Displays

Geomagnetic field models derived from satellite data cover now more than twenty years and are obtained through the processing and analyses of a massive amount of vector magnetic data. As our understanding of the geomagnetic field progresses, these models have to describe contributions of more and more sources with rather complex mathematical representation. In order to handle this increasing complexity and amount of available data, we use a sequential modelling approach (a Kalman filter), combined with a correlation based modelling step (Holschneider et al; 2016). In order to reach high temporal resolution for the core field, a sequence of snapshot models, 3-months apart, has been built. The main characteristics of the derived series of Gauss coefficients are the same as those of recently released field models based on classic modelling techniques. The results we obtained show the importance of a careful calibration of the Kalman prediction steps as well as applying Kalman smoother at the end of the modelling. We identify also the induced fields as the main limitation for an increase resolution of the core field. We will present how these induced fields have been handle in a recent version of the model and future steps to progress further in the representation of the different sources of the geomagnetic field.

How to cite: Lesur, V. and Ropp, G.: Sequential modelling of the Earth magnetic field, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6860, https://doi.org/10.5194/egusphere-egu2020-6860, 2020.

We usually think of crustal magnetic anomalies as static (barring some major seismic or thermal disruption).  But a significant portion of the crustal magnetic field is caused by the interaction of magnetic minerals with the Earth’s magnetic field.  This induced magnetic effect is dependent on the direction and magnitude of the ambient field.  So, of course, as the Earth’s magnetic field changes over time, the form and magnitude of induced magnetic anomalies will vary as well.  These changes will often be negligible for interpretation when compared with measurement and other interpretational uncertainties.  However, with the reduction of various sources of measurement noise and increased fidelity of interpretation, these temporal anomaly changes may need to be considered.

In addition to considerations relating to interpretation uncertainty, these temporal anomaly changes, if they are measured in multiple magnetic epochs, can theoretically provide valuable information for use in source inversion.  For example, since crustal magnetic anomalies arise from a combination of induced (dependent the ambient field) and remanent (not dependent on ambient field) magnetic sources, measurements of secular magnetic variation can assist in separating these two sources during inversion.

We will report modeling of the expected form and magnitude of predicted induced anomaly variations, the possible implications of these variations for data compilation and interpretation, and on the availability of relevant data for measuring them.  Recent research into the use of high-resolution magnetic anomaly maps for airborne magnetic navigation has also brought the issue of changing magnetic fields into focus.  Initial work indicates that changes in induced anomalies could affect navigation accuracy in certain situations.

How to cite: Saltus, R., Canciani, A., Meyer, B., and Chulliat, A.: Implications of magnetic secular variation for interpretation of crustal field anomalies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11741, https://doi.org/10.5194/egusphere-egu2020-11741, 2020.

How to cite: Ebbing, J., Yixiati, D., Baykiev, E., Haas, P., Ferraccioli, F., and Scheiber-Enslin, S.: Reprocessing of aeromagnetic data under consideration of satellite data for interpretation and modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2003, https://doi.org/10.5194/egusphere-egu2020-2003, 2020.

East Antarctica is the least understood continent on Earth due to its vast size, major ice sheet cover and remoteness. Coastal outcrops and glacial erratics have yielded cryptic but nevertheless fascinating clues into up to 3 billion years of East Antarctica’s geological and tectonic evolution. These geological constraints represent in turn the pillars to address global geodynamic linkages between East Antarctica, Australia, India, South Africa and Laurentia in the growth, assembly and dispersal of Gondwana, Rodinia and Nuna during the complex evolution of Earth's supercontinent cycles. However, due to the lack of drilling, our ability to project, test and augment such supercontinental linkages and several speculative geological interpretations in the interior of the continent beneath the East Antarctic Ice Sheet remains very limited.

While airborne and satellite gravity data and seismology are providing key new constraints on crustal and lithosphere thickness and help unveil large-scale heterogeneity in the East Antarctic lithosphere, detailed imaging of the architecture of individual crustal domains and their tectonic boundaries relies critically on magnetic anomaly data interpretation.

Here we exploit ongoing analyses of a recent continental-scale magnetic anomaly compilation (ADMAP 2.0) (Golynsky et al., 2018, GRL) augmented by major new datasets we recently collected, processed and compiled over the Recovery and South Pole frontiers and enhanced satellite magnetic imaging to:

1) reveal a more complex mosaic of distinct but in several places still cryptic Precambrian crustal provinces that represent the building blocks of interior East Antarctica;

2) provide new geophysical constraints that can be used to test different hypotheses of East-West Gondwana amalgamation along several candidate suture zones, including in particular the Shackleton suture zone, which provides a unique window on several distinct Precambrian terranes at the inferred leading edge of the composite Mawson Continent, as well as unique occurrences of Pan-African age rocks of ophiolitic affinity and

3) re-assess potential paths and the significance of the Kuunga suture zone between Greater India and East Antarctica and re-evaluate the tectonic origin of a major magnetic and gravity lineament previously thought to delineate the Indo-Australo-Antarctic suture and finally

4) propose new surveys in other frontier regions including in particular the under-explored interior of Princess Elizabeth Land and Recovery Subglacial Highlands that are critical in order to test the possible connectivity of the Kuunga, Gamburstev and potentially also Shackleton suture zones. 

Finally, we showcase examples of how we are combining aeromagnetic and gravity interpretations for East Antarctica with global magnetic and gravity datasets, geochronology, geochemistry, geology, tectonics and paleomagnetic data in an evolving plate kinematic framework (in GPlates) to re-assess supercontinent reconstructions with particular emphasis so far on Nuna and Gondwana.

How to cite: Ferraccioli, F., Eagles, G., Golynsky, A., Ebbing, J., Guochao, W., Green, C., Eglington, B., and Armadillo, E.: Tantalising new magnetic views of Precambrian and Pan-African age crustal architecture in interior East Antarctica , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11204, https://doi.org/10.5194/egusphere-egu2020-11204, 2020.

Natural resource exploration has advanced in recent years through integrating unmanned aerial vehicles (UAVs) with high-resolution magnetometer payloads. One design consideration when integrating these systems for mineral exploration applications is ensuring that the magnetic measurement quality is comparable to the previously established methods of terrestrial magnetic and aeromagnetic surveying. High-resolution optically pumped magnetometers, employing a resolution of 0.1 - 0.01 nT, are the standard magnetic sensors used in both manned terrestrial magnetic and aeromagnetic surveys. Integrating a high-resolution optically pumped magnetometer in a multi-rotor UAV payload bay will compromise the integrity of the total magnetic intensity (TMI) measurements due to the electromagnetic interference generated by the brushless permanent magnet synchronous motors and other onboard electromagnetic components. One solution involves physically suspending the high-resolution magnetometer below the resolvability limit of the electromagnetic interference via a semi-rigid mount. However, the swinging motions of the high-resolution magnetometer through the geomagnetic field while in this configuration have the potential to introduce periodic variations in the collected TMI data, compromising quality. Within this study, a UAV-borne aeromagnetic survey was conducted over a mineral exploration target to assess the potential impact of magnetometer swing on collected UAV-borne TMI data. A DJI-S900 multi-rotor UAV and a GEM Systems Potassium Vapour Magnetometer (GSMP-35U) were used to fly a 500 m by 700 m grid, using a line spacing of 25 m and a flight elevation of 35 m above the ground.The optically pumped magnetometer was suspended outside the resolvability limit of the electromagnetic interference below the UAV via a semi-rigid mount. A nine degrees of freedom inertial measurement unit (IMU) was fixed to the semi-rigid mount and a Kalman filter was applied to post-process the measurements calculating the positional variations (pitch, yaw and roll) of the magnetometer. Spectral analysis was applied to the UAV-borne TMI measurements and the IMU positional data assessing contributions to the TMI signal from the swinging, semi-rigidly mounted magnetometer. Periodic signals were observed within the recorded TMI data directly relating to the swinging frequency of the magnetometer in pitch and roll throughout flight. The amplitude of the periodic TMI variations was variable (< 1 nT – 5 nT) throughout the survey and depended on the horizontal gradient of the ambient magnetic field and the arc length of the magnetometer swing. The magnetometer swinging frequency (~0.35 Hz) was determined to be primarily dependant on the magnetometer suspension length. Overall, the wavelength of the periodic TMI variations due to the swinging motions was characterized with the IMU measurements and determined to be spectrally unique from the longer wavelength geological signals targeted within the survey area. Due to the wavelengths of the targeted and untargeted signals not spectrally overlapping, the TMI variations related to magnetometer swing noise were filtered out. The design factors controlling the wavelengths of the targeted geologic signals (flight speed) and untargeted magnetometer swing noise (suspension length) must be considered when integrating high-resolution magnetometers on multi-rotor UAVs, such that the wavelengths do not spectrally overlap and phase-based compensation algorithms are not required.

How to cite: Walter, C., Braun, A., and Fotopoulos, G.: On Compensating for Magnetometer Swing in UAV Magnetic Surveys, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12387, https://doi.org/10.5194/egusphere-egu2020-12387, 2020.

Reduction to pole and other transformations of total field magnetic intensity data are often challenging to perform at low magnetic latitudes, when remanence exists, and when large topographic relief exists. Several studies have suggested use of inversion-based equivalent source methods for performing such transformations under those complicating factors. However, there has been little assessment of the importance of erroneous edge effects that occur when fundamental assumptions underlying the transformation procedures are broken. In this work we propose a transformation procedure that utilizes magnetization vector inversion, inversion-based regional field separation, and equivalent source inversion on unstructured meshes. We investigated whether edge effects in transformations could be reduced by performing a regional separation procedure prior to equivalent source inversion. We applied our proposed procedure to the transformation of total field magnetic intensity to magnetic amplitude data, using a complicated synthetic example based on a real geological scenario from mineral exploration. While the procedure performed acceptably on this test example, the results could be improved. We pose many questions regarding the various choices and control parameters used throughout the procedure, but we leave the investigation of those questions to future work.

How to cite: Lelièvre, P., Fournier, D., Walker, S., Williams, N., and Farquharson, C.: Assessing and ameliorating edge effects in magnetic data transformations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5320, https://doi.org/10.5194/egusphere-egu2020-5320, 2020.

The measurement of the magnetic field has been a “backbone” geophysical method in mineral exploration since the 17th century. The existing instrumentation that measures Total Magnetic field Intensity (TMI) are a routinely used in ground, borehole and airborne surveys. In the TMI intensity data it is possible to observe certain signatures of magnetised objects, but retrieval of both magnetisation intensity and shape of 3-D magnetised objects from TMI can be difficult due to the vector nature of magnetisation and fundamental non-uniqueness of potential fields interpretation. Moreover, the presence of magnetic material in the host rock and/or presence of remanent magnetisation are challenges for TMI data interpretation.

Full Tensor Magnetic Gradiometry (FTMG) measurements provide the complete description of the magnetic field and hence an opportunity to get more information on the size, shape and material property of the magnetic rock mass. This is because the signatures in magnetic field originating from a specific magnetic object is observed in all independent components of magnetic field gradient tensor and thus, joint analysis of these tensor components constrains the number of possible magnetic models that fit the same data. In addition, observing the full tensor of magnetic field makes it possible to estimate the remanent magnetization with respect to the induced magnetization field if no a-priori information of remanent magnetization is available.

Highly sensitive magnetometers based on SQUID (Superconducting QUantum Interference Devices) technology has been successfully adopted in FTMG airborne measurements during the past decade. This achievement has given magnetic methods a new opportunity in terms of purely magnetic data modelling. In our work the benefits of interpretation of tensor airborne FTMG data are demonstrated through forward modelling and inversion with the grid search multiobjective global optimisation. As a case study, we consider airborne FTMG data measured with Supracon® JESSY STAR system in Northern Finland during the INFACT project.

Acknowledgements: This study has been done in the framework of EU Horizon 2020 funded INFACT project (webpage: https://www.infactproject.eu).

How to cite: Nevalainen, J., Kozlovskaya, E., Ranta, J.-P., Blanco, J. M., Kirsch, M., Gloaguen, R., Schneider, M., and Kobow, J.: Modelling of airborne Full Tensor Magnetic Gradiometry using data from the INFACT project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18945, https://doi.org/10.5194/egusphere-egu2020-18945, 2020.

A quick and effective technique is developed to diagnose the geomagnetic dipole field based on an unstrained single circular current loop model. In comparsion with previous studies, this technique is able to separate and solve the loop parameters successively. With this technique, one can search the optimum full loop parameters quickly, including the location of loop center, the loop orientation, the loop radius, and the electric current carried by the loop, which can roughly indicate the locations, sizes, orientations of the interior current sources. The technique tests and applications demonstrate that this technique is effective and applicable. This technique could be applied widely in the fields of geomagnetism, planetary magnetism and palaeomagnetism. The further applications and constrains are discussed and cautioned.

How to cite: Rong, Z., Wei, Y., Xu, W., Kong, D., Cui, J., Shen, C., Zhu, R., Wan, W., Yamauchi, M., Zhong, J., and Chai, L.: New technique to diagnose the geomagnetic field based on the single circular current loop model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13436, https://doi.org/10.5194/egusphere-egu2020-13436, 2020.

We examine the secular variations of global geomagnetic field on long temporal scales using the IGRF model given in Gauss coefficients for 1900 - 2020. We apply the Empirical Orthogonal Function (EOF) analysis to the geomagnetic field truncated at degree 6 and downward continue it to the core-mantle boundary (CMB) under the assumption of an insulating mantle. The first three EOF modes show the periods around 120, 75 and 60 years with corresponding spatial structures. These oscillational modes potentially support the manifestation of magnetic, Archimedes and Coriolis (MAC) waves in the stably stratified layer near CMB (Buffett, 2016). We also model and decompose the geomagnetic field to standing and drifting components according to trajectories of the Gauss coefficients similarly to Yukutake (2015). We then use the Complex EOF (CEOF) analysis on the drifting field. The results indicate the presence of the westward drift phenomenon but only weakly given the fact that the westward drift has only completed a fraction of a cycle during this time.

How to cite: Chung, C.-H. and Chao, B. F.: Analysis of Geomagnetic Variability by Empirical Orthogonal Functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-711, https://doi.org/10.5194/egusphere-egu2020-711, 2020.

The magnetic field of the Earth is generated in its core by the process called the geodynamo, which involves convection in the fluid and electrical conducting outer core. The evolution of this complex process is simulated by magnetohydrodynamic models, which provide the state of the core and the magnetic field at any point and any time of the simulation. Nevertheless, the complexity of these models implies a high computational cost. That is why conceptual simple models describing only the main mechanisms from a statistical perspective can also be useful.

In this work we present a conceptual model that reproduces the main features of the axial dipole moment (ADM) of the Earth magnetic field. It is based on the stochastic dynamics of two Brownian particles interacting with each other within a double-well potential. The obtained simulations are able to mimic the random temporal distribution of reversals and excursions and the asymmetric temporal evolution of ADM during reversals. The relation between the model features and the real mechanisms that lead to the observed behaviour is discussed.

How to cite: Molina Cardín, A., Dinis Vizcaíno, L., and Osete López, M. L.: Simplified model for axial dipole moment of the geomagnetic field from Brownian fluctuations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19012, https://doi.org/10.5194/egusphere-egu2020-19012, 2020.

Using the COV-OBS.x1 (Gillet et al., 2015) main geomagnetic field model, covering the time span 1840–2020, respectively IGRF-13 (1900-2020), we decomposed the geomagnetic field at Earth’s surface in oscillation modes by means of empirical orthogonal functions (EOF) as well into a long term and a cyclic component using HP filtering (Hodrick and Prescott, 1997). Further, the long term component is filtered using a Butterworth filter (1930) with different cut-off periods in order to obtain oscillation at inter-centennial (> 100 years) and sub-centennial (60-90 years) timescales. The EOF analysis shows that the first three oscillation modes are characterized by periodicities of >100 years while modes 4 and 5 are characterized by dominant periodicities of 70-90 year. Although the variance of the modes 4 and 5 is rather small compared to that of the first three modes, these two modes are responsible for the detailed structure of the geomagnetic field. A comparison between the results of both methods is done as well.

How to cite: Stefan, C., Dobrica, V., and Demetrescu, C.: Decomposing the geomagnetic field: oscillation modes and characteristics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16408, https://doi.org/10.5194/egusphere-egu2020-16408, 2020.

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The energy supply of geodynamic processes is one of the most important factors in the evolution of the planet earth system.The continuity and relatively stable regime of the planetary dipole magnetic field of the Earth is due, first of all, to the constant level of rotation energy continuously generated by the Earth during its rotation around its axis. In the mantle, asthenosphere, and the earth's crust, the determining energy factor is the density inhomogeneity of matter.

The Earth’s magnetic field, which is 99% generated by its internal sources, responds to phase transitions, which are the basis of the processes of self-organization of the planet Earth system.

The report presents ideas about energy sources, mechanisms and patterns of formation, transformation and replenishment of its reserves will significantly increase the reliability of the interpretation of cartographic information about structural and geophysical anomalies and related mineralogenesis processes.

How to cite: Litvinova, T., Кашик, Д., and Тихомиров, С.: Applied magnetic cartography is a tool for understanding the self-organization of the planet Earth system and its energy supply , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7054, https://doi.org/10.5194/egusphere-egu2020-7054, 2020.

Abstract:

The second part of the history of the Algerian magnetic repeat station network goes back to 1989 when the new one was started with 37 stations. It was then followed by three other networks in 1993, 1997 and 2005. The first part of this history started at the beginning of the XX^th and ended ca 1956.

After a 14-year break, we launched a new repeat stations network in February 2019. The number of carried out stations was increased to 51 to try to cover all the territory.

Each repeat station network consists of stations of periodically, say  5-6 years, measured of three components of the Earth's magnetic field. to try to derive the spatial distribution of the geomagnetic field of Algeria and it's secular variation. This periodicity is also very important for the need to update local as well as global geomagnetic field models such as the International Geomagnetic Reference Field (IGRF).

In this work we describe the new 2019 Algerian repeat station network. Then we will discuss the steps of the absolute measurements using two methods. The first one is called the ‘method of zero’ and the second one ‘method of residuals’. The accuracy and resolution of the instruments and data reduction used and their effect on the final results will as well be discussed. We derive the spatial distribution of the geomagnetic field, and its secular variation. Finally, we will show how local, for instance regional polynomial modeling, is the key issue.

Key words: geomagnetic repeat network, absolute measurements, zero method, residual method, magnetic maps of Algeria, secular variation.

How to cite: Lemgharbi, A., Abtout, A., Hamoudi, M., Bendekken, A., Annad, F., Hemmi, A., Mansouri, A., Aganou, E., Allili, M., and Mazari, A.: First Results of the 2019 Algerian Magnetic Repeat Station Network , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3915, https://doi.org/10.5194/egusphere-egu2020-3915, 2020.

The World Digital Magnetic Anomaly Map (WDMAM) is an initiative of the IAGA (International Association of Geomagnetism and Aeronomy) supported by the CGMW (Commission for the Geological Map of the World) of UNESCO. The second version was released in 2015 (Dyment et al., 2015; Lesur et al., 2016), and mandate was given to the authors to update this version 2.0 using the same methodology as often as newly available data would make it necessary. Five better datasets justify the preparation and release of version 2.1: (1) the complete digital aeromagnetic map of Brasil made available to CGMW by Agência Nacional do Petróleo, Gás Natural e Biocombustíveis; (2) an improved version of the aeromagnetic map of Russia prepared at VSEGEI; (3) the second version of the Antarctic Digital Magnetic Anomaly maP (ADMAP; Golynsky et al., 2018) which construction results from a remarkable international effort during and after the Second International Polar Year; (4) a new map of the Caribbean plate and Gulf of Mexico resulting from the compilation and re-processing of existing marine and aeromagnetic data in the area (Garcia, 2018); and (5) a new compilation of marine magnetic data worldwide. The new map shows significant improvements over the previous versions and will be shortly available at wdmam.org.

How to cite: Dyment, J., Choi, Y., Lesur, V., Garcia-Reyes, A., Catalan, M., Ishihara, T., Litvinova, T., and Hamoudi, M.: The World Digital Magnetic Anomaly Map: version 2.1, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22098, https://doi.org/10.5194/egusphere-egu2020-22098, 2020.

The modern epoch of ground magnetic surveying activity on the Czech territory was started by the Institute of Geophysics by setting up a fundamental network of the 1^st order in 1957-58. It consists of 199 points and was reoccupied in 1976-78 and 1994-96. The anomaly maps were constructed by subtraction of the IGRF model.

Extensive aeromagnetic measurements have been performed from 1959 to 1972 by permalloy probe of Soviet provenience. The accuracy of the instrumentation was about (and often above) 10 nT. The second period of airborne survey started in 1976. Thanks to the deployment of proton precession magnetometer, the accuracy improved to ~ 2 nT. Since 2004 the measurements were carried out by caesium magnetometer. The data were digitized, known anthropogenic anomalies were cleared away and data were transformed to the regular grid with step 250 m. The final data file of magnetic anomalies ΔT, administered by the Czech Geological Survey, represents a substantial contribution to the exploration of ore deposits and to the structure geology in general.

In view of the fact that data file of magnetic anomalies was compiled from data acquired by heterogeneous methods in the course of more than 50 years, our recent study is aimed at looking into the homogeneity of the data by comparison them with ground-based magnetic survey. A simple comparison of the contour maps showed good similarity of the large regional anomalies. For more detailed analysis, the variation of ΔT in the neighbourhood of all points of the fundamental network was inspected and the basic statistic characteristics were computed. Summary results as well as several examples will be presented accordingly as the INSPIRE compliant services and eventually as the user-friendly web map application and made available on the CGS Portal http://mapy.geology.cz/ and on the updated web of the CzechGeo/EPOS consortium www.czechgeo.cz. Incorporating the map into the World Digital Magnetic Anomaly Map (WDMAM – IAGA) is also under consideration. This data will also be interesting for the EPOS.

How to cite: Hejda, P., Čápová, D., Hudečková, E., and Kolejka, V.: Analysis of the Czech magnetic anomaly data obtained by ground-based and airborne magnetic surveys, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3314, https://doi.org/10.5194/egusphere-egu2020-3314, 2020.

Geological structure of western part of the Chukotka fold belt has been studied basing on the results of joint interpretation of geophysical data. The potential-field data, seismic and magnetotelluric data along two regional profile crossed the area and the off-shore seismic data obtained on the East Siberian sea were used in this study. The NE and NW oriented fault systems which control the mineragenous zones location were first detected and delineated. Joint interpretation of seismic and MT data along regional profiles allowed us to study: the deep structure of NW directed thrusts; the intrusion bodies morphology; the structural features of the Paleozoic and Mesozoic formations and the structure of volcanic deposits. The models of geological structure along regional profiles were used as a reference for potential field interpretation. Architecture of crystalline basement of the area was studied and several “steps” were detected. The depth of crystalline basement increases from north to south and reaches the largest depth under the volcanic deposits of Ochotsk-Chukotsk Volcanic Belt (OCVB).

How to cite: Antashchuk, K., Atakov, A., Mazurkevich, K., and Petrov, O.: Tectonic structure and metallogeny of the Western Chukotka: insights from comprehensive geophysical dataset interpretation., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11192, https://doi.org/10.5194/egusphere-egu2020-11192, 2020.

The wide range of anomalies caused by different geological structures from local to regional are studied by the heterogeneous datasets. They usually include the surveys of highly variable scales, resolution and quality. These parameters determine the methodology and technique used in further interpretation. The absence of detail and high quality surveys of geomagnetic field for large areas does not allow the implementation of the system analysis approach to full spectra of anomalies of magnetic field. The possibilities of system analysis using for various scale magnetic surveys to clarify of the tectonic settings and geological structure of the southeastern part of the Yano-Kolyma fold belt are considered. The geological structure of this area was studied earlier by the seismic and magnetotelluric investigations along 2DV regional profile. The tectonic settings are represented by several folded areas and cratons which are covered and knit together by Late Mesozoic bends and volcanic belt. The system interpretation of various scale magnetic surveys allowed us to obtain the geological and tectonic models of this area that include the following principal components: the deep structure of joint zones of different tectonic blocks; the structure and thickness of Paleozoic – Mesozoic deposits of sedimentary cover, crystalline basement and bends; the structure of volcanic belt deposits.

How to cite: Zubov, D., Antashchuk, K., Atakov, A., Mazurkevich, K., and Petrova, M.: The structure of the Southeastern part of the Yano-Kolyma Fold Belt by the results of interpretation of various scale magnetic data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11275, https://doi.org/10.5194/egusphere-egu2020-11275, 2020.

The Mt. Melbourne field is interpreted as a quiescent volcanic complex, located in Northern Victoria Land, Antarctica, at the boundary between the Transantarctic Mountains (TAM) and the West Antarctic Rift System (WARS). It is one of the handful Antarctic volcanoes with the potential for large–scale explosive eruptions [1], with resulting key effects on the local environment and potentially on climate.

The geological and geophysical structure of this volcanic field remains poorly known, despite its key relevance to better comprehend the Cenozoic tectonic and geodynamic processes responsible for the opening of the WARS and the uplift of the TAM rift flank.

Here we present results derived from a novel high–resolution aeromagnetic dataset, collected in the austral summer 2002/2003 during the XVIII Italian Expedition, with the aim of investigating the geophysical structure of the main volcanic centres of the field.

Aeromagnetic data were processed and Digital Enhancement and Depth to Magnetic Source analysis performed to reveal the distribution of the main fault systems affecting the Mt. Melbourne volcanic field, particularly beneath the ice–covered areas. The results highlight NNE–SSW, NW–SE and E–W trending structural systems, in agreement with the available tectonic information for the study area [2, 3]. Furthermore, similar NNW–SSE trending pervasive negative anomalies are detected beneath both the Mt. Melbourne edifice and Cape Washington, superimposed by positive ones forming radial patterns.

With the aid of laboratory magnetic susceptibility data from rock samples collected in the field [4], we carried out forward and inverse modeling across the volcanic centres in order to image their subglacial internal structure.

Based on our results, considering the ambiguity and narrowness of the available geochronological data [1, 5, 6], we propose two (non–mutually exclusive) interpretative models to explain the evolution steps of the Mt. Melbourne volcanic complex. In the former, a major volcanic phase responsible for building of the inner part of the main volcanic centres likely occurred prior to the last magnetic polarity reversal (i.e. before 0.78 Ma, Matuyama Chron), explaining the negative anomalies detected as due to remnant magnetisation. During the Pleistocene–Holocene period, a following second volcanic phase put in place at shallower levels, primarily with present–day magnetization. In the alternative model, magma pulses originated at the lithospheric step between the thick East Antarctic craton and the thinner Ross Sea crust [7] caused i) widespread volcanism at the surface of the volcanic complex, particularly with the building up of the Mt. Melbourne edifice, and ii) a regional upward of the Curie isotherm at depth, causing partial de–magnetisation of the underlying volcanic rocks.

References:

[1] Giordano et al. (2012). Bull. Volcanol., 74, 1985-2005.

[2] Storti et al. (2006). J. Struct. Geol., 28, 50-63.

[3] Vignaroli et al. (2015). Tectonophysics, 656, 74-90.

[4] Pasquale et al. (2009). Ann. Geophys., 52(2), 197-207.

[5] Armstrong (1978). New Zeal. J. Geol. Geophys., 21(6), 685-698.

[6] Armienti et al. (1991). Mem. Soc. Geol. Ital., 46, 427-452.

[7] Park et al. (2015). Earth Planet. Sci. Lett., 432, 293-299.

How to cite: Ghirotto, A., Armadillo, E., Crispini, L., Zunino, A., and Ferraccioli, F.: New insights into the evolution of the Mt. Melbourne volcanic field (Northern Victoria Land, Antarctica) from high–resolution aeromagnetic data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11576, https://doi.org/10.5194/egusphere-egu2020-11576, 2020.

The Rogaland Igneous Complex (RIC), in southwest Norway, is well known for its iron-titanium ore deposits (i.e. Storgangen and Tellnes), and potential apatite and vanadium-rich magnetite deposits. A better understanding of the subsurface structure of the complex and surrounding anorthosites will help to locate new mineral deposits, and in estimating the extent of the known mineralized zones. The RIC consists of anorthosites, leuconorites, mangerites, and the Bjerkreim-Sokndal (BK) layered intrusion. These igneous rocks were intruded into granulite facies rocks at 0.93-0.92 Ga, during the late-stage of the Sveconorwegian orogeny.

There is a strong correlation between the geology of the RIC and the magnetic and gravity anomaly patterns, with contrasting signatures between the three large anorthosite bodies (Egersund-Ogna, Haland-Helleren, and Ana-Sira) and the extensive BK layered intrusion.

Particularly, the Bouguer gravity map shows gravity lows over the anorthosites and the granulites, while a positive gravity anomaly ranging from 10 to 30 mGal correlates with the norite and mangerite rocks. In the aeromagnetic anomaly map, the anorthosites correlate with moderate to strong negative magnetic anomalies (below background) while mangerites and granulites have positive anomalies. More complex is the magnetic pattern over the BK layered intrusion. The latter is made up by 6 mega-cyclic units subdivided into a sequence of zones, defined by the presence or absence of certain index minerals which control the magnetic properties of the rocks and the magnetic pattern. This is clearly visible in the striking negative anomaly observed on the east limb of the Bjerkrem Lobe at Heskestad, with amplitude of -13000 nT in a high-resolution helicopter survey, and below -30000 nT in ground magnetic survey.

This area has long been explored, and a large set of geophysical data have been collected during multiple campaigns including gravity, seismic, airborne magnetic and radiometric data. Recently acquired ground magnetic data over the BK layered intrusion complement these data. Here, we used the geophysical data, and an extensive petrophysical dataset of over 1000 samples to investigate the shallow and deep structure of the RIC. A 3D gravity and magnetic model of the study area, built across multiple cross-sections, is presented. The BK layered intrusion is modeled in a doubly-plunging syncline structure and has a preliminary depth extent of approximately 4 km which agrees with previous seismic interpretations indicating the base at 4 to 5 km.

How to cite: Pastore, Z. and McEnroe, S.: 3D gravity and magnetic model of the Rogaland Igneous Complex in southwest Norway: a tank for ilmenite, apatite and magnetite resources , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18825, https://doi.org/10.5194/egusphere-egu2020-18825, 2020.

The West Antarctic Rift System (WARS) is known to have experienced distributed/wide mode extension in the Cretaceous, followed by narrow mode and variably oblique extension/transtension in the Cenozoic, the latter potentially linked to the onset of oceanic seafloor spreading within the Adare Basin (Davey et al., 2016, GRL). However, onshore the extent and impact of Cenozoic extension and transtension within the Transantarctic Mountains sector of East Antarctica is currently much less well-constrained from a geophysical perspective.

Here we combine aeromagnetic, aerogravity, land-gravity and bedrock topography imaging to help constrain the extent, architecture and kinematics of the largest Cenozoic pull-apart basin recognised so far in East Antarctica, the Rennick Graben (RG).

Enhanced potential field imaging reveals the extent of a Jurassic tholeiitic Large Igneous Province preserved within the RG and the inherited structural architecture of its basement, including remnants of uplifted ca 530-500 Ma arc basement in the northern Wilson Terrane and a ca 490-460 Ma subglacial thrust fault belt separating the Cenozoic western flank of the RG from the eastern margin of Wilkes Subglacial Basin (WSB).

The architecture of the RG is best explained in terms of a major composite right-lateral pull-part basin that extends from the Oates Coast to the Southern Cross Mountains block. We propose that Cenozoic strike-slip deformation kinematically connected the RG with both the western edge of the WARS and the eastern margin of the WSB. An earlier phase of left-lateral strike slip deformation is also emerging from recent geological field work in the study region but only relatively subtle offsets in aeromagnetic anomaly patterns are visible in currently available regional datasets.

We conclude that the RG is part of a wider distributed region of the continental lithosphere in East Antarctica that was deformed in response to an evolving Cenozoic transtensional tectonic setting that may have also affected enigmatic sub-basins such as the Cook Basins in the adjacent WSB region.

How to cite: Crispini, L., Ferraccioli, F., Armadillo, E., Läufer, A., and Ruppel, A.: Crustal architecture of the largest pull-apart basin in East Antarctica unveiled, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20023, https://doi.org/10.5194/egusphere-egu2020-20023, 2020.

Detection of land magnetic data spikes is an important issue in magnetic data processing. In addition, the presence of noisy contributions such as spikes, stripes and zigzag effects in magnetic data visualization represents the most common flaw that may degrade the image. Rendering the correct detection and identification of features very uncertain. In this study, a script called "Window_Despike" was written to mark the spike data points and give the index of each spike point. Spikes were replaced by linearly interpolating the adjacent "good" amplitudes, or replaced by Not a Number (NaN). Different windows size starting from window size of 3 till 9 compared and found that the best window size is 5.

How to cite: AlHumidan, S.: Detection and Removal of Noisy Land Magnetic Data Spikes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1647, https://doi.org/10.5194/egusphere-egu2020-1647, 2020.

Spectral analysis is among the most old and common techniques for the processing and interpretation of potential field data. This is related to the decay properties of the field power spectra which allows an easy estimation of the depths to the top and to the bottom of the sources of magnetic and gravity field anomalies. Such analysis can be accomplished however in different theoretical frameworks, assuming either a statistical ensemble of homogeneous sources or random fractal source distribution. Here, we present the many existing spectral analysis techniques to compare them with respect to estimating the depth to the source top and bottom. We evidence practical constraints on the depth estimation and inherent assumptions/limitations of the different approaches. Depth estimation using spectral methods requires a critical evaluation of window size, window location, and wavenumber range. Careful consideration of the merits and of the limitations of these different spectral techniques for different source distribution models may lead to robust and geologically meaningful outcomes. In fact, despite the several different approaches all the methods give quite consistent and often similar estimates of the source depths. However, due to ambiguities on the correction spectral factor, the best estimates are obtained if this factor is constrained by a priori information. Finally, we estimate the depth to the magnetic sources beneath Sicily, which may provide additional constraints to better understand the deep crustal geometry and thermal gradients of the region.

How to cite: Kelemework, Y. and Fedi, M.: Determination of depth to the magnetic sources using spectral analysis with application to Sicily, Southern Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8863, https://doi.org/10.5194/egusphere-egu2020-8863, 2020.

The depth to the top of the deepest magnetic source is a proxy for the depth to basement under the assumption that basement is magnetic rock covered by non-magnetic sediment. Spectral domain methods allow for rapid estimation of the depth of magnetic sources from a magnetic anomaly map by analysing the power spectra calculated from a window of magnetic field data. The choice of an appropriate window size is critical when employing these methods, due to the trade-off between robustness and locality. Larger windows are desirable as they include more data to average out randomness and noise, and larger windows are needed to observe the low-frequency components which relate to the deepest sources. But they may also include multiple objects which can confuse analysis and spatially smear results, so smaller windows are desirable for improved spatial resolution.

The three properties typically estimated are the depth to the bottom of the layer z_b, the depth to the top of the magnetic layer z_t and the magnetic fractal parameter β, which describes how the magnetic source changes with scale. For our purposes we are not interested in the depth to the underside of the layer and consequentially can use smaller windows as we do not require the low frequencies to constrain the depth to the bottom of the layer. Hence the minimum window size is now limited by the expected depth to the top of the layer. A wide frequency range is critical to best separate the effects of z_t and β and obtain robust depth estimates. Above a certain frequency the spectrum is dominated by shallow sources and different types of noise; it no longer contains information about the deepest magnetic source. For a given window size we can obtain more robust estimates be carefully identifying this upper limit for the frequency range. The frequency at which the spectrum changes from being dominated by the magnetic layer to dominated by other sources can vary from location to location.

Here we therefore introduce a methodology that selects a locally optimal upper limit for the frequency range by analysing the goodness of fit as a function of this frequency. For each location we identify candidate frequencies based on the R² value for a linear model for the power of the signal as a function of the frequency. From these candidate frequencies we chose the one resulting in the lowest root mean square error for the fitted spectral model. We use synthetic tests to derive an empirical relationship between the recoverable depth to the top of the magnetic layer and the window size; and illustrate the degree of undesirable spatial smoothing caused by an unnecessary large window for a given recoverable depth. Recovered trends for the depth to basement for Australia are comparable to solutions obtained from different sources of information. The true value of our improved spectral method though lies in its suitability for application in a real-time environment due to its efficiency.

How to cite: Westerlund, S., Chopping, R., Shao, Q., and Hauser, J.: Selecting Optimal Frequency Range for Estimating Depth to Magnetic Sources, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13181, https://doi.org/10.5194/egusphere-egu2020-13181, 2020.