Deep Magnetic Sources Research Articles

Analysis of Aeromagnetic Data for Enhancing Geologic Features Using Filtering Techniques Over the Congo Craton–Pan‐African Belt Contact, Centre‐East Cameroon

The Pan‐African mobile zone, situated at the margin of the Congo Craton in Cameroon, is characterized by a geological setting with polyphased tectonics, which can be summarised as the alternation of compression and extension events. The present study, conducted in a portion of the aforementioned zone, is aimed at investigating crustal structures using aeromagnetic data. To achieve this, a variety of advanced analysis and interpretation techniques were applied to reduce aeromagnetic data at the equator, including total horizontal derivative (THD), Centre for Exploration Targeting (CET), 3D Euler deconvolution (ED), spectral analysis, and 2D3/4 modeling. The results indicate that the study area can be divided into four magnetic units based on the magnetic properties (magnetic susceptibility, geometry, and depth) of the underlying rocks. The THD method reveals the presence of pseudocircular and linear structures, which are associated with mafic intrusions and faults, respectively. The structural trends of the lineament generated semiautomatically using the CET method suggest that the structures are oriented in order of priority: E‐W, WNW‐ESE, ENE‐WSW, and NE‐SW. These directions are probably related to the Pan‐African orogeny. According to the ED, the depths of these lineaments extend beyond 1250 m. A spectral analysis conducted on a profile intersecting some of the significant anomalies on the reduction to the equator (RTE) map indicates that the average depths of superficial and deep magnetic sources along this profile are approximately 0.5 and 6.5 km, respectively. The geological model obtained along this profile by direct 2D3/4 modeling indicates the presence of an ancient granitic basement, which is overlain by younger formations belonging to the Pan‐African.

Source Depth Determination of the Ningi-Burra Younger Granite Complex North-Eastern Nigeria from Aeromagnetic Data

Source Parameter Imaging (SPI) of aeromagnetic data covering an area within Birnin-Kudu Sheet 105 SE, Gwaram Sheet 106 SW, Kalatu Sheet 127 NE and Kaffin Madaki sheet 128 NW, bounded between Latitudes 10o 55’ N to 11o 10’ N and Longitudes 9o20’ E to 9o 35’ E. The application of Source Parameter Imaging (SPI) method revealed depth ranging from 64.704m to 538 to 777m, map of the depth estimates obtained from the SPI solution plot superimposed on the residual magnetic field intensity map of the area. The blue to cyan colours (64.704m to 111.520M) indicates the shallowest portions of the basement, the Green to Yellow colours (115.773m to 187.186m) represent the intermediate depth, while the Red to Pink coloured (201.698m to 538.77m) represent the highest peak of the area which are the deepest areas. The results from the SPI study indicate that deep seated bodies (dykes) dominated the area. Other Euler deconvolution solutions were also obtained by first selecting a magnetic structural index of zero (i.e., SI=0), followed by SI=0, SI=1, SI=2, and SI=3. This is based on the concept of Euler homogeneity, a description of scaling behavior in potential-field depth analysis. The magnetic structural index (SI=0) represents the depth of the contact with basement, which is as low as -131.093m and the highest is at 329.960m The best clustering solution was obtained by selecting a structural index of one (i.e., SI = 1) shows that the solution plotted as clustered in the region where the geological structures are located in the depth range of 8.707m to 1370.815m, which indicates that the deep magnetic bodies are concentrated at the peripheral side of the ring complex. The deeper magnetic sources are at the depth of -163.723m to the basement, while the highest depth to the shallower magnetic source in the area is 700.594m and represents the extrusive bodies in the area.

Structural and Tectonic Interpretation of Nkalagu and Environs Using High-Resolution Aeromagnetic Data

The aeromagnetic survey carried out by the Geological Survey of Nigeria between 2003 and 2010, provided high-resolution aeromagnetic data used in this study. The study area is Nkalagu and its environs. It lies within latitudes N6o00 and N6o30 and longitudes 8o00 E and E. The geology of the study area is predominantly the geology of the Lower Benue Trough. The purpose of the study is to determine the depth of the basement and delineate the basement morphology and structural features associated with the subsurface of the study area using the structural and tectonic interpretation of aeromagnetic data. This will create indebt knowledge of the geology of Nkalagu and its environs. The Aeromagnetic data obtained were subjected to various data preprocessing enhancement techniques these include relief imaging, vertical and horizontal derivatives, analytic signal, apparent susceptibility mapping, automatic gain control, and spectral analysis. Results of the study revealed lineament with trend directions in the N-S, NE-SW, NW-SE, and E-W, with the NW-SE trend being dominant. When subjected to 2-D spectral analysis, it revealed a two-layer depth model; the shallow magnetic sources and the deep magnetic sources with average depths of 1.041 km and 3.574km respectively. Similarly, the average sedimentary thickness of the study area was estimated at approximately 3.6km.

Aeromagnetic and structural characterization of dyke swarms in southeast Brazil: Evidence for Cenozoic reactivation of the Guapiara lineament, Ponta Grossa Arch

The Guapiara Lineament located on the northern edge of the Ponta Grossa Arch, Southeastern São Paulo state, Brazil, it is an important morphostructural feature defined by dykes oriented in NW-SE direction. The main objective of this work is applying new techniques for enhancing magnetic anomalies of the Guapiara Lineament and other structures, and using aeromagnetic data as support for the analysis of structure reactivation. Aerogeophysical techniques included qualitative and semiquantitative approaches (Euler Deconvolution and Radial Average Spectrum), we used the vertical and horizontal gradients, as well as deep magnetic sources and shallow magnetic sources filters, for an in-depth magnetic-structural characterization that was further correlated with the outcropping geology. We have also applied upward continuation (2000m) in the anomalous magnetic field map which was the basis for applying the Total Horizontal Gradient of the Analytical Signal (TDY), Analytic Signal Amplitude (ASA), Tilt angle (TDR), Total Horizontal Derivative of the Tilt Angle (TDR-THDR) and Tilt Angle of the Horizontal Gradient (TAHG) to improve our interpretations of the regional framework. The magnetic trends of the Guapiara Lineament are superimposed on the NE-SW (e.g.: Cubatão Lineament) and E-W (e.g.: São Sebastião Lineament) anomalies, which are considered structural inheritances of the Proterozoic basement. We report a set of reactivated N40–70W and E-W strike-slip faults showing the same orientation as the mapped magnetic lineaments. Moreover, they seem to be associated to deformational features in the dyke swarm, which was interpreted as evidence of Cenozoic tectonic reactivation.

Geological Structures Controlling Au/Ba Mineralization from Aeromagnetic Data: Harrat ad Danun Area, Saudi Arabia

Positive and negative magnetic anomalies occupied the total aeromagnetic (TM) map of the Harrat ad Danun area, Saudi Arabia. Reduction to the pole (RTP) maps display the range of magnetic values (−312.4 to 209.4 nT) that vary in shape, size, and magnitude. These anomalies generally follow the NNW–SSE (Red Sea axis trend), NE–SW, and NNE–SSW trends. The NNW-SSE linear negative and positive magnetic anomalies could be brought on by buried faults, shear zones, or subsurface dikes. In the central part, the position of Au and Ba mineralization was connected to this trend. It is concluded that the principal structures are represented by the NNW–SSE, NE–SW, and NNE–SSW tendencies. Based on gridded RTP magnetic data, the 2-D power spectrum was computed and revealed the frequency of the near-surface and deep magnetic components. It is believed that the depths of the shallow and deep magnetic sources are typically 80 m and 570 m, respectively. Additional negative and positive magnetic anomalies with varied amplitudes and frequencies, trending in the NNW–SSE, ENE–WSW, and NE–SW directions, are seen when the high-pass and low-pass maps are closely examined. Many faults in various directions cut into these anomalies. The occurrence of negative linear magnetic anomalies (−36.6 nT to −137.3 nT) at this depth (80 m) is also confirmed by this map. The TDR filter and the Euler deconvolution method were used to identify the horizontal variations in magnetic susceptibility as well as the source position and depth of magnetic sources. The linear clustering rings are thought to be caused by contacts or faults with depths between 1 m to 474 m that are oriented WNW–ESE, NNE–SSW, and NNW–SSE. These faults or contacts are thought to be particularly prominent in the western, eastern, southern, northern, and central zones. The majority of felsic and mafic dikes are found to be connected to subsurface structures, showing that three structural trends—WNW–ESE, NNE–SSW, and NNW–SSE—affect the studied area. This demonstrates that important features and shear zones control the majority of Saudi Arabia’s gold deposits. A negative magnetic anomaly that is centered in the area, trending NNW–SSE and crossing the NNE–SSW fault, is connected to the plotted gold and barite mineralization in the study area. This may imply that these two tendencies are responsible for mineralization. This result raises the possibility of mineralization in the NNW negative magnetic feature located in the western part of the area. The occurrence of gold and barite was significantly impacted by the NNW–SSE and NNE–SSW structural lineaments.

Upward Continuation and Reduction to Equator Filters on Aeromagnetic Data of Nkalagu and Abakaliki Regions of Lower Benue Trough, Southeastern Nigeria

This paper adopts two magnetic filtration techniques to enhance the Aeromagnetic maps of Nkalagu and Abakaliki regions of lower Benue trough, Southeastern Nigeria. This aim is to enhance the effectiveness of the map in characterizing the study area. Upward continuation (UPC) and reduction to magnetic equator (RTE) filters were applied to sufficiently improve the interpretation of the aeromagnetic data of the area in terms of recognition and understanding depth continuation and discrimination between shallow and deeper magnetic sources within the study area. The total magnetic intensity and reduction to the equator showed variations in anomalies. The variations were invariably related to magnetic susceptibility, depth, degree of strike, lithology or basement complex rocks harboring varying amounts of magnetic minerals.

Geometry of the Magma Chamber and Curie Point Depth Beneath Hawaii Island: Inferences From Magnetic and Gravity Data

This study used land gravity and airborne magnetic data to investigate the depth to the magmatic chamber and map the heat flow distribution beneath the active volcanoes of Hawaii Island using the Curie point depth (CPD) and gravity modeling. Obtaining some of the ground-based geophysical measurements was problematic due to accessibility limitations; therefore, this study used available data. The CPD and magnetic data were used to map the depth to the bottom of the magnetic layer by calculating the depth to the Curie isotherm (540°C) beneath Hawaii Island. The spectral peak method was used to calculate the depths to the shallow and deep magnetic sources for the entire island, and the CPD was calculated using the centroid method. A two-dimensional density model for two Earth layers was constructed using forward modeling of the gravity data. A large plume of dense intrusive material was observed beneath the three adjacent volcanoes of Mauna Loa, Mauna Kea, and Kilauea, and two small chambers were found to be located beneath the Kohala and Hualalai volcanoes. Based on the gravity modeling results, the depth to the magma layer varied from 0.5 to 10 km, and the heat flow was higher close to the volcanic eruption zones. The current study is informative and cost effective for the world’s most active volcanic areas.

Determination of depth to basement using spectral analysis of aeromagnetic data over Azare segment of Chad Basin

The high-resolution aeromagnetic data over part of the Bornu basin (sheet 84) north eastern (NE) Nigeria, was processed and interpreted using spectral analysis to determine the depth to the basement. The study area is bounded between longitude 10⸰ 00’ 00’’E to 10⸰ 30’ 00’’ E and latitude 11⸰ 00’ 00’’ N to 12⸰ 00’ 00’’. Regional residual separation of the total magnetic field intensity (TMI) was performed using polynomial fitting method. The residual map was divided into nine square spectral blocks using the filtering tool of Microsoft excel software. The Microsoft excel program employed Fast Fourier Transform (FFT) technique. It transforms the magnetic field data into the radial energy spectrum for each block. Then the average radial energy spectrum is computed in MATLAB. The result shows that depth to the deeper magnetic source ranges from 4503.9m to 1948.3m. However, it can be observed that NN, NE, and NW are having less deep magnetic sources ranging from 1948m to 2459.7m. The deepest sources happened at the major towns of interest which are Bidawa , Matsango , Yakiri and Fatara areas, Katagum Bauchi state ranging from 2632.8m to 4503.9m The maximum depth of the sedimentary unit was estimated as 4.5km because the isolated value beyond this depth cannot be connected , and this depth occurs around Matsango and Bidawa. The shallow depth magnetic source map shows that SS, SW, SE are having least shallow depth, central to northern part of the study area are having shallowest depth to the magnetic source.

Aeromagnetic anomalies and magnetic domains of the Jalisco Block, western Mexico

The Jalisco Block (JB) in western Mexico has been considered as a margin terrane being detached from the North American plate along the Tepic-Zacoalco rift and Colima graben. In this study, an aeromagnetic analysis of the JB and surrounding areas is performed applying a series of geophysical filters and VOXI inverse model. The JB is characterized by two aeromagnetic domains (AMD). AMD-Ia located throughout the south-central section that borders the continental margin is characterized by positive magnetic anomalies extending from the Armería river to Banderas Bay. The AMD-Ia corresponds to the Puerto Vallarta batholith emplaced in the late Cretaceous. The AMD-IIa in the northern section is marked by negative magnetic anomalies over volcanic and volcano-sedimentary rocks, extending to the southern Sierra Madre Occidental. We propose that JB tectonic boundaries correspond to the aeromagnetic AMD-Ia eastern limit and AMD-IIa northern limit. Estimated depths from the radially average power spectrum for deep magnetic sources in the AMD-Ia are in the range of 5.4 km, and for shallow sources from 0.8 to 1.2 km. For AMD-IIa depths are 10.45 km for deep sources and 0.13 km for shallow sources. Depths for deep sources from the Euler deconvolution for the AMD-Ia range from 3.3 to 4.5 km and from 3.5 to 4.9 km for the AMD-IIa. Shallow source depths in the AMD-Ia are 0.2–0.3 km and near surface for the AMD-IIa. The integrated analysis shows five aeromagnetic domains, two associated with the JB and three with the surrounding areas. The VOXI inverse model allows to estimate the dimensions and location of magnetic sources in the reduced to the pole and upward continuations.

Comparative depth estimates and modeling of magnetic anomalies over the Nkalagu area, Southeastern Nigeria

High-resolution aeromagnetic data of Nkalagu area, in the southern Benue Trough, Nigeria, were employed for the determination of sediment thickness and characterization of magnetic anomalies in the area for possible mineralization and hydrocarbon prospect. Four methods of depth estimates, namely Euler deconvolution, source parameter imaging (SPI), forward and inverse modeling and radially averaged power spectrum (RAPS), were utilized, and the results were compared. Standard Euler deconvolution for the structural index (SI) of 0.5, 1.0, 2.0 and 3.0 gave depths to magnetic sources that range from 0 to 1500 m, 300 to 3300 m, 0 to 4000 m and 200 to 4200 m, respectively. Depth estimates from source parameter imaging (SPI) range from 135 to 2876 m and spectral depth analysis from RAPS revealed deep and shallow magnetic sources with the shallow source depths ranging from 420 to 850 m, while the depth of deep magnetic sources ranges from 3170 to 5840 km. Four profiles were taken on the total magnetic intensity contoured map and were modeled using forward and inverse modeling techniques. The models suggest the existence of intermediate to basic intrusives at depths between 545 and 1977 m. Generally, it is deduced that pockets of thick sedimentary piles in the area are zones of possible hydrocarbon accumulation. Furthermore, the existence of clusters of straight and curvilinear magnetic lineaments with a predominant NE–SW trend deduced from the tilt derivative grid, reveals four zones of high lineament density which suggest favourable zones for follow-up exploration for base metal mineralization.

Redefining the boundary between crystalline and sedimentary rock of Eastern Dahomey Basin

This study defines a new boundary between the crystalline and sedimentary rocks of Eastern Dahomey Basin at the southwestern part of Nigeria using a geophysical approach that combines regional aeromagnetic and ground resistivity data. Aeromagnetic data covering the entire Eastern Dahomey Basin were acquired at 500 m line spacing, 80 m mean terrain, and processed into grids of Residual Magnetic Intensity (RMI) map. Filters and corrections such as upward continuation, and reduction to equator were applied to enhance deep magnetic sources and correct for magnetic inclination and declination. Tilt Derivative Angles (TDR) was applied for edge detection. To support the aeromagnetic analysis and interpretation, 104 Vertical Electrical Sounding (VES) surveys and 8 Electrical Resistivity Tomography (ERT) data were also acquired, processed and interpreted along the basement-sedimentary rock boundary. The TDR revealed a significant trend that corresponds to the edge between the basement complex and the sediments of the Eastern Dahomey Basin. A strong match was also noticed between the VES positions and the TDR map. Areas interpreted as basement rocks from the VES stations align with positive values on the TDR maps while the sedimentary terrains have negative TDR values. Our work demonstrates that areas that were previously fixed as sedimentary terrains on geological maps belong to the crystalline basement or transition zone. A new and reliable geological boundary is hereby drawn between the basement and sedimentary rocks. Thus, providing a revised map of the Eastern Dahomey Basin.

Assessment of depth to magnetic sources using high resolution aeromagnetic data of some parts of the Lower Benue Trough and adjoining areas, Southeast Nigeria

High resolution airborne magnetic data acquired between 2005 and 2010 were used to determine depth to shallow and deep magnetic sources in some parts of Southeastern Nigeria. Various depth estimation methods such as standard Euler deconvolution (SED), source parameter imaging (SPI), spectral depth analysis (SDA) and two dimensional (2-D) forward modeling were applied. Results obtained from SED, SPI and models of profiles 1 and 2 indicate that the Abakaliki Anticlinorium (AA) and Ikom-Mamfe Rift (IMR) regions are dominated by short wavelength magnetic anomalies caused by extensive tectonic events. The SED map showed depth to shallow and deep magnetic sources ranging from ~ 16.6 to ~ 338.3 m and ~ 394.3 to ~ 5748.1 m respectively. Likewise, depth estimates from the SPI map varies from ~ 147.1 to ~ 554.2 m (shallow magnetic sources) and ~ 644.2 to ~ 6141.6 m (deep magnetic sources). The result obtained from SDA revealed depths to deep magnetic basement in the range of ~ 769 to ~ 6666 m with an average of ~ 3449 m. Also, it showed that depth to shallow magnetic sources vary between ~ 119 and ~ 434 m with mean of ~ 269 m. The 2-D forward modelling showed maximum depth values of ~ 4700, ~4600 and ~ 6500 m in the models of profiles 1, 2 and 3 within the Anambra Basin (AB), Afikpo Syncline (AS) and Calabar Flank (CF) respectively. Generally, from all the various methods applied the results indicate that AB, AS and CF are dominated by long wavelength anomalies. The 2-D models indicated that the basement framework is undulant. Also, depth estimates involving the various methods used in this study correlate strongly with each other in the AB, AS and CF geological regions.

Subsurface Structural Mapping over Koton Karifi and Adjoining Areas, Southern Bida Basin, Nigeria, using High-Resolution Aeromagnetic Data

In this current work, we aim to delineate the subsurface structural trends, determine the depth to basement surface, and provide an illustrative 3D model for its subsurface structure. Four digitized aeromagnetic maps were acquired from the Nigerian Geological Survey Agency, Abuja. The total field aeromagnetic anomalies over Koton Karifi and adjoining areas have been evaluated. In order to map the subsurface structures and estimate the depth to basement surface the spectral analysis method was applied. To achieve such goals, a detailed analysis of the aeromagnetic data for the study area was performed. 2D interpretation was carried out for the aeromagnetic data. The processes used include contouring of the Total Magnetic Intensity (TMI) data, separation of regional and residual anomalies, structural detection methods such as analytic signal, vertical derivatives, and magnetic lineament mapping were used to map the contacts and faults within the study area. The first vertical derivative and the magnetic lineament maps show major geologic lineaments trending in East-West with minor ones trending Northeast-Southwest. In the south-eastern part of the study area, there is a dome-shaped linear feature. The result obtained using the spectral analysis method reveals two source depth models. The depths to deeper magnetic sources range from 2.81 km to 3.24 km with an average depth of 2.90 km. The deeper magnetic source bodies are identified with the magnetic basement. The shallower magnetic sources which range from 0.45 km to 1.81 km with an average depth of 1.13 km could be attributed to near surface magnetic sources which are magnetic rocks that intruded into the sedimentary formations or magnetised bodies within the sedimentary cover. Based on the sedimentary thickness range of 0.45 to 3.24 km, there is an indication that the possibility of hydrocarbon generation in the study area is feasible.

Determination of Hydro Geological and Petroleum Potential Using High Resolution Aeromagnetic Data, Over Matsina and Environs, Part of Nigerian Sector of Chad Basin, North Eastern Nigeria

Two dimensional spectral analyses of high resolution aeromagnetic data was carried out over matsina and environs North Eastern Nigeria to determine hydrocarbon and groundwater potentials. The analysis was done using Oasis Montaj version 8.2. The result obtained indicates the existence of two source depths. The shallower magnetic sources varies between 0.35 to 0.55 m and deeper magnetic sources also varies between 0.6 to 2000 m. The shallower magnetic sources could be as a result of basic intrusive within the Borno Basin, while the deeper sources correspond to the basement topography underlying the Chad formation. Comparing the result with what was obtained in Gubio, Maiduguri sub basin, it is apparent that the sedimentary thickness over Matsina and and Environs cannot be a potential site for oil exploration rather for ground water exploration.

Magnetism in cold subducting slabs at mantle transition zone depths.

The Earth's crust-mantle boundary, the Mohorovičić discontinuity, has been traditionally considered to be the interface between the magnetic crust and the non-magnetic mantle1. However, this assumption has been questioned by geophysical observations2,3 and by the identification of magnetic remanence in mantle xenoliths4, which suggest mantle magnetic sources. Owing to their high critical temperatures, iron oxides are the only potential sources of magnetic anomalies at mantle depths5. Haematite (α-Fe2O3) is the dominant iron oxide in subducted lithologies at depths of 300 to 600 kilometres, delineated by the thermal decomposition of magnetite and the crystallization of a high-pressure magnetite phase deeper than about 600 kilometres6. The lack of data on the magnetic properties of haematite at relevant pressure-temperature conditions, however, hinders the identification of magnetic boundaries within the mantle and their contribution to observed magnetic anomalies. Here we apply synchrotron Mössbauer source spectroscopy in laser-heated diamond anvil cells to investigate the magnetic transitions and critical temperatures in Fe2O3 polymorphs7 at pressures and temperatures of up to 90 gigapascals and 1,300 kelvin, respectively. Our results show that haematite remains magnetic at the depth of the transition zone in the Earth's mantle in cold or very cold subduction geotherms, forming a frame of deep magnetized rocks in the West Pacific region. The deep magnetic sources spatially correlate with preferred paths of the Earth's virtual geomagnetic poles during reversals8 that might not reflect the geometry of the transitional field. Rather, the paths might be an artefact caused by magnetized haematite-bearing rocks in cold subducting slabs at mid-transition zone depths. Such deep sources should be taken into account when carrying out inversions of the Earth's geomagnetic data9, and especially in studies of planetary bodies thatno longer have a dynamo10, such as Mars.

English

This study aimed to estimate the depth of magnetic source bodies in Lafia and Akiri areas which fall within the middle Benue trough, Nigeria. Aeromagnetic data were used and spectral analysis and source parameter imaging were used for the quantitative interpretation of the data. The total magnetic intensity (TMI) contour map obtained from gridding of the data ranging from -39.5 to 100.0 nT was separated into regional and residual contour maps; it was done by polynomial fitting to produce the residual aeromagnetic intensity contour map. The residual intensity varies from -78.9 to 55.0 nT while the regional intensity varies from -39.18 to 39.84 nT. Depth results obtained from spectral analysis revealed two depth sources: the shallower magnetic source bodies and the deeper magnetic source bodies. The depth of shallower magnetic sources ranges from 0.557 to 1.261 km, with an average depth value of 0.899 km, whereas the depth of deeper magnetic sources varies from 2.419 to 5.732 km with an average depth value of 4.105 km. The SPI depth result ranges from -0.5638 km (shallow magnetic bodies) to 5.8381 km (deep lying magnetic bodies). The two methods showed depth estimation within the same range. The sedimentary thickness obtained from the different methods indicates the possibility of hydrocarbon accumulation if other conditions for hydrocarbon generation are satisfied. Key words: Magnetic source bodies, depth estimation, spectral analysis, SPI, Lafia and Akiri areas, aeromagnetic data.

Interpretation of High Resolution Aeromagnetic Data for Hydrocarbon Potentials over Parts of Nasarawa and Environs North-Central Nigeria

Spectral depth analysis over parts of Nasarawa area was carried out using high resolution aeromagnetic data to determine the depth to bottom of magnetic sources, to enable build up a subsurface picture of basement configuration and possible sediments distribution that promote hydrocarbon exploration in the area. The study revealed one and in most cases two source model, these are the deep magnetic anomaly sources and the shallow magnetic anomaly sources. The deep sources anomalies vary between 2.23km and 5.11km with an average depth of 3.05km and this represents the magnetic basement depth. The shallow anomaly sources vary from 0.07km to 0.42km with an average of 0.21km and this may be regarded as the magnetic intrusions into the sediment, probably through the magmatic activities. The revelation from this study of 3.05km magnetic basement depth at the southern part of the area is synonymous to depth of Cretaceous sediment, which is very significant to hydrocarbon generation potential in the area. The result shows the graben nature and indicated the existence of a deeper sub basin in the area. The central and south-western parts with higher sedimentary thicknesses are the most probable sites for hydrocarbon accumulation.

Analysis of aeromagnetic anomalies and structural lineaments for mineral and hydrocarbon exploration in Ikom and its environs southeastern Nigeria

The purpose of this study is to analyze the aeromagnetic data of Ikom and its environs in Cross-River State, Nigeria in order to delineate the structural lineaments according to their spatial and directional attributes as well as define the depth to magnetic causative bodies. This was achieved with the aid of an acquired, digitized and processed aeromagnetic intensity contour map covering Ikom and its adjacent areas. The aeromagnetic data was subjected to diverse data enhancement and transformation routines. Regional-residual analysis was implemented on the total magnetic intensity map (contour interval = 20 nT) to generate a residual magnetic intensity contour map. Results show that the area is characterized by a very complex pattern of magnetic intensities of both long and short wave-lengths. Sequel to the regional-residual anomaly separation, azimuth orientation of the lineament structures delimited on the maps portray NS and NE-SW as the principal trends with E-W and NW-SE as the minor trends. Two depth sources were obtained by employing the commonly known Peter's half-slope method. Depth range for the deeper causative sources is 0.83 km–3.5 km with a magnetic intensity range of −30 nT to −220 nT while the superficial sources had a range of 0.2 km–0.75 km and a magnetic intensity that vary from −10 nT to 20 nT. The deeper magnetic sources are attributed to the underlying pre-Cambrian basement complex rocks and intra basement structures such as fractures and faults whereas the superficial sources are due to basement intrusives as well as ore-bearing veins present in the overlying Cretaceous rocks. The shallow basement depth and prevalence of intrusives and linear structures are indications of an area of good mineral prospects but unfavorable for hydrocarbon formation.

Integrated geophysical methods for subsurface characterisation and health hazard assessment in parts of southwestern basement Nigeria

Aeromagnetic and radiometric data of Ikole Sheet 245 were interpreted for structural mapping, rock characterization and radionuclides hazard indices assessment. The data acquired were processed, filtered and enhanced to improve signal to noise ratio of rocks magnetisation. The Total Magnetic Intensity (TMI) map revealed high and low intensity values between 65 and 180 nT and -150 to -65 nT respectively. The residual map revealed clearer the undistorted signatures of the area geology and the interpretive boundaries of the underlying geology. Reduction to Magnetic Equator (RTE) map showed NE-SW regional lineament (R-R’) that diagonally divides the study area into approximately two equal halves; a central ridge between two depressions trending in NW-SE direction within Akoko complex and N–S faults of F1-F’1, F2-F’2 and F3-F’3 within Ikole axis. The upward continued maps (1 km, 2 km and 4 km) revealed the attitude of the basement rocks and structures with depth, some of the identified structures are deep-seated beyond 1.6 km downward, and as well revealed a general trend of NW-SE for the basement rocks. The radioelement maps revealed varying radioelement concentrations. Areas with high concentrations implied that the rocks are crystalline, undeformed and rich in feldspar with U-Th bearing minerals majorly within Ikole complex. Other regions with low concentrations on the other hand depicted varying geologic framework compositions. The maps also revealed uranium deposit in Ikole and mineralized structures such as veins, joints, fractures and dykes. The Ternary map showed that the two depressions (A) and deep-seated faults (F-F’) contain low and high radioactive rich bearing-minerals respectively. The spectral depth analysis result showed that the shallow and deeper magnetic sources have an average depth estimate of 20 m and 3.60 km respectively. The Absorbed dose and Annual effective dose equivalent (AEDE) of the study area are below the world standard limit for radiological hazard. Hence, the study areas are mineralized and have several structures that retain important radioactive minerals with no hazardous risk.

Spectral depth analysis of some segments of the Bida Basin, Nigeria, using aeromagnetic data

Spectral depth analysis was carried out on ten (10) of the 2009 total magnetic field intensity data sheets covering some segments of the Bida basin, to determine the depth to magnetic basement within the basin. The data was obtained from the Nigerian Geological Survey Agency and used to produce a composite map of the study area which is situated within latitude 8 0 00'N - 10 0 00'N and longitude 4 0 30'E - 7 0 00'E. Sixteen (16) overlapping blocks were generated from the ten sheets and Spectral depth analysis was performed on each of the blocks. The results delineated two prominent magnetic zones of shallow and deeper depths with the depth to the shallow magnetic sources (h 1 ) ranging from 0.254km on Block 9 to 1.719km on Block 2, having an average depth value of 0.968km. The depth to the deeper magnetic bodies, h 2 , on the other hand, varies between 1.830km on Block 10 and 4.615km on Block 15, with an average value of 3.063km. The shallower magnetic sources were attributed to near surface magnetic sources which could be laterite, ironstones or ferruginous sandstones that intruded into the sedimentary basin close to the surface. The places with deeper magnetic sources were ascribed to intrusions of the magnetic basement into the basin at deeper depths, lateral discontinuities in the basement and other features differing in magnetic susceptibilities such as dykes, faults, fractures, horsts and grabens within the study area. Based on these results, the Kainji (1), Fashe (2), Mokwa (4), Egbako (5), Baro (7), Patigi/Baro (14) and Baro/Gulu (15) spectral blocks with sedimentary thickness values above 3.0km show the greatest potential for hydrocarbon accumulation. Keywords: Spectral Analysis, Depth to Magnetic Sources, Aeromagnetic data