Satellite Gradients for Lithospheric Modelling – Sensitivity Tests Over the Northern Segment of the Trans-European Suture Zone
1Kiel University, Germany; 2Bundesamt für Kartographie und Geodäsie, Frankfurt, Germany; 3NTNU, Trondheim, Norway; 4Geological Survey of Norway, Trondheim, Norway; 5NASA Goddard Space Flight Center, Greenbelt, MD, USA; 6Universities Space Research Association, Greenbelt, MD, USA; 7ESA-ESTEC
In the framework of the ESA Support To Science Element ‘SLIM – Satellite Magnetic Gradients for Lithospheric Modelling’, we explore if gradients calculated from the Swarm satellite data can improve modelling of lithosphere structures. Our test area is the northern segment of the Trans-European Suture Zone (TESZ), which shows a distinct magnetic anomaly on aeromagnetic and satellite data. In addition, we exploit a pre-existing model of the lithosphere, which provides structural (Depth to the Top Basement and to the Moho) constraints and isotherms (e.g. the depth to the Curie isotherm). In our approach we first invert aeromagnetic data for the susceptibility distribution within different geometries, where the deepest magnetic sources are limited by (i) 15 km depth, (ii) the Moho) and (iii) the Curie isotherm. Forward modelling of data at satellite height of 400km shows that the three models are not very distinct in the calculated fields. The gradients are more sensitive to the differences, especially the invariants and the so-called curvature component Txy.
In the following, we discuss the gain by lowering the satellite height to distinguish the different lithospheric models. A lower calculation height of 200 or 300 km increases the lithospheric signal significantly, and especially the gradients allow to differentiate the different models. Inverse modelling of the synthetic data allows to a certain degree to recover the effective induced magnetisation, when gradients are used, while the vector field is more sensitive to regional trends. However, Swarm is not measuring gradients directly, but these are calculated from the vector data. This means that only the gradients along and between the tracks are available. In our study area, already single-component inversion of the Txy component helps to establish a first-order model of the magnetisation within a pre-defined geometry. Such a model can be used as background or reference model for modelling of aeromagnetic data.
Constraining Lateral Variations of Upper-Mantle Electrical Conductivity Using Satellite-Detected Tidal Magnetic Signals
ETH Zurich, Institute of Geophysics, Switzerland
Data from CHAMP and Swarm satellites were shown to contain magnetic signals due to M2 tidal flow which were recently used to image the global electrical structure of the oceanic lithosphere and upper mantle down to a depth of about 250 km. This represents an important complement to the long-period magnetospheric responses, which lack resolution in the upper mantle. An open question is whether we can infer lateral variations in upper mantle conductivity from satellite-detected tidal magnetic signals and associate them with various tectonic processes? This study presents a comprehensive 3D feasibility study and 3D inversion results using real data.
Linking GIA and Lithospheric Structure of Antarctica with Satellite Gravity Gradients
1Kiel University, Germany; 2DTU Space, Copenhagen, Denmark; 3TU Delft, The Netherlands; 4British Antarctic Survey, Cambridge, UK; 5ESA-ESTEC
In the ESA Support to Science Element GOCE+Antarctica, we study the influence of the lithospheric structure on estimates of GIA. From recent geophysical, especially seismological, studies new insights on the deep structure of the Antarctic continents are available. However, the seismological models differ in resolution and do not provide a consistent image of the lithosphere. This is critical in analysing the feedback between the lithosphere and glacial loading or unloading.
To reduce such ambiguities, we combine the latest seismological models with gravity gradient data derived from the GOCE satellite mission. The gradients are in particular sensitive to the geometry and density variations of the main lithospheric layers, i.e. ice and sediment thickness, the Moho depth and the temperature and composition of the upper mantle. Initial results indicate that differences exist in the mode of compensation for West and East Antarctica related to different mantle properties.
The impact of an improved lithospheric model on GIA modelling is estimated by testing the sensitivity to the new temperature and density distribution and by comparing 1D and 3D viscosity models, especially in areas of low viscosity as in the Amundsen Sea sector.
Processing and Analysis of Satellite Gravity and Magnetic Data for Modelling the Lithosphere in Framework of 3D Earth
1CAU Kiel, Germany; 2Bundesamt für Kartographie und Geodäsie, Frankfurt, Germany; 3NASA Goddard Space Flight Center, Greenbelt, MD, USA
Shape index calculations and curvature analysis from GOCE gravity gradients are used successful for global tectonic and geological interpretations. The same techniques are applied to the Swarm magnetic data. Out test area is located over the northern part of the Trans-European Suture Zone (TESZ).
In a first step, the shape index is calculated from the spherical harmonic global model CHAOS 5 and 6. Low spherical harmonic degrees, related to the main field are suppressed, which results in ringing due the cut-off of the filter. Such data cannot be used for shape index calculation directly. We filter the data with different bandpass and cosine filters to remove those cut-off effects.
In a second step, we calculate magnetic gradients directly from track vector data for supposed magnetic quiet periods (Kp < 3). Still, some tracks compromise large outliers and a further quality analysis is performed to remove those outliers, e.g. lower noise level, only dark data, noise reduction by levelling. Next, a global reduction to pole techniques with varying inclination is applied to link the location of the magnetic anomalies to geological sources and to calculate meaningful gradient products.
This work has been performed in the framework of the ESA-STSE "SLIM - Swarm magnetic gradients for lithospheric modelling" and will be continued in the "3D Earth - a dynamic living planet".
Impact of Heat Flow and Laterally Varying Susceptibility on the Crustal Field
1Kiel University, Germany; 2Norwegian University of Science and Technology
The Curie isotherm should represent the lower limit of magnetization in the crust/lithosphere. This provides an opportunity to connect measurements of the magnetic field with thermal modelling. Using a simple thermal model, we infer the depth of the Curie isotherm from heat flow and LAB depth. In the modelling, we assume exponentially decaying heat production in the crust and a constant temperature of 1300 °C at the LAB. We calculate the magnetic crustal field caused by the estimated Curie isotherm assuming laterally constant susceptibilities for crust and mantle respectively.
The crustal/lithospheric field thus derived can explain some features of the magnetic field, namely those with a likely thermal origin. However, the overall fit is very poor.
For comparison we used a model of Vertically Integrated Susceptibility based on geological provinces to derive susceptibilities and ultimately the magnetic field. The model with laterally varying susceptibility has a better fit to the observed data.
We will discuss the uncertainties of the thermal modelling and their impact on the magnetic field.
Global Thermochemical Imaging of the Lithosphere Using Satellite and Terrestrial Observations
Dublin Institute for Advanced Studies, Ireland
Conventional methods of seismic tomography, topography, gravity and electromagnetic data analysis and geodynamic modelling constrain distributions of seismic velocity, density, electrical conductivity, and viscosity at depth, all depending on temperature and composition of the rocks within the Earth. However, modelling and interpretation of multiple data sets provide a multifaceted image of the true thermochemical structure of the Earth that needs to be appropriately and consistently integrated. A simple combination of gravity, electromagnetic, geodynamics, petrological and seismic models alone is insufficient due to the non-uniqueness and different sensitivities of these models, and the internal consistency relationships that must connect all the intermediate parameters describing the Earth involved.
Thermodynamic and petrological links between seismic velocities, density, electrical conductivity, viscosity, melt, water, temperature, pressure and composition within the Earth can now be modelled accurately using new methods of computational petrology and data from laboratory experiments. The growth of very large terrestrial and satellite (e.g., Swarm and GOCE ESA missions) geophysical data sets over the last few years, together with the advancement of petrological and geophysical modelling techniques, now present an opportunity for global, thermochemical and deformation 3D imaging of the lithosphere and underlying upper mantle with unprecedented resolution.
This project combines state-of-the-art seismic waveform tomography (using both surface and body waves), newly available global gravity satellite data (geoid and gravity anomalies and new gradiometric measurements from ESA's GOCE mission) and surface heat flow and elevation within a self-consistent thermodynamic framework. The aim is to develop a method for detailed and robust global thermochemical image of the lithosphere and underlying upper mantle. In a preliminary study, we convert a state-of-the-art global waveform tomography velocity model into a mantle density model based on thermodynamic considerations and compute its 3D synthetic gravity response to compare with satellite data. As part of work in progress we present a lithospheric model based on integrated geoophysical-petrological inversion of surface wave dispersion curves (Rayleigh and Love), topography, lithospheric geoid and surface heat flow. Broadband Rayleigh and Love fundamental mode phase velocity dispersion curves come from global phase velocity maps, computed in a broad period range using a large global dataset of phase velocity measurements, obtained using waveform inversion. The inversion is a non-linear gradient search combining steepest descent and local quadratic algorithms (Lavenberg-Marquardt) including dumping to a reference model and regularization for smoothness. The parameter space includes crustal structure (three layers with constant density, seismic velocities, heat production and thickness), mantle structure (Lithosphere-Asthenosphere boundary depth and temperature, composition and temperature distribution within the sublithosphere) and seismic radial anisotropy.