This thesis investigates geo-electric fields in the United Kingdom with particular regard to Geomagnetically Induced Currents (GIC) in the Scottish Power electricity transmission network (SPTN). The joint spectral characteristics of Scottish Power GIC and Eskdalemuir magnetic observatory data are analysed, and GIC are shown to be coherent with magnetic field variations over the period range 2-1100s. A bi-variate transfer function model of the physical link between magnetic field variations and GIC demonstrates that long-period (>200s) induction makes a first order contribution to the observed GIC at one SPTN site, and dominates the response at another. Thin-sheet modelling at a period of 750s is used to explore the relative influence of three factors on the size and spatial distribution of the calculated electric field: (i) the contrast in conductance between the sea and the land; (ii) variations in conductance due to sea depth; (iii) lateral variations in conductance representative of those in the geographic area occupied by the SPTN. The modelling suggests that a `coast-only' model (i) will over-predict electric field magnitudes in the SPTN region by a factor of 2-5 in comparison with model (iii). Distortion analysis of Magnetotelluric (MT) data at a period of 750s acquired over numerous field campaigns reveal pervasive galvanic distortion of the electric field in the SPTN region. GIC transfer functions of one site are consistently interpreted as proxy MT responses, and it is shown that galvanic distortion of the electric field modifies significantly the GIC amplitude response. A prototype model of the SPTN developed by the British Geological Survey and the Finnish Meteorological Institute is used to calculate GIC. It is shown that neglect of lateral variations of conductivity may lead to false conclusions about the direction of the external electric field that maximises GIC. Time derivatives of the Eskdalemuir horizontal magnetic field are used as an index of GIC activity, and to select events which may have led to large GIC in the time period (1983-2000) prior to the monitoring of GIC by Scottish Power. Backwards-prediction using the GIC transfer functions and observatory magnetic data suggests that GIC at the Scottish Power monitoring sites have amplitudes less than approximately 30A.

I implemented and tested the method of Non-Linear Conjugate Gradients (NLCG) to invert magnetotelluric (MT) and radiomagnetotelluric (RMT) data in two dimensions. The forward problem and the objective function gradients were computed using finite-difference methods. The NLCG algorithm was applied to three field data sets to test the performance of the code. It was then compared to the inversion techniques of Occam and damped Occam considering the quality of the output resistivity models and the computation times. The implemented code was further investigated by testing two line search techniques to reduce the objective function along a given search direction. The first line search procedure was constrained to the first Wolfe condition, leading to a rather inexact line search. The second, more thorough line search, was additionally constrained to the second Wolfe condition. Three preconditioners were applied to the NLCG algorithm and their performance was analysed. The first preconditioner was set to the diagonal of the approximate Hessian matrix and updated every 20-th iteration. Preconditioners two and three were updated with the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm using the identity matrix and the diagonal of the approximate Hessian matrix as start preconditioners, respectively. The tests showed that the method of NLCG is more efficient pertaining to computation times compared to the Gauss-Newton (GN) based techniques (Occam and damped Occam). For the two smaller data sets that were inverted, the NLCG inversion was two to four times faster than Occam and damped Occam. For the larger data set, the NLCG inversion converged more than one order of magnitude faster than the GN based inversion techniques. This is because GN methods require to evaluate the entire sensitivity matrix to update the model, whereas NLCG only needs to compute a matrix-vector product of the Jacobian. Moreover, expensive operations such as matrix products and direct inversions of linearised systems are avoided by NLCG. A limitation of the NLCG algorithm is that it is prone to converge to local minima due to the fixed Lagrange multiplier that is used in the penalty function. Occam inversion, which determines the optimal Lagrange multiplier as part of the inversion, did not show such problems. The line search tests of the NLCG algorithm showed that an inexact line search yields higher convergence per CPU time than a more exact line search. In accordance to previous studies, preconditioning accelerated the convergence of the NLCG algorithm considerably. The preconditioners updated with the BFGS algorithm achieved highest convergence. Choosing the identity matrix as a start preconditioner led to fast but unstable convergence. The reasons for that could not be determined completely. Taking the diagonal of the approximate Hessian as a start preconditioner instead of the identity matrix led to slower convergence for most of the inversion tests, but convergence could be stabilised. All the tests performed within this project led to a robust implementation of the NLCG algorithm. A default set-up pertaining to line search and preconditioning could be established. However, the NLCG set-up can be adjusted by the user to improve convergence for a specific data set. This makes the algorithm implemented in this thesis more flexible than previously introduced NLCG codes. Preconditioning can certainly still be improved with further tests. Moreover, a future project will be to extend the 2D code to 3D, where NLCG should perform especially well, because the number of model parameters is usually higher in 3D.

No description available.

The Skellefte District (SD) is one of the richest metallogenic mining areas in Sweden. The main deposits consist of volcanic-hosted massive sulphides (VHMS) rich in zinc, copper, lead, gold and silver, that have been explored and mined for more than a century. Considering that technological advancements allow deeper mining, and that today new discoveries rarely occur, renewed efforts are now directed at locating targets at greater depths.   Thus, current exploration strategies need to be adapted, and a better understanding of regional scale structures is necessary. To address these questions the project VINNOVA 4D modeling of the Skellefte District was launched. Its main purpose is to unravel the regional structures and tectonic setting of the SD. To accomplish this, new geological and geophysical data have been acquired in two key localities. This thesis presents the contribution from 2D and 3D inversion of magnetotelluric (MT) data and seismic reflection data. The main findings include: conductive hydrothermally altered zones within the otherwise resistive rocks of the Skellefte Group, the depth extension of early and postorogenic intrusions, prominent shear zones in the central part of the district, and enhanced reflectivity and conductivity at the base of the Skellefte Group throughout the SD. Even though the application of these methods is challenged by the complex geological setting of the SD, it is shown that after a careful processing and analysis of the data, they are able to provide a robust image of the deep subsurface. Additionally, the combination of reflection seismics and MT has proved to be a powerful tool for hypothesis testing and to develop the general understanding of the configuration and history of the SD. Furthermore, two 3D inversion models of MT data are presented and compared with the results of standard 2D determinant inversions. The 3D procedure shows significant improvements in data fit and is able to constrain better the observed model features. Although 3D inversion of MT data is not yet a run of the mill scheme and issues like model assessment and galvanic distortion effects need to be further addressed, results from complex environments with areal coverage, are already superior to those from 2D inversions. / Skellefteåfälten är ett av de viktigaste malmdistrikten i Sverige. Malmkropparna består av vulkaniskvärda Massiva Sulfider (VHMS) rika på Zink, Koppar, Bly, Guld och Silver, och har utforskats och brutits i mer än ett sekel. Med tanke på att de senaste tekniska framstegen tillåter djupare brytning, och att nya upptäckter är ovanliga idag, riktas nya ansträngningar mot att lokalisera malm på större djup. Aktuella prospekteringsstrategier måste därför anpassas, och en bättre förståelse av regionala strukturer är nödvändig. För att lösa dessa frågor lanserades projektet VINNOVA 4D modeling of the Skellefte District. Dess främsta syfte är att utreda de regionala strukturerna och det tektoniska läget av Skelleftefältet. För att uppnå detta, har nya geologiska och geofysiska data insamlats vid två viktiga platser i distriktet. Denna avhandling presenterar bidrag från inversionsmodellering i 2D och 3D av magnetotelluriska (MT) data samt resultaten av en reflektionsseismisk profil. De viktigaste resultaten är: bra ledande hydrotermiskt förändrade zoner inom de annars resistiva bergarterna i Skellefte-gruppen, djupet till tidiga och postorogeniska intrusioner, framstående skjuvzoner i den centrala delen av området, och ökad reflektionsförmåga och konduktivitet vid basen av Skellefte-gruppen i hela fältet. Även om tillämpningen av dessa metoder utmanas av fältens komplexa geologiska läge, visas det efter en noggrann bearbetning och analys av data att de ger en robust bild av den lite djupare berggrunden. Dessutom har kombinationen av reflektionsseismik och MT visat sig vara ett kraftfullt verktyg för hypotesprövning och för att utveckla den allmänna förståelsen av Skelleftefältet och dess historia. Därutöver presenteras två 3D inversionsmodeller av MT data och jämförs sedan med resultaten från 2D determinantinversioner. 3D tekniker visar betydande förbättringar av datapassform och begränsar observerade anomalier bättre. Även om 3D inversion av MT data ännu inte är en vanlig teknik och frågor som modellbedömning och galvaniska distorsionseffekter måste behandlas ytterligare, är resultat från komplexa miljöer med lagom yttäckning redan överlägsna. / VINNOVA 4D modeling of the Skellefte District

Neste trabalho foram abordados dois temas geofísicos visando o avanço no conhecimento da litosfera do sul do cráton São Francisco. O primeiro tema teve como objetivo a proposição de um método alternativo para a definição de bordas de placas litosféricas, utilizando dados gravimétricos. Utilizou-se o mapa da primeira derivada vertical da anomalia gravimétrica Bouguer continuada para cima à altura de 100 km. Resultados crustais, ao longo de um perfil de sísmica de refração profunda, localizado no estado de Goiás e a modelagem gravimétrica 2-D de um perfil localizado entre os estados de São Paulo e Minas Gerais foram utilizados para calibrar os resultados da derivada vertical. Recentes dados geoquímicos, petrológicos e de tomografia sísmica reforçam o resultado de que, nas bordas sul e oeste, a placa São Franciscana estende-se sob a Faixa Brasília e parte da bacia do Paraná. O segundo tema foi a interpretação de uma anomalia gravimétrica Bouguer positiva, aproximadamente circular com amplitude da ordem de 25 mGal. Nesta mesma região existe uma anomalia positiva do geóide, também circular, com raio de aproximadamente 250 km e uma amplitude de +7 m. O método sísmico da função do receptor não mostra a existência de afinamento crustal, eliminando assim, uma das possíveis causas da anomalia gravimétrica. Perfis magnetotelúricos radiais à anomalia gravimétrica indicam a presença de uma região no manto com resistividade elétrica mais baixa (< 100 .m), com origem a 200 km de profundidade e estendendo até a crosta inferior. Os resultados da inversão gravimétrica 3-D do alto gravimétrico indicam que a crosta inferior mais densa atinge uma espessura de 12 km, se assumirmos um contraste de densidade de +50 kg/m3. Coincidentemente com a região de crosta inferior mais densa, as sondagens magnetotelúricas indicam uma crosta eletricamente mais condutora, reforçando a existência de um underplating magmático. A impregnação de material derivado do manto, na base da crosta, é evidenciado em superfície, pela presença de diversos diques de basaltos toleíticos de composição similar aos basaltos da parte norte da Bacia do Paraná (alto TiO2) de idades do Cretáceo Inferior. Os diques toleíticos estão distribuídos por toda região sul do cráton. O manto litosférico com menor resistividade elétrica estende-se até profundidade de 200 km, sugerindo que o manto litosférico do sul do cráton São Francisco sofreu um rejuvenescimento composicional e leve aumento de densidade (50 a 70 kg/m3), responsável pelo alto do geóide. A diminuição de resistividade elétrica no manto, abaixo de profundidade de 100 km, é mais facilmente explicada pela presença de fundidos carbonatíticos no manto superior, provenientes de partes mais profundas do manto e resultantes de um menor grau de fusão. O magmatismo carbonatítico foi anterior ao magmatismo toleítico, uma vez que dados petrológicos e geoquímicos mostram que os diques toleíticos do Cretáceo Inferior na região do Espinhaço apresentam de 5% a 10% de carbonatitos em sua composição. A forma semi-circular do limite sul erosional do Grupo Bambuí deve ser resultado da erosão dos sedimentos mais antigos, devido ao soerguimento da litosfera, durante o magmatismo carbonatítico. / This thesis comprises of two parts in which two geophysical studies are described improving the knowledge of the lithosphere of the south São Francisco craton. In the first part, we propose an alternative method for defining the limits of lithosphere plate using gravity data. We used the first vertical derivative of Bouguer anomaly upwarded to 100 km height. Crustal results along a deep refraction seismic profile in the Goiás state together with a 2-D gravity model along a profile between Minas Gerais and São Paulo states were used to calibrate the vertical derivative results. Recent geochemical, petrological data and seismic topography data reinforce the gravity results that the southern and western parts of the São Francisco plate extends under parts the Brasília Belt and parts of the Paraná basin. In the second part, the interpretation of a positive Bouguer gravity anomaly, circular shaped and + 25 mGal amplitude gravity anomaly is presented. This gravity anomaly coincides with a positive, circular shaped, 7 m amplitude geoid anomaly with 250 km of radius. Receiver function results do not show a crustal thinning, excluding this possibility as the cause of gravity high. Deep magnetotelluric soundings along two profiles radial to gravity anomaly indicate a region in the mantle with lower electrical resistivity (< 100 .m) from the depth of 200 km and extending upward and reaching the lower crust. The residual Bouguer anomalies are inverted using a 3-D algorithm and indicate that the denser lower crust reaches 12 km of thickness for + 50 kg/m3 of density contrast. Coincident with a denser lower crust, magnetotelluric soundings reveal a low electrical resistivity crust, reinforcing the presence of magmatic underplating. At the surface, the contamination of magmatic material in the lower crust is supported by the presence of basaltic tholeiitic dike swarms similar, in composition, to the basalts of northern Paraná basin (high TiO2) of Early Cretaceous age. Tholeiitic dikes swarms are distributed throughout the southern region of the craton. Lithospheric mantle with lower electrical resistivity reaches 200 km of depth, indicating that the lithospheric mantle in the southern São Francisco craton underwent compositional rejuvenation accompanied by a mild increase in density (+ 50 to + 70 kg/m3) responsible for the positive geoid anomaly. The decrease of electrical resistivity in the mantle at depths beyond 100 km is easier explained by the presence of carbonatitic melts in the upper mantle, derived from a low degree of partial melt of a metasomatized deeper mantle. The carbonatitic magmatism was prior to tholeiitic magmatism since petrological and geochemical data show that the Early Cretaceous Espinhaço tholeiitic dikes contain 5 to 10% of carbonatites in their composition. The circular shaped erosional limit of the Bambui Group may have been produced by lithosphere uplift at the time of carbonatitic magmatism.

Neste trabalho foram abordados dois temas geofísicos visando o avanço no conhecimento da litosfera do sul do cráton São Francisco. O primeiro tema teve como objetivo a proposição de um método alternativo para a definição de bordas de placas litosféricas, utilizando dados gravimétricos. Utilizou-se o mapa da primeira derivada vertical da anomalia gravimétrica Bouguer continuada para cima à altura de 100 km. Resultados crustais, ao longo de um perfil de sísmica de refração profunda, localizado no estado de Goiás e a modelagem gravimétrica 2-D de um perfil localizado entre os estados de São Paulo e Minas Gerais foram utilizados para calibrar os resultados da derivada vertical. Recentes dados geoquímicos, petrológicos e de tomografia sísmica reforçam o resultado de que, nas bordas sul e oeste, a placa São Franciscana estende-se sob a Faixa Brasília e parte da bacia do Paraná. O segundo tema foi a interpretação de uma anomalia gravimétrica Bouguer positiva, aproximadamente circular com amplitude da ordem de 25 mGal. Nesta mesma região existe uma anomalia positiva do geóide, também circular, com raio de aproximadamente 250 km e uma amplitude de +7 m. O método sísmico da função do receptor não mostra a existência de afinamento crustal, eliminando assim, uma das possíveis causas da anomalia gravimétrica. Perfis magnetotelúricos radiais à anomalia gravimétrica indicam a presença de uma região no manto com resistividade elétrica mais baixa (< 100 .m), com origem a 200 km de profundidade e estendendo até a crosta inferior. Os resultados da inversão gravimétrica 3-D do alto gravimétrico indicam que a crosta inferior mais densa atinge uma espessura de 12 km, se assumirmos um contraste de densidade de +50 kg/m3. Coincidentemente com a região de crosta inferior mais densa, as sondagens magnetotelúricas indicam uma crosta eletricamente mais condutora, reforçando a existência de um underplating magmático. A impregnação de material derivado do manto, na base da crosta, é evidenciado em superfície, pela presença de diversos diques de basaltos toleíticos de composição similar aos basaltos da parte norte da Bacia do Paraná (alto TiO2) de idades do Cretáceo Inferior. Os diques toleíticos estão distribuídos por toda região sul do cráton. O manto litosférico com menor resistividade elétrica estende-se até profundidade de 200 km, sugerindo que o manto litosférico do sul do cráton São Francisco sofreu um rejuvenescimento composicional e leve aumento de densidade (50 a 70 kg/m3), responsável pelo alto do geóide. A diminuição de resistividade elétrica no manto, abaixo de profundidade de 100 km, é mais facilmente explicada pela presença de fundidos carbonatíticos no manto superior, provenientes de partes mais profundas do manto e resultantes de um menor grau de fusão. O magmatismo carbonatítico foi anterior ao magmatismo toleítico, uma vez que dados petrológicos e geoquímicos mostram que os diques toleíticos do Cretáceo Inferior na região do Espinhaço apresentam de 5% a 10% de carbonatitos em sua composição. A forma semi-circular do limite sul erosional do Grupo Bambuí deve ser resultado da erosão dos sedimentos mais antigos, devido ao soerguimento da litosfera, durante o magmatismo carbonatítico. / This thesis comprises of two parts in which two geophysical studies are described improving the knowledge of the lithosphere of the south São Francisco craton. In the first part, we propose an alternative method for defining the limits of lithosphere plate using gravity data. We used the first vertical derivative of Bouguer anomaly upwarded to 100 km height. Crustal results along a deep refraction seismic profile in the Goiás state together with a 2-D gravity model along a profile between Minas Gerais and São Paulo states were used to calibrate the vertical derivative results. Recent geochemical, petrological data and seismic topography data reinforce the gravity results that the southern and western parts of the São Francisco plate extends under parts the Brasília Belt and parts of the Paraná basin. In the second part, the interpretation of a positive Bouguer gravity anomaly, circular shaped and + 25 mGal amplitude gravity anomaly is presented. This gravity anomaly coincides with a positive, circular shaped, 7 m amplitude geoid anomaly with 250 km of radius. Receiver function results do not show a crustal thinning, excluding this possibility as the cause of gravity high. Deep magnetotelluric soundings along two profiles radial to gravity anomaly indicate a region in the mantle with lower electrical resistivity (< 100 .m) from the depth of 200 km and extending upward and reaching the lower crust. The residual Bouguer anomalies are inverted using a 3-D algorithm and indicate that the denser lower crust reaches 12 km of thickness for + 50 kg/m3 of density contrast. Coincident with a denser lower crust, magnetotelluric soundings reveal a low electrical resistivity crust, reinforcing the presence of magmatic underplating. At the surface, the contamination of magmatic material in the lower crust is supported by the presence of basaltic tholeiitic dike swarms similar, in composition, to the basalts of northern Paraná basin (high TiO2) of Early Cretaceous age. Tholeiitic dikes swarms are distributed throughout the southern region of the craton. Lithospheric mantle with lower electrical resistivity reaches 200 km of depth, indicating that the lithospheric mantle in the southern São Francisco craton underwent compositional rejuvenation accompanied by a mild increase in density (+ 50 to + 70 kg/m3) responsible for the positive geoid anomaly. The decrease of electrical resistivity in the mantle at depths beyond 100 km is easier explained by the presence of carbonatitic melts in the upper mantle, derived from a low degree of partial melt of a metasomatized deeper mantle. The carbonatitic magmatism was prior to tholeiitic magmatism since petrological and geochemical data show that the Early Cretaceous Espinhaço tholeiitic dikes contain 5 to 10% of carbonatites in their composition. The circular shaped erosional limit of the Bambui Group may have been produced by lithosphere uplift at the time of carbonatitic magmatism.

The thesis presents the application of the magnetotelluric (MT) sounding method to image Earth’s crust in Oman and South Australia. The aim of these MT surveys is to provide constraints on the geological interpretation of emplacement scenarios and the tectonic evolution of the geological domain. The thesis concentrates on the methodological aspects of the MT technique, e.g. the data analysis and modelling of electromagnetic fields. The phase tensor approach by Caldwell et al. (2004) is applied to the data and provides insights into the dimensionality of the MT data in even complex and electrically distorted terranes. Modelling and inversion of the MT data is performed with various 2-D and 3-D codes to show how the interpretation of the data can benefit from multiple modelling approaches. Data collected in a 2-D survey across the Oman ophiolite mountains show complex behaviour and 2-D inversion and 3-D forward modelling resolve ambiguities in the emplacement scenario of the Oman ophiolite. It is believed that initial underthrusting of the Jurassic-Cretaceous oceanic lithosphere was followed by exhumation. Further oceanic thrusting subsequently led to rising of lower-plate eclogites and eventually gravitational collapse of the ophiolite onto the margin (Gray et al., 2000). The 3-D inversion code by (Siripunvaraporn et al., 2005a) was expanded to incorporate static shift corrections and inversion model misfits have therefore improved significantly compared to inversion models without static shift correction. 2-D and 3-D surveys across the South Australian Gawler Craton reveal deep crustal conductors which are connected to near surface mineralisation systems of the IOCG Olympic Dam deposit in the north-eastern part of the craton and the Au-dominated central Gawler Craton provinces. / Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2008

Radio magnetotellurics (RMT), crosshole ground penetrating radar (GPR), and crosshole electrical resistance tomography (ERT) were applied in a range of hydrogeological applications where geophysical data could improve hydrogeological characterization. A profile of RMT data collected over highly resistive granite was used to map subhorizontal fracture zones below 300m depth, as well as a steeply dipping fracture zone, which was also observed on a coinciding seismic reflection profile. One-dimensional inverse modelling and 3D forward modelling with displacement currents included were necessary to test the reliability of features found in the 2D models, where the forward models did not include displacement currents and only lower frequencies were considered. An inversion code for RMT data was developed and applied to RMT data with azimuthal electrical anisotropy signature collected over a limestone formation. The results indicated that RMT is a faster and more reliable technique for studying electrical anisotropy than are azimuthal resistivity surveys. A new sequential inversion method to estimate hydraulic conductivity fields using crosshole GPR and tracer test data was applied to 2D synthetic examples. Given careful surveying, the results indicated that regularization of hydrogeological inverse problems using geophysical tomograms might improve models of hydraulic conductivity. A method to regularize geophysical inverse problems using geostatistical models was developed and applied to crosshole ERT and GPR data collected in unsaturated sandstone. The resulting models were geologically more reasonable than models where the regularization was based on traditional smoothness constraints. Electromagnetic geophysical techniques provide an inexpensive data source in estimating qualitative hydrogeological models, but hydrogeological data must be incorporated to make quantitative estimation of hydrogeological systems feasible.

A three-dimensional inversion was implemented for magnetotellurics, which is a passive electromagnetic method in geophysics. It exploits natural electromagnetic fields of the Earth, which function as sources. Their interaction with the conductive parts of the subsurface are registered when components of the electric and the magnetic field are measured and evaluated. The all-at-once approach is an inversion scheme that is relatively new to geophysics. In this approach, the objective function – the basis of each inversion – is called the Lagrangian. It consists of three parts: (i) the data residual norm, (ii) the regularisation part, and (iii) the forward problem. The latter is the significant difference to conventional inversion approaches that are built up of a forward calculation part and an inversion part. In the case of all-at-once, the forward problem is incorporated in the objective function and is therefore already taken into account in each inversion iteration. Thus, an explicit forward calculation is obsolete. As an objective function, the Lagrangian shall reach a minimum and therefore its first and second derivatives are evaluated. Hence, the gradient of the Lagrangian and its Hessian are constituent parts of the KKT system – the Newton-type system that is set up in the all-at-once inversion. Conventional inversion approaches avoid the Hessian because it is a large, dense, not positive definite matrix that is challenging to handle. However, it provides additional information to the inversion, which raises hope for a high quality inversion result. As a first step, the inversion was programmed for the more straightforward one-dimensional magnetotelluric case. This was particularly suitable to become familiar with sQMR – a Krylov subspace method which is essential for the three-dimensional case to be able to work with the Hessian and the resulting KKT system. After the implementation and validation of the one-dimensional forward operator, the Lagrangian and its derivatives were set up to complete the inversion, which successfully solved the KKT system. Accordingly, the three-dimensional forward operator also needed to be implemented and validated, which was done using published data from the 3D-2 COMMEMI model. To realise the inversion, the Lagrangian was assembled and its first and second derivatives were validated with a test that exploits the Taylor expansion. Then, the inversion was initially programmed for the Gauss-Newton approximation where second order information is neglected. Since the system matrix of the Gauss-Newton approximation is positive definite, the solution of this system of equations could be carried out by the conventional solver pcg. Based on that, the complete KKT system (Newton\\\'s method) was set up and preconditioned sQMR solved this system of equations.

Dans ce manuscrit, nous développons des méthodologies de détection et caractérisation de sources géomagnétiques et atmosphériques en se basant sur la transformée en ondelettes continues. Les techniques introduites se basent sur les caractéristiques temps-fréquence des ondes observées dans les séries temporelles magnétotelluriques (MT). A partir de ces procédures de détection, nous détaillons l'implémentation d'une stratégie de détermination des fonctions de réponse MT basée sur les statistiques robustes, et du bootstrap hiérarchique pour le calcul des incertitudes. Deux études MT sont également détaillées. La première étude MT concerne la caractérisation de la structure géoélectrique situé sous l'observatoire magnétique de Chambon-La-Forêt, France. La seconde étude concerne des mesures effectuées dans la vallée de Trisuli au Népal en mars 2016. L'objectif de cette campagne est la comparaison avec une étude effectuée en 1996. Nous discutons des effets topographiques sur les sondages MT. Nous présentons également une nouvelle interprétation de la distribution de conductivité dans le sous-sol de vallée de Trisuli. / In this manuscript, we detail the application of continuous wavelet transform to processing schemes for the detection and the characterisation of geomagnetic and atmospheric sources. Presented techniques are based on time-frequency properties of electromagnetic (EM) waves observed in magnetotellurics (MT) time series. We detail the application of these detection procedures in a MT processing scheme. To recover MT response functions, we use robust statistics and a hierarchical bootstrap approach for uncertainties determination. Interpretation of two datasets are also presented. The first MT study deals with the caracterisation of the resistivity distribution below the French National magnetic observatory of Chambon-la-Forêt. The second study details the interpretation of new MT soundings acquired in March 2016 in the Trisuli valley, Nepal. The main objective of this campaign was to compare the new soundings with an old campaign in 1996. We discuss topography effects on MT soundings and their implication on the resistivity distribution. We also introduce a new interpretation of the resistivity distribution in Trisuli valley.