Magnetic Anomaly Grid Research Articles

Assessment of global geomagnetic models from aeromagnetic data in Cameroon, Central Africa

ABSTRACT Global geomagnetic models are a compilation of existing magnetic data which were reprocessed and freely made available as 2 or 3 arc minute resolution grids at 0-km or 4–5-km altitude relative to the geoid. Thus, many researchers rely on those data due to their open access. This study aims at evaluating freely released global geomagnetic models with respect to aeromagnetic data which are the original data, usually subjected to third party or commercial restrictions in Cameroon. Hence, both global geomagnetic models and aeromagnetic data were sampled along 4 profiles for assessment. The extracted data were analysed using Pearson’s correlation coefficient depicted in tables. The Power Spectrum Density curves of both aeromagnetic and their respective global geomagnetic grids were computed and displayed as graphs. The results show that the so-claimed 4–5-km altitude relative to the geoid of global geomagnetic models is heterogeneous in Cameroon since the spectral content of those models vary from 5–20 km. Besides, considerable discrepancies were observed with Earth Magnetic Anomaly Grid version 3 (EMAG2v3) data, indicating that they are not reliable for scientific studies based in Cameroon. This study provides insights on global geomagnetic models and could aid in optimizing the use of those models in Cameroon.

Determination of thermal structure of the crust beneath the Gongola Basin, Upper Benue Trough, Nigeria

Geothermal energy resources have been established globally to be among the sustainable and environmentally harmless means of energy generation. The data for this study came from the Earth Magnetic Anomaly Grid 2 version 3 (EMAG2V3). It was acquired at a flying elevation of 4 km above the ellipsoid and has a resolution of 2-arc minutes (3.66 km). This research aim to study the thermal structure of the crust beneath the Gongola Basin so as to determine the heat flow, geothermal gradient and the Curie point depth. In this study, the creation of an air satellite magnetic map of the Gongola Basin is accomplished through the use of a fractal magnetization approach. However, Curie point depth (commonly known as bottom most of magnetic sources depth) measure the distance between two points in space, depth to bottom magnetic source (DBMS). Geothermal characteristics of the area was evaluated using Oasis Montaj. So the earth magnetic field is utilized to determine the depth of anomalous sources that ranges from few meters to tens of kilometers. The top depth ranges from 3.4465km to 10.942km, the centroid depth ranges from 14.045km to 31.338km, the geothermal gradient ranges from 10.2566Oc/km to 23.4445 Oc/km and the heat flow ranges from 25.6415mW/m2 to 58.84 mW/m2. The study identify a point (11°63E, 10°12N) with optimal values (26.44OC/km, 66.11mW/m2) at a distance 21.9km which is above the typical optimal values obtained in stable continental craton as obtainable in African plate.

Nature and Origin of Magnetic Lineations Within Valdivia Bank: Ocean Plateau Formation by Complex Seafloor Spreading

AbstractValdivia Bank (VB) is a Late Cretaceous oceanic plateau formed by volcanism from the Tristan‐Gough hotspot at the Mid‐Atlantic Ridge (MAR). To better understand its origin and evolution, magnetic data were used to generate a magnetic anomaly grid, which was inverted to determine crustal magnetization. The magnetization model reveals quasi‐linear polarity zones crossing the plateau and following expected MAR paleo‐locations, implying formation by seafloor spreading over ∼4 Myr during the formation of anomalies C34n‐C33r. Paleomagnetism and biostratigraphy data from International Ocean Discovery Program Expedition 391 confirm the magnetic interpretation. Anomaly C33r is split into two negative bands, likely by a westward ridge jump. One of these negative anomalies coincides with deep rift valleys, indicating their age and mechanism of formation. These findings imply that VB originated by seafloor spreading‐type volcanism during a plate reorganization, not from a vertical stack of lava flows as expected for a large volcano.

Spatial Relations between Geological Structures and Precious and Base Metal Deposits from Magnetic Investigation of the Pangar-Djérem Zone, Cameroon

The version three of Earth Magnetic Anomaly Grid (EMAG2-v3) used in this work is aimed at establishing the spatial relationships between tectonic structures and gold and associated mineralization in the Pangar-Djérem area, central and southern Cameroon. Various processing and transformation of image data such as the reduction to the equator, horizontal gradient, generalized derivative, and pseudogravity allowed the identification and delineation of numerous faults related to Syn-D1, Syn-D2, and Syn-D3 deformations. The main orientations, W-E, SW-NE, and SE-NW, obtained from these faults define the intensity of the Pan-African and Eburnian tectonics in the Cameroonian basement. Correlations between mapped faults and mineral occurrences define a structural control of gold mineralization in the study area. Furthermore, the spatial relationships between these faults and the igneous/metasedimentary rocks demonstrate that the Eburnian and Pan-African orogenies favoured the circulation of gold-enriched fluids along faults and shear zones. Gold mineralization in the Pangar-Djérem zone is of the vein type (primary gold), resulting from the intrusion of type I granites and the circulation of enriched fluids in a brittle to ductile shear context. The pseudogravity anomalies associated with the sedimentary deposits/rocks and the SW-NE structures of the Sanaga and Bétaré-Oya Faults define an environment of detrital sedimentary deposits, consisting of metavolcanic and metasedimentary rocks, and justify the existence of gold-bearing quartz veins.

Determination of Curie Point Depth Distribution and Heat Flow Regime Characteristics in Eratosthenes Seamount, Eastern Mediterranean Sea

The Curie point depth (CPD) has been established to enhance the determination of magnetic sources average bottom depth. In this study, the CPD and heat flow regime of the Eratosthenes Seamount in the Eastern Mediterranean were investigated in order to understand the relationship between them. When CPD and heat flow values are determined together, we can understand the types and processes of geothermal resources formation. CPD and heat flow were obtained by applying the spectral analysis method to magnetic data, which was obtained from the Earth Magnetic Anomaly Grid (EMAG2). The result shows that CPD is approximately 22 km across the Seamount and is approximately 9 km in the northern part of the Eratosthenes Seamount near Cyprus. The heat flow regime manifested from CPD is about 95 mW/m2 in the investigated region, where the average thermal conductivity value (k) is considered to be 2.5 W/m °C. The heat flow regime is characterized by an increase in the northward part of the Eratosthenes Seamount and decreases towards the south.

Mapping of major tectonic lineaments across Cameroon using potential field data

The cartography of lineaments across a territory can be optimized using geophysical potential field data. In this study, land gravity and EMAG2 (Earth Magnetic Anomaly Grid) data were simultaneously used to identify and characterize the major lineaments that spread across Cameroon. The data were filtered using a multi-scale approach including horizontal and vertical gradient analyses. The Euler Deconvolution method was later applied to the filtered data to estimate the extension and depth of the identified lineaments. Results show that the main lineaments across Cameroon are laterally extended with a dominant N45°E orientation. Some of these lineaments correlated well with the geographical location of some known major tectonic structures found across the country. The depth of these lineaments varies between 1 and 35 km. Some of the identified faults are still active as their location correlated with the location of some recent earthquakes that occurred in Cameroon. This work, therefore, highlights some hidden tectonic features which knowledge generally precedes exploration for subsurface resources.Graphical

Arctic geothermal structures inferred from Curie-point depths and their geodynamic implications

We applied the modified centroid method with a fractal magnetization model on a new magnetic anomaly grid for the Circum-Arctic area (CAMP) to obtain the first high-resolution Arctic Curie-point model and to infer geothermal structure. The shallowest Curie-point depths, found in the oceanic domain, reveal the cooling pattern of oceanic lithosphere and asymmetric mantle thermal activities across spreading ridges. The Cretaceous High Arctic large igneous province (HALIP), the most prominent magmatic feature in the Arctic, shows slightly lower heat flow and higher thermal conductivities than the surrounding area, indicating a complete cooling of the HALIP, but HALIP also has shallow Curie-point depths. We attribute this contradiction to the high titanium content of HALIP that lowers the Curie temperature. Therefore, Curie-point depths constrain well the boundary of the HALIP. On the Arctic continental shelves, the largest Curie-point depths are found in deep sedimentary basins, where lower crust eclogitization was previously interpreted. Furthermore, we found that this positive correlation between large Curie-point depths and large sediment thickness is prevalent for global continental margin sedimentary basins.

High geothermal heat flow beneath Thwaites Glacier in West Antarctica inferred from aeromagnetic data

Geothermal heat flow in the polar regions plays a crucial role in understanding ice-sheet dynamics and predictions of sea level rise. Continental-scale indirect estimates often have a low spatial resolution and yield largest discrepancies in West Antarctica. Here we analyse geophysical data to estimate geothermal heat flow in the Amundsen Sea Sector of West Antarctica. With Curie depth analysis based on a new magnetic anomaly grid compilation, we reveal variations in lithospheric thermal gradients. We show that the rapidly retreating Thwaites and Pope glaciers in particular are underlain by areas of largely elevated geothermal heat flow, which relates to the tectonic and magmatic history of the West Antarctic Rift System in this region. Our results imply that the behavior of this vulnerable sector of the West Antarctic Ice Sheet is strongly coupled to the dynamics of the underlying lithosphere.

Estimating Curie-point depths using both wavelet-based and Fourier spectral centroid methods in the western Pacific marginal seas

SUMMARY The Curie-point depth is an important magnetic parameter for studying the geological structure and thermal evolution of the lithosphere. In this study, we first apply wavelet transform to the centroid method, and compare it with the conventional Fourier spectral centroid method based on a 3-D fractal magnetization model. We find that the Fourier transform method is suitable for estimating smaller Curie-point depths (less than 20 km), whereas the wavelet transform method has better accuracy in estimating large Curie-point depths (more than 30 km). The application of both methods to the western Pacific magnetic anomalies from the Earth Magnetic Anomaly Grid of 2-arcmin resolution (EMAG2) proves their reliability and comparability. Our results show small Curie-point depths in areas with active magmatism and high heat flow, like mid-ocean ridges and island arcs of subduction zones, but large Curie-point depths in sedimentary basins and forearcs of subduction zones with low surface heat flow. We wrote our Curie-point depth inversion software CPDINV using FORTRAN language, which includes completed modules of minimum-curvature gridding, projection, wavelet and Fourier transforms, least-squares fitting and depth estimation.

Depth to the bottom of the magnetic layer, crustal thickness, and heat flow in Africa: Inferences from gravity and magnetic data

Data from the Earth Gravitational Model (EGM2008) and the Earth Magnetic Anomaly Grid (EMAG2) were used to develop a continental scale crustal thickness model for Africa, and to estimate the depth to the bottom of the magnetic layer (DBML) and the geothermal gradient and heat flow. The results are: (1) the estimated DBML from the magnetic data varies from ~23.0 to ~37.2 km. The shallowest DBML values are located in the northern, eastern, and western parts of the continent, whereas the deepest values are observed in the central and southern regions. (2) The estimated crustal thickness based on gravity data varies from ~29.9 km in the northern and western parts of Africa to ~48.0 km in its southern regions, with an average thickness of 35.1 km for the whole continent. (3) The estimated heat flow varies between high values of 46–59 mW/m2, observed in the northern, eastern, and western regions to low values of ~< 41 mW/m2, observed in the central and southern parts of the continent. (4) The geothermal gradient values vary between 14.5 and 23.6 °C/km (5) The East African rift zone is underlain by shallow DBML characterized by high heat flow values that vary between 42 and 59 mW/m2 (6) The heat flow anomalies in Egypt and Libya may be associated with the zone of the Pelusium megashear system, and it shows heat flow values that vary between 36.3 and 59.0 mW/m2. The current study has taken advantage of the availability of the EGM2008 and EMAG2 datasets to map crustal thickness variations and DBML beneath the continental landmass of Africa.

Late Pleistocene–Holocene stress in the South American intraplate evidenced by tectonic instability in central Amazonia

Abstract Documenting neotectonic instabilities and determining the style and time of deformation in the vast and difficult to access central Amazonia region is challenging. We focus on these issues by investigating a large area of the Negro River drainage basin, applying morphostructural analysis based on synthetic aperture radar data. The digital elevation models of the C-band Shuttle Radar Topography Mission and the L-band Protection System of Amazonia were used as the database. We also used subsurface magnetic information from the Earth Magnetic Anomaly Grid global model to validate the morphostructures. The results revealed NW-oriented morphostructural lineaments bounding multiple depositional valley fills. These were extensively fragmented to form regularly distributed en échelon rectangular blocks commonly offset horizontally by several kilometers. Strike-slip faults and oblique, either normal or reverse, faults are present. These structures were reactivated along the main NE- and SW-oriented regional structural trends due to N-S-oriented horizontal compression and E-W-oriented horizontal extension in the late Pleistocene and Holocene. The extensive neotectonic faulting results from the interplay of plate motion and Andean uplifting since the late Pleistocene, combined with local stresses.

The igneous rock intrusion beneath Ambon and Seram islands, eastern Indonesia, based on the integration of gravity and magnetic inversion: its implications for geothermal energy resources

Ambon and Seram islands in eastern Indonesia are located in the convergence zone of three major plates, causing this area to have many geothermal manifestations on the surface. A subsurface geological study on Ambon and Seram has been conducted based on the integration of gravity and magnetic inversion modeling to determine the intrusion of igneous rock as a potential source of geothermal energy. The gravity and magnetic inversion utilized gravity and magnetic anomaly data from the World Gravity Map (WGM) and the Earth Magnetic Anomaly Grid (EMAG), as well as the CRUST 1.0 global earth crust model, as the constraints. In general, gravity and magnetic anomalies in the study area are compatible, in which the southern part of Ambon is dominated by high anomalies and the northern part is covered by low anomalies associated with the Seram trench. The high anomaly is thought to be related to the intrusion of igneous rocks underneath the surface that have cooling and are rich in magnetic minerals, while the low anomaly is related to the destruction zone due to subduction in the north of Seram. The gravity inversion shows the distribution of subsurface rock density ranging from 1.86 g cm-3 to 2.82 g cm-3, while magnetic inversion produces rock susceptibility contrast ranging from -0.0034 SI to 0.0058 SI. Based on Curie point depth analysis of magnetic data, the bottom depth of the magnetic source is estimated at 25 km. The 3D subsurface model of Ambon and Seram based on gravity and magnetic inversion shows a massive and evident intrusion pattern with high rock density and susceptibility contrast down to a depth of 10 km. The distribution of high density and susceptibility is thought to have implications for geothermal resources in Ambon and Seram.

Crustal and Mantle Structure Beneath the Southern Payenia Volcanic Province Using Gravity and Magnetic Data

AbstractThe Auca Mahuida volcanic field lies on the southernmost Payenia Volcanic Province, one of the broadest retroarc volcanic plateaux in the southern Central Andes (~38°S). This voluminous basaltic flooding of Quaternary age was originated from a deep asthenospheric source, interpreted as a mantle plume product of changing slab dynamics. The geometry of this source is deduced from magnetotelluric data, but the limited spatial coverage of this array does not allow a detailed resolution of this anomaly. In order to present a detailed geometry of the conductive anomaly and related crustal magmatic bodies, we used multiple data sources. We combined Magnetic and Bouguer anomalies, Curie isotherm depth (Tc), Elastic Thickness (Te) and Moho depth derived from the Global Earth Magnetic Anomaly Grid (EMAG2) and terrestrial gravity measurements, all together in a holistic geophysical analysis. The magnetic data depict a nearly 200‐km‐in‐diameter circular anomaly that would correspond to a dense body according to the Bouguer anomaly. Geoid data from the Gravity Field Model (EIGEN‐6c4) have been filtered in order to isolate deeper mass influences and visualize the asthenospheric upwelling previously described from magnetotelluric data. Moho inversion yields a crustal attenuation at 36‐ to 32‐km depth coinciding with Te below 20‐km depth and a shallow Tc (≤15‐km depth) at the site where Geoid positive undulation was calculated. Finally, surface analysis allowed defining a topographic swell, compatible with the dimensions of the identified magnetic anomaly, where the main rivers deviated, potentially due to a recent base level change.

Comparison of global geomagnetic field models and evaluation using marine datasets in the north-eastern Atlantic Ocean and western Mediterranean Sea

Many models describing the Earth’s magnetic field exist and are maintained by international research organizations. However, no clear general overview of the cross-compared quality of these models is available. An evaluation of the internal component and of the anomaly field as estimated by a selection of nine geomagnetic models with global coverage is presented here. Anomaly field models are also compared with an external, independently processed marine dataset extracted from Shom’s (the French Hydrographic and Oceanographic Service) geophysical database. The comparative study shows that internal geomagnetic fields computed outside the provisional domain, up to degrees 12 or 13 of spherical harmonics (SH), can be considered as equivalent. Relative amplitude differences with the World Magnetic Model 2015 (WMM2015) and the 12th generation of the International Geomagnetic Reference Field (IGRF-12) taken as references are lower than 10 nT. For an extent reduced to the north-eastern Atlantic Ocean and western Mediterranean Sea, anomaly field models up to SH degree 720 and the 3rd version of the Earth Magnetic Anomaly Grid (EMAG2v3) at the equivalent 0.5° resolution are the closest models to the filtered marine data. However, they are significant spatial discrepancies and standard deviations of differences between models and data are about 65 nT. The 3-arc-minute World Digital Magnetic Anomaly Map (WDMAM) best reflects the unfiltered marine data, but with significant standard deviations of about 70 nT. Results demonstrate that any model can be used with insignificant errors to compute raw anomalies from total field measurements by removing the internal component of the Earth’s magnetic field. In addition, they suggest that the priority should be given to WDMAM for levelling and evaluating marine datasets and for computing enhanced high-resolution regional magnetic maps.

Mapping of Deep Tectonic Structures of Central and Southern Cameroon by an Interpretation of Surface and Satellite Magnetic Data

The aim of this study is to determine the depth of deep tectonic structures observed in the Adamawa-Yadé zone (central part of Cameroon) and propose a new structural map of this area. The horizontal gradient associated with upward continuation and the 3D Euler deconvolution methods have been applied to the Earth Magnetic Anomaly Grid 2 (EMAG2) data from the study area. The determination of the maximum magnitude of the horizontal gradient of the total magnetic intensity field reduced to the equator, in addition to the main contacts deducted by Euler solution, allowed the production of a structural map to show the fault systems for the survey area. This result reveals the existence of two structural domains which is thus confirmed by the contrast of magnetic susceptibility in the Central Cameroon Zone. The suggested depths are in the range of 3.34 km to 4.63 km. The structural map shows two types of faults (minors and majors) with W-E, N-S, NW-SE, NE-SW, ENE-WSW, WNW-ESE, NNE-SSW, and NNW-SSE trending. The major faults which are deepest (3.81 km to 4.63 km) with NE-SW, W-E, and N-S direction are very represented in the second domain which includes the Pangar-Djerem zone. This domain which recovers many localities (Ngaoundéré, Tibati, Ngaoundal, Yoko Bétaré-Oya, and Yaoundé) is associated with the Pan-African orogeny and the Cameroon Volcanic Line.

Derivation and Error Analysis of the Earth Magnetic Anomaly Grid at 2 arc min Resolution Version 3 (EMAG2v3)

AbstractThe Earth Magnetic Anomaly Grid at 2 arc min resolution version 3, EMAG2v3, combines marine and airborne trackline observations, satellite data, and magnetic observatory data to map the location, intensity, and extent of lithospheric magnetic anomalies. EMAG2v3 includes over 50 million new data points added to NCEI's Geophysical Database System (GEODAS) in recent years. The new grid relies only on observed data, and does not utilize a priori geologic structure or ocean‐age information. Comparing this grid to other global magnetic anomaly compilations (e.g., EMAG2 and WDMAM), we can see that the inclusion of a priori ocean‐age patterns forces an artificial linear pattern to the grid; the data‐only approach allows for greater complexity in representing the evolution along oceanic spreading ridges and continental margins. EMAG2v3 also makes use of the satellite‐derived lithospheric field model MF7 in order to accurately represent anomalies with wavelengths greater than 300 km and to create smooth grid merging boundaries. The heterogeneous distribution of errors in the observations used in compiling the EMAG2v3 was explored, and is reported in the final distributed grid. This grid is delivered at both 4 km continuous altitude above WGS84, as well as at sea level for all oceanic and coastal regions.

Break-up and seafloor spreading domains in the NE Atlantic

Abstract An updated magnetic anomaly grid of the NE Atlantic and an improved database of magnetic anomaly and fracture zone identifications allow the kinematic history of this region to be revisited. At break-up time, continental rupture occurred parallel to the Mesozoic rift axes in the south, but obliquely to the previous rifting trend in the north, probably due to the proximity of the Iceland plume at 57–54 Ma. The new oceanic lithosphere age grid is based on 30 isochrons (C) from C24n old (53.93 Ma) to C1n old (0.78 Ma), and documents ridge reorganizations in the SE Lofoten Basin, the Jan Mayen Fracture Zone region, in Iceland and offshore Faroe Islands. Updated continent–ocean boundaries, including the Jan Mayen microcontinent, and detailed kinematics of the Eocene–Present Greenland–Eurasia relative motions are included in this model. Variations in the subduction regime in the NE Pacific could have caused the sudden northwards motion of Greenland and subsequent Eurekan deformation. These events caused seafloor spreading changes in the neighbouring Labrador Sea and a decrease in spreading rates in the NE Atlantic. Boundaries between major oceanic crustal domains were formed when the European Plate changed its absolute motion direction, probably caused by successive adjustments along its southern boundary. Supplementary material: Figures showing the long wavelength of the NAG-TEC magnetic anomaly grid, detailed magnetic anomalies and isochrons, and a Table documenting aeromagnetic surveys for NAG-TEC magnetic compilation are available at https://doi.org/10.6084/m9.figshare.c.3661925

A Preliminary Assessment of the Hydrocarbon Potential of Kerio Valley Basin: Gravity and Magnetic Interpretation

The aim of the study was to assess the hydrocarbon prospectivity of the Kerio Basin in the Kenya Rift. Regional gravity and magnetic data sourced from the International Gravity Bureau (BGI) and Earth Magnetic Anomaly grid (EMAG2) respectively, were processed and analysed for major structures and geological anomalies based on variations in density magnetism and or chemistry of the rocks.Gravity Isostacy data shows low gravity anomalies of up to -100mGals that depict a deeper basement prominent to the southern part of the study area. Gravity profiles show that sediment thickness gradually increases to the south where we expect hydrocarbon accumulation. The magnetic anomaly map reveals low susceptibility rocks of between - 20nT to -200nT to the south of the basin. Magnetic tilt depth indicates sediment thickness of 2-3.5Km above the basement. The results agree with the observed gravity values of -100mGals to -200mGals in the same area. Hence the southern part of the study area shows the highest potential for hydrocarbon accumulation. Integration of these data with seismic and other geophysical methods in the southern area of the Kerio Basin can help gauge the hydrocarbon potential and reduce the uncertainty in the preliminary results.

Curvature of a geometric surface and curvature of gravity and magnetic anomalies

Curvature describes how much a line deviates from being straight or a surface from being flat. When curvature is used to interpret gravity and magnetic anomalies, we try to delineate geometric information of subsurface structures from an observed nongeometric quantity. In this work, I evaluated curvature attributes of the equipotential surface as functions of gravity gradients and analyzed the differences between the theoretical derivation and a practical application. I computed curvature of a synthetic model that consisted of representative structures (ridge, valley, basin, dome, and vertical cylinder) and curvature of the equipotential surface, gravity, and vertical gravity gradient (which is equivalent to the magnetic reduction-to-the-pole result) due to the same model. A comparison of curvature of such a geometric surface and curvature of different gravity quantities was then made to help understand these curvature differences and an indirect link between curvature of gravity data and actual structures. Finally, I applied curvature analysis to a magnetic anomaly grid in the Gaspé belt of Quebec, Canada, to illustrate its useful property of enhancing subtle features.

Depth to magnetic sources in the offshore northern Perth Basin

During 2009-11 Geoscience Australia completed a petroleum prospectivity study of the offshore northern Perth Basin. Basement in the northern Perth Basin is deep and generally not resolved in the reflection seismic data. Recent improvements to the magnetic ship-track database and magnetic anomaly grid allowed an assessment of depth to magnetic sources, and estimation of sediment thickness, providing new insight into basement depth and trends. 2D models along seismic transects and analysis using spectral methods indicate that penetration of the deepest sediments by high-susceptibility bodies is probable. The reflection seismic evidence for these bodies is not clear, though in some cases they may be associated with faults and structural highs. Where the modelled bodies penetrate the sediments they are mostly below or within the Permian strata. A moderate positive magnetic anomaly (the Turtle Dove Ridge) is modelled by massive bodies whose tops are 5-15 km below sea floor. The depth to magnetic basement map highlights sub-basins and structural highs within the northern Perth Basin, with up to 12 km of sediment in the Zeewyck sub-basin.