Joint inversion of the latest seismic and satellite gravity datasets, also including topography and other relevant data, is beginning to produce much more detailed and robust 3D models of temperature and composition within the Earth than possible previously. The satellite-measured gravity field (free-air anomalies, geoid anomalies, and gravity gradients) and terrestrial seismic measurements provide complementary constraints on 3D Earth structure. The explosive recent growth in the amount of broadband seismic data now enables us to resolve the structure of the lithosphere and underlying mantle in much finer detail than only a few years ago. Tomographic models resulting from the inversion of seismic data alone, however, are characterized by non-uniqueness. This is especially so when we target the temperature and composition of the rock at depth, essential for our understanding of the Earth’s dynamics and evolution. The recent gravity field measurement from GOCE offer essential complementary information. Here, we show how we utilize the massive datasets now available, by building state-of-the-art seismic datasets and then combining the information from surface-wave, gravity and other data by means of iterative pertrological inversion.

We use waveform inversion of over a million seismograms to constrain the crustal and upper mantle structure globally. Inversion of waveforms directly for shear-wave velocities involves solving large linear systems of many millions of equations, which produces high-resolution models but makes it challenging to combine different data types or to use probabilistic approaches. An alternative is to use phase velocities of the fundamental (and, ultimately, higher) modes measured by waveform inversions, compute global or regional phase-velocity maps in broad period ranges, and then split the 3D problem into a series of small, 1D inverse problem, one for every point at the Earth’s surface. Following this approach, we invert local phase-velocity curves jointly with other data in a petrological inversion for temperature and composition of the rock at depth, and then fine-tune the final 3D model using the gravity field data. Our global model reveals patterns of the evolution of the oceanic lithosphere in different geodynamic environments and the regional-scale structure of the lithosphere of continents.

Focussing on Africa in a regional case study, we use a large, newly available dataset for waveform tomography of the continent. It reveals a much more complex, fragmented cratonic lithosphere than seen previously. In west Africa, two separate thick lithospheric units underlie the Reguibat and Man-Leo Shields. The mantle root of the Congo Craton comprises three main parts, beneath and around the Gabon-Cameroon, Bomu-Kibali and Kasai Shields, with no high-velocity root beneath the Angola Shield. We also identify and map cratons unexposed at the surface (buried under sedimentary cover), including the Niassa Craton, SE Congo, and the Arabian Platform. In northern Africa, strong low-velocity anomalies beneath the Afar Hotspot and the East African Rift Zone are accounted for by high temperatures and partial melting and extend down to the bottom of the upper mantle. Relatively high temperatures are also detected beneath the Ahaggar, Tibesti and Darfur Hotspots in the Sahara Desert at 56-260 km depth. In the South Atlantic, the model reveals the patterns of interaction between the Mid-Atlantic Ridge (MAR) and nearby hotspots, i.e. Ascension, Tristan da Cunha, Shona and Bouvet. High-temperature anomalies beneath major hotspots extend substantially deeper than those beneath the MAR. The Vema Hotspot, in particular, displays a pronounced seismic low-velocity anomaly under the thick, cold lithosphere of the Cape Basin. A strong anomaly also underlies the offshore segment of the Cameroon Line, between Africa and the MAR.

This study is a part of the STSE 3D Earth.

We introduce a formalism for estimating local spatial averages of the core-mantle boundary (CMB) radial magnetic field and its time derivatives, based on magnetic field observations collected by low-Earth-orbit satellites. This provides a useful alternative to conventional core field modelling based on global spherical harmonic basis functions, where noise in the polar regions maps into all harmonics, and model regularization and spectral truncation are required. A powerful perspective offered by the proposed technique is formal appraisal of the spatial resolution and variance of the resulting field averages. We use the Green's functions for the Neumann boundary value problem to link the satellite observations to the radial magnetic field on the CMB and estimate field averages using a modified Backus-Gilbert inversion approach. Our approach builds on the Subtractive Optimally Localized Averages (SOLA) method developed in helioseismology, that seeks averaging kernels as close as possible to a chosen target kernel. We are able to account for both internal and external field sources and can easily incorporate data error covariance information, for example describing along-track serial error correlation. As a proof of concept we present a global map collecting local estimates of the radial main field (MF) constructed on a grid at the CMB with one degree spacing in latitude and longitude, derived from one month of three component vector magnetic field data collected by the Swarm satellite trio, using data from dark and geomagnetically quiet times. Using sums and differences of the field components taken along track and in the east-west direction we obtain estimates with spatial resolution kernel widths varying between 18 and 54 degrees depending on the latitude, and a standard deviation of approximately 10muT (i.e. 5% of the mean CMB field amplitude). The morphology of our CMB radial field map agrees well with results from conventional spherical harmonic field models. In a second application, we determine local estimates of the average rate of change, or secular variation (SV), of the radial field at the CMB, initially considering two year time windows, and performing the analysis on data collected by either the Swarm or CHAMP satellites. We are able to obtain stable local estimates of the SV at the CMB, and present maps of estimates having averaging kernel widths of approximately 42, 33 and 30 degrees on the equator, with corresponding standard derivations of 0.25, 2.5 and 5muT/yr respectively. By subtracting SV estimates constructed at different epochs we are able to calculate the local aggregated secular acceleration (SA) and to study its time changes. Differencing SV estimates 2 years apart, and considering an averaging kernel width of 42 degrees on the equator, we obtain SA maps very similar to those found in the CHAOS-6 field model truncated at SH degree 10. Here, however, we are able to directly control the width of the spatial averaging kernel and the length of the time window under study, enabling study of the robustness of the inferred SA. Pushing to higher resolution in time, considering one year differences of SV estimates constructed using one year windows, we are able to track the evolution of coherent SA structures in time-longitude plots at the equator. At 25W in mid 2007 we find a distinctive SA 'cross-over' event, with strong, oppositely signed and adjacent, SA features rapidly changing sign within a year. Our method is well suited for studying such spatio-temporally localized SA events at high resolution; there will be further opportunities for such investigations as the time series of data provided by the Swarm mission lengthens.

We present our two-step approach for determining electrical conductivity of the upper mantle using solar quiet (Sq) variations. Due to their frequency content (1, 2, 3, 4 … cycles per day), Sq variations are sensitive to Earth’s mantle conductivity at about 150 to 500 km depth.

We start by determining the inducing part of the Sq variations (the source). We work with magnetically quiet equinoctial days from nearly 20 years of magnetic fields from mid-latitude geomagnetic observatories. The source is parameterized by spherical harmonics (SH). Using Fourier spectra of horizontal magnetic field components, we determine a frequency-specific set of SH coefficients for the selected days. This procedure requires a knowledge about Earth's conductivity distribution, which we specify as 1-D mantle overlain by non-uniform continents and oceans. The latter allows us to account for the ocean induction effect (OIE).

Next, we estimate a new type of transfer functions (TFs) which relate at a specific frequency SH coefficients, describing the source, with Fourier spectra of a local vertical magnetic field. With this approach, we can estimate TFs even at sites not used for the source determination, e.g. at sea-bottom sites.

Using 3-D forward operator to account for OIE, we invert the estimated TFs for local 1-D conductivity profiles using stochastic formalism which, along with the model itself, provides the model uncertainties. We recover local 1-D conductivity profiles at multiple stations and discuss the detected differences.

In our presentation, we present our efforts to combine model products from the ESA Support to Science Elements GOCE+Antarctica, 3D Earth and CryoSMOS in order to estimate geothermal heat-flux in Antarctica. The thermal structure of the Antarctic continent is one of the most unknown parameters, but critical for a variety of applications on different scales, e.g. ice-sheet modelling or glacial-isostatic adjustments. In recent years, a variety of methods (e.g. seismology, gravity and Curie isotherm modelling) has been used to estimate the thermal structure and most specific the geothermal heat flux. Due to the limited amount of direct measurements, estimates are often derived from modelling of satellite data or seismological models, or based on a combination of these. All of these approaches indicate the clear differences between the geothermal heat-flux in East and West Antarctica, but result as well in local differences of up to 50 %.

In order to reconcile these differences, we present an inverse modelling framework, where data products from the GOCE, Swarm and SMOS satellite missions are integrated with seismological results. Our set-up allows us to quantify how the uncertainties of the different data products is affecting the reliability of the derived models. First results show that especially the use of a specific Curie isotherm model is heavily influencing the inversion results and not necessarily leading to a reasonable distribution of petrophysical parameters (e.g. radiogenic heat production). Therefore, Curie isotherm estimates have to go hand in hand with interpretation of airborne geophysical data to be suitable for interpretation. The combination of the different data sets by inverse modelling reveals as well the crucial role of seismological or ideally seismological models integrated with gravity to define the background heat-flow. The information on the ice temperature has a minor influence, but is crucial in deriving a realistic estimate for ice-sheet modellers. Our approach is a step towards a more consistent model of the temperature of the Solid Earth in Antarctica.

Despite significant advancements in our knowledge of the crustal architecture and tectonic history of Antarctica, aided by major new continent-wide magnetic and gravity anomaly compilations such as ADMAP 2.0 and AntGG respectively, and tantalising new views of the Antarctic lithosphere derived from GOCE satellite gravity gradient data, the South Pole region itself has remained poorly known, due to the gap in satellite observations and the paucity of modern airborne geophysical research.

Here we present some highlights from the PolarGAP project of ESA that are helping unveil the subglacial topography and deeper crustal architecture of the South Pole region and provide new views of the tectonic and erosional processes that affected this part of Antarctica from the Precambrian to the Cenozoic.

The new bedrock topography map shows that the previously inferred Pensacola-Pole Basin (PPB) stretches from the Weddell Sea Rift System to South Pole. The new free-air gravity data indicate that it is superimposed upon a system of glacially over-deepened grabens, which were likely active during Jurassic rifting and inferred strike-slip faulting in association with the wider Transantartic Rift System. The overdeeping is however attributed to much later Cenozoic ice sheet development, including preferential steering of ice streaming along the inferred rifts and fault systems.

The aeromagnetic data contain subtle imprints of the Jurassic magmatism associated with this rift system, but are mostly reflecting older basement rocks, including both arc and back-arc intrusions linked to the Cambrian stages of the Ross Orogen.

We then combined the PolarGAP aeromagnetic data with ADMAP 2.0 data and this sheds new light into the extent of Precambrian basement provinces hidden beneath the East Antarctic Ice Sheet in the South Pole region. This enables us in turn to image an inherited crustal boundary between a proposed back-arc region of the Ross Orogen and an inferred composite Precambrian microplate referred to here as the Recovery Province. The inherited boundary was likely reactivated during Jurassic rifting and strike slip faulting, as part of a more distributed plate boundary between East and West Antarctica and exerted key controls on the location and early development of the Pensacola-Pole basin.

Finally, englacial layers revealed from PolarGAP radar imaging provide unexpected hints for potential areas of high geothermal heat flux (GHF) near South Pole. We infer that such elevated geothermal heat flux may be caused by buried high heat producing Precambrian basement rocks. This has important implications for the search for the oldest ice in the South Pole region and for future ESA studies addressing the wider role of the Solid Earth, and in particular GHF, in influencing sublacial hydrology beneath the Antarctic ice sheets.

In recent years, a couple of studies, based on different geophysical methods, provided new images of the lithospheric structure of the Antarctic continent. However, the findings are not always in agreement, and uncertainties regarding the thermal and density structure of the lithosphere are high. Together with flexural rigidity, mantle viscosity, and surface heat flow, knowledge of Antarctica’s lithospheric build-up is critical in glacial isostatic adjustment modelling. Comprehensive lithospheric models can help to reduce ambiguities of individual methods and datasets, and are thus suitable to better constrain the interactions between the solid earth and the cryosphere. Here we combine the latest seismological findings with gravity gradient data from ESA´s GOCE satellite mission, which are in particular sensitive to the geometry and density variations of the main lithospheric layers.
Our 3D lithospheric model of Antarctica accounts for thermodynamically consistent temperature and petrology of the upper mantle, seismic body wave velocities, and isostasy. Our results indicate that Antarctica is largely in isostatic equilibrium, however, the topography of some regions seem to have components which cannot be explained by pure isostasy. A new Moho depth map of the continent is derived that is consistent with the gravity gradient field and satisfies seismic crustal thickness estimates. It exhibits detailed thickness variations in East Antarctica, also in regions with sparse seismic station coverage where crustal thickness estimates in previous studies were unreliable.
The subsurface thermal field of our model can be used to infer the mantle’s viscosity for the purpose of GIA modelling. We find that viscosity values at shallow depth (150 km) in the south part of the Ross Sea, part of the Antarctic Peninsula, and East Antarctic coastal areas are lower than previous estimates of 3D mantle viscosity. This increases the importance of late Holocene ice mass changes in these areas and underlines the requirement of improving mantle viscosity estimates.