The Shillong Plateau (SP) in NE India is one of the most debatable proterozoic basement in the world due to its complex tectonics. Though it had some collisional histories during the formation of the Gondwanaland associated with the Indo-Australo-Antarctic suture, but the signature of crustal and moho depth models gives a different idea about the modified crust under the SP. Despite its peak elevation at around 2000 m, the moho depth observed from the seismic tomography and satellite gravity data under the SP is not more than 34 km, which is remarkably smaller than the surrounding Bengal basin (⁓44 km) and the Brahmaputra basin (⁓44 km). We have tried to solve the problem related to the moho variation, taking into account the field gravity and magnetic anomaly. The major trends in the gravity anomaly predominant along EW direction conforming to the trends of regional geological structures across most of the SP. As our study area concentrates along an NS profile across two different litho units restricted to the central part of the plateau. The corrected field magnetic anomaly across the study area has a little variation between 0 to -500 nT, although some change in anomaly pattern can be seen along the northern side of the SP reaching towards -3500 nT. Moreover, the southern side of the plateau has very little magnetic anomaly variation. The bouguer gravity anomaly varies from ⁓ -70 mGal at the northern boundary to ⁓ +10 mGal with a steep gradient found across the southern side. The gradual change over to positive anomaly under SP, strong -ve anomaly under the Brahmaputra basin to the north and moderate negative anomaly under the Bengal basin towards the south suggested an uplifted moho under SP, which is demonstrated by the 2D gravity modelling. Closely spaced bouguer anomaly contours along the southern part and EW trending magnetic anomaly along the northern part of the SP, indicating two boundary faults viz. Oldham fault/ Brahmaputra valley fault in the North and Dauki fault in the south, dipping towards each other supported the SP for the formation of the pop-up tectonics.

How to cite: Samantaray, S., Pathak, P., Mohanty, W. K., and Gupta, S.: Sub-surface characteristics of pop-up tectonics through field gravity and magnetic modelling: An example of the Shillong Plateau, NE India, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6315, https://doi.org/10.5194/egusphere-egu23-6315, 2023.

The paper is mainly aimed at presenting some results of the geophysical investigations focused on the Gurghiu Mountains volcanism.

The Gurghiu Mountains are located in the central inner (western) part of Eastern Carpathians, Romania. They represents the middle segment of the approx. 160 km long Neogene to Quaternary volcanic chain Călimani-Gurghiu-Harghita (CGH), the southeastern end of the magmatic arc adjoining the Carpathians from Slovakia to Romania. CGH is a typical andesite-dominated calc-alkaline volcanic range. As part of it, Gurghiu Mountains exhibits (with minor exceptions) monotonous volcanic rocks, clearly dominated by andesites and pyroxene andesites.

Several years ago, CGH volcanism was subject to research within a specific project funded by the Romanian National Agency for Scientific Research. During the project, gravity and geomagnetic investigations were conducted in the Gurghiu Mountains areal to help unveiling the composition and structure of the volcanic edifices. Thus, consistent gravity and geomagnetic data sets over the studied area were obtained. Furthermore, various data mining techniques (e.g., Bouguer anomaly for various reference densities, geomagnetic and reduced-to-the-pole geomagnetic anomaly, regional-residual separation through upward/downward continuation and/or polynomial regression, high-order derivatives) were applied in order to create more intuitive images helping in the qualitative interpretation of the geophysics results.

In a second stage, quantitative approaches were employed for unveiling the hidden structure of the shallow part of the crust. Consequently, 2D and 3D models of the density and magnetic structure of the main volcanic forms in the area (e.g., Fâncel-Lăpusna caldera, Seaca-Tătarca, Sumuleu and Ciumani-Fierăstraie crater areas) were inferred from joint inversion of gravity and geomagnetic data.

Finally, based on the inversion results, attempts to construct 3D models of the shallow crust architecture were made by employing the forward modelling approach under constraints provided by rock physics studies and exploration wells.

Key words:  gravity, geomagnetism, density, magnetic susceptibility, inversion, forward modelling, volcanism, Gurghiu Mountains, Eastern Carpathians

How to cite: Besutiu, L., Zlagnean, L., Isac, A., and Romanescu, D.: Density and magnetic architecture of the Gurghiu Mountains volcanoes as inferred from geophysical data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8177, https://doi.org/10.5194/egusphere-egu23-8177, 2023.

The Mediterranean Sea crust has been intensely studied both for scientific reasons and for economic activities such as natural resources exploration and exploitation. However, a complete high-resolution numerical model of the crust over the whole region is still missing. In fact, from the one hand, we have global crustal models, which however are usually too coarse to accurately describe this complex area, while on the other hand we have continental scale models, which are obtained by merging different datasets, without an homogeneous analysis.

In the current study we perform a joint inversion of gravity and magnetic field measurements, constrained with seismic profiles, on the whole Mediterranean Sea Area with a spatial resolution of about 15 km in the planar direction and ranging from 200 m to 1200 m in the vertical one, for a total of more than 2-million cells.

The inversion has been carried out within the XORN project (https://xorn-project.eu/) funded by the European Space Agency. The result of the study is a complete three-dimensional (3D) model of the crust beneath the Mediterranean Sea region in terms of density and magnetic susceptibility distributions and geological horizons, completed by an estimate of the predicted accuracy.

Several maps, such as depth of main geological horizons (namely the base of Plio-Quaternary and Messinian sediments, the basement, the Curie isotherm, and the Moho), have been derived, from the 3D model.

The model has been validated through comparisons with local studies, seismic information, heat flow data not used within the inversion.

How to cite: Sampietro, D., Capponi, M., Thébault, E., and Gailler, L.: Mediterranean Sea Crustal Structure from Potential Fields, Results of XORN Project, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11094, https://doi.org/10.5194/egusphere-egu23-11094, 2023.

The northern of Algeria is located between the limit of the African and Eurasian plates. It is known for its geological heterogeneity. This part is experienced with few geophysical data. Among this data, the gravimetric survey can reveal several pieces of information about geological complexity. Although, before any achievement of gravimetric data, it is imperative to perform a gravimetric network. In this work, we present the full steps of implementing the gravimetric network located in the eastern part of the north of Algeria, combined with the processing in detail, using a manual method. The new gravimetric network is situated in the main area of the Guelma basin and its surrounding area (07° 00’; 08° 00 ’E and 36° 00’; 36° 45’N). This network encompasses thirty-nine gravimetric reference stations, linked to the Algerian gravity network. It forms one polygon that is built with 61 triangular loops connected to each other with 99 links. The initiation of the method used, and all stages of the gravity data are described. The average of the gap of gravity values at each station is about 11 µGal. the campaign was carried out using a terrestrial Scintrex CG3 gravimeter. The new gravimetric network of the north eastern part of Algeria is adjusted by means of the ginning method. The principal purpose of the realization of this gravimetric network is to provide a high quality for all future works with respect to the gravimetric studies in the north eastern part of Algeria.

How to cite: bayou, Y., Bouyahiaoui, B., Abtout, A., Berguig, M. C., and A. Renaut, R.: Gravimetric network of the Eastern part of the North of Algeria, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2629, https://doi.org/10.5194/egusphere-egu23-2629, 2023.

The Kunene Anorthosite Complex (KAC), located in SW Angola, is one of the largest anorthosite structures in the world. Dating from the Mesoproterozoic, its installation process is still not clear. Several mafic and ultramafic outcrops can be found surrounding the KAC. Once considered related with its emplacement, the study of these bodies may help us understand the history of this unique geological feature. While geochronological data show that they are synchronous, or possibly a bit younger, than the embedding granites and migmatites of Paleoproterozoic age, the question arises of whether they are intrusions installed in the host rock or if they are instead recycled remains of older Arch crust. The development of these outcrops in depth provides relevant clues regarding the origin of these bodies and their relationship with the Eburnean (~1.93-2.04 Ga) and Epupa-Namibe (~1.83-1.74 Ga) events. One of these mafic outcrops, designated the Hamutenha outcrop (Huíla Province) exhibits an elongated shape and a NW-SE orientation and is characterized by an internal zonation.  Generally, the innermost part is composed of ultramafic rocks of (mostly harzburgites and dunites), with diorites outcropping in its NW and SE borders. The Hamutenha outcrop was previously identified for potentially bearing Cr, Ni and PGE mineralization.

Therefore, the aim of this study is two-fold. Firstly, it attempts to determine the development at depth of the mafic body to better understand its origin. Secondly, it tries to clarify the emplacement mechanisms responsible for the potential mineralization and to evaluate the likelihood of its economic potential. Aeromagnetic and ground gravimetric data acquired in the framework of project PLANAGEO (National Geology Plan for Angola) of which the National Laboratory of Energy and Geology (Portugal) was one of the partners, was used to create a magnetic vector model and a density contrast model of the Hamutenha body. These 3D models were interpreted in combination with the detailed geological observations and aeroradiometric data also from the PLANAGEO project, providing new insights on the underground lithological differentiation and geometry of this geological structure.

How to cite: Represas, P., Sousa, P., Morais, I., Cordeiro, D., Carvalho, J., Batista, M. J., Dito, M., Llorente, J. M., Marques, F., Mateus, T., Rodrigues, J. F., Lobón, J. L., and Oliveira, D.: New insights on the ultramafic intrusions surrounding the Kunene Anorthosite Complex (SW Angola) from gravity, magnetic and radiometric data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15641, https://doi.org/10.5194/egusphere-egu23-15641, 2023.

The Wilkes Subglacial Basin hosts potentially the largest unstable sector of the East Antarctica Ice Sheet due to the depth of the ice bed below sea level. Ice covering such basins poses a potentially high, but poorly constrained risk for future sea-level rise, as it is more vulnerable to melting by warming of the surrounding ocean. Such melting could potentially trigger mechanisms of unstable retreat. The neighbouring Transantarctic Mountains are the largest non-contractional mountain range on Earth. Traditionally, the Transantarctic Mountains are viewed as dividing the ancient East Antarctic craton from the younger West Antarctic Rift system. However, petrological samples and previous geophysical mapping suggest that the craton boundary is further west, following the western edge of the Wilkes Subglacial Basin. Subglacial geology influences geothermal heat flow and bed roughness, and therefore to better understand the past, present and possible future behaviour of the East Antarctic Ice Sheet improved understanding of the subglacial geology on which it flows, especially in the Wilkes Subglacial Basin and Transantarctic Mountains region, is important.

We present a new 3D crustal model of the Wilkes Subglacial Basin and the Transantarctic Mountains based on joint inversion of airborne gravity and magnetic data using the mutual information inversion algorithm incorporated in the software JIF3D. Our model shows a large intrusive body located in the interior of the Wilkes Subglacial Basin and suggests a tectonically complex area west of the Basin, which could potentially indicate the transition zone at the margin of the Terre Adélie Craton. Geological units are inferred by clustering of inverted susceptibility and density distribution and are validated against sparse petrological samples from the Transantarctic Mountains region and along the George V Land and Terre Adélie coasts. Our inferred crustal properties model can provide crucial insight into the heterogeneity of subglacial geology in terms of thermal conductivity and crustal heat production, which could influence the geothermal heat flow in this area and therefore make the overlying ice sheet more vulnerable than commonly thought. 

How to cite: Lowe, M., Jordan, T., Moorkamp, M., Ebbing, J., Ruppel, A., Koglin, N., Green, C., Lösing, M., and Larter, R.: Constraining subglacial geology using mutual information inversion of gravity and magnetic data in the Wilkes Subglacial Basin and Transantarctic Mountains of East Antarctica, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-368, https://doi.org/10.5194/egusphere-egu23-368, 2023.

The Gamburtsev Subglacial Mountains (GSM) in central East Antarctica are  completely buried beneath the East Antarctic Ice Sheet. The GSM are known to be underlain by anomalously thick crust (~50–60 km) and ~200 km thick Precambrian lithosphere, but their crustal-scale geology remains less well- studied. Little is known about the 3D heterogeneity in crustal architecture beneath the GSM, and how this may relate to larger-scale tectonic processes responsible for Gondwana amalgamation.

Here, we use airborne gravity and aeromagnetic anomalies to explore the crustal architecture of the GSM in unprecedented detail. The gravity and magnetic images show three distinct geophysical domains, and a dense lower crustal root is modelled beneath the northern and central domains. We propose that the root may reflect magmatic underplating, associated with Pan-African age back-arc basin formation and inversion, followed by the collision of Australo-Antarctica and Indo-Antarctica. The high frequency linear magnetic patterns parallel to the Gamburtsev Suture zone, suggest that the upper crustal architecture is dominated by thrust and strike-slip faults, formed within a large-scale transpressional fault system.

We calculated a 2D gravity and magnetic model along a passive seismic profile to investigate the crustal architecture of the GSM, with the aid of depth to magnetic source estimates.   By combining the crustal model with  geological constraints, we propose a new evolutionary model suggesting that the crust of the northern and central GSM domains formed part of a cryptic accretionary orogen, of proposed Pan-African (~650-550 Ma?) age. The inferred accretionary stage was followed by continental collision (~540-520 Ma?) along the Gamburtsev suture, which is linked here to Gondwana amalgamation.

How to cite: Wu, G., Ferraccioli, F., Gao, J., and Tian, G.: Accretionary orogen unveiled beneath the Gamburtsev Sublglacial Mountains in East Antarctica, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16138, https://doi.org/10.5194/egusphere-egu23-16138, 2023.

By checking the magnetic data measured from New Ocean Reacher 3 at 2020/9/21 to 29, we can easily find that there are two anomalies localized south-west of Penghu. In the west, it is located around 119.08°E, 23.46°N and with defect amount 620 nT. The other site is located at 119.34°E, 23.45°N but not as solid as the first one (340 nT).

To understand the magnetic structure below these two sites, this study will use the iTilt-Euler method as the primary method for calculating the depth of these two magnetic anomalies. Before applying iTilt-Euler method, I’ll calculate total horizontal gradient and only use “quality of local maximum” larger than 3 to make sure the input data are around the edge of the source. Following the iTilt-Euler method, we will use the zero-order analytical signal as a constraint to select solutions that are above the structure. Finally, we will use the average of the selected solutions representing the properties of this anomalous site.

After going through the whole process, we discovered that the structure of the western site could be the fault with the top 0.88 km depth and the mean structural index 0.23. And the other site could be the dike, which is 1.8 km depth and has an average structural index of 1.4.

How to cite: Hsu, C., Hsu, S.-K., and Lo, C.-L.: Apply iTilt-Euler method on the magnetic anomaly at southwest of Penghu., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4843, https://doi.org/10.5194/egusphere-egu23-4843, 2023.

In practical problems concerned with an exploration of geological mineral resources, to enhance the resolution for its geological interpretation, downward continuation of the gravity anomaly is usually performed, as downward continuation can highlight local and shallow gravitational sources related to the ore body, which plays a very important role in the following processing and interpretation of gravity data. However, downward continuation is an ill-posed issue and has been a research topic for gravity exploration.

General classical methods for the downward continuation of gravity anomalies mainly include spatial-domain methods, of which, however, their convolution calculations are complicated; frequency-domain methods, the product calculations by Fourier transform from spatial-domain convolution, according to which not only do downward continuation factors have amplification effects, but also errors from the discretization and truncation of the Fourier transform cause oscillations in results. Improved methods, such as regularization filtering methods and generalized inverse methods, according to which although the stabilities of these downward continuations are improved, their downward continuation depths are not significant (generally no more than 5 times the measured interval); the integral iteration method, according to which stable results can be achieved for noise-free data and the depths of its downward continuation are large, but its number of iteration is giant, resulting in the reducing of computational efficiency and the accumulation of noises; Adams-Bashforth methods and Milne methods established by numerical solutions of the mean-value theorem, according to which they are easy to calculate and with greater depth of downward continuation (more than 15 times the measured interval). However, measured vertical derivatives are needed use to improve their accuracy.

As the coverage of measured vertical derivatives is low and their costs are high in real gravity explorations of geological mineral resources, which means it is not always possible to utilize measured vertical derivatives. To widen the real application for downward continuation methods of numerical solutions, instead of the measured vertical derivatives, we use the calculated ones by the ISVD (integrated second vertical derivative) method. At the same time, to improve the accuracy of the result using calculated derivatives, we present two new methods, Adams-Moulton and Milne-Simpson, based on implicit expressions of numerical solutions of the mean-value theorem for gravity anomaly downward continuation. These two methods have mathematical significance for improving the accuracy of numerical solutions. To demonstrate their effectiveness, we compare these four methods for downward continuation in the same degree including an Adams-Bashforth method, a Milne method, an Adams-Moulton method and a Milne-Simpson method by texting on the synthetic and real data of gravity exploration. The results show that the two implicit methods have higher accuracy, which has practical significance for the resolution improvement of gravity anomaly downward continuation in exploration interpretation.

How to cite: Zhang, C., Qin, P., Yan, J., Chen, L., and Wu, L.: Two new methods for gravity anomaly downward continuation based on implicit expressions of numerical solutions of mean-value theorem and their comparison, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8713, https://doi.org/10.5194/egusphere-egu23-8713, 2023.

Keywords: salt structure, gravity inversion, density model

A gravity survey is a good choice to investigate various subsurface structures, including salt domes. We performed numerous gravity survey simulations based on synthetic and analogue geological models, but also on real survey data. We started with forward gravity/density modelling of various shapes of salt diapirs (intrusions), using not only usually measured gravity data, but also gravity gradients. The resulting data mixed with certain levels of noise was then used for the gravity inversion process. We found some limits of sensitivity to selected starting models and extreme significance of the realistic definition of starting models for geologically plausible inversion results.

We applied this experience to real data – we digitized published gravity maps with negative anomalies related to salt structures. Contrary to the publication, we developed a more complex 3D model of the principal salt structure.

Currently, we follow analogue modelling of a simulated salt intrusion process in a laboratory and perform gravity modelling according to the digitized shape of salt (special silicon) intruding homogenous sedimentary (sand) formations.

Besides other methods, we apply 3D deterministic inversion coupled with the estimation of the starting model parameters based on the gravity gradients analysis. These parameters are mainly the dip, depth, and lateral extent. The problem is defined on a discrete rectangular mesh with the possibility of localized refinement to increase or decrease the resolution in certain parts of the model. The results provide a detailed density model of the diapir allowing the estimation of the spatial extent of the salt sheet. The usage of gravity gradients leads to the construction of more reliable starting models of near-surface salt structures for gravity inversion. Our aim is also to achieve a suitable geometrical correlation with magnetotellurics (MT), as such a twin gravity-MT response for various types of salt structures may encourage the application of such twin geophysical methodology.

How to cite: Beránek, R. and Mrlina, J.: Modelling 3D subsurface structures using gravity and enhanced gravity gradient method, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6032, https://doi.org/10.5194/egusphere-egu23-6032, 2023.

Flexural isostasy is commonly used to understand the relationship between the observed topography, the crustal structure, and the gravity. Compared to local isostasy, flexural models behave like low-pass filters on the crust-mantle interface. Using this methodology different internal structures are revealed showing the geometry of crustal and lithospheric structures. In the current flexure studies it is assumed that the lithosphere has uniform densities. The misfit between this method and the observed gravity data could be used to invert for lateral densities in the lithosphere.  

In this study spectral analysis on the topographic and gravity results from the flexural models is performed to study the effect of lateral variations. For the inversion we use the full tensor of the gravity gradient as they show more sensitivity to the lithosphere structures. The inversion technique is based on spectral kernel models that are able to depict the sensitivity of satellite gravity data. Extensive synthetic analysis is been performed to acquire the best inversion settings and to study the uncertainty of the inversion results with respect to the chosen flexural model. A two-layer lateral density model (crust – upper mantle) is applied to the Sunda Subduction zone to yield more insights into the density structure of the subducting plate.

How to cite: Root, B.: Inversion of the lateral density variations of the lithosphere using the full gravity gradient tensor, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5562, https://doi.org/10.5194/egusphere-egu23-5562, 2023.

Lithological interpretation of remote sensing and geophysical data plays a vital role in mineral resource mapping, especially in areas of the limited outcrop. This study applied a Random Forest (RF) classifier to obtain the refined lithological map of the Mundiyawas-Khera mineralized belt of the Alwar basin, India, from remote sensing and potential field data. A total of 540 samples covering the major lithologies were fed to RF for training (80%) and testing (20%), and its performance was evaluated using precision, recall, and accuracy. The degree of uncertainty associated with RF was also computed using the information entropy technique to pinpoint the regions where the refined lithology map is incorrectly classified. The results indicate that RF yields an overall accuracy of 73.15% in classifying all the major lithological units in the region, such as felsic volcanic, carbon phyllite, mica schist, quartzite, and tremolite-bearing dolomite. Among all the five lithologies, RF showed the best precision (84.62%) and recall (90.91%.) for quartzite and M-mica schist respectively and comparable precision/recall values for the felsic volcanic rocks that host Cu mineralization. Whereas other lithologies, dolomite and carbon phyllite, were not accurately predicted by RF, which might be due to the limited number of samples. The results of the class membership probabilities indicate that not all the litho-units predicted by the model are absolute. The study illustrates that RF can be used as a viable alternative in regions with limited outcrops and geochemical information to prepare the new lithology map or refine the existing geological maps. 

Keywords: Machine Learning, Lithology Classification, Gravity and Magnetic Data

How to cite: Singh, B. K. and Rao, G. S.: Random Forest classifier for lithological mapping of the Mundiyawas-Khera mineralized belt of the Alwar basin, India, from remote sensing and potential field data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8232, https://doi.org/10.5194/egusphere-egu23-8232, 2023.