Acquisition and processing of gravity and magnetic field data and their integrative interpretation

Gravity and magnetic field data contribute to a wide range of geo-scientific research, from imaging the structure of the earth and geodynamic processes (e.g. mass transport phenomena or deformation processes) to near surface investigations. The session is dedicated to contributions related to spatial and temporal variations of the Earth gravity and magnetic field at all scales. Contributions to modern potential field research are welcome, including instrumental issues, data processing techniques, interpretation methods, innovative applications of the results and data collected by modern satellite missions (e.g. GOCE, GRACE, Swarm), potential theory, as well as case histories.

Co-organized by EMRP2/GD8
Convener: Jörg Ebbing | Co-conveners: Carla Braitenberg, Alexandra Guy, Mikhail Kaban, Leonardo UiedaECSECS
vPICO presentations
| Fri, 30 Apr, 15:30–17:00 (CEST)

vPICO presentations: Fri, 30 Apr

Alexander Minakov and Carmen Gaina

We explore the mantle density structure of the northeast Atlantic region by performing constrained linear inversion of the satellite gravity gradient tensor data using statistical prior information. The residual gravity gradient signal and the prior covariance matrix are obtained using a crustal model constrained by updated database of seismic reflection and refraction profiles. We construct a 3D reference density distribution in the upper mantle assuming a pure shear model for lithospheric rifting. The mantle reference density model is consistent with mineral phase equilibria assuming a pyrolitic bulk composition. The forward modeling of the gravity gradients in the 3D reference model is performed on a global scale using a spherical harmonics approach. The northeast Atlantic model is represented using a spherical shell covering the study region down the depth of 410 km. We use tesseroids as mass elements for solving the forward and inverse gravity problem at the regional scale. The relationship between the seismic velocity and density anomalies in the Iceland-Jan Mayen region and the low-density corridor across central Greenland are discussed for understanding the origin of heterogeneities in the upper mantle of the northeast Atlantic region and their possible connections with the Cenozoic Iceland plume activity.

How to cite: Minakov, A. and Gaina, C.: Using satellite data to decipher geodynamics of the northeast Atlantic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-438, https://doi.org/10.5194/egusphere-egu21-438, 2021.

Bart Root, Javier Fullea, Jörg Ebbing, and Zdenek Martinec

Global gravity field data obtained by dedicated satellite missions is used to study the density distribution of the lithosphere. Different multi-data joint inversions are using this dataset together with other geophysical data to determine the physical characteristics of the lithosphere. The gravitational signal from the deep Earth is usually removed by high-pass filtering of the model and data, or by appropriately selecting insensitive gravity components in the inversion. However, this will remove any long-wavelength signal inherent to lithosphere. A clear choice on the best-suited approach to remove the sub-lithospheric gravity signal is missing. 

Another alternative is to forward model the gravitational signal of these deep situated mass anomalies and subtract it from the observed data, before the inversion. Global tomography provides shear-wave velocity distribution of the mantle, which can be transformed into density anomalies. There are difficulties in constructing a density model from this data. Tomography relies on regularisation which smoothens the image of the mantle anomalies. Also, the shear-wave anomalies need to be converted to density anomalies, with uncertain conversion factors related to temperature and composition. Understanding the sensitivity of these effects could help determining the interaction of the deep Earth and the lithosphere.

In our study the density anomalies of the mantle, as well as the effect of CMB undulations, are forward modelled into their gravitational potential field, such that they can be subtracted from gravity observations. The reduction in magnitude of the density anomalies due to the regularisation of the global tomography models is taken into account. The long-wavelength region of the density estimates is less affected by the regularisation and can be used to fix the mean conversion factor to transform shear wave velocity to density. We present different modelling approaches to add the remaining dynamic topography effect in lithosphere models. This results in new solutions of the density structure of the lithosphere that both explain seismic observations and gravimetric measurements. The introduction of these dynamic forces is a step forward in understanding how to properly use global gravity field data in joint inversions of lithosphere models.

How to cite: Root, B., Fullea, J., Ebbing, J., and Martinec, Z.: Combining the deep Earth and lithospheric gravity field to study the density structure of the upper mantle, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2950, https://doi.org/10.5194/egusphere-egu21-2950, 2021.

Nils-Peter Finger, Mikhail K. Kaban, Magdala Tesauro, Walter D. Mooney, and Maik Thomas

The presented model describes the lithospheric state of the cratonic regions of Africa in terms of temperature, density and composition based on joint analysis of gravity and seismic data. In addition, a new model of depth to the Moho was calculated from available seismic data. It was then used in combination with data on topography, sediments, and deep mantle anomalies to obtain residual mantle gravity and residual topography. These residual fields were corrected for thermal effects based on S-wave tomography and mineral physics constraints, assuming a juvenile mantle. Afterwards, the thermally corrected fields are jointly inverted to uncover potential compositional density variations. Following the isopycnic hypothesis, negative variations in cratonic areas are interpreted to be caused by iron depletion. Adapting the initially juvenile mantle composition allows to iteratively improve the thermal and compositional variations, culminating in a self-consistent model of the African lithosphere. Deep depleted lithospheric roots exist under the Westafrican, northern to central Congo, and Zimbabwe Cratons. The temperatures in these areas range from below 800 °C at 100 km depth to 1200 °C at 200 km depth. Higher temperatures and absence of depletion at depths below 100 km in wide areas of the eastern to southern Congo and the Kaapvaal Cratons indicate a thinner and strongly reworked lithosphere.

How to cite: Finger, N.-P., Kaban, M. K., Tesauro, M., Mooney, W. D., and Thomas, M.: An integrated thermo-compositional model of the African cratonic lithosphere from gravity and seismic data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15215, https://doi.org/10.5194/egusphere-egu21-15215, 2021.

Mikhail Kaban, Alexei Gvishiani, Roman Sidorov, Alexei Oshchenko, and Roman Krasnoperov

A new model has been developed for the density and thickness of the sedimentary cover in a vast region at the junction of the southern part of the East European Platform, the Pre-Caucasus and some structures adjacent to the south, including the Caucasus. Structure and density of sedimentary basins was studied by employing the approach based on decompensation of gravity anomalies. Decompensative correction for gravity anomalies reduces the effect of deep masses providing compensation of near-surface density anomalies, in contrast to the conventional isostatic or Bouguer anomalies. . The new model of sediments, which implies their thickness and density, gives a more detailed description of the sedimentary thickness and density and reveals new features which were not or differently imaged by previous studies. It helps in better understanding of the origin and evolution of the basins and provides a background for further detailed geological and geophysical studies of the region.

How to cite: Kaban, M., Gvishiani, A., Sidorov, R., Oshchenko, A., and Krasnoperov, R.: A new sedimentary cover model for the southern area of the East European Platform and the Pre-Caucasus based on decompensation gravity anomalies data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15625, https://doi.org/10.5194/egusphere-egu21-15625, 2021.

David Arutyunyan, Ivan Lygin, Kirill Kuznetsov, Tatiana Sokolova, Tatiana Shirokova, and Alexey Shklyaruk

The 3D gravity inversion was realized in order to reveal the density features of the Earth's crust the Barents Sea. The original 3D density model of the region includes both lateral and depth density`s changes.
The main steps of the modelling are:

- The calculation of the anomalies of the gravity field in Bouguer reduction with the three-dimensional gravitational effect correction of the seabed.

- Gravity field correction for the three-dimensional influence of the Moho boundary (according to the GEMMA model). The excess density at the Moho picked by minimizing the standard (root-mean-square) deviation of the gravity effect from GEMMA Moho boundary and Bouguer anomalies. So, the regional density jump at the Moho border is 0.4 g / cm3.

- Based on regional geological and geophysical data about the deep structure of the Barents Sea, it was developed generalized dependence of density changes by depth in the sedimentary cover and the consolidated part of the earth's crust.

- Compilation of 3D original model of the base of the sedimentary cover on predictive algorithms of neural networks. The neural network was trained on several reference areas located in different parts Barents area using a number of potential fields transformations and the bottom of the sedimentary cover from model SedThick 2.0.

- Using the resulted dependence of the crust density change by depth and a new model of the sedimentary cover bottom, the gravitational field corrected for the impact of the sedimentary cover with variable density.

- The finally stripped gravity field is used to create density model above and below the base of the sedimentary cover. Frequency filtering on Poisson wavelets [Kuznetsov et al., 2020] had been used for the final separation of the gravitational field into its components.

- The inverse task was solved using specialized volumetric regularization [Chepigo, 2020].

As a result, the crust of the Barents Sea density inhomogeneities were localized by depth and laterally in 3D model, which became the basis for further structural-tectonic mapping.


Chepigo L.S. GravInv3D [3D density modeling software]. Patent RF, no. 2020615095, 2020. https://en.gravinv.ru/

Kuznetsov K.M. and Bulychev A.A. GravMagSpectrum3D [Program for spectral analysis of potential fields]. Patent RF, no. 2020619135, 2020.

How to cite: Arutyunyan, D., Lygin, I., Kuznetsov, K., Sokolova, T., Shirokova, T., and Shklyaruk, A.: The latest 3D density model of the Barents Sea crust, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12151, https://doi.org/10.5194/egusphere-egu21-12151, 2021.

Anirban Biswas and Srinivasa Rao Gangumalla

Indo-Burma subduction zone is one of the seismically active regions in India where the Indian plate is underthrusting the Burmese arc. However, the nature of the slab subduction in this region and its associated stress-regime are less understood due to the lack of deep crustal information. In the present study, we analyze the vertical gravity component of the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) and topography data to model the Moho depth interface and flexure parameters of the Indo-Burmese subduction region. Here, Moho depths are obtained by performing the non-linear gravity inversion using tesseroids in spherical coordinates. It is observed that the Moho interface in the Bay of Bengal (Indian plate) lies at a depth of 20-30 km and then deepens to a depth of 50-60 km towards the Burmese region. Beneath the Shan Plateau, Moho depth varies gently from 35 to 40 km and shows an eastward dip at Sagaing fault.  We also constructed eight profiles across the subduction zone to model the flexure parameters such as effective elastic thickness (Te), forebulge, and bending moments (Mo). The modelling results indicate that both Te (15-55 km) and Mo (1.12×10-19 to 2.84×10-19 N.m) values vary significantly along the subduction zone and show correlation with slab depth. Larger values of Te (55 km) and Mo (2.84×10-19 N.m) are noticed in the central Indo-Burmese subduction zone, where the slab depth is around 110-120 km. Whereas the lowest values of Te (15 km) and Mo (1.12×10-19 N.m) are inferred for the profiles lying in the southern Indo-Burmese subduction.

How to cite: Biswas, A. and Gangumalla, S. R.: Modelling the Moho depth and Flexure parameters across the Indo-Burma subduction zone., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6208, https://doi.org/10.5194/egusphere-egu21-6208, 2021.

Mitsuhiro Hirano, Hiroyuki Nagahama, and Jun Muto

In the northeastern Japan arc with the active compressive stress field since ~3 Ma, it is reported that active faults have a characteristic distribution on the short-wavelength (< 160 km) Bouguer anomalies: Active faults tend to be located in negative regions. It suggests that they do not simply correspond to geologic distributions, and also reflect active crustal deformation in the northeastern Japan arc. Although previous studies proposed that cracks and volumetric strain caused by faulting contribute to negative gravity anomalies, the quantitative effect of active faults on the short-wavelength Bouguer anomalies in the northeastern Japan arc has been unclear in previous studies because of the low resolution of the gravity map. So, we evaluated the quantitative effect of active faults in the northeastern Japan arc using the latest digital datasets for gravity measurements. First, we created a new short-wavelength (< 160 km) Bouguer anomaly map with high spatial resolution and redrew the geologic map to the mass-density distribution map. On our map, active faults are accompanied by negative regions or grooves. The negative regions or grooves with active faults cannot be only explained by the existence of a low mass-density layer (e.g., sedimentary layer) based on the mass distribution map and cylinder's model with a mass-density depending on the depth. We then showed that gravity anomalies due to accumulated cracks and volumetric strain caused by faulting over the past three million years, which is estimated at around -10 mGal, should also be taken into account. Our result indicates accumulated crustal deformation can generate negative gravity anomaly zones along the strain concentration zones, impacting the pattern of short-wavelength Bouguer anomalies throughout in the entire northeastern Japan arc. Moreover, the earthquakes occur near the crustal bending regions in Niigata-Kobe Tectonic zone, which is a strain concentration field. Since active crustal deformation with large dislocation is associated with the curvature of crustal bending, gravity anomalies can be related to the crustal geometry including the curvature. Finally, we would reveal that the relationship between gravity anomaly and crustal deformation originates from the correspondence among differential geometric objects in space-time and material space, and the short-wavelength Bouguer anomalies are the result of its projection.

How to cite: Hirano, M., Nagahama, H., and Muto, J.: Short-wavelength Bouguer anomaly and active faults in the northeastern Japan arc from the viewpoint of differential geometry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14053, https://doi.org/10.5194/egusphere-egu21-14053, 2021.

Alexandra Guy, Christel Tiberi, and Saandar Mijiddorj

This study integrates gravity modelling and analysis with seismic constraints through the prism of seismic anisotropy to characterize the structures of southern Mongolia, in particular at the lower crustal but also the upper mantle levels. Recently, gravity signal analysis and forward modelling combined with magmatic geochemistry and thermodynamic modelling demonstrate that relamination of allochtonous felsic to intermediate lower crust played a major role in southern Mongolia structure. Relamination of material induces a homogeneous layer in the lower crust, which contrasts with the highly heterogeneous upper crustal part composed of different lithotectonic domains. The seismic signals of the seven southernmost stations of the MOBAL2003 experiment were analyzed to get the receiver functions. The data treatment was performed following a new protocol, which reduces the noise on the different components. This treatment reveals the variation of the crustal thickness of cca. 10 km along the first 450 km of the profile. In addition, some seismic stations display significant signals related to the occurrence of a low velocity zone (LVZ) at lower crustal and upper mantle levels. The depth of the Moho discontinuity and the dips of the seismic interfaces obtained from the seismic inversions as well as the boundaries of the different tectonic zones constitute the starting points from the 2D forward gravity modelling along the southern part of the MOBAL 2003 profile. Moreover, the density values applied to the different blocks were determined according to the global lithological composition of the different units and the vergences of the tectonic contacts were constrained by the geodynamic studies. The gravity modelling reveals the occurrence of a low density zone in the lower crust beneath the four southernmost seismic stations, which corresponds to the LVZ observed with the receiver function analysis. The combination of the independent methods enhances the occurrence of a low velocity and a low density zone (LVLDZ) at lower crustal level beneath the southernmost part of the MOBAL 2003 seismic profile. These LVLDZ may demonstrate the existence of the relamination of a hydrous material in southern Mongolia.

How to cite: Guy, A., Tiberi, C., and Mijiddorj, S.: Crustal structures of southern Mongolia from seismic anisotropy and gravity analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-867, https://doi.org/10.5194/egusphere-egu21-867, 2021.

Nils Holzrichter, Alexandra Guy, and Jörg Ebbing

Machine learning applications in geophysical studies are often used to predict geophysical observations in areas with sparse or not data or recognize patterns and similarities in data. In our study, we test different techniques to improve the information of constraining data by machine learning and to improve strengthen the modelling of lithospheric structures with potential field data. Constraining data like seismic information, surface geology, rock classifications etc. is often used during the interpretation step of lithospheric modelling to aid the qualitative interpretation. Consensus between additional data and the own model is assessed by comparison and used to describe the model goodness consistency. First Wwe test, how this additional data can be used before the modelling by using machine learning techniques to quantify the data. We focus on supervised learning to predict crustal structure in areas with little constraints, on trained learning in data-rich areas. Second, we test the spatial analysis of surface data to determine lithospheric boundaries in depth. These tests are performed in North America and the Central Asian Orogenic belt (CAOB) to compare the results in areas with respectively good and spare data coverage. That approach can be used to link the large variety of surface and deep information in the CAOB region.

The combination of the different geophysical data available with the geological data should improve our tectonic modelling.

How to cite: Holzrichter, N., Guy, A., and Ebbing, J.: Predict depth constraints for lithospheric modelling by machine learning , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2921, https://doi.org/10.5194/egusphere-egu21-2921, 2021.

Santiago Rubén Soler and Leonardo Uieda

The equivalent source technique is a well known method for interpolating gravity and magnetic data. It consists in defining a set of finite sources that generate the same observed field and using them to predict the values of the field at unobserved locations. The equivalent source technique has some advantages over general-purpose interpolators: the variation of the field due to the height of the observation points is taken into account and the predicted values belong to an harmonic field. These make equivalent sources a more suited interpolator for any data deriving from a harmonic field (like gravity disturbances and magnetic anomalies). Nevertheless, it has one drawback: the computational cost. The process of estimating the coefficients of the sources that best fit the observed values is very computationally demanding: a Jacobian matrix with number of observation points times number of sources elements must be built and then used to fit the source coefficients though a least-squares method. Increasing the number of data points can make the Jacobian matrix to grow so large that it cannot fit in computer memory.

We present a gradient-boosting equivalent source method for interpolating large datasets. In it, we define small subsets of equivalent sources that are fitted against neighbouring data points. The process is iteratively carried out, fitting one subset of sources on each iteration to the residual field from previous iterations. This new method is inspired by the gradient-boosting technique, mainly used in machine learning solutions.

We show that the gradient-boosted equivalent sources are capable of producing accurate predictions by testing against synthetic surveys. Moreover, we were able to grid a gravity dataset from Australia with more than 1.7 million points on a modest personal computer in less than half an hour.

How to cite: Soler, S. R. and Uieda, L.: Gradient-boosted equivalent sources for gridding large gravity and magnetic datasets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1276, https://doi.org/10.5194/egusphere-egu21-1276, 2021.

Mohamed Sobh, Khaled Zahran, Nils Holzrichter, and Christian Gerhards

Widespread Cenozoic volcanisms in the Arabian shield including “Harrats” have been referring to lithospheric thinning and/or mantle plume activity as a result of Red Sea rift-related extension.

A fundamental key in understanding the deriving mechanism of these volcanic activities and its relationship to 2007-2009 seismic swarms required a reliable model of the present-day lithospheric thermo-chemical structure.

In this work, we modeled crustal and lithospheric thickness variation as well as the variations in thermal, composition, seismic velocity, and density of the lithosphere beneath the Arabian shield within a thermodynamically self - consistent framework.

The resulting thermal and density structures show large variations, revealing strong asymmetry between the Arabian shield and Arabian platform within the Arabian Plate.

We model negative density anomalies associated with the hot mantle beneath Harrats, which coincides with the modelled lithosphere thinned (~ 65 km) as a result of the second stage of lithospheric thinning following the initial Red Sea extension.

How to cite: Sobh, M., Zahran, K., Holzrichter, N., and Gerhards, C.: Thermal Imaging of the Lithosphere beneath Arabian Shield and Implications for "Harrats" Volcanic Field, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7785, https://doi.org/10.5194/egusphere-egu21-7785, 2021.

Rama Chandrudu Arasada and Srinivasa Rao Gangumalla

Eastern Indian shield comprises rocks that well persevered the Archean to the Proterozoic history of the earth. However, the lithospheric evolution of the region is poorly understood due to the scanty of seismological observations. In the presented study, an integrated approach is adopted to analyze the satellite gravity (GOCE), aeromagnetic, and topography data complemented with seismological constraints to understand the thermal evolution of the region. Wavelet based Bouguer-topography coherence method was used to compute spatial variations of effective elastic thickness (Te) in the region. We noticed high Te values of 27-31 km over EGMB and low to moderate Te values of 22-30 km over SC and CGGC. Results of 3-D forward gravity modeling of Complete Bouguer anomalies show that the Moho boundary lies at a depth of 35-38 km below the Eastern Ghats Mobile Belt (EGMB) and 38-40 km below Singhbhum Craton (SC), and it increases gradually towards the Chotanagpur granite gneiss complex (CGGC) to a depth of 40-44 km. Curie depth point (CDP) values obtained based on the spectral analysis of aeromagnetic data range from 25-30 km beneath the EGMB, 23-26 km over SC, and 30-36 km beneath the CGGC. Further comparison of CDP values with Moho depths (35-44 km) from 3-D forward gravity modeling and available deep seismic sounding/receiver function data in this region indicate that CDP values are shallower than the Moho. Unlike other cratonic regions, the shallowest CDP and low Te values observed over the Eastern Indian Shield suggests thermal reworking of the cratonic lithosphere in this region.

How to cite: Arasada, R. C. and Gangumalla, S. R.: Curie depth point, effective elastic thickness, and 3-D crustal structure of Eastern Indian shield based on the interpretation of satellite gravity (GOCE) and aeromagnetic data., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6042, https://doi.org/10.5194/egusphere-egu21-6042, 2021.

Richard Saltus, Arnaud Chulliat, Brian Meyer, and Christopher Amante

Magnetic maps depict spatial variations in the Earth’s magnetic field.  These variations occur at a wide range of scales and are produced via a variety of physical processes related to factors including structure and evolution of the Earth’s core field and the geologic distribution of magnetic minerals in the lithosphere.  Mankind has produced magnetic maps for 100’s of years with increasing fidelity and accuracy and there is a general understanding (particularly among the geophysicists who produce and use these maps) of the approximate level of resolution and accuracy of these maps.  However, few magnetic maps, or the digital grids that typically underpin these maps, have been produced with accompanying uncertainty quantification.  When uncertainty is addressed, it is typically a statistical representation at the grid or survey level (e.g., +- 10 nT overall uncertainty based on line crossings for a modern airborne survey) and not at the cell by cell local level.

As magnetic map data are increasingly used in complex inversions and in combination with other data or constraints (including in machine learning applications), it is increasingly important to have a handle on the uncertainties in these data.  An example of an application with need for detailed uncertainty estimation is the use of magnetic map information for alternative navigation.  In this application data from an onboard magnetometer is compared with previously mapped (or modeled) magnetic variations.  The uncertainty of this previously mapped information has immediate implications for the potential accuracy of navigation.

We are exploring the factors contributing to magnetic map uncertainty and producing uncertainty estimates for testing using new data collection in previously mapped (or modeled) map areas.  These factors include (but are likely not limited to) vintage and type of measured data, spatial distribution of measured data, expectation of magnetic variability (e.g., geologic or geochemical environment), statistics of redundant measurement, and spatial scale/resolution of the magnetic map or model.  The purpose of this talk is to discuss the overall issue and our initial results and solicit feedback and ideas from the interpretation community.

How to cite: Saltus, R., Chulliat, A., Meyer, B., and Amante, C.: Uncertainty Estimation for Magnetic Maps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3541, https://doi.org/10.5194/egusphere-egu21-3541, 2021.

Yixiati Dilixiati, Wolfgang Szwillus, and Jörg Ebbing

We apply a Bayesian inversion based on the Monte Carlo Markov chain sampling scheme to magnetic anomaly data of Australia. In our inversion, we simultaneously solve for the susceptibility distribution and the thickness of the magnetic layer. Due to the excellent data coverage, we test our method for Australia. As data source, we use aeromagnetic data of Australia, which are conformed to the recent satellite magnetic model, LCS-1, by an equivalent dipole source approach combined with a spherical harmonic representation. The data are presented in different heights in order to minimize local scale features and to maximize sensitivity to the thickness of the magnetic layer. As constraint, we use estimates of the magnetic layer based on measurements of geothermal heat flow and crustal rock properties. Hereby, we assume that the Curie isotherm does coincide with the deepest magnetic layer. We systematically explore, the effect of increasing model resolution and of the geothermal heat flow values considering their accuracy and quality. The set-up will in the next step be applied to other continental areas of the Earth.

How to cite: Dilixiati, Y., Szwillus, W., and Ebbing, J.: Bayesian inversion of magnetic data: A case study of Australia , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3252, https://doi.org/10.5194/egusphere-egu21-3252, 2021.

Fausto Ferraccioli, Aisling Dunn, Chris Green, Tom Jordan, Rene Forsberg, Graeme Eagles, Kenichi Matsuoka, and Tania Casal

An Andean-style convergent margin was active between ca 580 and 460 Ma along the margin of Gondwana. It led to the emplacement of a major magmatic arc, which is in parts exposed along the much younger Transantarctic Mountains (TAM). Arc magmatism, thrusting, deformation and metamorphism are hallmarks of the long-lived subduction-related Ross Orogen (RO).

Despite the wealth of knowledge on the RO, the location, structure and evolution of the unexposed boundary between the Precambrian Mawson Craton and the RO remains very poorly known, particularly in the South Pole (SP) region- one of the largest poles of ignorance in the whole of East Antarctica.  

Here we combine new aeromagnetic data collected during the ESA PolarGAP campaign with vintage ADMAP 2.0 (Golynsky et al., 2018- GRL) aeromagnetic datasets in the SP region and level these using the satellite magnetic LCS-1 model to investigate the craton margin and RO. The final levelled data were draped at 2800 m above the bedrock topography (Morlighem et al., 2020, Nature Geo.) and reduced to the pole.  

To enhance magnetic signatures and reveal subglacial basement terranes we applied pseudo-gravity transforms, derivatives and upward continuation. We also computed new airborne gravity residual maps and compared these with enhanced magnetic anomaly images. We applied a variety of depth to source of the magnetic and gravity residual anomalies, including tilt depth, Werner and Euler Deconvolution and constructed simple 2D models of the crustal architecture of the RO and the adjacent Precambrian craton margin.  

Using the information from enhanced aeromagnetic imaging and combined magnetic and gravity modelling we propose a new tectonic model for the region. In our model, a former late Neoproterozoic rifted margin that developed along an irregular cratonic margin of the Mawson continent evolved during the Ross Orogen in a wide back-arc basin tectonic setting, linked to a predominantly retreating accretionary subduction-related setting from ca 530 Ma to 500 Ma. This led to the emplacement of magnetite-rich ribbons of arc crust, which are magnetically imaged for the first time in this sector of the active margin. Complex deformation of these ribbons is also imaged from aeromagnetic signatures and appears to resemble some of the deformation patterns observed in the Tasmanides in Australia in evolving retreating accretionary arc and back arc systems (e.g Moresi et al., 2014, Nature).

How to cite: Ferraccioli, F., Dunn, A., Green, C., Jordan, T., Forsberg, R., Eagles, G., Matsuoka, K., and Casal, T.: Back arc basin unveiled at South Pole along an irregular East Antarctic craton margin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13233, https://doi.org/10.5194/egusphere-egu21-13233, 2021.

Theresa Schaller, Mirko Scheinert, Philipp Zingerle, Roland Pail, and Martin Willberg

The gravity field reflects mass inhomogeneities (mostly) inside the Earth. Therefore, gravity inversion and geophysical gravity field modelling are important tools to study the Earth's inner structure and tectonic evolution. In Antarctica, it is extremely challenging to carry out geoscientific studies due to its harsh environment and difficult logistics. Additionally, the continent is covered by an up to 5 km thick ice sheet. However, in the framework of IAG Subcommission 2.4f “Gravity and Geoid in Antarctica” (AntGG) a large database of airborne, shipborne and ground based gravity data has been compiled. Especially airborne data have been acquired during recent years, among others in the area of the polar gap of satellite gravity data. Now, in a joint project funded by the German Research Foundation (DFG) all existing and new gravity data were processed to infer an enhanced gravity field solution for Antarctica (see contribution by Scheinert et al., session G1.5). Processed data e.g. gravity disturbances on the ground or a constant height and other functionals will be provided on a regular grid with 5 km grid spacing. Subsequently, the new Antarctic gravity field solution can now be used for further geophysical and tectonic studies. We use the newly calculated gravity disturbances to study subglacial topography, sediment thickness and Moho depth and to improve respective existing models in Antarctica. For this, we apply 2D Parker-Oldenburg inversion in combination with results from other gravity based studies and further constraining data (e.g. seismic data and ice penetrating radar). We investigate how the higher resolution (5 km) of the new Antarctic gravity field solution facilitates the study of smaller regions in more detail, specifically parts of Wilkes Land, Dronning Maud Land and the Weddell Sea. Additionally, we will infer accuracy estimates for the resulting boundaries in terms of the used inversion parameters (density contrast, average density and filter wavelengths) and their respective gravity signal. Thus, the challenges of gravity field inversion in Antarctica will be discussed in detail and first results of the subsurface modelling will be presented.

How to cite: Schaller, T., Scheinert, M., Zingerle, P., Pail, R., and Willberg, M.: Geophysical subsurface modelling based on the updated, enhanced regional gravity field solution in Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10590, https://doi.org/10.5194/egusphere-egu21-10590, 2021.

Dimitrios A. Natsiopoulos, Elisavet G. Mamagiannou, Eleftherios A. Pitenis, Georgios S. Vergos, Ilias N. Tziavos, and Vassilios N. Grigoriadis

Within the GeoGravGOCE project, funded by the Hellenic Foundation for Research Innovation, a main goal has been the densification of the available land gravity database around the eastern part of the city of Thessaloniki, Greece, where the core International Height Reference Frame (IHRF) station AUT1 is located in order to improve regional geoid and potential determination. Hence it was deemed necessary to densify the available gravity data within radiuses of 10 km, 20 km, 50 km and 100 km from the AUT1 core IHRF site. In that frame, and given the geological complexity of the region surrounding Thessaloniki and the significant variations of the terrain, gravity campaigns were appropriately designed and gravity measurements were carried out in order to densify the database and cover as much as possible traverses of varying altitude. The measurements have been carried out with the CG5 gravity meter of the GravLab group and dual-frequency GNSS receivers in RTK mode for orthometric height determination. In this  study we provide details of the gravity campaigns, the measurement principle and the finally derived gravity and free-air gravity anomalies. The mean measurement accuracy achieved was at the ~20 μGal level for the gravity measurements and ~3 cm for the orthometric heights. In all cases the final derived gravity value was based on the absolute point established by the GravLab team at the AUTH seismological station premises with the A10 (#027) absolute gravity meter.

How to cite: Natsiopoulos, D. A., Mamagiannou, E. G., Pitenis, E. A., Vergos, G. S., Tziavos, I. N., and Grigoriadis, V. N.: Gravity data collection with a CG5 gravitymeter for densification of the gravity data around the AUT1 IHRF station, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1677, https://doi.org/10.5194/egusphere-egu21-1677, 2021.

Luigi Sante Zampa, Emanuele Lodolo, Nicola Creati, Martina Busetti, Gianni Madrussani, Edy Forlin, and Angelo Camerlenghi

In this study, we present a comparative analysis between two types of gravity data used in geophysical applications: satellite altimeter-derived gravity and sea-bottom gravity.

It is largely known that the marine gravity field derived from satellite altimetry in coastal areas is generally biased by signals back-scattered from the nearby land. As a result, the derived gravity anomalies are mostly unreliable for geophysical and geological interpretations of near-shore environments.

To quantify the errors generated by the land-reflected signals and to verify the goodness of the geologic models inferred from gravity, we compared two different altimetry models with sea-bottom gravity measurements acquired along the Italian coasts from the early 50s to the late 80s.

We focused on the Gulf of Manfredonia, located in the SE sector of the Adriatic Sea, where: (i) two different sea-bottom gravity surveys have been conducted over the years, (ii) the bathymetry is particularly flat, and (iii) seismic data revealed a prominent carbonate ridge covered by hundreds of meters of Oligocene-Quaternary sediments.

Gravity field derivatives have been used to enhance both: (i) deep geological contacts, and (ii) coastal noise. The analyses outlined a “ringing-noise effect” which causes the altimeter signal degradation up to 17 km from the coast.

Differences between the observed gravity and the gravity calculated from a geological model constrained by seismic, showed that all datasets register approximately the same patterns, associated with the Gondola Fault Zone, a major structural discontinuity traversing roughly E-W the investigated area.

This study highlights the importance of implementing gravity anomalies derived from satellite-altimetry with high-resolution near-shore data, such as the sea-bottom gravity measurements available around the Italian coasts. Such analysis may have significant applications in studying the link between onshore and offshore geological structures in transitional areas.

How to cite: Zampa, L. S., Lodolo, E., Creati, N., Busetti, M., Madrussani, G., Forlin, E., and Camerlenghi, A.: A comparison between sea-bottom gravity and satellite altimeter-derived gravity in coastal environments: A case study of the Gulf of Manfredonia (SW Adriatic Sea), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3101, https://doi.org/10.5194/egusphere-egu21-3101, 2021.

Pauline Le Maire, Denis Thieblemont, Marc Munschy, Guillaume Martelet, and Geoffroy Mohn

Continent-Ocean Transitions (COT) and ultra-slow spreading ridges, floored by wide area of exhumed serpentinized mantle, bear strong amplitude magnetic lineations. However, whether these anomalies are linked to inversions of the direction of the magnetization (therefore characterized as isochrones of seafloor spreading) or to structural and lithological contrasts remains an open question. Generally, marine magnetic data acquired at sea surface along profiles, are too low resolution to image the intensity variations of the magnetic field at a kilometric scale. Performing a dense deep tow magnetic survey at a present-day COT or ultra-slow spreading system would be better to determine the sources of the magnetic signal but remains expensive. To go ahead, a valuable alternative to address these questions is to record the magnetic signal on ophiolite representing remnants of COT and oceanic systems sampled in orogenic system. We worked on the Chenaillet Ophiolite (French Alps), which represents a fossil COT or ultra-slow spreading system integrated to the Alpine orogeny. This ophiolite escaped high-pressure metamorphism and has only been weakly deformed during Alpine orogeny, preserving its pre-orogenic structure.

We performed an UAV magnetic survey using fluxgate magnetometers in complex conditions due to the altitude (> 1800 m), the strong topography variations and the weather conditions (negative temperatures, snow). Despite these difficulties, which highlight the viability of UAV for geophysical measurements, a survey of 20 square kilometers with 219 km of profiling was completed 100 m above ground level. Flight line spacing is 100 m above the ophiolitic basement and 200 m above the sedimentary units. Another magnetic UAV survey was flown with another UAV to map a small area 10 m above ground level. Magnetic anomaly maps were computed after standard processing (e.g., calibration/compensation, temporal variation and regional magnetic field corrections, levelling).

Our first results evidence well-defined magnetic anomalies clearly linked to serpentinite. This shows that the magnetic signal is of sufficient resolution to contribute to a revision of the cartography of the massif combining geological observations and magnetic data.

In addition, the magnetic susceptibility was measured on 60 outcrops, to support interpretation.

In this presentation, we focus on the magnetic acquisition campaigns, processing and 2D/3D interpretations by forward modelling and data inversion. Lastly, two items are discussed: 1) contribution of magnetic UAV surveys for geological mapping; and 2) implication of the results on the Chenaillet massif to discuss the contribution of magnetic mapping to the understanding of the TOC or ultra-slow spreading system.

How to cite: Le Maire, P., Thieblemont, D., Munschy, M., Martelet, G., and Mohn, G.: UAV high-resolution magnetic mapping of the Chenaillet ophiolite, in the Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7050, https://doi.org/10.5194/egusphere-egu21-7050, 2021.

hugo reiller, marc munschy, jean-françois oelher, sylvain lucas, and didier rouxel

Since the 70’s, ship-mounted three-component magnetometers are used for marine geophysical mapping, with the benefits of being able to be operated permanently with a minimum of technical maintenance. However, to obtain accuracies similar to those of ship-towed absolute scalar magnetometers, the intense interfering magnetic fields generated by the hull and steel parts of the ship have to be removed. The most common correction method, called “vector compensation”, uses high precision inertial navigation systems in order to correct the measured data for the ship’s magnetic field and calculate the vector of the compensated magnetic field in the Earth coordinated system.

This work alternatively uses the “scalar compensation” method applied in airborne magnetism since the 60’s. The aim is to compute the intensity of the compensated magnetic field without measurements of the attitude of the vector and using linear least-square regression analysis. This correction method is applied to shipboard three-component magnetometer data acquired on different vessels during different surveys. Results are compared to those obtained with ship-towed absolute scalar magnetic measurements.

Keywords: shipboard three-component magnetic measurements; magnetic compensation; marine magnetics.

How to cite: reiller, H., munschy, M., oelher, J., lucas, S., and rouxel, D.: Scalar compensation of shipboard three-component magnetic measurements and applications for marine geophysical mapping, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5326, https://doi.org/10.5194/egusphere-egu21-5326, 2021.

Denis Anikiev, Hans-Jürgen Götze, Judith Bott, Angela Maria Gómez-García, Maria Laura Gomez Dacal, Christian Meeßen, Cameron Spooner, Constanza Rodriguez Piceda, Christian Plonka, Sabine Schmidt, and Magdalena Scheck-Wenderoth

We introduce a modelling concept for the construction of 3-D data-constrained subsurface structural density models at different spatial scales: from large-scale models (thousands of square km) to regional (hundreds of square km) and small-scale (tens of square km) models used in applied geophysics. These models are important for understanding the drivers of geohazards, for efficient and sustainable extraction of resources from sedimentary basins such as groundwater, hydrocarbons or deep geothermal energy, as well as for investigation of capabilities of long-term underground storage of gas and radioactive materials.

The modelling concept involves interactive fitting of potential fields (gravity and magnetics) and their derivatives within IGMAS+ (Interactive Gravity and Magnetic Application System), a well-known software tool with almost 40 years of development behind it. The core of IGMAS+ is the analytical solution of the volume integral for gravity and magnetic effects of homogeneous bodies, bounded by polyhedrons of triangulated model interfaces. The backbone model is constrained by interdisciplinary data, e.g. geological maps, seismic reflection and refraction profiles, structural signatures obtained from seismic receiver functions, local surveys etc. The software supports spherical geometries to resolve the first-order effects related to the curvature of the Earth, which is especially important for large-scale models.

Currently being developed and maintained at the Helmholtz Centre Potsdam – GFZ German Research Centre, IGMAS+ has a cross-platform implementation with parallelization of computations and optimized storage. The powerful graphical interface makes the interactive modelling and geometry modification process user-friendly and robust. Historically IGMAS+ is free for research and education purposes and has a long-term plan to remain so.

IGMAS+ has been used in various tectonic settings and we demonstrate its flexibility and usability on several lithospheric-scale case studies in South America and Europe.

Both science and industry are close to the goal of treating all available geoscientific data and geophysical methods inside a single subsurface model that aims to integrate most of the interdisciplinary measurement-based constraints and essential structural trends coming from geology. This approach presents challenges for both its implementation within the modelling software and the usability and plausibility of generated results, requiring a modelling concept that integrates the data methods in a feasible way together with recent advances in data science methods. As such, we present the future outlook of our modelling concept in regards to these challenges.

How to cite: Anikiev, D., Götze, H.-J., Bott, J., Gómez-García, A. M., Gomez Dacal, M. L., Meeßen, C., Spooner, C., Rodriguez Piceda, C., Plonka, C., Schmidt, S., and Scheck-Wenderoth, M.: Interdisciplinary data-constrained 3-D potential field modelling with IGMAS+, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2964, https://doi.org/10.5194/egusphere-egu21-2964, 2021.