Lithospheric structure from forward and inverse modeling of satellite gravity and magnetic data

Satellite missions have provided the Earth's gravity and magnetic field at resolutions sufficient for large-scale geophysical applications. While satellite data do not possess the same resolution as ground data, their homogeneous coverage and low error make them ideal for studying large-scale lithospheric structure and processes. In this thesis, the sensitivity of satellite potential field data to structures in the lithosphere is investigated. Furthermore, a crustal model is derived based on a data base of active seismic experiments. The calculation of the crustal model is data-driven, unlike previous crustal models, which typically rely on expert knowledge. Finally, a simple probabilistic joint inversion of gravity gradients and topography is developed, that is capable of estimating a two-layer density model of the lithosphere and its uncertainty.Die global verfügbaren Messungen des Schwere- und Magnetfeldes mithilfe von Satelliten liegen heute in einer Auflösung vor, die großräumige geophysikalische Anwendungen ermöglichen. Zwar ist es mit Satelliten nicht möglich, die Auflösung von Bodenmessungen zu erreichen. Um aber großkalige lithosphärische Strukturen zu untersuchen, sind Satellitendaten ideal, da die Datenqualität sehr homogen ist und sie einen geringen Messfehler aufweisen. In dieser Dissertation wird untersucht, wie groß die Sensitivität von Satellitendaten tatsächlich ist. Darüber hinaus wird ein globales Krustenmodell basierend auf einer Datenbank aktiver seismischer Profile erstellt, das -- anders als vorherige Krustenmodelle -- weitgehend ohne manuelle Eingabe von Expertenwissen auskommt. Zuletzt werden die Ergebnisse der verschiedenen Kapitel in einer gemeinsamen, probabilistischen Inversion von Schweregradienten und Topografie kombiniert, mit der ein einfaches Zwei-Schicht Dichtemodell der Lithosphäre und dessen Unsicherheit bestimmt werden kann

Precursory worldwide signatures of earthquake occurrences on Swarm satellite data

The study of the preparation phase of large earthquakes is essential to understand the physical processes involved, and potentially useful also to develop a future reliable short-term warning system. Here we analyse electron density and magnetic field data measured by Swarm three-satellite constellation for 4.7 years, to look for possible in-situ ionospheric precursors of large earthquakes to study the interactions between the lithosphere and the above atmosphere and ionosphere, in what is called the Lithosphere-Atmosphere-Ionosphere Coupling (LAIC). We define these anomalies statistically in the whole space-time interval of interest and use a Worldwide Statistical Correlation (WSC) analysis through a superposed epoch approach to study the possible relation with the earthquakes. We find some clear concentrations of electron density and magnetic anomalies from more than two months to some days before the earthquake occurrences. Such anomaly clustering is, in general, statistically significant with respect to homogeneous random simulations, supporting a LAIC during the preparation phase of earthquakes. By investigating different earthquake magnitude ranges, not only do we confirm the well-known Rikitake empirical law between ionospheric anomaly precursor time and earthquake magnitude, but we also give more reliability to the seismic source origin for many of the identified anomalies.Publishedid 202872A. Fisica dell'alta atmosferaJCR Journa

Constraints on the thermal state of the continental lithosphere

The thermal state of the lithosphere is an important driver of many physical and chemical processes within the Earth. Understanding the distributions of heat flow and radiogenic heat production provides an important constraint on lithospheric thermal models. By some estimates nearly 40% of continental heat flow is produced by radioactive decay in the crust, however the distribution of heat producing elements is poorly constrained. Creating robust models of radiogenic heat production requires an understanding of its natural variability. The creation of a global whole-rock geochemical database provides a framework for discussing global distributions of thermal parameters. I have collated over one million digital rock entries with a range of sample data including major and trace element concentrations, isotopic ratios, and other metadata. Associated naming schema and physical parameter estimates are also computed in a standardised manner, including radiogenic heat production. I then present a new model for continental igneous heat production from ~4 Ga to the present using a novel silica-normalised igneous data set and compare to previous discussions of granitic and sedimentary trends in the literature. Crude normalization for composition indicates lithological control is the dominant factor on heat production after the influence of decay is removed. I find that heat production at formation for different rock types has been relatively constant through time except for the early Archean to ~2.7 Ga. I suggest the heat production–age pattern does not significantly reflect the influences of erosion, secular cooling, depletion, or the supercontinent cycle as suggested by some previous studies, but instead either reflects a shift in the bulk composition of the crust or evidence for bias in the rock record due to thermal stability. Geophysical proxies provide additional constraints on the crustal thermal state. I have developed a global Curie Depth model from the latest satellite-derived lithospheric magnetic model using the equivalent source magnetic dipole method. Forward modelling was conducted to simulate the observed lithospheric magnetic field. Our updated methodology involves additional vector components utilised in the forward modelling calculations, a differing long-wavelength model, and inclusion of a spatially variable magnetic susceptibility estimate. Resultant continental Curie depth estimates show good agreement with observed heat flow observations and provide further evidence that Curie depth estimates can assist in estimates of the thermal state of the lithosphere. Finally, I assess various heat flow models for Antarctica derived from geophysical proxies. Extrapolation from isotherm estimates at depth require models of heat production and thermal conductivity to model surface heat flow. Differences in models can have non-trivial influences on the results produced. Quantifying the uncertainty associated with these thermal parameters is also critical for understanding and interpreting the heat flow solutions. I propose a set of models derived from whole-rock geochemical data, and guided by compositional studies of the crust. Uncertainties associated with this model are estimated via the Monte Carlo method. I show that applying models guided by global insights provides a reasonable fit to the Antarctica continent, and a method of estimating uncertainty in thermal parameters for regions lacking basement geology constraints.Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, YEA

Modelización del campo geomagnético principal y su variación secular en el Atlántico Norte mediante el uso de datos de naturaleza heterogénea

En aquesta tesi doctoral es presenta un estudi detallat de l’ús i utilitat de la tècnica regional de modelització basada en el Anàlisi Harmònic sobre un Casquet Esfèric (SCHA). L’esmentada tècnica, y sobre tot, la seva revisió posterior (R-SCHA), van ser desenvolupades especialment per modelar les longituds d’ona curtes de les components geomagnètiques. Aquest fet ens deixa una qüestió a resoldre: l’ús del R-SCHA per a modelar longituds d’ona llargues pròpies del camp geomagnètic principal i la seva variació secular. Amb la intenció de explorar aquesta opció per primer cop s’han desenvolupat diverses estratègies que es plasmen al present treball. La distribució espacial i temporal de les dades emprades (dades escalars i vectorials provinents d’observatoris, estacions seculars, marines i de satèl·lit) ha permès constrènyer la zona d’estudi de l’Atlàntic Nord, entre 1960 i 2000. Fent imprescindible el desenvolupament, per primer cop, de la norma espaciotemporal del camp geomagnètic en termes de la tècnica R-SCHA, propiciant l’obtenció de longituds d’ona espacial i temporal més realistes. El model obtingut, NAGRM, es capaç d’ajustar amb gran precisió les dades emprades, especialment el camp principal i la seva variació secular proporcionat per les mitges anuals i mensuals dels observatoris geomagnètics. Fet que ressalta la bondat del NAGRM, al se capaç de reproduir amb precisió les dades geomagnètiques més fiables sobre la superfície terrestre. A més, el model regional mostra una adequada i continua descripció en altura per als diferents elements geomagnètics. Amb la finalitat d’augmentar el valor i la utilitat del NAGRM, aquest es constreny al final del seu període mitjançant el model global CHAOS-6, permetent una continuïtat suau entre ambdós models per a l’any 2000. Durant el procés de modelització s’ha posat de rellevància les avantatges d’emprar dades marines de creuament en grans zones oceàniques per a èpoques sense dades geomagnètiques de satèl·lit. Per aquest motiu el període temporal seleccionat ha estat de 1960 a 2000, època a partir de la qual es disposa d’una bona cobertura espacial gracies a les dades de satèl·lits como el Ørsted, el CHAMP i els de la constel·lació Swarm. Per mostrar les esmentades avantatges s’ha generat un model regional de variació secular de la intensitat geomagnètica a la zona de l’Atlàntic Nord, NAGSVM-F. En aquest cas particular totes les dades emprades estan localitzades sobre la superfície terrestre, pel que la tècnica utilitzada ha estat el SCHA. Per a la correcta aplicació, de la mateixa forma que per al model NAGRM, s’ha desenvolupat la norma del camp geomagnètic en termes del SCHA. Els models regionals obtinguts, NAGRM i NAGSVM-F, permeten determinar les incerteses gracies a l’aplicació del mètode bootstrap. A més, ambdós models poden ser emprats per detectar els jerks geomagnètics succeïts dintre de la finestra temporal de 1960 - 2000. Aquests fenòmens queden registrats al sintetitzar la variació secular al voltant dels anys on s’han produït. En aquesta tesi es constata que la tècnica R-SCHA proporciona models regionals de camp geomagnètic principal i de la seva variació secular comparables amb els proporcionats pels models globals.En esta tesis doctoral se presenta un estudio detallado del uso y utilidad de la técnica de modelización regional basada en el Análisis Armónico sobre un Casquete Esférico (SCHA). Dicha técnica, y sobre todo su posterior revisión (R-SCHA), fueron especialmente desarrolladas para modelar cortas longitudes de onda de los elementos geomagnéticos. Esto dejó, por tanto, una cuestión a resolver: el uso del R-SCHA para modelar largas longitudes de onda características del campo geomagnético principal y su variación secular. Para explorar por primera vez esta opción, se ha desarrollado diferentes estrategias que se plasman en el presente trabajo. La distribución espacial y temporal de los datos usados (datos escalares y vectoriales procedentes de observatorios, estaciones seculares, marinos y de satélite) ha permitido constreñir el área de estudio en la zona del Atlántico Norte, entre 1960 y 2000. Haciendo necesario el desarrollado, por primera vez, de la norma espaciotemporal del campo geomagnético en términos de la técnica R-SCHA, propiciando la obtención de longitudes de onda espacial y temporal más realistas. El modelo obtenido, NAGRM, es capaz de ajustar con alta precisión los datos usados, especialmente el campo principal y su variación secular proporcionados por las medias anuales y mensuales de los observatorios geomagnéticos. Hecho que resalta la bondad de NAGRM, al ser capaz de reproducir con precisión los datos geomagnéticos más fiables sobre la superficie terrestre. Además, el modelo regional también muestra una adecuada descripción continua en altura para los diferentes elementos geomagnéticos. Con la intención de aumentar el valor y la utilidad de NAGRM, éste se constriñó en su periodo final mediante el reciente modelo global CHAOS-6 permitiendo una continuidad suave entre ambos modelos en el año 2000. Durante el proceso de modelización se ha puesto de manifiesto las ventajas de emplear datos marinos de cruce en grandes zonas oceánicas para épocas desprovistas de datos geomagnéticos de satélites. Por ese motivo, la ventana temporal seleccionada ha sido de 1960 hasta 2000, época a partir de la cual ya se dispone de buena cobertura espacial gracias a los datos de satélites como el Ørsted, el CHAMP y los de la constelación Swarm. Para mostrar dichas ventajas, se ha generado un modelo regional de variación secular de la intensidad geomagnética de la zona del Atlántico Norte, NAGSVM-F. En este caso particular todos los datos empleados se localizan sobre la superficie terrestre, por lo que la técnica utilizada ha sido el SCHA. Para su aplicación se ha requerido, de la misma forma que para el modelo NAGRM, desarrollar la norma del campo geomagnético en términos SCHA. Los modelos regionales obtenidos, NAGRM y NAGSVM-F, permiten también determinar sus incertidumbres gracias a la aplicación del método bootstrap. Cabe destacar que ambos modelos pueden ser usados para detectar los jerks geomagnéticos acontecidos en la ventana temporal 1960 - 2000. Dichos fenómenos se ven plasmados al sintetizar la variación secular alrededor de los años en que estos eventos se han producido. Todo lo mostrado en este trabajo constata que la técnica R-SCHA proporciona modelos regionales del campo geomagnético principal y su variación secular que se comportan comparablemente con los modelos globales.This thesis presents a detailed study of the regional modelling technique based on Spherical Cap Harmonic Analysis (SCHA). This technique, and especially its later revision (R-SCHA), were developed to model geomagnetic short wavelengths. Therefore, this fact leads to an issue yet to be resolved: the use of the R-SCHA to model characteristic long wavelengths of the main geomagnetic field and its secular variation. In order to explore this option, this study aims to introduce different strategies that have been developed. The spatial and temporal data distribution (scalar and vector data from observatories, repeat stations, and marine and satellite surveys) enables us to study the North Atlantic area between 1960 and 2000. Thus, it was necessary to develop a spatiotemporal norm of the geomagnetic field in terms of the R-SCHA technique, which provided more realistic spatial and temporal wavelengths. The developed model, NAGRM, is able to reproduce with high accuracy the data used, specially the main field and its secular variation provided by the observatory annual and monthly means. This fact highlights the efficiency of NAGRM, being able to reproduce accurately the most reliable geomagnetic data on the Earth's surface. In addition, the regional model also shows a suitable continuous description in height for the different geomagnetic components. In the interest of increasing the value of NAGRM, at the end of its period of validity it was constrained to the recent global model CHAOS-6. This gave a smooth continuity between both models. During the modelling process, the advantages of using marine crossover data in large oceanic zones when having poor geomagnetic satellite data distribution have been demonstrated. As a consequence, the time chosen is from 1960 to 2000. During these years a good spatial coverage was available thanks to satellite data provided by Ørsted, CHAMP and Swarm constellation. A secular variation regional model of the geomagnetic intensity for the North Atlantic area, NAGSVM-F, has been generated to show these advantages. In this particular case, the data come from the Earth surface and, therefore, the technique used was the SCHA. As for the NAGRM model, a spatiotemporal norm of the geomagnetic field in terms of SCHA was developed. The regional models NAGRM and NAGSVM-F use the bootstrap algorithm to predict uncertainty. It should be pointed out that both models can be used to detect geomagnetic jerks that occurred between 1960 and 2000 time interval. These phenomena are captured when the secular variation are synthesized around the time these events have occurred. This study shows that the R-SCHA technique provides regional models which behave as global models for the main geomagnetic field and its secular variation

Magnetotelluric studies across the Damara Orogen and Southern Congo craton

A thesis submitted to the Faculty of Science, University of the Witwatersrand, in fulfilment of the requirements for the degree of Doctor of Philosophy University of the Witwatersrand School of Geosciences and Dublin Institute for Advanced Studies School of Cosmic Physics Geophysics Section February 2016Archean cratons, and the Proterozoic orogenic belts on their flanks, form an integral part of the Southern Africa tectonic landscape. Of these, virtually nothing is known of the position and thickness of the southern boundary of the composite Congo craton and the Neoproterozoic Pan African orogenic belt due to thick sedimentary cover. In this work I present the first lithospheric-scale geophysical study of that cryptic boundary and define its geometry at depth. The results are derived from two-dimensional (2D) and three-dimensional (3D) inversion of magnetotelluric data acquired along four semi-parallel profiles crossing the Kalahari craton across the Damara-Ghanzi-Chobe belts (DGC) and extending into the Congo craton. Two dimensional and three-dimensional electrical resistivity models show significant lateral variation in the crust and upper mantle across strike from the younger DGC orogen to the older adjacent cratons. The Damara belt lithosphere is found to be more conductive and significantly thinner than that of the adjacent Congo craton. The Congo craton is characterized by very thick (to depths of 250 km) and resistive (i.e. cold) lithosphere. Resistive upper crustal features are interpreted as caused by igneous intrusions emplaced during Pan-African magmatism. Graphite-bearing calcite marbles and sulfides are widespread in the Damara belt and account for the high crustal conductivity in the Central Zone. The resistivity models provide new constraints on the southern extent of the greater Congo craton, and suggest that the current boundary drawn on geological maps needs revision and that the craton should be extended further south. The storage possibilities for the Karoo Basins were found to be poor because of the very low porosity and permeability of the sandstones, the presence of extensive dolerite sills and dykes. The obvious limitation of the above study is the large spacings between the MT stations (> 10km). This is particularly more limiting in resolving the horizontal layers in the Karoo basin. However the 1D models provide layered Earth models that are consistent with the known geology. The resistivity values from the 1D models allowed porosity of the Ecca and Beaufort group lithologies to be calculated. It is inferred that the porosities values are in the range 5-15 % in the region below the profile. This value is considered too low for CO2 storage as the average porosity of rock used for CO2 is generally more than 10 to 12 percent of the total rock unit volume

Combining CHAMP and Swarm Satellite Data to Invert the Lithospheric Magnetic Field in the Tibetan Plateau

CHAMP and Swarm satellite magnetic data are combined to establish the lithospheric magnetic field over the Tibetan Plateau at satellite altitude by using zonal revised spherical cap harmonic analysis (R-SCHA). These data are integrated with geological structures data to analyze the relationship between magnetic anomaly signals and large-scale geological tectonic over the Tibetan Plateau and to explore the active tectonic region based on the angle of the magnetic anomaly. Results show that the model fitting error is small for a layer 250–500 km high, and the RMSE of the horizontal and radial geomagnetic components is better than 0.3 nT. The proposed model can accurately describe medium- to long-scale lithospheric magnetic anomalies. Analysis indicates that a negative magnetic anomaly in the Tibetan Plateau significantly differs with a positive magnetic anomaly in the surrounding area, and the boundary of the positive and negative regions is generally consistent with the geological tectonic boundary in the plateau region. Significant differences exist between the basement structures of the hinterland of the plateau and the surrounding area. The magnetic anomaly in the Central and Western Tibetan Plateau shows an east–west trend, which is identical to the direction of the geological structures. The magnetic anomaly in the eastern part is arc-shaped and extends along the northeast direction. Its direction is significantly different from the trend of the geological structures. The strongest negative anomaly is located in the Himalaya block, with a central strength of up to −9 nT at a height of 300 km. The presence of a strong negative anomaly implies that the Curie isotherm in this area is relatively shallow and deep geological tectonic activity may exist