Enter Zoom Meeting

G4.3

EDI
Geoid determination, gravity and magnetic field data and their 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 near surface investigations. The first part of this 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, machine learning, interpretation methods, innovative applications of the results and data collected by modern satellite missions, potential theory, as well as case histories.
The second part of this session will focus on the practical solution of various formulations of geodetic boundary-value problems to yield precise local and regional high-resolution (quasi)geoid models. Contributions describing recent developments in theory, processing methods, downward continuation of satellite and airborne data, treatment of altimetry and shipborne data, terrain modeling, software development and the combination of gravity data with other signals of the gravity field for a precise local and regional gravity field determination are welcome. Topics such as the comparison of methods and results, the interpretation of residuals as well as geoid applications to satellite altimetry, oceanography, vertical datums and local and regional geospatial height registration are of a special interest.

Co-organized by EMRP2/GD9/TS12
Convener: Jörg Ebbing | Co-conveners: Hussein Abd-Elmotaal, Leonardo UiedaECSECS, Alexandra Guy, Carla Braitenberg, Mikhail Kaban, Xiaopeng Li, Georgios S. Vergos
Presentations
| Wed, 25 May, 10:20–11:47 (CEST), 13:20–15:48 (CEST)
 
Room K1

Wed, 25 May, 10:20–11:50

Chairpersons: Hussein Abd-Elmotaal, Georgios S. Vergos, Riccardo Barzaghi

10:20–10:27
|
EGU22-11302
|
ECS
|
|
On-site presentation
Hergeir Teitsson and René Forsberg

A gravimetric quasi-geoid model, based on the latest FAMOS database release, has been computed for the Baltic Sea region, aiming for a best-possible model on the sea, while not focusing on the surrounding land.

 The geoid computation is based on the FFT remove-compute-restore method. XGM2019 is used as global reference field, with a Wong-Gore linear tapering from 180 to 200. No terrain corrections are included in the computation, since these are not expected to contribute to the accuracy of the model on the sea.

The gravimetric quasi-geoid model is compared to a GNSS-levelled ITRF2008 zero-tide dataset, the altimetry based DTU21 Mean Sea Surface dataset, and to a few tide gauge stations distributed throughout the region. Some preliminary comparisons to the GNSS-levelling dataset indicates that the gravimetric geoid has an accuracy of ±25 mm in the region surrounding the Baltic Sea.

How to cite: Teitsson, H. and Forsberg, R.: Gravimetric quasi-geoid of the Baltic Sea and comparison to GNSS levelling, DTU21 and tide gauges, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11302, https://doi.org/10.5194/egusphere-egu22-11302, 2022.

10:27–10:34
|
EGU22-6671
|
ECS
|
Virtual presentation
Ismael Foroughi et al.

In this contribution, we estimate the uncertainty (error) of the input gravity measurements needed for the determination of the geoid with an internal sub-centimetre accuracy. The accuracy of the geoid height is a function of the resolution/accuracy of the input gravity and topographical data, and the methodology used to solve a geodetic boundary value problem. The purpose of this study is to estimate the maximum allowable error in the terrestrial gravity measurements based on a required standard deviation of the error in the geoid heights (e.g., ≤1cm). This is done with an assumption of a known Digital Elevation Model (DEM), and an Earth Gravitational Model (EGM) along with their error estimates.

 

We use the one-step integration method (one-step kernel) for the determination of the geoid. In this method, the anomalous gravity at any surface above the geoid is estimated by integrating over the geoid-level disturbing potentials in harmonic space. By applying the covariance law to the one-step integration method, the error of the gravity measurements at the Earth's surface can be estimated using the expected error of the geoid heights. Taking advantage of the remove-compute-restore technique, we estimate the error of the residual surface gravity measurements using the (known) error estimates of the topographical and EGM corrections.  

 

We select the Colorado test area (35°N - 40°N, 250°E - 258°E) to generate a 1¢×1¢ grid of geoid random errors with a standard deviation of 1cm. We use the topographical data from the Shuttle Radar Topography Mission (SRTM) Ver. 3.0. and the global model of DIR_R5 up to degree/order 140 to apply the remove-compute-restore technique. The uncertainty estimate of the SRTM heights and the covariance matrix of the spherical harmonic coefficients of the DIR_R5 are used to calculate the errors of the topographical gravitational attraction and low-degree EGM signals on the geoid heights and surface anomalous gravity data.

 

Our preliminary results show that to achieve a sub-centimetre accuracy in the Colorado area, we require grid surface gravity measurements with a standard deviation of less than 2.5mGal. This result is optimistic as in the geoid determination process, the anomalous gravity data are downward continued from the Earth’s surface to the geoid, whereas this step is not required in our experience. Besides, we assume a constant standard deviation of 1cm for all the errors of the geoid heights, whereas such high accuracy may not be needed in high mountains. We will provide further results for the elevation-dependent geoid error and also investigate the effect of downward continuation on our results.     

How to cite: Foroughi, I., Pagiatakis, S., Goli, M., and Ferguson, S.: Accuracy requirements of the gravity measurements for sub-centimetre geoid, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6671, https://doi.org/10.5194/egusphere-egu22-6671, 2022.

10:34–10:41
|
EGU22-13101
|
Virtual presentation
Jordan Krcmaric

The U.S. National Geodetic Survey (NGS), an office of the National Oceanic and Atmospheric Administration (NOAA), is preparing for the release of a new vertical datum, the North American-Pacific Geopotential Datum of 2022 (NAPGD2022). This new datum will be based on a high degree spherical harmonic model of the Earth’s gravitational potential, and will yield a geoid undulation model (GEOID2022) to calculate orthometric heights from GNSS-derived ellipsoid heights.

As part of the preparation for the new vertical datum, NGS has computed annual experimental geoid models (xGEOID) since 2014. The xGEOID model released in 2020 (xGEOID20) uses an updated digital elevation model (DEM) composed of TanDEM-X, MERIT, and USGS 3DEP data. The DEMs are merged together to create a seamless elevation model across the extent of the xGEOID20 model. The accuracy of the merged DEM is tested using independent datasets such as GPS observations on leveled bench marks and ground elevations from ICESat-2. The effect of the updated DEM on the geoid model is also determined by comparing geoid models computed with previous DEMs to the new xGEOID20 model, and with comparisons to the NGS Geoid Slope Validation Survey lines.

How to cite: Krcmaric, J.: Development and evaluation of the xGEOID20 Digital Elevation Model at NGS, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13101, https://doi.org/10.5194/egusphere-egu22-13101, 2022.

10:41–10:48
|
EGU22-1899
Xiaopeng Li et al.

Bathymetric data over lake areas are not included in previous NGS (National Geodetic Survey) geoid model computations. Mean lake surfaces are used as the bare rock surface during the modeling. This approximation treats the water body as rocks with the same size, and causes errors that can be avoided. This study uses the bathymetric model to rigorously compute the volume of water bodies instead of treating them as rocks, during geoid modeling. To make fair comparisons and show the effects clearly, three sets of geoid models are generated with the same theory currently used at NGS, and with the same parameters. Model-Base is computed without bathymetric information of the water body. In this model, the real water bodies are simply replaced by rocks. Model-Condensed and Model-Density are generated with bathymetric information. The treatments of water bodies are different between the two models, but both are based on the hypothesis of mass conservation. The water bodies are condensed into the equivalent rocks in the Model-Condensed, leading to the geometrical shape changes in the lake area. In the Model-Density, the density of each topographical column bounded by the lake surface and geoid is taken as the average of the density of water and rock bodies included in this column, resulting in the density changes in the lake area. The study area is focused on the Great Lakes area of North America. The geoid model differences between Model-Condensed and Model-Base range from -18 to 25 mm, forming a Gaussian distribution. The distribution of the geoid model differences between Model-Density and Model-Base are not in a Gaussian form, and their values are in the range between -1 and 18 mm. Both the nearby GNSS/Leveling bench marks from US and the multi-year averaged altimetry data are used to validate the results. Consistent geoid model precision improvements of about 2 mm are confirmed around the Lake Superior, which is the deepest and largest lake, over all selected frequency bands of the Stokes’s kernel. The numerical results prove the importance of considering water bodies in the determination of a high-accuracy geoid model over the Great Lakes area.

How to cite: Li, X., Lin, M., Krcmaric, J., Jia, Y., Shum, C., and Roman, D.: Bathymetric Effects on Geoid Modeling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1899, https://doi.org/10.5194/egusphere-egu22-1899, 2022.

10:48–10:55
|
EGU22-650
|
Virtual presentation
Vassilios Grigoriadis et al.

In the frame of the “Modernization of the Hellenic Gravity Network” project, we aim at computing a high resolution and accuracy geoid for Greece. For this reason, we selected initially two test areas in northern and southern Greece covering an area of about 100 km2 each, where gravity and GNSS/leveling measurements were carried out. Based on these recent, well documented and reliable measurements, we investigate the use of different techniques for the determination of the geoid, including Least-Squares Collocation, FFT and Input-Output Systems, following the Remove-Compute-Restore approach. For the remove/restore part, we examine different Residual Terrain Modeling schemes along with the use of older and recent Global Geopotential Models. Moreover, we compute the geoid-quasigeoid separation term using different approaches. We then validate the results obtained against the new GNSS/leveling measurements across the test areas and draw conclusions towards the determination of a regional geoid for Greece.

How to cite: Grigoriadis, V., Andritsanos, V., Natsiopoulos, D., and Vergos, G.: Investigation of different geoid computation techniques in the frame of the ModernGravNet project, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-650, https://doi.org/10.5194/egusphere-egu22-650, 2022.

10:55–11:05
|
EGU22-927
|
solicited
Riccardo Barzaghi and Georgios Vergos

With the definition of the International Height Reference System (IHRS) and the development of a roadmap for its implementation through the International Height Reference Frame (IHRF), an analytical evaluation of the various approaches for the practical determination of potential values at IHRF is necessary. In this work we focus on two main approaches to estimate geopotential values at IHRF stations. The first approach resides on the use of either local gravity anomalies and gravity disturbances around each site and the geopotential determination based on Stokes’ and Molodensky’s boundary value problems, respectively. In this scheme, the influence of the classical residual terrain model (RTM) reduction as well as that of RTM effects based on spherical harmonics expansion of the topographic potential are investigated. Furthermore, the introduction of possible biases within the various pre- and post-processing steps are thoroughly investigated, as e.g., during the estimation of station geometric heights, along with the influence of the quasi-geoid to geoid separation estimation. In the second approach, we investigate the determination of geopotential values based on either national and regional geoid models, i.e., resembling the case that access to local gravity data is not available, and the determination has to be based on some available geoid model. In the present work we analyze the theoretical and methodological steps that need to be followed in each approach, identifying the possible sources of biases. Finally, some early results are presented aiming at providing a roadmap and an error assessment for the practical realization of the IHRF.

How to cite: Barzaghi, R. and Vergos, G.: Practical implementation of the IHRF employing local gravity data and geoid models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-927, https://doi.org/10.5194/egusphere-egu22-927, 2022.

11:05–11:12
|
EGU22-780
|
On-site presentation
Hussein Abd-Elmotaal and Norbert Kühtreiber

The coverage of the gravity data plays an important role in the geoid determination. This paper tries to answer whether different geoid determination techniques would be affected similarly by such gravity data coverage. The paper presents the determination of the gravimetric geoid in two different countries where the gravity coverage is quite different. Egypt (representing the same situation in Africa) has sparse gravity data coverage over relatively large area, while Austria has quite dense gravity coverage in a significantly smaller area. Two different geoid determination techniques are tested. They are Stokes’ integral with modified Stokes kernel, for better combination of the gravity field wavelengths, and the least-squares collocation technique. The geoid determination has been performed within the framework of the non-ambiguous window remove-restore technique (Abd-Elmotaal and Kühtreiber, 2003). For Stokes’ geoid determination technique, the Meissl (1971) modified kernel has been used with numerical tests to obtain the best cap size for both geoids in Egypt and Austria. For the least-squares collocation technique, a modelled covariance function is needed. The Tscherning-Rapp (Tscherning and Rapp, 1974) covariance function model has been used after being fitted to the empirically determined covariance function. The paper gives a smart method for such covariance function fitting. All geoids are fitted to GNSS/levelling geoids for both countries. For each country, the computed two geoids are compared and the correlation between their differences versus the gravity coverage is comprehensively discussed.

How to cite: Abd-Elmotaal, H. and Kühtreiber, N.: Effect of Gravity Data Coverage on the Gravity Field Recovery: Case Study for Egypt (Africa) and Austria, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-780, https://doi.org/10.5194/egusphere-egu22-780, 2022.

11:12–11:19
|
EGU22-787
|
On-site presentation
Georgios S. Vergos et al.

The combination of GOCE Satellite Gravity Gradiometer (SGG) data with local free-air gravity anomalies, towards the estimation of improved geoid and gravity field models, requires their downward continuation to the Earth’s surface (ES). Within the GeoGravGOCE project, which aims to explore the local improvements in geoid and gravity field modeling offered by GOCE, optimal combination of GOCE and surface data was sought in order to acquire insights of their contribution especially over poorly surveyed areas. GOCE SGG data are first pre-processed, to filter out noise and reduce long-wavelength correlated errors, and are consequently reduced to a mean orbit (MO) so that downward continuation to the Earth’s surface can be carried out. The reduction from the orbit level to a MO was performed by estimating GGM gradient grids per 1 km from the MO to the maximum orbital level, and then linearly interpolating for the reduction from the actual satellite height. Having determined the filtered GOCE filtered SGG data to a MO, the next step referred to their downward continuation to the ES. Gravity anomalies from XGM2016 generated on the ES have been used as ground truth and were upward continued to the MO in the spectral domain through the input output system theory method. The evaluation of GOCE SGG data to the MO with GGM-derived gradients is performed using a Monte-Carlo annihilation method finding the global minimum of a cost function that may possess several local minima. The GOCE data that satisfy the aforementioned criteria of this simulated annealing method are frozen and the steps mentioned above are repeated until all generated SGG data meet the criterion. The developed procedure can be successfully applied for downward continuation of GOCE SGG from a MO to the ES for regional gravity field applications. The present work summarizes the results achieved while the evaluation is performed against local free-air gravity anomalies and residuals to XGM2019.

How to cite: Vergos, G. S., Pitenis, E. A., Mamagiannou, E. G., Natsiopoulos, D. A., and Tziavos, I. N.: GOCE SGG data downward continuation to the Earth’s Surface, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-787, https://doi.org/10.5194/egusphere-egu22-787, 2022.

11:19–11:26
|
EGU22-12613
|
ECS
|
On-site presentation
Modelling of Mercury's crustal structures from gravity data inversion
(withdrawn)
Salvatore Buoninfante et al.
11:26–11:33
|
EGU22-8228
|
Virtual presentation
Pilar Clariana et al.

The acquisition and interpretation of gravity and magnetic data represents a cost-effective tool in geophysics since it allows to determine the geometry and distribution of the density and magnetic properties at depth of the subsurface rocks. The study area, where gravity and magnetic data have been interpreted, is the La Cerdanya basin (Eastern Pyrenees), a Neogene ENE-WSW oriented half graben located in the Axial Zone, the central part of the Pyrenees mainly formed by Paleozoic rocks. It is situated in the NW block of the La Tet fault and its Neogene sediments lie unconformably on top of the Paleozoic basement. Its dimensions are approximately 30 km long and 7 km wide. The tectonic evolution and geometry of the La Cerdanya basin is not well known and this work aims to add new constraints to help solving the Neogene tectonic evolution of the Eastern Pyrenees and to improve the knowledge of its 3D geometry. 

The magnetic anomaly map of the study area, based on airborne magnetic data, shows very little contrasts of the magnetic properties between the Neogene rocks of the La Cerdanya basin and the Paleozoic rocks surrounding it. Gravity data consist of previous and new acquired gravimetric stations and the residual Bouguer anomaly map shows density contrasts big enough to model the geometry of the basin and the neighbor intrusive bodies. They have been incorporated into a 3D geological model based on available geological and petrophysical data using the 3D GeoModeller software. The 3D potential fields model has been made taking into account the three most representative units outcropping in the study area: the Neogene rocks, the Late Carboniferous intrusive bodies and the Paleozoic basement. The resulting potential fields response of the model is consistent with the observed data. The 3D model shows a basin slightly deeper than shown in previous works and has helped to better define the 3D geometry of the basin and the along-strike geometry of the La Tet fault.

How to cite: Clariana, P., Muñoz, R., Ayala, C., Bellmunt, F., Piña-Varas, P., Soto, R., Gabàs, A., Macau, A., Rubio, F., Rey-Moral, C., and Martí, J.: New insights to characterize the La Cerdanya basin structure from 3D gravity modelling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8228, https://doi.org/10.5194/egusphere-egu22-8228, 2022.

11:33–11:40
|
EGU22-3409
|
ECS
|
Virtual presentation
Mitsuhiro Hirano and Hiroyuki Nagahama

In the northeastern Japan arc with the active compressive stress field since ~3 Ma, it is reported that a characteristic relationship between crustal deformation including faulting and short-wavelength (< 160 km) Bouguer anomalies. According to previous studies, active faults tend to be located in negative regions, which are caused by cracks and volumetric strain due to accumulated fault dislocation. Especially, it is shown that in strain concentration zones with active faults and muti folding, the effect of accumulated fault dislocation forms the negative zones of gravity anomaly along the northeastern Japan arc, impacting the pattern of short-wavelength Bouguer anomalies throughout the entire arc. In this presentation, we extend this concept further and discuss the positive and negative zones of gravity zones along the entire northeastern Japan arc from the geometrical viewpoint of folding with one of the defect, disclination. Folding is described by Euler-Schouten curvature tensor, which defines the protrusion of included space (e.g., two-dimensional Riemannian space) from enveloping space (e.g., three-dimensional Euclid space). Based on previous studies, the density of earthquake occurrence is proportional to the curvature of the plastic folding deformation of the crust, which is related to Euler-Schouten curvature, and fault dislocation also accumulates at the regions with its high curvature. The row (accumulation) of fault dislocation can be replaced by the disclination, and Riemann-Christoffel curvature, derived from Euler-Schouten curvature tensor, also expresses disclination density. In particular, angular folding with local curvature accompanied by a pair of disclination is called Kink folding, forming the mass-loss or mass-excess regions around disclination. Since Kink folding can approximately be the same as the undulating region bounded by several faults (fault block) in strain concentration zones, it is expected that the northeastern Japan arc has not only negative zones of gravity anomaly but also positive zones along the arc due to the mass-loss or mass-excess regions around disclination. Therefore, we conclude that the positive and negative zones of gravity anomaly along the northeastern Japan arc reflect the geometric condition of the crust with disclination.

How to cite: Hirano, M. and Nagahama, H.: Short-wavelength Bouguer anomaly and folding with disclination in the northeastern Japan, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3409, https://doi.org/10.5194/egusphere-egu22-3409, 2022.

11:40–11:47
|
EGU22-4230
|
Virtual presentation
György Hetényi et al.

Modelling of geophysical data is often subject to choices made by the researcher undertaking the work. The level of structural complexity in the model, the bounds on parameters imposed by a priori knowledge, the thoroughness and efficiency in exploring the parameter space may all lead to bias in determining what the best fitting models can be.

To avoid bias from our own ideas in constraining the subsurface shape of a given density anomaly, we hereby invite anyone interested to create their own models. This is planned by sharing the same gravity data measured in the field, the same digital elevation model, the main features of the local geological maps, and bounds on the encountered rock density values. These data will be shared openly, in the form of a modelling challenge: each participating researcher or group is expected to submit their solution(s). All these will be compared during a dedicated workshop, ultimately resulting in a joint publication.

The target of this modelling challenge is the world-famous Balmuccia peridotite body (45.84°N, 8.16°E) in the Ivrea-Verbano Zone (IVZ). Here mantle rocks are naturally exposed at the surface, in the broader context of the IVZ, a middle- to lower crustal terrain along the Europe-Adria plate boundary’s eastern side. The surface exposure of the Balmuccia peridotite is ~ 4.4 km N-S by 0.6 km E-W, with outcrop elevation changes exceeding 1000 m. About 150 new gravity data points have been measured within a radius of 3 km from the centre of the peridotite body, along more or less accessible paths and slopes. The measurements have been carried out with a Scintrex CG-5 relative gravimeter, tied to a reference point, and all points located via differential GPS with typical vertical precision of a few cm. Farther away regional gravity data is available at few km spacing.

Beyond the modelling challenge, the interest in constraining the subsurface shape of the Balmuccia peridotite body is its future target role in the ICDP DIVE continental drilling project (www.dive2ivrea.org).

How to cite: Hetényi, G., Baron, L., Scarponi, M., Subedi, S., Michailos, K., Dal, F., Gerle, A., Petri, B., Langone, A., Greenwood, A., Ziberna, L., Pistone, M., Zanetti, A., and Müntener, O.: Crowd modelling: Launching an open gravity-modelling call to challenge the Balmuccia peridotite body, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4230, https://doi.org/10.5194/egusphere-egu22-4230, 2022.

Wed, 25 May, 13:20–14:50

Chairpersons: Alexandra Guy, Carla Braitenberg

13:20–13:30
|
EGU22-2658
|
ECS
|
solicited
|
On-site presentation
Renata Regina Constantino et al.

The presence of subglacial sediments is important in enabling streaming ice flow and may be a critical controlling factor in determining the onset regions of ice streams. Improving our knowledge of the location of sedimentary basins underlying large ice sheets will improve our understanding of how the substrate influences the ice streams.  Advancing our understanding of the interaction between subglacial sediments and ice flow is critical for predictions of ice sheet behavior and the consequences on future climate change. To date, no comprehensive distribution of onshore and offshore sedimentary basins over Antarctica has been developed. The goal of this project is to use a combination of large-scale datasets to characterize known basins and identify new sedimentary basins to produce a continent-wide mapping of sedimentary basins and provide improved basal parametrizations conditions that have the potential to support more realistic ice sheet models. The proposed work is divided into three main steps. In the first step, the Random Forest (RF), a supervised machine learning algorithm, is used to identify sedimentary basins in Antarctica. In the second step, a regression analyses between aerogravity data and topography is done to evaluate the gravity signal related to superficial heterogeneities (i.e. sediments) and compare the results to the depth of magnetic sources using the Werner deconvolution method. Last, the correlation between sedimentary basins and ice streams is investigated. Here, we will present the preliminary results from Step 1. The Random Forest uses ensemble learning method for classification and regression. The classification rules for this present work are based on the geophysical parameters of major known sedimentary basins. First we classify the known basins with all available geophysical compilations including topography, gravity and magnetic anomalies, sedimentary thickness, crustal thickness, geothermal heat flux, information on the geology, rocky type and bedrock geochemistry, and then use the Random Forest machine learning algorithm to classify the geology underneath the ice into consolidated rock and sediments based on these parameters.

How to cite: Constantino, R. R., Tinto, K. J., and Bell, R. E.: Using random forest machine learning algorithm to help investigating the relationship between subglacial sediments and ice flow in Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2658, https://doi.org/10.5194/egusphere-egu22-2658, 2022.

13:30–13:37
|
EGU22-13231
|
|
On-site presentation
Jonas Liebsch et al.

Sub-ice-bathymetry is an important boundary condition when modelling the evolution of ice shelves and ice sheets. Radar sounding is a proven method to reveal the sub-ice-topography beneath grounded ice. However, it fails to image the bathymetry beneath the floating ice shelves due to the strong radar reflectivity of sea water. As an alternative, the inversion of gravity measurements has been used increasingly frequently in recent years. To overcome the ambiguity of inverse modelling, this method benefits from independent depth constraints derived from direct measurements distributed throughout the model area, such as by active seismic, hydroacoustic, and radar methods.

Here, we present a novel geostatistical approach to gravity inversion and compare it to the classical and more commonly used FFT approach. Instead of only fitting individual points, we also include the spatial continuity of the sub-ice morphology. To do so, we calculate a variogram that fits the available depth measurements and derive a covariance matrix from it. The covariance matrix and an initial bathymetry model obtained by kriging together describe an a-priori probability density. For the inversion, the model bathymetry is related to the measured gravity using a quasi-Newton method, for which the derived probability density serves as the inversion’s regularization term. We successfully apply the algorithm to airborne gravity data across the Ekström ice shelf (Antarctica) and compare our results with those of previous studies based on the classical approach. The simplified addition of constraints both for the geometry and the density structure in our approach proves to be advantageous.

How to cite: Liebsch, J., Ebbing, J., Eisermann, H., and Eagles, G.: Geostatistical Gravity Inversion for Estimating Sub-Ice-Bathymetry, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13231, https://doi.org/10.5194/egusphere-egu22-13231, 2022.

13:37–13:44
|
EGU22-13195
|
Virtual presentation
Fausto Ferraccioli et al.

Geothermal heat flux (GHF), coupled with subglacial topography and hydrology, influences the flow of the overlying Antarctic ice sheet. GHF is related to crustal and lithospheric structure and composition and tectonothermal evolution, and is also modulated by subglacial sedimentary basins and bedrock morphology. Despite its importance for both solid earth and cryosphere studies, our knowledge of Antarctic GHF heterogeneity remains limited compared to other continents- especially at regional scale. This is due to the paucity of direct measurements and the spatial gap wrt much larger scale geophysical proxies for GHF, based on continental-scale magnetic and seismological predictions that also differ considerably from each other in several regions. To reduce this major knowledge gap, the international community is increasingly active in analysing geophysical, geological and glaciological datasets to help constrain GHF (e.g. Burton-Johnson et al., SCAR-SERCE White Paper, 2020). Here we focus on 4D Antarctica- an ESA project that aims to help link bedrock, crust, lithosphere and GHF studies, by analysing recent airborne and satellite-derived potential field datasets. 

We present our recent aeromagnetic, aerogravity and satellite data compilations for 5 study regions, including the Amundsen Sea Embayment sector of the West Antarctic Ice Sheet (e.g. Dziadek et al., 2021- Communications Earth & Environment) and the Wilkes Subglacial Basin (WSB), the Recovery glacier catchment, the South Pole and Gamburtsev Subglacial Mountains and East Antarctic Rift region. We apply Curie Depth Point (CDP) estimation on existing aeromagnetic datasets and compilations in our study regions conformed with SWARM satellite magnetic data (Ebbing et al., 2021- Scientific Reports). We tested the application of different methods, including the centroid (e.g. Martos et al., 2017, GRL) and Bayesian inversion approaches of Curie depth and uncertainty (e.g. Mather and Fullea, 2019- Solid Earth) and defractal and geostatistical methods (e.g. Carrillo-de la Cruz et al., 2021- Geothermics). We then compare our CDP results with crust and lithosphere thickness and interpretations of crustal and lithospheric setting.

Using our new aeromagnetic interpretations we define Precambrian and early Paleozoic subglacial basement in East Antarctica that is mostly concealed beneath Phanerozoic sedimentary basins and ice sheet cover. This enables us to discuss whether different basement provinces differ in terms of CDP estimates (as expected), or if these are either not or only partially resolved. A particularly informative case is the WSB. Here our magnetic assessments of GHF heterogeneity for the Terre Adelie Craton, Wilkes Terrane and Ross Orogen can be indirectly tested by exploiting independent geological and geophysical information derived from their Australians correlatives, namely the Gawler and Curnamona cratons and the Delamerian Orogen. 

Our Curie depth estimates yield geologically reasonable thermal boundary conditions required to initialise new thermal modelling efforts in several study areas. However, developing 3D models of crust and lithosphere thickness and intracrustal composition (as a proxy for the ranges of radiogenic heat production and thermal conductivity) with reasonably detailed crustal architecture, derived from both potential field and seismological datasets is a key next step to constrain Antarctic geothermal heat flux heterogeneity at higher-resolution ice stream scale.  

How to cite: Ferraccioli, F., Mather, B., Armadillo, E., Forsberg, R., Ebbing, J., Ford, J., Gohl, K., Eagles, G., Green, C., Fullea, J., Verdoya, M., and Carillo de la Cruz, J. L.: 4D Antarctica: recent aeromagnetic, aerogravity and satellite data compilations provide a new tool to estimate subglacial geothermal heat flux , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13195, https://doi.org/10.5194/egusphere-egu22-13195, 2022.

13:44–13:51
|
EGU22-1673
Alexandra Guy et al.

3D forward gravity modelling combined with receiver function analysis characterize the structures of the southern part of the Mongolian collage. Recently, a multidisciplinary approach integrating potential field analysis with geology and magmatic geochemistry demonstrate that relamination of an 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 48 stations of the MOBAL2003 and the IRIS-PASSCAL experiments were analyzed to get the receiver functions. The resulting crustal thickness variation is first compared with the topography of the Moho determined by the 3D forward modeling of the GOCE gravity gradients. In addition, seismic stations south of the Hangay dome display significant signal related to the occurrence of a low velocity zone (LVZ) at lower crustal level. The receiver function analysis also revealed a significant difference between the crustal structures of the Hangay dome and the tectonic zones in the south. Finally, these seismic analysis inputs such as crustal thickness, strike and dips of the seismic interfaces as well as the boundaries and the lithologies of the different tectonic zones constitute the starting points from the 3D forward gravity modelling. The combination of these two independent methods enhances the occurrence and the extent of a low velocity and a low density zone (LVLDZ) at lower crustal level beneath central Mongolia. 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 from receiver functions and gravity analysis in central Mongolia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1673, https://doi.org/10.5194/egusphere-egu22-1673, 2022.

13:51–13:58
|
EGU22-12704
|
ECS
|
Virtual presentation
Matthew Joseph Comeau et al.

Joint inversion of complementary datasets is an important tool to gather new insights and aid interpretation, especially in regions which show structural complexity. Using the joint inversion framework jif3D [1] with a newly developed coupling for density and resistivity, based on a variation of information approach which is a machine-learning method that constructs a possible relationship between the properties [2], we combine satellite gravity measurements with electromagnetic data, from broadband and long-period magnetotellurics [3,4,5,6].

Central Mongolia is located in the continental interior, far from tectonic plate boundaries, yet has a high-elevation plateau and enigmatic widespread low-volume basaltic volcanism [7,8,9]. The processes responsible for developing this region remain unexplained and there are questions about its tectonic evolution. A recent project employed thermo-mechanical numerical modeling [10] to simulate the temporal evolution of various tectonic scenarios, offering an opportunity to test hypotheses and determine which are physically plausible mechanisms. Constraints on lithospheric properties, e.g., density distribution, are important for evaluating the geodynamic models. Furthermore, they can help shed light on questions regarding the nature of lower crustal electrical conductors [11], which may be related to tectonically-significant low-viscosity zones.

We will present preliminary results that provide new constraints on the 3-D structure of the lithosphere beneath Central Mongolia, as well as a roadmap for moving towards integrating geophysical results into geodynamic modeling to better understand the evolution of the lithosphere.

 

References:

[1]  Moorkamp, M. et al. 2011. A framework for 3-D joint inversion of MT, gravity and seismic refraction data. Geophysical Journal International, 184(1). https://doi.org/10.1111/j.1365-246X.2010.04856.x 

[2]  Moorkamp, M., 2021. Deciphering the state of the lower crust and upper mantle with multi-physics inversion. ESSOAr. https://doi.org/10.1002/essoar.10508095.1 

[3]  Comeau, M.J., et al., 2018. Evidence for fluid and melt generation in response to an asthenospheric upwelling beneath the Hangai Dome, Mongolia. Earth and Planetary Science Letters, 487. https://doi.org/10.1016/j.epsl.2018.02.007 

[4]  Käufl, J.S., et al., 2020. Magnetotelluric multiscale 3-D inversion reveals crustal and upper mantle structure beneath the Hangai and Gobi-Altai region in Mongolia. Geophysical Journal International, 221(2). https://doi.org/10.1093/gji/ggaa039 

[5]  Becken, M., et al., 2021a. Magnetotelluric Study of the Hangai Dome, Mongolia. GFZ Data Services. https://doi.org/10.5880/GIPP-MT.201613.1 

[6]  Becken, M., et al., 2021b. Magnetotelluric Study of the Hangai Dome, Mongolia: Phase II. GFZ Data Services. https://doi.org/10.5880/GIPP-MT.201706.1 

[7]  Comeau, M.J., et al., 2021a. Images of a continental intraplate volcanic system: from surface to mantle source. Earth and Planetary Science Letters, 587. https://doi.org/10.1016/j.epsl.2021.117307 

[8]  Papadopoulou, M., et al., 2020. Unravelling intraplate Cenozoic magmatism in Mongolia: Reflections from the present-day mantle or a legacy from the past? Proceedings of the EGU. https://doi.org/10.5194/egusphere-egu2020-12002 

[9]  Ancuta, L.D., et al., 2018. Whole-rock 40Ar/39Ar geochronology, geochemistry, and stratigraphy of intraplate Cenozoic volcanic rocks, central Mongolia. Geological Society of America Bulletin, 130. https://doi.org/10.1130/b31788.1 

[10]  Comeau, M.J., et al., 2021b. Geodynamic modeling of lithospheric removal and surface deformation: Application to intraplate uplift in Central Mongolia. Journal of Geophysical Research: Solid Earth, 126(5). https://doi.org/10.1029/2020JB021304

[11]  Comeau, M.J., et al., 2020. Compaction driven fluid localization as an explanation for lower crustal electrical conductors in an intracontinental setting. Geophysical Research Letters, 47(19). https://doi.org/10.1029/2020gl088455 

 

 

 

How to cite: Comeau, M. J., Moorkamp, M., Becken, M., and Kuvshinov, A.: Joint inversion of gravity and electromagnetic data — New constraints on the 3-D structure of the lithosphere beneath Central Mongolia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12704, https://doi.org/10.5194/egusphere-egu22-12704, 2022.

13:58–14:05
|
EGU22-3615
|
Virtual presentation
Carlo Iapige De Gaetani et al.

Investigating the crustal architecture, specifically the discontinuity interface between the upper mantle and lower crust of the Earth, so-called Moho, can be done in three prevailing techniques, namely lithology, seismicity, and gravity. In contrast to using the information from analyzing the characteristics of rocks and seismic waves, which are sparsed and expensive, inverting gravity data of satellite missions such as GOCE and GRACE is a suitable alternative for such purposes.

The present paper attempts to map the Moho surface using the gravity data as we considered a simplified Earth model based on three shells including the core, mantle and crust with a potential T on a given sphere outside this body. In this notation, by subtracting the topographic effects, compensating for density anomalies in the crust, and other known constants from the observation that are given on and outside the mean Earth radius, one is left with the potential of a single layer on the mean Moho sphere by taking into consideration the Helmert condensation approach. In planar approximation, this is to say that the topography is formally referred to an xy plane and also the condensation surface which is a plane, situated at a depth D below the previous one. Therefore, relating the topographic load of a mass column with height h over the same elementary area element at depth d, the measure of how deep the crust is sinking into the mantle material as a consequence of the load, we can interpret the Moho variations with respect to some mean crustal thickness.

To do this inversion, we applied the Least Square Collocation (LSC) approach which uses the functional relationships between the quantities, the auto-covariance and cross-covariance matrices based on a covariance function between observations and the unknowns. Practically, after constructing the required residual data, an empirical covariance is estimated, then fitted to analytical one to define the required covariance models.

Finally, the Moho variations has been estimated in an active tectonic zone created by the continental collision of the Arabian plate from South-West and Turan shield from North-East with respect to a mean Moho depth equal to 45 km. Results of this study are comparable and much the same with other studies so that different rheological zones of Iranian plateau can be seen in this estimated map of Moho. For instance, a maximum depth is estimated for Sanandaj-Sirjan zones in South-East and minimum depth for Caspian Sea in North.

How to cite: De Gaetani, C. I., Heydarizadeh Shali, H., Ramouz, S., Safari, A., and Barzaghi, R.: Moho depth evaluation using GOCE gradient data and Least Square Collocation over Iran, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3615, https://doi.org/10.5194/egusphere-egu22-3615, 2022.

14:05–14:12
|
EGU22-12459
|
Virtual presentation
Carla Braitenberg et al.

The objective of this work is to investigate the geologic and tectonic units in the Iranian plateau in relation to the information that can be obtained from the gravity field observed from space. The objective requires to collect seismologic tomography, seismicity, geodetic observations of crustal movements, a database of active faults, active seismic investigations of sediment depths, heat flow measurements and to use this information as a constraint for gravity inversion with the present available satellite-derived gravity field. The gravity field correlated to the topography defines blocks of the plateau, which indicates varying crustal rigidity (Pivetta and Braitenberg, 2020). We find that mechanisms of vertical growth are tied to crustal thickening, coherently identified from the gravity field, seismic tomography and isostasy. Persistent high density crustal blocks are identified for instance SE of Isfahan, which require further investigation and validation, also in relation to magmatism. The study is embedded in a major project addressing the “Intraplate deformation, magmatism and topographic evolution of a diffuse collisional belt: Insights into the geodynamics of the Arabia-Eurasia collisional zones” financed by the Italian Ministry (PRIN 2017). When defining the density structure and its uncertainties, the question appears, what improvements on the knowledge of the structure, seismic faults, and on the block-structure can be expected from future gravity missions, with a payload of quantum gradiometers and atom-clocks in a multi satellite configuration. The geophysical sensitivity to quantum gravimetry in space is of interest to the MOCAST+ ASI project, a follower project of the MOCASS ASI project, in which the geophysical sensitivity of the quantum gradiometer payload has been studied (Pivetta et al., 2021).

Pivetta, T., & Braitenberg, C. (2020). Sensitivity of gravity and topography regressions to earth and planetary structures. Tectonophysics, 774, 228299. https://doi.org/10.1016/j.tecto.2019.228299

Pivetta, T., Braitenberg, C. & Barbolla, D.F. (2021) Geophysical Challenges for Future Satellite Gravity Missions: Assessing the Impact of MOCASS Mission. Pure Appl. Geophys. https://doi.org/10.1007/s00024-021-02774-3

How to cite: Braitenberg, C., Pivetta, T., Pastorutti, A., and Tesauro, M.: Geologic and Tectonic units in the Iranian Plateau from present and future satellite missions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12459, https://doi.org/10.5194/egusphere-egu22-12459, 2022.

14:12–14:19
|
EGU22-12634
|
On-site presentation
Alberto Pastorutti et al.

Regional-scale geophysics is a central tool in improving the knowledge on geologic and tectonic units and on their structural relationships in a complex convergent setting. Harmonization, reduction, and integrated modelling of data such as gravity models and seismic tomographies allows to constrain the geometry and properties of geologic bodies at depth and to test hypotheses on their evolution. In the context of an interdisciplinary project involving multiple Italian institutions, “Intraplate deformation, magmatism and topographic evolution of a diffuse collisional belt: Insights into the geodynamics of the Arabia-Eurasia collisional zones”, we present the result of an integrated analysis across the Zagros Orogen. It represents the most active collisional zone in the Iranian plateau, consequent to the NE-ward subduction of the Neo-Tethyan Ocean.

 

We integrate models of surface topography and gravity through isostatic analysis, i.e. by enquiring the relationship connecting the two observables – the former expressing the load on the lithosphere, the latter a proxy of the crust-mantle boundary undulations. We developed and employed two independent methods, one relying on plate flexure and providing estimates on the spatial distribution of the integrated rigidity of the lithosphere, the other a non-parametric residualization method, based on topo-gravity regression analysis (Pivetta and Braitenberg, 2020). We refine their estimates by including the additional information provided by locally available models of sedimentary infills, in order to correct the loads, and by seismological Moho depth data (e.g. Gvirtzman et al., 2016), to mitigate ambiguities in the crustal thickness inferred from gravity inversion. This analysis allowed the isolation of different rigidity domains - which reflect the assemblage of tectonic provinces and the shallow expression of deep structures - and to obtain the anomalous quantities (e.g. residual gravity disturbance, residual topography) which the initial model does not explain. These include intra-crustal loads, which correlate with areas affected by magmatism and can provide further constrain on the geometry of buried structures.

 

We then improve these estimates with the data derived from seismic tomographies, including the recent shear-wave velocity model by Kaviani et al. (2020). By employing a velocity-to-density conversion strategy and gravity forward modelling, we show the impact of prior reduction of gravity data for upper-mantle signal sources. In addition to that, we use tomography-derived temperature modelling to estimate the variations of lithospheric strength profiles throughout the study area, comparing it with the independently estimated flexural rigidity.

 

Pivetta, T., & Braitenberg, C. (2020). Sensitivity of gravity and topography regressions to earth and planetary structures. Tectonophysics, 774, 228299. https://doi.org/10.1016/j.tecto.2019.228299

Gvirtzman, Z., Faccenna, C., & Becker, T. W. (2016). Isostasy, flexure, and dynamic topography. Tectonophysics, 683, 255–271. https://doi.org/10.1016/j.tecto.2016.05.041

Kaviani, A., Paul, A., Moradi, A., Mai, P. M., Pilia, S., Boschi, L., Rümpker, G., Lu, Y., Zheng, T., Sandvol, E. (2020). Crustal and uppermost mantle shear wave velocity structure beneath the Middle East from surface wave tomography. Geophysical Journal International, 221(2), 1349–1365. https://doi.org/10.1093/gji/ggaa075

How to cite: Pastorutti, A., Braitenberg, C., Pivetta, T., and Tesauro, M.: Lithospheric architecture across the Zagros Orogen as sensed by the integration of isostatic analysis, gravity inversion, and seismic tomography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12634, https://doi.org/10.5194/egusphere-egu22-12634, 2022.

14:19–14:26
|
EGU22-4803
|
On-site presentation
Natalia-Silvia Asimopolos and Laurentiu Asimopolos

The gravity and magnetic anomalies separation operation consists in determining the number of sources, the characteristics of each (depth, density, shape, and dimensions) so as to result in cumulative total anomaly, measured at the Earth’s surface. This separation has to be done in the context of the fundamental ambiguity of gravimetric and magnetic information, based on the cause-effect ratio. There are various methods for achieving this separation of anomalies. This paper presents some examples of the use of the moving average method and the polynomial trend surfaces. In particular, we presented the results of the mobile mediation with different windows compared to the tendency surfaces with different degrees, for a case study in Eastern Carpathians mountains area. For this study we used data available from several sources.

From the International Gravimetric Bureau we used gravimetric data for the WGM2012 geoglobal model: Bouguer anomaly for density 2.67 g / cm3, outdoor anomaly, isostatic anomaly, gravitational disturbance and altitude.

From the geophysics portal of the Geological Institute of Romania we used the magnetic data resulting both from the scanning of the national geomagnetic maps and from the catalogs of measurements from the archive. We also used the deep geological sections made on the basis of seismic data, corroborated with gravimetric and magnetic data that cross the Eastern Carpathians.

Other data used for depth correlations were the isobath map of the Moho surface, the Conrad surface, the geoid, and the quasigeoid.

For the study of deep tectonics based on all the data used we used the correlation coefficient between various parameters, calculated in movable windows of different sizes both in plan and in space. For this we have developed specific calculation programs.  The moving average is a direct method for separating regional effects and local (residual) effects. Polynomial trend surfaces analysis contributes to the recognition, isolation and measurement of trends that can be calculated and represented by analytical equations, thus achieving a separation in regional and local variations. The analytical expressions of the polynomial trends based on the least squares method were calculated, highlighting the regional trend caused by the deep structures. Then, by calculating the residual values resulting from the difference between the initial values and the trend values from the network nodes used, we highlighted the superficial local effects. We also obtained information about the regional trend caused by geological structures at medium and large depths, by calculating the difference between gravity parameters, obtained with different moving average windows or tendency surfaces with different degrees, interpolated in same network.

How to cite: Asimopolos, N.-S. and Asimopolos, L.: Separation of gravimetric and magnetic anomalies with different degrees of regionality in the Eastern Carpathians, Romania, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4803, https://doi.org/10.5194/egusphere-egu22-4803, 2022.

14:26–14:33
|
EGU22-7769
|
ECS
|
On-site presentation
Małgorzata Ponikowska et al.

Defining a transition zone between the Precambrian East European Craton (EEC) and the Palaeozoic West European Platform (WEP) is still a matter of discussion despite a large body of geophysical and geological data. The main tectonic feature of the transition zone is the Teisseyre-Tornquist Zone (TTZ), which has been variously interpreted over the past decades mainly because of a thick (c. 10 km) Palaeozoic and Mesozoic sedimentary cover masking its crustal architecture.  We investigated the crustal structure of the TTZ using a 270-km long wide-angle reflection/refraction profile (WARR) measured along 15 ocean-bottom seismometers and 2 land stations during the course of the RV MARIA S. MERIAN expedition ‘MSM52’. This NE to SW profile is oriented nearly parallel to the Polish coast, located ~ 48 km south of the Danish island of Bornholm. We prepared a two-dimensional gravity and magnetic forward model along this profile, using the Geosoft GM-SYS software with layers of infinite length. The basis for the potential field modelling is a seismic velocity model that has been prepared through trial-and-error forward modelling.

The seismic velocity model shows a continuity of the lower and middle crust of the EEC towards the basement of the WEP. The synthetic magnetic profile is smooth and indicates that the seismic data accurately revealed the geometry and depth of the magnetic (crystalline) basement. However, the model was unable to replicate short-wavelength, high-amplitude magnetic anomalies in the ENE section of the profile, probably representing iron oxide mineralisation in the crystalline basement of the EEC. The gravity model shows 3 areas of misfit between the synthetic and observed gravity profile. The most prominent misfit coincides with the NE boundary of the TTZ. To remedy the misfit, we produced two alternative gravity models that deviate from the seismic velocity model in the problematic area. One model postulates a crustal keel underneath the NE section of the TTZ and the other suggests the presence of a middle crust magmatic intrusion. Both models equally and adequately reduce the misfit of the gravity model.

Our models suggest a SW-ward continuation of the Baltica middle and lower crust through the TTZ and seem to preclude the coincidence of the Caledonian Thor suture with the TTZ. An important perturbation of the upper crust and sedimentary cover within the latter is mostly associated with the superimposed effects of Devonian-Carboniferous and Permian-Mesozoic extension. The only conspicuous compressional event confirmed by our data is the Late Cretaceous-Paleogene inversion of the Permian-Mesozoic basin. Due to limited resolution, our models did not reveal the effects of Caledonian nor Variscan shortening, including the Caledonian Deformation Front.

This study was funded by the Polish National Science Centre grant no UMO-2017/27/B/ST10/02316.

How to cite: Ponikowska, M., Mazur, S., Janik, T., Wójcik, D., Malinowski, M., Hübscher, C., and Heyde, I.: Two-dimensional gravity and magnetic model along a new WARR profile in the transition zone from the Precambrian to Palaeozoic platform in the southern Baltic , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7769, https://doi.org/10.5194/egusphere-egu22-7769, 2022.

14:33–14:40
|
EGU22-711
|
ECS
|
|
Virtual presentation
El Houssein Abdeina et al.

The famous circular structure of Richat, sometimes referred to as “the eye of Africa”, is located in the northwestern part of the Taoudeni basin, in the central part of the Mauritanian Adrar plateaus. It is expressed at the surface as a slightly elliptical depression, about 40 kilometers in diameter, marked by concentric ridges of Proterozoic-Lower Paleozoic sediments. Its origin as resulting from either a meteorite impact or a deep magmatic intrusion, has been long debated. Modelling of high-resolution airborne magnetic data as well as satellite gravity data reinforces the intrusion hypothesis. Geophysical modelling has been calibrated by determinations of rock properties from various types of magmatic lithologies sampled in the field. The three complementary types of geophysical data allow us to image at various scales and depths the buried structures of the Richat magmatic complex, to determine the areas most affected by hydrothermal alteration and finally to elaborate a kinematic model for its emplacement. We emphasize that : (1) the Richat intrusion is characterized by the presence of two important circular magnetic signals that coincide with gabbroic ring dykes partly exposed at the surface, (2) its overall circular structure rests above a deep mafic (gabbroic) body, (3) the upwelling of magma at the surface has been facilitated by the presence of concentric faults and (4) the central zone of the complex recorded intense hydrothermal alteration. This case study aims to provide insights for similar types of magma-induced ring structures observed worldwide.

How to cite: Abdeina, E. H., Bazin, S., Chazot, G., Bertrand, H., Le Gall, B., Youbi, N., Sabar, M. S., Bensalah, M. K., and Boumehdi, M. A.: The deep structure of the Richat magmatic intrusion (northern Mauritania) from geophysical modelling. Insights into its kinematics of emplacement, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-711, https://doi.org/10.5194/egusphere-egu22-711, 2022.

14:40–14:47
|
EGU22-2625
|
ECS
|
|
On-site presentation
Gabriela Camargo et al.

The Monchique alkaline complex (MAC) crops out in southern Portugal with a roughly elliptical shape of about 80 km2 elongated along ENE-WSW direction. The MAC dates to Late Cretaceous (69-72 Ma) and intrudes the Carboniferous Flysh formation of the South Portuguese Zone. At the surface, it comprises two main types of syenites: a central homogeneous nepheline syenite surrounded by a heterogeneous syenite unit, and some less expressive outcrops of mafic rocks (gabbros, hornfels, breccia and basalts). This igneous complex belongs to the Upper Cretaceous West Iberia alkaline magmatic event, characterized by alkaline magmatism of sublithospheric origin and active from approximately 100 Ma to 69 Ma.

The Monchique region hosts the most active seismic cluster of mainland Portugal, with low magnitude earthquakes (M < 4) that occur along lineations with NNE–SSW and WNW–ESE preferred orientation.

In this work we study the Monchique region through gravimetric and magnetic methods in order to: 1) better understand how the MAC influences the geomagnetic and gravimetric field in the region; 2) to create a new and consistent 2D and 3D model for the intrusion; and 3) to help constraining the origin of the observed seismicity and its possible relation with the existence of subcropping magmatic bodies.

We process recently acquired data - ground gravity survey (49 points) and drone-borne aeromagnetic survey – and integrate it with existing data. The interpretation of gravimetric results is complemented by density analysis of magmatic and host rocks. We perform 3D magnetic and gravity inversion to model the geometry of gravity and magnetic sources, and 2D magnetic forward modeling along a representative profile.

The calculated Bouguer gravity anomaly shows a positive gradient towards the southwest with a negative peak in the center of the Monchique mountain. However, when applied the terrain correction (complete Bouguer anomaly), this peak vanishes. This is justified by the similar mean density values for the syenite and host rocks, respectively 2560 kg/m3 and 2529 kg/m3.

The new aeromagnetic data allows for mapping the Monchique magnetic anomaly with unprecedented detail and reveal a 10 km elongated anomaly with 30 m wavelength with maximum 1707 nT amplitude. 3D susceptibility inversion models show a 15km long body with maximum depth between 5-10km, and susceptibility >0.02 SI, in agreement with previous susceptibility analysis in the region. The highest magnetic signal is found at Picota hill (east), but the deepest parts of the intrusion seem to be bellow Foia hill (west). It is noteworthy that earthquake hypocenters concentrate at depths of 5-20 km, thus below most of the modeled magmatic intrusion.  

This work was developed for the MSc thesis of GCC, in the frame of ATLAS project (PTDC/CTA-GEF/31272/2017), POCI-01-0145-FEDER-031272, FEDER-COMPETE/POCI 2020) partly funded by FCT. FCT is further acknowledged for support through projects UIDB/50019/2020-IDL, PTDC/CTA-GEF/1666/2020 and PTDC/CTA-GEF/6674/2020.

How to cite: Camargo, G., Neres, M., Bos, M., Martins, B., Custódio, S., and Terrinha, P.: Magnetic and gravimetric modeling of the Monchique magmatic intrusion in south Portugal, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2625, https://doi.org/10.5194/egusphere-egu22-2625, 2022.

Wed, 25 May, 15:10–16:40

Chairpersons: Carla Braitenberg, Alexandra Guy

15:10–15:17
|
EGU22-12321
|
Virtual presentation
Martina Capponi and Daniele Sampietro

The Earth crust represents less than 1% of the volume of our planet but is exceptionally important as it preserves the signs of the geological events that shaped our planet. This thin layer is the place where the natural resources we need can be accessed (e.g.  critical raw materials, geothermal energy, water, oil and gas, minerals, etc.). For these reasons, a thorough understanding of its structure is crucial for both scientific and industrial future activities. It is well known that potential fields methods, exploiting gravity and magnetic fields, are among the most important tools to recover fundamental information on the Earth crust. In recent years, thanks to the increasing availability of seismic/seismological data and to gravity and magnetic satellite missions, the crust has been thoroughly investigated and modelled at global and continental scales. However, despite this progress, it remains poorly understood in many regions as global models are often too coarse to provide detailed information about the regional and local dynamics.  

With this respect, the challenge to be faced nowadays is represented by the development of ad-hoc techniques to fully exploit these different geophysical global data and to merge them with regional datasets compiled at the Earth’s surface. Currently, the different sources of information when analysed individually suffer from non-uniqueness. Magnetic and gravity signals detect different crustal parameters and rarely coincide because various combinations of geological structures generate similar observations outside the sources. A promising solution is represented by the joint processing in a consistent way of both gravity and magnetic fields data, possibly incorporating the available geological knowledge and constraints coming from seismic acquisitions, in such a way to reduce the space of possible solutions. 

In the eXperimental jOint inveRsioN (XORN) project, funded by the European Space Agency through the EO4society program, Geomatics Research & Development srl (GReD) together with Laboratoire Magmas et Volcans (LMV) of Clermont Auvergne University will develop an innovative algorithm aiming at performing complete 3D joint inversion of gravity and magnetic fields properly constrained by geological a-priori qualitative information. The developed algorithm will be used within the project to recover a 3D regional model of the Earth crust in the Mediterranean Area in terms of density and magnetic susceptibility distribution within the volume, and in terms of depths of the main geological horizons. Within this regional case study particular attention will be given to the bathymetric layer thus defining and testing a strategy that could potentially be applied worldwide to improve our knowledge of this layer which is fundamental for every application that aims at studying (e.g. for tsunami hazards), conserving and sustainably using the oceans, seas and marine resources. 

The first results about technical developments will be here presented together with preliminary modelling aspects of the Mediterranean test case. 

How to cite: Capponi, M. and Sampietro, D.: eXperimental jOint inveRsioN (XORN) project: first results of a 3D joint gravity and magnetic inversion, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12321, https://doi.org/10.5194/egusphere-egu22-12321, 2022.

15:17–15:24
|
EGU22-6489
|
ECS
|
|
Virtual presentation
Diogo Rodrigues et al.
 
 

How to cite: Rodrigues, D., Neres, M., Terrinha, P., Bos, M., and Martins, B.: Drone-magnetic survey along the Alentejo coast (SW Portugal): a quest for the intruded Messejana fault, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6489, https://doi.org/10.5194/egusphere-egu22-6489, 2022.

15:24–15:31
|
EGU22-11258
|
ECS
|
|
Virtual presentation
Leon Kaub et al.

Magnetic surveys employing Uncrewed Aerial Systems (UAS) allow a fast and affordable acquisition of high-resolution data. We developed a self-built carbon-fiber frame which can be used to attach magnetometers 0.5 m below an UAS. In order to remove undesired signals from the magnetic recordings that originate from the aircraft and that can cause strong heading errors, we apply calibration processes often referred to as magnetic compensation. These processes are usually applied for manned aerial surveys for both scalar and vector magnetometer data and require flying a calibration pattern prior to a survey. We recently published open-source software written in Python to process data and compute compensations for both scalar and vector magnetometers. We tested our method with two commercially available magnetometer systems (scalar and vector) by flying dense grid patterns over a test site using different suspension methods (magnetometer system attached to 2.8 m long tethers, fixed on the landing gear of the UAS, and fixed on our frame configuration). The accuracy of the magnetic recordings was assessed using both standard deviations of the calibration pattern and tie-line cross-over differences from the grid survey. Our frame configuration resulted after magnetic compensation in the highest accuracy of all configurations tested. The frame also allows for the acquisition of aeromagnetic data under a wide range of flight conditions. This is of great advantage compared to the often-used tethered solutions to avoid recording the aircraft’s signals. Since tethered payloads are prone to rotations and swing motions, they require skilled pilots and can be difficult to fly safely. In contrast to that, our system is easy to use and due to its high in-flight stability, even fully autonomous flights are possible. Since the calibration flights that are required for magnetic compensation need to be collected in areas with low magnetic gradients, it can be difficult to find suitable locations in areas with strong magnetic gradients – such as in volcanic and geothermally active regions. However, a survey collected at the location of the calibration site can be used to evaluate the geological magnetic signal. The compensation process involves then two successive evaluations of the compensation parameters. First, an approximate evaluation of the compensation parameters is done assuming a constant value of the magnetic field at the calibration site. The resulting compensation parameters are then used to compensate the survey data collected over the calibration site and evaluate the magnetic field along the calibration pattern trajectory. Second, the compensation parameters are reevaluated taking the magnetic field variations into account. We tested this double calibration scheme on recordings that were collected over the Krafla geothermal area in the Northern Volcanic Zone of Iceland. The double calibrated data resulted in higher accuracy than a single calibration showing that this method can improve magnetic compensation in magnetically high-gradient areas.

How to cite: Kaub, L., Bouligand, C., and Glen, J. M. G.: Collecting and calibrating magnetic data from surveys with Uncrewed Aerial Systems (UAS) and an approach for regions with strong magnetic gradients, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11258, https://doi.org/10.5194/egusphere-egu22-11258, 2022.

15:31–15:38
|
EGU22-1602
|
ECS
|
On-site presentation
Hugo Reiller et al.

We compare marine magnetic measurements simultaneously acquired with absolute and three-component fluxgate sensors to evaluate their respective benefits for marine geophysical mapping and detection surveys.

Shom collected the data in shallow waters, in the Bay of Brest (France) and in the Iroise Sea, during two cruises in the Fall 2021. As per standard practice, an absolute Overhauser magnetometer was towed 180 m behind the 60 m-long Laplace and Lapérouse hydrographic vessels. In addition, two vector magnetometers were temporarily installed at the top of the ship’s mast and on the roof of a 10 m-long launch. Scalar data were processed following Shom’s standards: shift to sensor position, layback adjustments, removal of gyrations and spikes, filtering and calculation of magnetic anomalies by removing the IGRF model (Alken et al., 2021) and reducing external variations measured at a local reference station. Vector data were corrected for the strong magnetic fields generated by the hull and other steel components of the ship by the application of a “scalar compensation” using a least-squares regression analysis (Leliak, 1961) on data from figures of merit. The compensated vector data then need to be low-pass filtered to remove uncorrected variations of attitude and heading. Magnetic anomalies were finally computed by removing the median value for each profile and reducing external variations from the same local reference station.

Our first results show that maps of total-field anomalies derived from vector data acquired on the ship are very close to those of the absolute data upward-continued to the altitude of the mast. This similarity suggests that it is possible to perform good-quality magnetic surveys without the constraint of having to tow an instrument. The different processing steps however raise the detection threshold for anthropogenic objects lying on the seafloor or partially buried. Vector data acquired on smaller launchs are much more complicated to compensate as ranges of pitch, roll and heading variations are greater than for a large ship and potentially imperfectly sampled by the figures of merit.

How to cite: Reiller, H., Oehler, J.-F., Lucas, S., Marquis, G., Rouxel, D., and Munschy, M.: Comparison between towed absolute and shipborne 3C fluxgate magnetic measurements in shallow water. Applications for marine geophysical surveys., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1602, https://doi.org/10.5194/egusphere-egu22-1602, 2022.

15:38–15:48
|
EGU22-6615
|
solicited
Richard Saltus et al.

To paraphrase a common model aphorism: “all magnetic maps are wrong, some are useful”. In other words, all maps of the Earth’s magnetic field are subject to uncertainty, both observationally and dynamically. Depending on the intended use of the map, this uncertainty will have varying implications. For those of us who build and use magnetic maps it is important to gain understanding of the uncertainty in these maps to ensure that they are clearly presented and suitable for a given use.

Uncertainty evaluation is a general challenge that affects all magnetic maps and models, but here we concentrate on maps of magnetic anomalies (i.e., perturbations of the Earth’s main field primarily due to variations in magnetic minerals in the crust and shallow mantle) in oceanic areas.

Magnetic anomaly maps for oceanic regions are typically representations of gridded data. The grids are built from available data which generally consists of marine trackline data with a range of ages, collection parameters and uncertainty in original observations. Data coverage and trackline geometries are highly variable around the world. For example, near-shore regions in the Northern Hemisphere tend to be well sampled, whereas open ocean portions of the Southern Hemisphere are poorly sampled.

Quantification of cell by cell uncertainty for magnetic anomaly grids can be subdivided into two regimes: cells containing data and cells without data. For cells containing data, factors such as point-wise observation uncertainty, number of observations, and spatial distribution of data, can be analysed to estimate grid value uncertainty. For interpolated cells, factors such as distance to nearest data cells, local field behavior, and uncertainty in surrounding cells are relevant.

Using NOAA/NCEI trackline marine data for portions of the Caribbean Sea and North Atlantic we are constructing and testing uncertainty models and methods for representing this uncertainty for a variety of magnetic map uses. For a marine magnetic anomaly grid of a portion of the North Atlantic at a 4 km grid interval (the same grid interval used by our global EMAG2 magnetic anomaly compilation), the calculated cell level uncertainty ranges from 20 nT to 150 nT with a mean value of 90 nT. This mean value is similar to the average grid uncertainty of 100 nT/cell that we estimated for marine areas of EMAG2v3. Different gridding approaches, including kriging or minimum curvature algorithms, yield variations in individual cell values, but these variations fall within our estimated uncertainty ranges. 

How to cite: Saltus, R., Chulliat, A., Meyer, B., and Bates, M.: Examination of magnetic map variability and uncertainty: crustal magnetic anomalies in oceanic areas, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6615, https://doi.org/10.5194/egusphere-egu22-6615, 2022.