Global High-Resolution Magnetic Field Inversion using Spherical Harmonic Representation of Tesseroids as Individual Sources

In this study, we present a novel approach combining the advantages of tesseroids in representing geophysical structures though their voxel-like discretization features with a spherical harmonic representation of the magnetic field. Modelling of the Earth lithospheric magnetic field is challenging since part of the spectra is hidden by the core field and the forward modeled field of a lithospheric magnetization is always biased by the spectral range used. In our approach, a spherical harmonic representation of the magnetic field of spherical prisms (tesseroids) is used for high-resolution magnetic inversion of lithospheric field models. The use of filtered spherical harmonic models of the magnetic field of each tesseroid ensures that the resulting field matches the spectral range of the input data. For the inversion, we use the projected gradient method. The projected gradient method easily allows us to assign an initial guess (i.e., a-priori assumption) for the inversion and avoids negative values of susceptibilities. The latter is providing more plausible models since induced magnetization is assumed to be dominant over the continents and, for the oceans, a remanence model can be subtracted. We show an application of the technique to a synthetic dataset and a satellite-derived lithospheric field model where the model geometry is based on seismic information. We also demonstrate a proof-of-concept for high-resolution tile-wise inversion for the Bangui anomaly in Africa

Spectral analysis and inverse modeling of satellite data and aeromagnetic data

A series of Earth observation satellite missions has opened a new era for the study of Earth’s magnetic field. Due to the homogeneous global coverage and high accuracy of satellite data, magnetic models derived from those provide reliable estimates of the long-wavelength components of the crustal magnetic field. How such satellite magnetic models can contribute to our understanding of the characteristic of the crustal structures is the main topic of this thesis. First, a comparison of conventional filtering methods was made and a new method for regional spherical harmonic analysis is presented and a thorough discussion is provided by considering the case study of the Australian continent. Next, together with reduction to the pole of satellite data and long-wavelength corrected aeromagnetic compilations, correlated tectonic signatures over the neighboring continents in a Gondwana framework are shown. Finally, a positivity constraint was applied to the global magnetic susceptibility inversion and a globally inverted susceptibility model for a reconstructed Gondwana framework is presented and discussed

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

Towards a Digital Twin of the Earth System: Geo-Soft-CoRe, a Geoscientific Software & Code Repository

[Abstract] The immense advances in computer power achieved in the last decades have had a significant impact in Earth science, providing valuable research outputs that allow the simulation of complex natural processes and systems, and generating improved forecasts. The development and implementation of innovative geoscientific software is currently evolving towards a sustainable and efficient development by integrating models of different aspects of the Earth system. This will set the foundation for a future digital twin of the Earth. The codification and update of this software require great effort from research groups and therefore, it needs to be preserved for its reuse by future generations of geoscientists. Here, we report on Geo-Soft-CoRe, a Geoscientific Software & Code Repository, hosted at the archive DIGITAL.CSIC. This is an open source, multidisciplinary and multiscale collection of software and code developed to analyze different aspects of the Earth system, encompassing tools to: 1) analyze climate variability; 2) assess hazards, and 3) characterize the structure and dynamics of the solid Earth. Due to the broad range of applications of these software packages, this collection is useful not only for basic research in Earth science, but also for applied research and educational purposes, reducing the gap between the geosciences and the society. By providing each software and code with a permanent identifier (DOI), we ensure its self-sustainability and accomplish the FAIR (Findable, Accessible, Interoperable and Reusable) principles. Therefore, we aim for a more transparent science, transferring knowledge in an easier way to the geoscience community, and encouraging an integrated use of computational infrastructure.This research has been funded by the Projects EPOS IP 676564, EPOS SP 871121, SERA 730900, GeoCAM (PGC2018-095154-B-I00, Spanish Government) and the Center of Excellence for Exascale in Solid Earth (ChEESE) under the Grant Agreement 823844. IDF was funded by a FEDER-Junta de Castilla y León Postdoctoral contract (SA0084P20). JA and M-GL are funded by the Spanish Ministry of Science and Innovation through the Juan de la Cierva fellowship (IJC 2018-036074-I and IJC 2018-036826-I, respectively), funded by MCIN/AEI /10.13039/501100011033. AH is grateful for his Ramón y Cajal contract (RYC 2020-029253-I). Additional funding was provided by the Spanish Ministry of Science and Innovation (RTI 2018-095594-B-I00, PGC 2018-095154-B-100) and the Generalitat de Catalunya (AGAUR, 2017SGR1022). AP’s work was supported by: a Science Foundation Ireland Career Development Award (17/CDA/4695); an investigator award (16/IA/4520); a Marine Research Programme funded by the Irish Government, co-financed by the European Regional Development Fund (Grant-Aid Agreement No. PBA/CC/18/01); European Union’s Horizon 2020 research and innovation programme InnoVar under grant agreement No 818144; SFI Centre for Research Training in Foundations of Data Science 18/CRT/6049, and SFI Research Centre awards I-Form 16/RC/3872 and Insight 12/RC/2289_P2. AH and SG thank the Spanish research project PaleoModes (CGL2016-75281-C2-1-R) which provided some of their financial support. JF is supported by an Atracción de Talento senior fellowship (2018-T1/AMB/11493) funded by Comunidad Autonoma de Madrid (Spain), and a project funded by the Spanish Ministry of Science and Innovation (PID2020-114854GB-C22)Junta de Castilla y León; SA0084P20Generalitat de Catalunya; 2017SGR1022Science Foundation Ireland; 17/CDA/4695Science Foundation Ireland; 16/IA/4520Ireland. Marine Institute; PBA/CC/18/01Science Foundation Ireland; 18/CRT/6049Science Foundation Ireland; 16/RC/3872Science Foundation Ireland; 12/RC/2289_P2Comunidad Autonoma de Madrid; 2018-T1/AMB/1149

Integrated geophysical investigation of the Karoo Basin, South Africa

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy Johannesburg, August 2015 School of Geosciences, University of the WitwatersrandThe possibility of extensive shale gas resources in the main Karoo Basin has resulted in a renewed focus on the basin, and particularly the Whitehill Formation. The main Karoo Basin has been the subject of geological studies since before the 1920s, but geophysical data provides an opportunity to shed new light on the basin architecture and formation. In this thesis, I use regional gravity, magnetic and borehole data over the basin, as well as vintage seismic data in the southern part of the basin. Modern computational capacity allows for more information to be extracted from these seismic data, and for these data to be better integrated with potential field data. The integration of datasets in a three-dimensional model (3D) has allowed for a better understanding of the shape of the basin and its internal structure, in turn shedding light on basin formation. A new depth map of the basin constructed using this extensive database confirms that the basin deepens from on- to off-craton. The basin is deepest along the northern boundary of the Cape Fold Belt (CFB), with a depth of ~4000 m in the southwestern Karoo and ~5000 m in the southeastern part of the basin. Sediment thickness ranges from ~5500 to 6000 m. The Whitehill Formation along this boundary reaches a depth of ~ 3000 m in the southwest and ~4000 m in the southeast. Despite limited boreholes in this region, the basin appears to broadly deepen to the southeast. These seismic and borehole data also allow for mapping of the Cape Supergroup pinch-out below the Karoo basin (32.6°S for the Bokkeveld and 32.4°S for the Table Mountain Group), with the basin reaching a thickness of around 4 km just north of the CFB. The gravity effect of these sediments in the south is not sufficient to account for the low of the Cape Isostatic Anomaly near Willowmore and Steytlerville. This ~45 mGal Bouguer gravity low dominates the central region of the southern Karoo at the northern border of the CFB. The seismic data for the first time show uplift of lower-density shales of the Ecca Group (1800 – 2650 kg/m3) in this region, and structural and seismic data suggest that these lower density sediments continue to depth of 11 to 12 km along normal and thrust faults in this region. Two-dimensional density models show that these shallow crustal features, as well as deeper lower crust compared to surrounding regions, account for the anomaly. These seismic and borehole data also allow for constraints to be placed on the distribution and geometry of the dolerite intrusions that intruded the basin after its formation, and in some cases impacted on the shale layer, to be constrained. The highest concentrations of dolerites are found in the northwest and east of the basin, pointing towards two magma sources. The region of lowest concentration is in the south-central part of the basin. Here the intrusions are confined to the Beaufort Group, ~1000 m shallower than the shale reservoir, suggesting it should be the focus of exploration efforts. These dolerite sills are shown to be between 5 and 30 km wide and are saucer-shaped with ~ 800 m vertical extent, and dips of between 2° and 8° on the edges. The sheets in the south of the basin extend for over 150 km, dipping at between 3° and 13°, and are imaged down to ~ 5 km. This change in dip of the sheets is linked to deformation within the Cape Fold Belt, with greater dips closer to the belt, although these sheets do not appear to intrude strata dipping at more than 15 to 20°. In order to understand the shape of the Karoo basin and construct a 3D model of the basin, an understanding is needed of the underlying basement rocks. The Beattie Magnetic Anomaly (BMA) that stretches across the entire southern part of the basin forms part of the basement Namaqua-Natal Belt. Filtered magnetic data confirm that the Namaqua and Natal Belts are two separate regions with different magnetic characteristics, which is taken into account during modelling. The BMA is shown to be part of a group of linear magnetic anomalies making up the Natal Belt. The anomaly itself will therefore not have an individual effect on basin formation, and the effect of the Natal Belt as a whole will have to be investigated. An in-depth study of outcrops associated with one of these linear magnetic anomalies on the east coast of South Africa suggest the BMA can be attributed to regions of highly magnetic (10 to 100 x 10-3 SI) supracrustal rocks in Proterozoic shear zones. Along two-dimensional magnetic models in the southwestern Karoo constrained by seismic data, these magnetic zones are modelled as dipping slabs with horizontal extents of ~20-60 km and vertical extents of ~10-15 km. Body densities range from 2800- 2940 kg/m3 and magnetic susceptibilities from 10 to 100 x 10-3 SI. These, as well as other geophysical and geological constraints, are used to construct a 3D model of the basin down to 300 km. Relatively well-constrained crustal structure allows for inversion modelling of lithospheric mantle densities using GOCE satellite gravity data, with results in-line with xenolith data. These results confirm the existence of lower density mantle below the craton (~3270 kg/m3) that could contribute to the buoyancy of the craton, and an almost 50 kg/m3 density increase in the lithospheric mantle below the surrounding Proterozoic belts. It is this change in lithospheric density along with changes in Moho depths that isostatically compensate a large portion of South Africa’s high topography (<1200 m). The topography higher than 1200 m along the edge of the plateau, along the Great Escarpment, are shown to be accommodated by an asthenospheric buoyancy anomaly with a density contrast of around 40 kg/m3, while still mimicking the Bouguer gravity field. These findings are in line with recent tomographic studies below Africa suggesting an “African Superplume” or “Large Low Velocity Seismic Province” in the deep mantle. The basin sediment thickness maps were further used to investigate the formation of the main Karoo Basin. This was accomplished by studying the past flexure of the Whitehill Formation using north-south two-dimensional (2D) profiles. Deepening of the formation from ~3000 m in the southwest to ~4000 m in the southeast is explained using the concept of isostasy, i.e., an infinite elastic beam that is subjected to an increasing load size across the Cape Fold Belt. Load height values increase from 4 km in the southwest to 8 km in the southeast. This larger load is attributed here to “locking” along a subduction zone further to the south. The effective elastic thickness (Te) of the beam also increases from around 50 km over the Namaqua and Natal Belts in the southwest to 80 km over the Kaapvaal Craton and Natal Belt in the southeast. The changes in Te values do not correlate with changes in terrane, i.e., a north to south change, as previously though. The large extent and shape of the Karoo basin can therefore, in general, be explained as a flexural basin, with the strength of the basement increasing towards the southeast. Therefore, while factors such as mantle flow could have contributed towards basin formation, reducing the load size needed, it is no longer necessary in order to account for the large extent of the basin. This flexure model breaks down further to the southeast, most likely due to a very high Te value. This could be the reason for later plate break in this region during Gondwana breakup. It is inferred that this increase in Te is linked to the buoyancy anomaly in the asthenospheric mantle

Comparison of Different Coupling Methods for Joint Inversion of Geophysical data: A case study for the Namibian Continental Margin

Integration of multiple geophysical data is a key practice to reduce model uncertainties and enhance geological interpretations. Electrical resistivity models resulting from inversion of marine magnetotelluric (MT) data, often lack depth resolution of lithological boundaries and distinct information for shallow model parts. This is due to the diffusive nature of electromagnetic fields, enhanced by deficient data sampling and model regularization during inversion. Thus, integrating data or models to constrain layer thicknesses or structural boundaries is an effective approach to derive better constrained and more detailed resistivity models. We investigate the different impacts of three cross-gradient coupled constraints on 3D MT inversion of data from the Namibian passive continental margin. The three constraints are a) coupling with a fixed structural density model; b) coupling with satellite gravity data; c) coupling with a fixed gradient velocity model. Here we show that coupling with a fixed model (a and c) improves the resistivity model the most. Shallow conductors imaging sediment cover are confined to a thinner layer in the resulting resistivity models compared to the MT-only model. Additionally, these constraints help to suppress vertical smearing of a conductive anomaly attributed to a fracture zone, and clearly show that the seismically imaged Moho is not accompanied by a change in electrical resistivity. All of these observations help to derive an Earth model, which will form the basis for future interpretation of the processes that lead to continental break-up during the early Cretaceous