Controlled-Source Electromagnetics (CSEM) is a method that has been used since the 1980’s in the marine environment for determining electrical properties of the subsurface. Receivers on the seafloor collect total electric and magnetic fields which are produced as a result of interaction of the transmitter generated primary fields with the seawater and subsurface. Badea et al. (2001) coded an existing algorithm for solving Maxwell’s equations. This finite element 3D forward modeling algorithm is used to simulate CSEM experiments. The objective of the present study is to model the changes in electromagnetic response for a resistive disk and a more geometrically complex structure, which are rough approximations of hydrocarbon reservoirs. The parameters that are varied in studying these subsurface structures are the disk radius, disk depth, the transmitter frequency, the transmitter location, and the structure orientation.The results showed that a disk of finite radius behaves similar to an infinite disk at short range and grades into double half-space behavior at longer ranges. The frequency of the transmitter must be tuned to the disk depth as certain frequencies will penetrate too shallow or too deep to probe the disk. Moving the transmitter away from the receivers causes a decrease in signal strength, but exhibits a greater capacity to distinguish between the double half-space and infinite disk scenarios. The disk was then replaced by a more complex structure. To determine if the 3D nature of the structure may be located a study was undertaken to probe the structure from different perspectives using different transmitter locations and azimuths. It is determined that the 3D nature of the structure could not be observed until the structure’s thickness is sufficiently large.The goal of the study is to better understand the effect of subsurface parameters on the total fields and show the usefulness of the 3D forward modeling code. Understanding the relationships between these parameters and the resulting signals is important in terms of setting up a real experiment. Marine CSEM studies are costly and using a valuable tool such as an accurate finite element 3D forward modeling algorithm may save time and money.

Controlled-Source Electromagnetics (CSEM) is a method that has been used since the 1980’s in the marine environment for determining electrical properties of the subsurface. Receivers on the seafloor collect total electric and magnetic fields which are produced as a result of interaction of the transmitter generated primary fields with the seawater and subsurface. Badea et al. (2001) coded an existing algorithm for solving Maxwell’s equations. This finite element 3D forward modeling algorithm is used to simulate CSEM experiments. The objective of the present study is to model the changes in electromagnetic response for a resistive disk and a more geometrically complex structure, which are rough approximations of hydrocarbon reservoirs. The parameters that are varied in studying these subsurface structures are the disk radius, disk depth, the transmitter frequency, the transmitter location, and the structure orientation.The results showed that a disk of finite radius behaves similar to an infinite disk at short range and grades into double half-space behavior at longer ranges. The frequency of the transmitter must be tuned to the disk depth as certain frequencies will penetrate too shallow or too deep to probe the disk. Moving the transmitter away from the receivers causes a decrease in signal strength, but exhibits a greater capacity to distinguish between the double half-space and infinite disk scenarios. The disk was then replaced by a more complex structure. To determine if the 3D nature of the structure may be located a study was undertaken to probe the structure from different perspectives using different transmitter locations and azimuths. It is determined that the 3D nature of the structure could not be observed until the structure’s thickness is sufficiently large.The goal of the study is to better understand the effect of subsurface parameters on the total fields and show the usefulness of the 3D forward modeling code. Understanding the relationships between these parameters and the resulting signals is important in terms of setting up a real experiment. Marine CSEM studies are costly and using a valuable tool such as an accurate finite element 3D forward modeling algorithm may save time and money.

Masaya volcano, Nicaragua, has been the site of tremendous Plinian basaltic eruptions. Two eruptions ~6,500 and 2,250 BP formed the 6 kilometer (km) x 11 km, northwest trending Masaya caldera. The present day active Santiago Crater within the caldera is the site of persistent volcano degassing and occasional phreatic explosions. While the mechanism responsible for these phreatic explosions is unclear, one possible explanation is the interaction of groundwater with the shallow magma chamber beneath Masaya. This interaction with meteoric water is supported by the substantial steam discharge from the vent, which is significantly larger than other similar volcanoes in the world. To better understand these interactions, the distribution of groundwater was characterized for the volcano based on interpretation of 29 Transient Electromagnetic (TEM) soundings. The TEM data were modeled using two independent methods to estimate resistivity as a function of depth. Results from modeling the TEM data indicate an overlying highly resistive layer throughout the caldera that is underlain by one or more conductive layers. The implied water table of the caldera is expressed as a subdued replica of the topography in the higher vent regions in the central and southern portions of the caldera and decreases to a level that coincides with the elevation Lake Masaya in the lower sections of the caldera. The water table elevation in the caldera also shows a marked difference from the regional groundwater flow system as there is a large gradient in head values suggesting a sharp change in transmissivity along the caldera boundaries, which indicate the caldera is hydraulically isolated from the surrounding region. In order to better understand the hydrologic processes at Masaya caldera, a 3-D finite difference groundwater model was created using the 29 estimated water levels and two groundwater flux measurements to simulate the hydrologic system The model calibration revealed that a deep, highly permeable layer must feed the active vent in order for the steam emissions to be maintained at their current levels. This information about the caldera provides a baseline for forecasting the response of this isolated groundwater system to future changes in magmatic activity.

Active marine electromagnetic methods have proven to be a powerful tool to detect resistivity anomalies associated with gas hydrate. However, because the propagation of electromagnetic fields for these methods works in the diffusive regime the spatial resolution of the resistivity structure is limited. So far only bulk electrical properties have been estimated from measured data, although hydrate bearing layers are found to be highly heterogeneous. We computed response curves for synthetic one- and two-dimensional models to investigate the resolution capabilities for various measurement geometries with respect to resistive features. Electric dipole transmitters (TXs) are used as sources. In the marine case, the in-line electric dipole-dipole configuration has proven its capabilities to detect the shallow resistive gas-hydrate. Our model study demonstrates that both the depth to a resistive feature can be resolved nicely using data for multiple TX-RX offsets. However, resolving smaller features of the resistive zone, for example if the zone is split in separate resistive layers, is extremely difficult. The resolution of the target can be improved using electrical downhole transmitters. So far there have been no reports of the detection of permafrost gas hydrate deposits with surface electromagnetic methods. Our calculations show that a similar setup to that used in the marine case is capable of detecting gas hydrate on land. The resolution, however, is lower than for the marine case, because of the significantly greater depths to the target.

In this thesis, four inversion problems of different scale and difficulty are solved. Two of them are electromagnetic inverse problems. Two more are joint inversion problems of potential field data and other types of data. First, a linear approximation, which is a generalization of the low-induction-number approximation standard in slingram dual-loop interpretation is developed and used for rapid two and three dimensional inversion. The approximation takes induction within a background half-space into account and can thus be applied in conductive scenarios, where otherwise a rigorous electromagnetic modeling would be required. Second, a three-dimensional inversion of airborne tensor very-low-frequency data with a rigorous forward modeling at its core is developed. For dealing with the large scale of the forward problem, a nested fast-Fourier-transform-based integral equation method is introduced, wherein electromagnetic interactions are arranged according to their range and larger ranges are treated with less accuracy and effort. The inversion improves the traditional interpretation through data derived maps by providing a conductivity model, thus constraining the upper few hundred meters of the crust down to the shallowest conductor and allowing the study of its top in three dimensions. The third inversion problem is the the joint inversion of refraction and geoelectric data. By requiring the velocity and resistivity models to share a common, laterally variable layered geometry, easily interpretable models, which are reasonable in many geological near surface situations (e.g., groundwater exploration in Quaternary sediments), are produced directly from the joint inversion. Finally, a joint inversion of large scale potential field data from a gabbro intrusion is presented. Gravity and magnetic data are required to abide to a petrophysical constraint, which is derived from extensive field sampling. The impact of the constraint is maximized under the provision that both data sets are explained equally well as they would be through individual inversions. This leads to a simple and clearly defined intrusion geometry, consistent for both the density and magnetic susceptibility distribution. In all presented inversion problems, field data sets are successfully inverted, the results are appraised through synthetic tests and, if available, through comparison with independent data. / Diese Arbeit hat die Lösung von vier geophysikalischen Umkehraufgaben, sogenannten Inversionsproblemen, zum Gegenstand. Zwei dieser Aufgaben befassen sich mit der Inversion elektromagnetischer Daten, zwei weitere sind Probleme der kombinierten Inversion von Datensätzen aus unterschiedlichen geophysikalischen Messverfahren. Im ersten Problem wird die für die Auswertung elektromagnetischer Zweispulensystemdaten typische lineare Näherung der kleinen Induktionszahlen als Bornsche Näherung verallgemeinert, ihre Anwendbarkeit durch exakte Berücksichtigung der Induktionsvorgänge in einem beliebigen homogenen Halbraum von schlechtleitenden auf gutleitende Untergründe ausgedehnt und schließlich zur zwei- und dreidimensionalen Inversion eingesetzt. Dadurch kann auch im leitfähigen Untergrund eine aufwändige exakte Modellierung vermieden werden. Im zweiten Problem wird eine dreidimensionale Inversion von flugzeuggestützten Längstwellenmessungen entwickelt und als ihre Grundlage eine exakte elektromagnetische Rechnung erdacht. Damit wird traditionelle kartengestützte Dateninterpretation durch ein dreidimensionales Leitfähigkeitsmodell ergänzt, welches die oberen hundert bis dreihundert Meter der Erdkruste bis hin zur Tiefe des obersten Leiters abbildet, so dass dessen Oberflächenform erkundet werden kann. Die enorme Problemgröße wird durch eine Fouriertransformationsmethode bewältigt, welche die elektromagnetischen Wechselwirkungen nach ihrer Reichweite einteilt, die Fernwirkungen mit entsprechend verringerter Genauigkeit behandelt und dadurch eine erhebliche Anzahl an Rechnungen einspart. Im dritten Problem werden refraktionsseismische und geoelektrische Messungen kombiniert, indem sowohl das Geschwindigkeits- als auch das Widerstandsmodell mit einer gemeinsamen, lateral veränderlichen und durch beide Datensätze bestimmten Schichtstruktur versehen werden. Ein solches, durch Schichten definiertes Inversionsergebnis, stellt in vielen oberflächennahen Anwendungen, beispielsweise im Grundwasserbereich, ein sinnvolles Abbild der Erde dar. Im vierten Problem werden Schweremessungen und Magnetfeldmessungen, die über einer Gabbrointrusion aufgenommen wurden, mittels einer empirischen petrophysikalischen Beziehung vereinigt, welche aus Labormessungen an einer großen Anzahl von Gesteinsproben abgeleitet wurde. Hierbei wird der Einfluss dieser Modellkopplung solange maximiert, wie beide Datensätze mit derjenigen Genauigkeit angepasst werden können, welche vorher in Einzelinversionen erreicht wurde. Das Ergebnis ist ein einfaches, geometrisch konsistentes Modell der Verteilungen von Dichte und magnetischer Suszeptibilität. In allen vier Aufgaben wurden erfolgreich reale Felddaten invertiert. Die Güte der Ergebnisse wurde mittels synthetischer Experimente untersucht und, so vorhanden, mit unabhängigen Informationen verglichen.

We have numerically investigated the influence of a nanoscale silicon tip in proximity to an illuminated gold nanoparticle. We describe how the position of the high-permittivity tip and the size of a nanoparticle impact the absorption, peak electric field and surface plasmon resonance wavelength under different illumination conditions. We detail the finite element method (FEM) approach we have used for this, whereby we specify a volume excitation field analytically and calculate the difference between this source field and the total field (i.e., scattered-field formulation). We show that a nanoscale tip can locally enhance the absorption of the particle as well as the peak electric field at length scales far smaller than the wavelength of the incident light.

The feasibility of predicting porewater salinity based on calibrated surface electromagnetic methods is discussed in a coastal wetland on the southern banks of Tampa Bay in West-Central Florida. This study utilizes a new method to float commercial land based electromagnetic (EM) instruments in shallow marine waters of less than 1.5 meters. The floating EM-31 (Geonics, Ltd.) effectively sensed the magnitude and lateral extent of high and low salinity porewaters within mangrove lined ditches and ponds. Resistivity and EM geophysical methods are merged with direct sampling data to calibrate layers in electromagnetic models to infer shallow (<30m) groundwater salinity patterns. Initial marine resistivity surveys are necessary to discriminate between equivalent EM model solutions for seafloor conductivities beneath shallow (0.1-1.5m) marine (~30 ppt) waters. Using formation factors computed from nearby resistivity surveys, porewater conductivity predictions based on surface EM-31 and EM-34 measurements are successful at distinguishing overall porewater salinity trends. At the Tampa Bay study site, the most distinctive terrain conductivity anomalies are associated with mangroves bordering marine waters. Highly elevated porewater conductivities are found within 5m of the mangrove trunks, falling sharply off within 10m, presumably due to saltwater exclusion by mangrove roots. Modeling indicates the shallow water EM-31 measurements probably lack the resolution necessary to image more subtle porewater conductivity variations, such as those expected in association with diffuse submarine groundwater discharge. However, the technique has potential application for locating high contrast zones of freshwater discharge and other salinity anomalies in shallow and nearshore areas not accessible to conventional marine resistivity or land-based arrays, and hence may be useful for interdisciplinary studies of coastal wetland ecosystems.

The interaction of an evanescent wave and plasmonic nanostructures are simulated in Finite Element Method. Specifically, the optical absorption cross section (Cabs) of a silver nanoparticle (AgNP) and a gold nanoparticle (AuNP) in the presence of metallic (gold) and dielectric (silicon) atomic force microscope (AFM) probes are numerically calculated in COMSOL. The system was illuminated by a transverse magnetic polarized, total internally reflected (TIR) waves or propagating surface plasmon (SP) wave. Both material nanoscale probes localize and enhance the field between the apex of the tip and the particle. Based on the absorption cross section equation the author was able to demonstrate the increment of absorption cross section when the Si tip was brought closer to the AuNP, or when the Si tip apex was made larger. However, the equation was not enough to predict the absorption modification under metallic tips, especially for a AgNP's Cabs; neither it was possible to estimate the optical absorption based on the localized enhanced field caused by a gold tip. With the help of the driven damped harmonic oscillator equation, the Cabs of nanoparticles was explained. In addition, this model was applicable for both TIR and Surface Plasmon Polaritons illuminations. Fitting the numerical absorption data to a driven damped harmonic oscillator (HO) model revealed that the AFM tip modifies both the driving force (F0), consisting of the free carrier charge and the driving field, and the overall damping of the oscillator beta. An increased F0 or a decreased beta will result in an increased Cabs and vice versa. Moreover, these effects of F0 and beta can be complementary or competing, and they combine to either enhance or suppress absorption. Hence, a significantly higher beta with a small increment in F0 will result in an absorption suppression. Therefore, under a Si tip, Cabs of a AuNP is enhanced while Cabs of a AgNP is suppressed. In contrast, a Au tip suppresses the Cabs for both Au and Ag NPs. As an extension of this absorption model, further investigation of the guided mode and a close by nanostructure is proposed, where the scattered wave off the structure attenuates the guided mode with destructive interference.

Dans ce travail, les propriétés des réseaux diffractifs ordonnés de nanoparticules d'or sont étudiées numériquement et expérimentalement. Ces résonances sont beaucoup plus étroites que celles observées dans le cas d'une nanoparticule isolée. D'après les simulations numériques, deux régimes distincts de réponse sont identifiés, l'un correspond à l'anomalie de Rayleigh (RA) l'autre au mode plasmon de réseau 2D (LPM). Dans la partie expérimentale nous avons fabriqué des réseaux de nanoparticules d'or en utilisant la lithographie d'électronique. La transmission spectrale a été mesurée dans le domaine optique pour caractériser ces réseaux. Toutes les caractéristiques essentielles des spectres expérimentaux sont en bon accord avec les simulations numériques. Les distributions du champ électrique pour différents paramètres de réseau sont étudiées pour obtenir le maximum d'augmentation du champ à la surface de la nanoparticule. L'excitation des résonances plasmon dans les réseaux diffractifs de nanoparticules d'or en condition asymétrique de l'indice de réfraction est examinée expérimentalement. L'excitation des modes plasmon à profil spectral étroit dans l'environnement asymétrique a été expérimentalement vérifiée. La possibilité d'accorder la longueur d'onde de ces résonances dans le proche infrarouge en changeant les paramètres structurels des réseaux périodiques en combinant taille et forme des nanoparticules est discutée. Ces résultats sont importants pour les applications telles que les spectroscopies en champ électrique exalté et la détection en biologie ou en chimie. / The properties of ordered diffractive arrays of gold nanoparticles are studied numerically and experimentally. Using numerical simulations I identify, two distinct regimes of lattice response, associated with two-characteristic states of the spectra: Rayleigh anomaly and lattice plasmon mode. In experimental part gold nanoparticle arrays were fabricated using e-beam lithography. Spectroscopic transmission measurements then were carried out to optically characterize these arrays. All the essential features of the experimental spectra were reproduced well by numerical simulations. Electric field distributions for different lattice parameters are studied in order to maximize the enhancement of electric field at the nanoparticle surface. The excitation of plasmon resonances in diffractive arrays of gold nanoparticles placed in asymmetric refractive index environment is examined experimentally. The excitation of the plasmon modes with narrow spectral profile in asymmetric environment was experimentally verified. The ability to tune the wavelength of these resonances in the near infrared range by varying the structural parameters of the periodic arrays in combination with size and geometry of the constituent nanoparticles is discussed. The presented results are of importance for the field enhanced spectroscopy as well as for plasmonic bio and chemical sensing.

Le travail de recherche présenté dans ce mémoire de thèse, qui s’est déroulée dans le cadre d’un dispositif CIFRE (Conventions Industrielles de Formation par la Recherche) en collaboration avec le pôle géophysique du CEBTP à clermont-ferrand et le laboratoire Magmas et Volcans de l’UCA. Ce travail porte sur l’amélioration du rendement de la méthode géophysique électromagnétique de sub-surface. L’idée est de parvenir à extraire plus d’informations à partir des données électromagnétiques en gardant une méthode de prospection proche des méthodes classiques actuelles. Les techniques électromagnétiques (EM) sont des méthodes géophysiques fondées sur la mesure des variations de champs magnétiques et sont utilisé pour réaliser des mesures des caractéristiques électriques des sols. Ces appareils EM sont dits inductifs et ne nécessitent donc pas de contact avec le sol. Ils peuvent donc être mis en œuvre avec une vitesse d’acquisition plus importante que la plupart des autres méthodes géophysiques (profils électriques, gravimétrie, sismique...).Dans le cadre de ce travail, différents développements ont été effectués pour répondre à la problématique de l’amélioration des outils de prospection EM. La prospection sur le terrain a été améliorée grâce au développement d’un prototype de système d’acquisition automatisé. Celui ci est composé d’un conductivimètre (EM-31), d’un chariot support en fibre de verre, d’un GPS et d’un ordinateur assurant un enregistrement continu et géo-référencé des données à l’aide d’un programme spécialement conçu en Python.Ce mémoire présente également une procédure de correction des valeurs du conductivimètre EM-31, notamment la correction des effets de la hauteur de l’appareil par rapport au sol.Un programme Matlab a également spécifiquement été conçu pour le traitement automatisé de données EM. Ce programme permet de disposer rapidement des outils de base pour le traitement et la bonne visualisation des données.Deux études de cas ont été réalisées dans le cadre de ce travail doctoral.La première concerne une prospection linéaire d’une centaine de kilomètres sur des digues de protection contre les crues le long du fleuve Loire. Cette étude met en évidence les difficultés rencontrées lors d’une prospection de grande envergure et permet d’identifier les problématiques d’une étude géophysique à grande échelle, notamment la gestion du grand nombre de données. Cela contraint le choix de la méthodologie de prospection et permet de mettre en place les procédures d’automatisation des traitements.La seconde étude concerne la mise en œuvre des outils EM sur des terrains de nature volcanique. La prospection EM a su s’avérer très efficace pour la cartographie de nombreux sites archéologiques. Cependant les sols et les roches en région volcaniques sont connus pour avoir des effets magnétiques forts. Il s’agit ici dans le cadre d’une prospection archéologique, de déterminer plus précisément les effets magnétiques du sous-sol sur la mesure EM. / This study has been conducted in the framework of the CIFRE doctoral contract, in collaboration with the Ginger CEBTP Geophysics pole and the “Laboratoire Magmas et Volcans”. The main objective of this work is to improve the performance of the sub-surface electromagnetic induction method.The principle is to retrieve more information from electromagnetic data keeping a prospecting method close to current field methods. Electromagnetic (EM) methods are based upon magnetic fields variations in order to to measure electrical characteristics of soils. EM devices are inductive and so don’t need contact with the ground. Thus they can be implemented faster than most of others geophysical methods (seismic, electrical profiles, gravimetry). In this framework, several developments have been performed to respond to the improvement of EM prospecting methods. The field survey was improved by the development of an automated acquisition system including a conductivimeter (EM-31) mounted on a fibreglass cart with a GPS receiver and a computer running a special Python program which ensure continuous data recording and geo-reference. In this thesis we present a correction procedure for the EM-31 conductivimeter values, in particular the device height above the ground. A Matlab program was also specifically conceived for automated EM data processing. It combines basic data processing and visualization tools. Two case studies was conducted during this doctoral work. The first one is about a hundred kilometers of linear prospecting along the Loire protection dykes (France). This study highlights the difficulties of large scale geophysical prospecting and allows to identify specific issues such as management of large data number. This influence the prospecting methodology and allows implementation of adapted automatized data processing. The second case study is about the implementation of EM devices on volcanic fields. Several archaeological sites has been mapped using EM methods. However in volcanic regions, soils and rocks are known to have strong magnetic effects. In this specific case of an archaeological study the purpose is to determine precisely the magnetic effects of soils on EM measures.