Interpretation of high resolution aeromagnetic data over southern Benue Trough,southeastern Nigeria

High resolution airborne magnetic data of parts of the southern Benue Trough were digitally processed and analyzed in order to estimate the depth of magnetic sources and to map the distribution and orientation of subsurface structural features. Enhancement techniques applied include, reduction to pole/equator (RTP/RTE), first and second vertical derivatives, horizontal gradients and analytic signal. Results from these procedures show that at least 40% of the sedimentary basin contain shallow (<200 m) magmatic bodies, which in most cases are intermediate to mafic intrusive and hyperbysal rocks, and may occur as sills, dikes or batholiths. Magnetic lineaments with a predominant NE–SW trend appear to be more densely distributed around the basement rocks of the Oban Hills and metamorphosed rocks around the Workum Hills. 3D standard Euler deconvolution and Source Parameter Imaging (SPI ^TM) techniques were employed for depth estimation. Results from the two methods show similar depth estimates. The maximum depth to basement values for 3D Euler and SPI are 4.40 and 4.85 km with mean depths of 0.42 and 0.37 km, respectively. Results of 2D modelling of magnetic profiles drawn perpendicular to major anomalies in the study area reveal the existence of deep seated faults which may have controlled the emplacement of intrusive bodies in the basin. The abundance of intrusive bodies in the study area renders this part of the southern Nigerian sedimentary basins unattractive for petroleum exploration. However, the area possesses high potential for large accumulation of base metal mineralization.     

Analysis of subsurface magnetic features of the southeast of Al-Muwayh quadrangle,Saudi Arabia,using aeromagnetic data

Aeromagnetic data have been utilized to investigate the subsurface features of the southeast of Al-Muwayh quadrangle. Several techniques have been comprehensively used in an integrative way to reach the goals. Local phase and normalized standard deviation filters are used in this study as edge detectors, showing the possible occurrences of structural lineaments/faults in the quadrangle. Magnitude magnetic transform filters are used to produce anomalies that are closer to the true horizontal position of magnetic sources to enhance the interpretation. Among these transforms, a transform which has been used as edge detectors and the other two transforms are used to show the shallow and the shallowest magnetic sources within the study area. Tilt angle is mainly used to delineate the main magnetic contacts (faults), their locations, and their expected depths. The integration between these different filters show clearly the possible occurrences of edges (contacts/faults), the direction of these lineaments, the source locations of magnetic anomalies, the shallow and the shallowest causative targets, and the location and the depths of the main faults deduced from the tilt angle approach.     

Aeromagnetic signature analysis of Gasus area and its surrounding,central eastern desert,Egypt

This study presents analysis and interpretation of aeromagnetic data to construct subsurface structure maps and determine the depths to magnetic basement. Subsequent analysis steps are applied to the total magnetic intensity (TMI) data. These steps include Reduction to the Pole (RTP) map that was derived from the TMI map then subjected to spectral frequency analysis to isolate RTP map into high and low frequencies and determine pseudo-depth for each map from power spectrum in which there were two main average levels (interfaces) at depth 1.3 and 0.7 km for the deep-seated and near-surface magnetic structures, respectively. 3D Euler deconvolution technique is applied to calculate the depth to the magnetic contacts; the results show clustering of solutions, the shallow solutions are related to the outcrops less than 80 m from flight level, and the solutions which are related to sedimentary cover locations (Um al-Huweitat, Wasif, and Mohamed Rabah basins) are ranged between 200 and to more than 1100 m. Moreover, anomaly enhancement techniques such as tilt derivative (TD), horizontal tilt derivative (TDX), and source edge detect (SED) are applied to enhance shallow features, and determining structure element boundary for both shallow- and deep-seated anomalies and revealing possible faults such as 2D forward modeling are applied to selected profiles to adjust the interpreted structures and map the boundaries and basement relief of the three basins (Mohammed Rabah, Wasif, and Um al-Huweitat). The analysis shows that the study area is affected with network of dip–slip faults due to high shearing and compression stress which formed grabens and horsts at different depth levels in the study area. The trends of predominant faults related to deep-seated structures are NW–SE with depths ranged between 1 and 1.4 km and in regard to the shallow-seated structure, the predominant fault trends are NW–SE, NNW–SSE, and NNE–SSW with depths ranged between 0.3 and 0.8 km.     

Aeromagnetic data interpretation of east Qattara Depression,Northwest Desert,Egypt

The study area is located at the east of Qattara Depression at the north of the Western Desert of Egypt. The study area contains Abu Gharadig basin, which is the most petroliferous basin in the Egyptian Western Desert. Only three exploratory wells are presented in the study area, showing a thick sediment section overlying basement rocks. Magnetic data have been frequently used in geophysical exploration. Aeromagnetic data are mainly utilized to estimate the depth to the magnetic basement as well as to delineate the possible structures of the study area. The depth to magnetic basement has been estimated using the analytical solution of exponential equations obtained from the Fourier transformation of magnetic data, assuming multi-prisms. The depths obtained from this technique vary from 0.70 to 2.91 km with an average depth of 2.08 km. Local phase filters have been mainly used as edges detector where the possible occurrences structures can be delineated. Hyperbolic tilt angle, second-order tilt angle, and normalized total horizontal derivative (TDX) provide the best results for delineating the possible structures, showing the possible contacts within the basement of the study area. The edge enhancement filters show that the study area has been affected by different structural trends taking E-W, NE-SW, NNE-SSW, N-S, and ENE-WSW directions.     

Subsurface structural features of the basement complex and mineralization zone investigation in the Barramiya area,Eastern Desert of Egypt,using magnetic and gravity data analysis

In the Barramiya area, the majority of gold deposits are generally related with the quartz veins that associated with shear zones cutting the crystalline basement rocks. The quartz vein system is controlled by shear zone and general faults. The present study is to delineate the general faults, shear zones, geological limits, and basement rock relief, using airborne magnetic and gravity data analysis at the Barramiya gold mine and surrounding area, Eastern Desert of Egypt. To achieve our goal, we have applied on magnetic and gravity data the following techniques: reduction to pole (RTP), analytical signal, tilt derivative, total horizontal derivative, 3D Euler deconvolution, downward continuation, and source parameter imagining power spectrum techniques. The analytical signal used to map the types of rock boundaries. Tilt derivative and total horizontal derivative filters helped to delineate fractures and the contact zones of the formations that host the main Barramiya shear zone. 3D Euler deconvolution techniques helped to delineate the fault trends which represented at the following direction: NNE–SSW and NNW–SSE. The average depths of both regional and residual causes have been estimated by applying downward continuation, source parameter imagining, and power spectrum techniques. According to the results of the present study, the depth of the basement rocks is relatively high (~?80-m depth) in the western part of the study area and the basement rocks cropped out in the surface at the rest of the area. Our results are coinciding with the previous geological studies.     

Structural features of South-Adamawa (Cameroon) inferred from magnetic anomalies: Hydrogeological implications

Structural directions of the southern part of the Adamawa plateau in Cameroon are highlighted by filters applied on the magnetic anomaly map. Determination of the magnitude maxima of (1) the horizontal gradient of the field reduced to the pole, and (2) the analytic signal, allowed a map to be produced showing various lineaments interpreted as the fault system of the studied area. In particular, the faults on the shaded relief map of the horizontal gradient magnitude suggest a slide to the left of the Foumban shear zone in the studied area. Interpretation of the observed anomaly along a profile directed SSE-NNW shows the presence of a magnetized body considered as an intrusion into the granitic basement of probably basaltic volcanic rocks, put in place during the reactivation of the Foumban shear zone. In addition, the observed correlation between the mapped structural directions, the sites of thermo-mineral springs and the hydrographical network shows that the results of this study will be helpful for further hydrogeological research in the studied area.     

Integrated potential field study on the subsurface structural characterization of the area North Bahariya Oasis, Western Desert, Egypt

The present study aims mainly to delineate and outline the regional subsurface structural and tectonic framework of the buried basement rocks of Abu El Gharadig Basin, Northern Western Desert, Egypt. The potential field data (Bouguer gravity and total intensity aeromagnetic maps) carried out in the Abu El Gharadig Basin had been analyzed together with other geophysical and geological studies. The execution of this study is initiated by transformation of the total intensity aeromagnetic data to the reduced to pole (RTP) magnetic map. This is followed by applying several transformation techniques and various filtering processes through qualitative and quantitative analyses on both of the gravity and magnetic data. These techniques include the qualitative interpretation of gravity, total intensity magnetic and RTP magnetic maps. Regional–residual separation is carried out using the power spectrum. Also, the analytic signal and second vertical derivative techniques are applied to delineate the hidden anomalies. Aeromagnetic anomalies in the area reflect significant features on the basement tectonics, on the deep-seated structures and on the shallow-seated ones. Major faults and intrusions in the area are indicated to be mainly along the NE–SW, NW–SE, ENE–WSW and E–W directions. The Bouguer gravity map indicates major basement fracturing, as well as variations in the sedimentary basins and ridges and subsequent tectonic disturbances. The most obvious anomalous trends on the gravity map, based on their frequencies and amplitudes, are along the NE–SW, ENE–WSW, E–W and NW–SE trends. The main of Abu EL Gharadig Basin depositional center does not show sharp variations, because of the homogeneity of the marine rocks and the great basement depths.     

Contribution of the geomagnetism to the region of the massif Jbel Saghro geology,Eastern Anti-Atlas,Morocco

Our investigation relates to the interpretation of the magnetic data of Jbel Saghro, by aeromagnetic processing using Geosoft and ArcGis software. The data were obtained from digitizing the preexisting maps. From the magnetic field anomaly maps, different anomalies were identified with differences in intensities, sizes, and shapes and in being isolated or interfered. The directions are in general ENE-WSW, NNW-SSE, W-E, and NW-SE which are in good agreement with the general tectonic structure of the region. In order to correspond the anomalies to their natural sources and their geometrical configuration, it was necessary to calculate the residual polar. This avoids the presence of two lobes for the same source, what makes it possible to delimit the geological sources laterally. Thus, the attribution of each magnetic anomaly to its geological source becomes possible. Indeed, generally magnetic anomaly of strong intensities and big wavelengths is associated to very magnetized structures related to the Precambrian basement, and the negative a.m. is connected to structures of weak magnetization located in general in the cover. The application of the tilt angle filter allowed the localization of the outcrops of the inliers and new geophysical lineaments. On the other hand, the local maxima of the horizontal gradient made it possible to reveal the large structural features concerning the main tectonic events of the area.     

Fault pattern delineation and structural interpretation of the Gafsa trough (onshore central Tunisia) using gravity data

The purpose of this investigation was to identify subsurface lineaments in Gafsa trough (onshore central Tunisia) after gravity data analysis. The Bouguer and residual gravity maps show a gravity values decrease from west to east associated with subsidence variation and confirmed by a regional seismic reflection profile. The deep structural map of the study area is elaborated after the application of two methods: (1) the automatic lineament tracing after horizontal gravity gradient and (2) 3D Euler method. The dominant trends show approximately NW–SE, E–W, and NE–SW directions. Some of these trends are well correlated with the major faults systems. We can qualify the deep structuration model as a mosaic of quadratic blocks bounded by significant deep flower fault corridors. The elaborated structural map of the study area constitutes also a useful document for rationalizing the future petroleum exploration in the Gafsa trough.     

Basement-controlled faulting of Paleozoic strata in southern Ontario, Canada: new evidence from geophysical lineament mapping

Basement fault reactivation is now recognized as an important control on sedimentation and fault propagation in intracratonic basins. In southern Ontario, the basement consists of complexly structured mid-Proterozoic (ca. 1.2 Ga) crystalline rocks and metasedimentary rocks that are overlain by up to 1500 m of Paleozoic sedimentary strata. Reactivation of basement structures is suspected to control the location of Paleozoic fault and fracture systems, but evaluation has been hindered by a limited understanding of the regional structural characteristics of the buried basement. New aeromagnetic- and gravimetric-lineament mapping presented in this paper better resolves the location of basement discontinuities and provides further evidence for basement controls on the distribution of Paleozoic fault and fracture systems. Lineament mapping was facilitated by reprocessing and digital image enhancement (micro-levelling, regional residual separation, derivative filtering) of existing regional gravity and aeromagnetic datasets. Reprocessed images identify new details of the structural fabric of the basement below southern Ontario and delineate several previously unrecognized aeromagnetic and gravity lineaments and linear zones. Linear zones parallel the projected trends of mid-Proterozoic terrane boundaries identified by field mapping on the exposed shield to the north of the study area, and are interpreted as zones of shearing and basement faulting. Mapped aeromagnetic and gravity lineaments show similar trends to Paleozoic faults and fracture networks and broad zones of seismicity in southern Ontario. These new data support an ‘inheritance model’ for Paleozoic faulting, involving repeated reactivation and upward propagation of basement faults and fractures into overlying cover strata.     

Using airborne geophysical data in identifying tectonic lineaments in east of Iran

In this paper we tried to identify the main tectonic lineaments in Eastern Iran including Lut block and Sistan suture zone from the airborne geomagnetic data together with tilt filter. As the map of obtained lineaments from airborne geomagnetic data has been studied, four distinct set of lineaments has been identified: (i) north–south, (ii) east–west, (iii) northeast–southwest, and (iv) northwest–southwest that are concurrently with structural zones and area’s big faults. New faults which have been identified in this investigation are lineaments with trend northeast–southwest and east–west. The depth of these lineaments has been calculated through Euler modeling. Magnetic lineaments trending east–west have the most depth, so these lineaments are related to basement faults.     

Analyses of Aeromagnetic Data to Delineate Basement Structures and Reveal Buried Igneous Bodies in Kaladgi Basin,Karnataka

Aeromagnetic data overcome constrain of inadequate exposures and provide signatures of bodies beneath sediment cover. Present work on analysis of aeromagnetic data over western part of Kaladgi basin provided insight into the basement structures and their role in basin evolution. In the study area, the NW-SE and NE-SW are the major trends of magnetic lineament followed by E-W and N-S trends.Archean to Paleoproterozoic basement is manifested by two structural zones, NW-SE trends related to major lineaments within the basement and the NE-SW trends presumed intra-basinal fault systems which controlled the local depressions. The basin configuration deduced from depth to basement show that the Kaladgi basin is an open deep basin and divided into several sub-basins, separated by fault-controlled NE-SW and NW-SE oriented basement ridges. An intriguing find in the western part are the numerous scattered smaller-scale, circular or semicircular, distinct magnetic anomalies of moderate to strong magnetic signal with strong remenance. Analyses coupled with 3D inversions in combination with sub-surface probing reveal in-homogeneities within basement gneisses and supracrustal rocks of the Kaladgi basin, Dharwar craton. 3D inversions of these circular bodies, suggest that they are apophyses of the intrusions or alternatively as younger intrusive stocks. Sub-surface probing by boreholes over circular bodies revealed leucocratic granite with porphyritic texture emplaced as intrusive within the Chitradurga metasediments. This implies that these intrusives are post-Chitradurga schist and pre Badami sediments as they have not affected the latter. However, they can be presumed to be coeval to potassic granites, which intrude the eastern part of the western Dharwar craton in southern India, until geochronological data are available.     

基于重磁场特征的敦化盆地构造格架研究

依据重力、磁力异常数据及其处理结果( 水平梯度模和斜导数) ,对敦化盆地边界、基底起伏、断裂位置及以火成岩为代表的磁性体分布进行了研究。盆地重力异常的分析和水平梯度模及斜导数的计算结果表明,盆地基底具南部凹陷、中央凸起和北部凹陷的“两凹一凸”的起伏形态特征,盆地内断裂以SW--NE 向为主,盆地为单断半地堑式盆地。依据航磁异常,将盆地划分为4 个异常区: 东北部磁异常区、中部低磁异常区、西南高磁异常区和西南边部相对低磁异常区。结合磁异常水平梯度模和斜导数的计算结果显示,以火成岩为代表的磁性体受SW--NE 向构造控制。     

松辽盆地布格重力异常小波多尺度分解与解释

松辽盆地是中国东北盆地群的代表性盆地之一。松辽盆地及其邻区的布格重力异常分布特征以北东—北北东向为 主,其次为东西向,重力场分区分块特征明显,显示了明显的拼贴构造背景,依据布格重力异常特征对研究区构造单元进 行分区,共分成松辽盆地异常区、大兴安岭重力梯级带等 5 个特征区。利用小波多尺度分解方法对松辽盆地重力场进行分 离,选择最佳的分离阶数(4 阶),随后利用对数功率谱确定每阶异常的视深度,提取区域异常,分层研究地壳顶部(D1G)、基 底(D2G,D3G)及深部密度界面(A4G)的起伏特征,共圈定基底凹陷 14 个,并绘制了研究区的 Moho 面埋深图。地球物理特征 揭示了松辽盆地的深部构造,为研究盆地油气勘探及构造演化提供了重要依据。     

基于航磁资料的大兴安岭南部区域成矿构造填图尝试

大兴安岭地区是中国重要的成矿区带之一,具备大规模成矿的条件和找矿潜力。对大兴安岭地区的大型断裂、次级断裂及基底构造系统深入研究是揭示控制该地区构造-岩浆活动和成矿作用的重要内容。对大兴安岭南部的航磁数据进行处理和解释,查明了研究区北东向、东西向、北北东向和北西向深大断裂为主的深部基底构造特征,圈定了火山机构分布位置,分析了大兴安岭南部区域构造特征,揭示了反映岩浆岩活动的线性或面状正磁异常和磁异常梯级带,探讨了研究区金属成矿的有利区带。为大兴安岭南部地区的区域地质、构造特征三维地质填图、找矿远景区的圈定提供了基础性信息。     

秦岭造山带中段重磁异常与成矿带的关系

成矿带的展布受地质构造的控制,而地质构造在重磁异常上有明显反映。为了研究秦岭造山带中段重磁异常与成矿 带的关系,该文系统整理、处理了秦岭造山带中段已获得的重磁力测量资料,分析研究了重磁异常的展布特征、推断了研 究区的断裂构造,讨论了结晶基底的起伏特征,并结合区域地质资料及矿床分布资料讨论了重磁场特征及其与地质构造、 成矿带的关系。结果表明,秦岭造山带中段成矿带均分布在重、磁异常梯度带上或几组不同方向异常的交汇部位,尤其在 局部重力高异常范围内及其边部梯度带上矿体富集。这一结果为研究秦岭造山带的地质演化、地质构造（尤其是深部构 造）、断裂分布及下一步的成矿有利区预测提供重要的参考信息。     

Integrated interpretation of satellite imagery, aeromagnetic, aeroradiometric and ground exploration data-sets to delineate favorable target zones for unconformity related uranium mineralization, Khariar basin, Central India

The Mesoproterozoic Khariar basin, to the SE of Chhattisgarh basin, comprises 1000 m thick arenaceous-argillaceous sediments. For the first time, a multidisciplinary approach has been made to the integrate interpreted satellite imagery, aero-magnetic and aero-radiometric data with available ground exploration data sets with an objective to understand structural fabric and to establish various parameters for unconformity related uranium mineralization in the environs of Khariar basin. Total Magnetic Intensity (TMI) anomaly image has been useful to mark major faults (ENE-WSW), magnetic bodies and overall basement characteristics. Combination of first vertical derivative (1^VD) and tilt derivative magnetic images brought out presence of NW-SE magnetic linear (dominant) with minor ENE-WSW and NNE-SSW trends. Basic dykes and quartz veins are the surface manifestations of NW-SE trend in basement. Radially averaged power spectrum indicates the approximate basement configuration. Enhanced Thematic Mapper satellite imagery (Landsat ETM⁺) interpretation has shown lineaments along NW-SE, NNE-SSW, ENE-WSW and ENW-WSE directions. These observations are corroborated by interpreted results of magnetic data. Analysis of both results indicate NW-SE and ENE-WSW trends as post depositional. Aero-radiometric images (U, Th, K and ternary U-Th-K) show overall radio-elemental distribution for various litho-units. Besides, Th and K images along with interpreted ETM⁺ satellite imagery (RGB: 432/752/751) are useful to map small outliers and to modify basement-sediment contact. Geochemical data from basement rocks around Khariar basin suggests the younger Bundeli granitoids and its equivalents are good source of uranium in the western margin. Presence of labile uranium is inferred from higher concentration of uranium in water samples. The Airborne gamma-ray spectrometry (AGRS) and hydro-geochemical anomalies fall along fault zones and intersection of fault zones. The western and southern margin of Khariar basin are also characterized by presence of paleosol at unconformity, which are favorable factors for unconformity type uranium mineralization. Based on the present study, part of western and southern margin emerge as potential target areas for further exploration of uranium.     

Magnetic mapping of geologic contacts in the Taourirt–Oujda corridor–Horst belt (northeastern Morocco)

From a geologist's perspective, magnetic data are most useful in mapping contacts and providing insight into the 3D geometry of the geology. Since the early 1970s, automatic methods based on the horizontal and vertical derivatives are applied to potential field data in order to map geological lineaments. In this paper, three methods for locating magnetic contacts are applied to magnetic anomaly data of the study area. The local maxima of horizontal gradient determined on the magnetic map and its upward continued maps to several heights delineate the contacts and indicate their dips. The amplitude of the 3D analytic signal of magnetic data provides information on the location of the edge of the sources in both the horizontal and vertical dimensions. The Euler deconvolution is also used to estimate the source depth at the contact location. These results allow the production of a structural map showing the magnetic lineaments for the survey area. This map is consistent with many faults already recognised or supposed by the traditional structural studies and highlights new ones by specifying their layout and dips.     

福建东洋地区地球物理场特征

为了研究东洋地区深部地质结构,在福建东洋地区开展了综合地球物理勘查,对福建东洋地区地球物理场进行了分析,结合已有的地质资料,分析研究了区域地球物理场特征及区域深部地质结构特征.研究认为:福建东洋地区位于巨型环形构造外环带西南部,研究区处在东部沿海磁场剧烈变化带和西部内陆磁场相对平缓带的过渡区域,区域航磁ΔT异常以北东向条带状异常带为主,区内分布有2处剩余重力异常高;地球物理综合剖面范围被两条北西向深大断裂分割为3部分,结合区域地质特征,推断区域南部的浅层是一套推覆无根的变质岩系高阻体,深部为一套以中生代沉积岩石为主的低阻体,区域北部浅部主要为中生代沉积岩夹薄层火山岩组成的低阻体,深部为下古生代和元古界基底岩系共同反应的高阻体,区域中部是火山岩主要发育区.      

Magnetic characterization of a retroarc extensional basin: The Loncopué Trough

The Loncopué Trough is a Pliocene to Quaternary extensional basin developed over the hinterland area of the Southern Central Andes. This basin is bounded by two neotectonic extensional fault systems delimiting a narrow topographic low. Previous studies have mostly focused on structural and geochemical aspects of this feature. However, geophysical surveys, aimed to unravel deep structure beneath a thick-younger than 5 My volcanic coverage, are scarce and based their interpretations on low resolution data sets. In this study, we collected new aeromagnetic data with the objective of characterizing the magnetic properties of the crust in the Loncopué Through and nearby zones. Additionally, we analyze the spatial relation between geological structures, volcanic fields and magnetic data. In order to highlight the boundaries of the magnetic sources and calculate the basement depth, we applied derivative techniques and the source parameter imaging. Also, we estimated an effective susceptibility model using the Magnetization Vector Inversion method, which takes into account the combined effects of remanence and induced magnetization. To determine the thermal structure of the area, we calculated the Curie depth points through the spectral analysis technique. From the analysis of magnetic data we were able to characterize the main structures and lineaments associated with this retroarc extensional trough. Notably, only the Loncopué eastern fault system seems to be a crustal-scale tectonic feature, while E-W-, ENE- and ESE-trending lineaments are interpreted as relatively minor structures segmenting the basement. Finally, our susceptibility model, together with the analysis of the Curie point, revealed potential magmatic/hydrothermal reservoirs in the Copahue volcano, and the Codihue and Cajón de Almanza regions that could be connected at depth forming a regional magmatic body.