider data errors and the fact that geological structures can not always be approached to the ideal cases. In this sense, a program, WALDIM, has been created, which allows performing the dimensionality analysis in real data. With reference to the Bahr method, its limitations and inconsistencies have been analysed, and have been solved through the redefinition of the threshold limits of the invariants, and the use of an additional parameter, Q, also used by the WAL method. In the second part, the magnetotelluric study of the geoelectric structures of the Central Betics crust is presented. The data show a complex dimensionality, for which these have been modelled using a 3D model at crustal level. The model shows a good correlation between the resistivity structures and the surface geology, as well as the continuation of them at lower depths. Below the Internal Zone, at upper crustal level, the model shows a conductive body between the Nevado-Filabride complex and the Betics detachment level. This conductor is interpreted as a differentiated lithologic unit, formed by basic or low crustal rocks, containing a conducting mineral phase, such as graphite or pyrite.[cat] El mètode magnetotel·lúric és un mètode geofísic que permet caracteritzar les propietats elèctriques dels materials de la terra. Dins el marc d'aquest mètode, aquesta tesis es divideix en dues parts. La primera consisteix en una sèrie d'aportacions metodològiques a l'anàlisi de la dimensionalitat de les dades, posant èmfasi en els efectes produïts pels errors. D'aquesta manera, s'han estudiat els ja coneguts mètodes d'anàlisi de la dimensionalitat de WAL i Bahr, basats en invariants rotacionals. En el primer, s'ha ampliat el mètode per tal de considerar els errors de les dades i el fet que les estructures geològiques no sempre es poden relacionar amb els casos ideals. Així, s'ha creat el programa WALDIM, que permet realitzar l'anàlisi de la dimensionalitat per a dades reals. Pel que fa al mètode de Bahr, s'han analitzat les seves limitacions i inconsistències, que s'han resolt mitjançant la redefinició dels valors llindar, i la utilització d'un paràmetre adicional, Q, també utilitzat en el mètode de WAL. En la segona part, es presenta l'estudi magnetotel·lúric de l'estructura geoelèctrica de l'escorça de la zona central de la Serralada Bètica. Les dades mostren una dimensionalitat complexa, i es modelitzen mitjançant la creació d'un model 3D a nivell cortical. Aquest model mostra una bona correlació entre les estructures resistives i la geologia superficial, així com la seva continuïtat a majors profunditats. A nivell d'escorça superior, el model mostra un cos conductor a la Zona Interna, localitzat entre els complexes Nevado-Filàbride i el nivell de desenganxament de les Bètiques. Aquest conductor s'interpreta com una unitat litològica diferenciada, formada per roques bàsiques o del mantell inferior, que conté una fase mineral conductora, com pot ser grafit o pirita.[spa] El método magnetotelúrico es un método geofísico que permite caracterizar las propiedades eléctricas de los materiales terrestres. En el marco de este método, esta tesis se divide en dos partes. La primera consiste en una serie de aportaciones metodológicas al análisis de dimensionalidad de los datos, enfatizando en los efectos producidos por los errores. Así, se han estudiado los ya conocidos métodos de análisis de dimensionalidad de WAL y Bahr, basados en invariantes rotacionales. En el primero, se ha ampliado el método de manera que se consideren los errores de los datos, y el hecho que las estructuras geológicas no se correspondan siempre con casos ideales. De esta manera, se ha creado el programa WALDIM que permite realizar el análisis de la dimensionalidad en datos reales. En cuanto al método de Bahr, se han analizado sus limitaciones e inconsistencias, que se han solucionado mediante la redefinición de los valores lindar, y la adición de un nuevo parámetro, Q, también utilizado en el método de WAL. En la segunda parte, se presenta el estudio magnetotelúrico de la estructura geoeléctrica de la corteza de la zona central de la Cordillera Bética. Los datos muestran una dimensionalidad compleja, y se modelizan mediante la creación de un modelo 3D a nivel cortical. Este modelo muestra la buena correlación entre las estructuras resistivas y la geología superficial, así como su continuidad a profundidades mayores. A nivel de corteza superior, el modelo muestra un cuerpo conductor en la Zona Interna, localizado entre el complejo Nevado-Filábride y el nivel de despegue de las Béticas. Este conductor se interpreta como una unidad litológica diferenciada, formada por rocas básicas o del manto inferior, que contiene un mineral conductor tal como grafito o pirita

The role of electrical anisotropy in magnetotelluric responses: From modelling and dimensionality analysis to inversion and interpretation

The study of electrical anisotropy in the Earth, defined as the electrical conductivity varying with orientation, has experienced important advances in the last years regarding the investigation of its origins, how to identify and model it, and how it can be related to other parameters, such as seismic and mechanical anisotropy. This paper provides a theoretical background and a review of the current state of the art of electrical anisotropy using electromagnetic methods in the frequency domain, focusing mainly on magnetotellurics. The aspects that will be considered are the modelling of the electromagnetic fields with anisotropic structures, the analysis of their responses to identify these structures, and how to properly use these responses in inversion and interpretation. Also, an update on the most recent case studies involving anisotropy is provided

WALDIM: A code for the dimensionality analysis of magnetotelluric data using the Rotational Invariants of the Magnetotelluric Tensor

In the magnetotelluric (MT) method, the analysis of geoelectric dimensionality has acquired special importance in the last years, because numerical codes have made it possible to model and invert data using either one-dimensional (1D), two-dimensional (2D) or three-dimensional (3D) approaches. We present a FORTRAN code termed WALDIM to perform the dimensionality analysis of a set of MT data, according to the WAL rotational invariants criteria. These criteria are based on the possible annulment of the invariants of the MT tensor, which allow retrieval of as much information as possible from this tensor, without taking any a priori dimensionality assumption. When determining the dimensionality of real and therefore noisy data, two problems arise. The first is due to the data errors, which propagate into the invariants values, and therefore, to the determination of the dimensionality. The second is the fact that the invariants are rarely precisely zero, and the definition of a threshold is necessary. To solve these problems, WALDIM takes into account the data errors. Additionally, the dimensionality results can be grouped into frequency bands. Thus, we provide a software utility that allows providing a robust description of the dimensionality, and the parameters necessary for data correction prior to modeling. Given its completeness at analyzing the MT tensor for both individual and bands of frequencies, this code is meant to be a practical tool for MT data analysis

Dimensionality imprint of electrical anisotropy in magnetotelluric responses.

Dimensionality analysis of magnetotelluric data is a common procedure for inferring the main properties of the geoelectric structures of the subsurface such as the strike direction or the presence of superficial distorting bodies, and enables the most appropriate modeling approach (1D, 2D or 3D) to be determined. Most of the methods currently used assume that the electrical conductivity of individual parts of a structure is isotropic, although some traces of anisotropy in data responses can be recognized. In this paper we investigate the imprints of anisotropic media responses in dimensionality analysis using rotational invariants of the magnetotelluric tensor. We show results for responses generated from 2D synthetic anisotropic models and for field data that have been interpreted as showing the effects of electrical anisotropy in parts of the subsurface structure. As a result of this study we extend the WAL dimensionality criteria to include extra conditions that allow anisotropic media to be distinguished from 2D isotropic ones. The new conditions require the analysis of the strike directions obtained and take into account the overall behavior of different sites in a survey

Improving Bahr's invariant parameters using the WAL approach.

In the magnetotelluric technique, several methods exist to perform dimensionality analysis of the measured data using rotational invariants of the impedance tensor. Among these methods there is some dilemma on the different criteria established, which sometimes lead to non-equivalent interpretation

Geodynamic implications for the formation of the Betic-Rif orogen from magnetotelluric studies

Magnetotelluric data from the central Betics mountains (Spain) have been used to determine the electrical resistivity of the crust after a three‐dimensional (3D) interpretive approach. At shallow levels (3 km), the most striking and well‐resolved feature of the model is an upper‐middle crust conductive body, located at the core of the Internal Betics antiform. This approximately 14‐km‐thick body is interpreted as basic or ultrabasic rocks containing a conducting mineral phase. Its structural location above the sole thrust of the Betic orogen and beneath the Nevado‐Filábride complex confirms the presence of a major suture zone between this complex and the autochthonous Iberian plate. This suture may correspond to an ancient oceanic or transitional domain developed between Iberia and the Alboran Domain during the opening of the Tethys Ocean, partially subducted and closed during the development of the Betic orogen. The possible geodynamic scenarios for the Betics have been reconsidered, taking into account this new constraint

Three-Dimensional Magnetotelluric Characterization of the Travale Geothermal Field (Italy) Remote Sens. 2022, 14, 542.

The geoelectrical features of the Travale geothermal field (Italy), one of the most productive geothermal fields in the world, have been investigated by means of three-dimensional (3D) magnetotelluric (MT) data inversion. This study presents the first resistivity model of the Travale geothermal field derived from derivative-based 3D MT inversion. We analyzed MT data that have been acquired in Travale over the past decades in order to determine its geoelectrical dimensionality, directionality, and phase tensor properties. We selected data from 51 MT sites for 3D inversion. We carried out a number of 3D MT inversion tests by changing the type of data to be inverted, the inclusion of static-shift correction at some sites where new time-domain electromagnetic soundings (TDEM) were acquired, the grid rotation, as well as the starting model in order to assess the connection between the inversion model and the geology. The final 3D model herein presents deep elongated resistive bodies between the depths of 1.5 and 8 km. They are transverse to the Apennine structures and suggest a correlation with the strike-slip tectonics. Comparison with a seismic velocity model and well log data suggests a highly-fractured volume of rocks with vapor-dominated circulation. The outcome of this study provides new insights into the complex geothermal system of Travale

The North Maladeta fault (Spanish Central Pyrenees) as the Vielha 1923 earthquake seismic source: recent activity revealed by geomorphological and geophysical research.

The Spanish Central Pyrenees have been the scenario of at least two damaging earthquakes in the last 800 years. Analysis of macroseismic data of the most recent one, the Vielha earthquake (19 November 1923), has led to the identification of the North Maladeta Fault (NMF) as the seismic source of the event. This E-W trending fault defines the northern boundary of the Maladeta Batholith and corresponds to a segment of the Alpine Gavarnie thrust fault. Our study shows that the NMF offsets a reference Neogene peneplain. The maximum observed vertical displacement is ~ 730 m, with the northern downthrown sector slightly tilting towards the South. This offset provides evidence of normal faulting and together with the presence of tectonic faceted spurs allowed us to geomorphically identify a fault trace of 17.5 km. This length suggests that a maximum earthquake of Mw=6.5 ± 0.66 could occur in the area. The geomorphological study was improved with a resistivity model obtained at Prüedo, where a unique detritic Late Miocene sequence crops out adjacent to the NMF. The section is made up of 13 audiomagnetotelluric soundings along a 1.5 km transect perpendicular to the fault trace at Prüedo and reveals the structure in depth, allowing us to interpret the Late Miocene deposits as tectonically trapped basin deposits associated with normal faulting of the NMF. The indirect age of these deposits has been constrained between 11.1-8.7 Ma, which represents a minimum age for the elevated Pyrenean peneplain in this part of the Pyrenees. Therefore, we propose the maximum vertical dip slip rate for the NMF to be between 0.06-0.08 mm/a. Normal faulting in this area is attributed to the vertical lithospheric stress associated with the thickened Pyrenean crust

Volcanic monitoring of the 2021 LaPalma eruption using long‑period magnetotelluric data

Between September and December 2021, the frst subaerial volcanic eruption in the Canary Islands in 50 years took place on the island of La Palma. Since November 2021, we have been conducting a long-period magnetotelluric (MT) monitoring experiment at a site located 2.4 km east of the volcanic cone. Having continuously recorded data since then, the obtained dataset shows signifcant changes in resistivity over the fourteen months following the eruption: more than± 20% in apparent resistivity and± 2 degrees in phase. These temporal variations in electrical resistivity, recorded continuously using long-period MT during both the syn- and post-eruptive stages, have not been reported to date, making this dataset unique. Four estimated impedances have been selected as representatives of the major temporal changes observed and inverted to generate new 3-D resistivity models. The results provide novel key information on the spatiotemporal evolution of the subsoil’s electrical resistivity, enabling the characterization of a set of structures acting as preferred magmatic fuid pathways. Therefore, our study highlights the strong potential of MT as a volcanic monitoring tool and provides new insights about the evolution of the fuid pathways during the post-eruptive stage. These fndings enhance our understanding of the magmatic system and may contribute to volcanic hazard mitigation in the future

Imaging leachate runoff from a landfill using magnetotellurics: The Garraf karst case

Electrical and active source electromagnetic geophysical methods have been traditionally employed to approach and tackle environmental problems, such as those caused by landfills. However, since these problems are more consequential and cover broader areas, it is necessary to use deeper penetration methods, such as magnetotellurics. In the Garraf Massif (Catalan Coastal Ranges, NE Spain), an urban waste disposal landfill had been in operation from 1974 to 2006, during which more than 26 million metric tons of garbage had been deposited. This landfill overlies karstic terrain, thus principally impacting groundwater circulation. Previous electrical resistivity tomography profiles had partially imaged the infill but were not able to penetrate below the base of the original landfill. During 2019 and 2020 we performed a magnetotelluric study over the landfill and its surrounding with the goals of characterizing the electrical resistivity of the infill and below it. The 2D and 3D resistivity models confirmed the highly conductive nature of the leachate and allowed us to identify its presence below the landfill base, which we quantified with maximum thicknesses of 90 m. This proved that landfill leachate had filtered through the original impermeable layer, enhanced by the karstic drainage structure