Horizontal Gradient Magnitude Research Articles

Gravity study of the Douala sub-basin (Cameroon) using Euler 3D deconvolution, source edge detection (SED) and special function analysis

Gravity data over an extending section of the north west portion of the Douala sedimentary sub-basin have been processed for both shallow and deep geologic contacts. Qualitative analysis, Euler 3D deconvolution, source edge detection (SED) and analysis of some special functions (horizontal gradient magnitude, analytic signal and the local wavenumber) have been carried out. One major peak and two minor peaks were highlighted on the anomaly maps as well as three major zones of anomaly which move from a very high positive anomaly zone in the center through an average positive zone to an average negative zone. Results of Euler 3D deconvolution and SED emphasized the presence of the underlying intrusive body to depths greater than 4 km in this study area and also suggested other geologic contacts above it. Solutions of special function analysis indicated that these geological contacts consist of some major faults with N–S, NE–SW, NW–SE and E–W orientations as well as some minor faults with NW–SE, E–W and NE–SW orientations having dip directions generally pointing away from the center of the study area. The faults and contacts were shown to be buried at a minimum depth of about 2 km with the majority having a structural index value in the range − 0.8 to 0.

Edge detection and magnetic basement depth of Danko area, northwestern Nigeria, from low-latitude aeromagnetic anomaly data

This study aims at estimating depths and boundaries of magnetic source bodies, in order to provide an indirect insight into subsurface geologic features within Danko area of Kebbi state. The study area is located within low magnetic latitude. Hence, the total magnetic intensity anomaly map was reduced to the magnetic equator. This is to center magnetic anomalies over their respective magnetic source bodies, which equally makes the interpretation of sources easier. Matched filtering of the reduced-to-equator (RTE) magnetic anomaly map indicates that the anomalies could be represented by two equivalent layers with some additional noise. The estimated mean depths of burial for the regional and residual sources are 1000 m and 500 m, respectively. Horizontal gradient magnitude and tilt derivative methods carried out on the RTE aeromagnetic anomaly map facilitated the identification of two distinct geologic trends bounded by a prominent fault suspected to be the documented Yelwa fault. Furthermore, the analytic signal map showed magnetic basement depth values between 50 and 900 m with an average value of 475 m, indicating a compelling alignment of results with the 500 m depth value for the residual matched filter layer. Several mineral occurrences have been found within the outcropping basement rocks of the study area. Hence, the combined appraisal of source edge locations and magnetic basement depth estimates offers useful information of the underlying basement configuration, which is highly essential for mineral exploration targets that are mostly obscured by regolith cover.

Interpretation of gravity data using 3D inversion and 2D continuous wavelet transform in Hedil deformed structures, northern Tunisia

The Hedil region in northern Tunisia shows several geological structures associated with intense tectonic deformations that induced genesis of overthrusts and tectonic imbrications with rising of Triassic evaporites. This region constitutes a transition zone, between the Atlassic and Tellian domains, and exhibits multi-scale tectonic deformations related to the basins structure and the regional geodynamic of the Northern African margin. Gravity data processing and continuous wavelet transform, coupled with field data, lead to underline the subsurface geometric configuration and to propose an evolutionary structural framework for the studied area.The horizontal gradient magnitude (HGM) and its maxima highlight several lineaments trending NE-, E−, NW and N however the NE-SW direction is predominantly expressed. Depths of the main buried structures were estimated using the Euler deconvolution method. The highest values that exceed 3 km seem to underlines the structures associated to NE-SW, NW-SE and rarely E-W inherited features. The NE-SW oriented structures are mainly represented by overthrusts and tectonic imbrication zones that were occurred along inherited faults in the southern part of the area, as a result of the Atlassic compressional tectonics of Late Miocene and Quaternary ages. However the other lineaments correspond mainly to strike-slip and reverse. The application of 3D inversion method allows identification and characterization of geological structures in subsurface. The most deformed structures are marked by positive anomalies and are sometimes underlined by intrusion of Triassic evaporite rocks. However, the collapsed structures are underlined by negative anomalies testifying a material of low density. The extracted inversion sections highlight the deep rooted geometry of the Triassic bodies, locally associated with overturn of pierced strata.The continuous wavelet transform method was used to get an idea about the deep structures of the region, to estimate the boundary between the sedimentary filling and the Precambrian basement, and determining the depth and dipping of some identified contacts. Some uplifted structures and Triassic evaporite intrusions especially that of the Bazina are marked by risings of pre-Triassic deep causative sources. These zones of significant anomalies in the Atlassic domain are probably areas of intense deformation related to the ancient Hercynian tectonics that affected the North African margin. The identified structures should be related to the regional tectonic inversion, related to the African and European plates subduction.

New Insights from Gravity Data on the Geodynamic Evolution of Northern African Passive Margin, Case Study of the Tajerouine Area (Northern Tunisian Atlas)

In Northern Tunisia, geological structuring is very diverse. This diversification is due to its localization, which is situated within the convergence zone between Africa and Eurasia plates. This situation allows to a complicated geodynamic evolution from Permian Tethy’s opening to the quaternary. Our study area is a key zone because we find several structures (Triassic extrusions, reefs, folds, grabens). The Surface studies (structural, sedimentology, etc.) leave controversies and many directions are only indicated as supposed faults. Thus, to understand geodynamic evolution is very important for petroleum and mining exploration. For this purpose geophysical method, which corresponds to a gravity data interpretation, is used to explore the subsurface structures. In this study, different techniques (regionalresidual separation, Horizontal gradient magnitude, upward continuation, Euler DE convolution) were applied to the gravity anomaly map. The results obtained allowed to draw up a structural map showing faults system responsible for structuring the study area. The obtained structural map is consistent with several faults already identified in previous studies and shows new directions. This map leads to better understanding the geological structures and the geodynamic evolution of the region and is a very useful document to guide future mining and hydrocarbons operations research.

Edge position detection and depth estimation from gravity data with application to mineral exploration

A method is applied to residual gravity data taken from the Ortaklar mine, Turkey to delineate the edge position and to estimate the depth of a buried ore body. The horizontal gradient magnitude (HGM) and tilt angle map (TAM) techniques are applied to the first vertical derivative (FVD) of the residual gravity data. The maxima contours of the HGM data and the zero contours of the TAM correspond to the edges of the ore body. The half distance between ± 45° (± 0.785 rad) TAM contours is used to determine the depths of the body. The obtained results are then compared against the depths retrieved from the Grav2dc inversion program, which inverted two profiles taken across the maximum response of the gravity anomaly map. The depths of the ore body estimated by the two methods are found in good agreement as reveal from drilling information.

Geostatistical Analysis of Mesoscale Spatial Variability and Error in SeaWiFS and MODIS/Aqua Global Ocean Color Data

AbstractMesoscale (10–300 km, weeks to months) physical variability strongly modulates the structure and dynamics of planktonic marine ecosystems via both turbulent advection and environmental impacts upon biological rates. Using structure function analysis (geostatistics), we quantify the mesoscale biological signals within global 13 year SeaWiFS (1998–2010) and 8 year MODIS/Aqua (2003–2010) chlorophyll a ocean color data (Level‐3, 9 km resolution). We present geographical distributions, seasonality, and interannual variability of key geostatistical parameters: unresolved variability or noise, resolved variability, and spatial range. Resolved variability is nearly identical for both instruments, indicating that geostatistical techniques isolate a robust measure of biophysical mesoscale variability largely independent of measurement platform. In contrast, unresolved variability in MODIS/Aqua is substantially lower than in SeaWiFS, especially in oligotrophic waters where previous analysis identified a problem for the SeaWiFS instrument likely due to sensor noise characteristics. Both records exhibit a statistically significant relationship between resolved mesoscale variability and the low‐pass filtered chlorophyll field horizontal gradient magnitude, consistent with physical stirring acting on large‐scale gradient as an important factor supporting observed mesoscale variability. Comparable horizontal length scales for variability are found from tracer‐based scaling arguments and geostatistical decorrelation. Regional variations between these length scales may reflect scale dependence of biological mechanisms that also create variability directly at the mesoscale, for example, enhanced net phytoplankton growth in coastal and frontal upwelling and convective mixing regions. Global estimates of mesoscale biophysical variability provide an improved basis for evaluating higher resolution, coupled ecosystem‐ocean general circulation models, and data assimilation.

Geophysical mapping of buried faults in parts of Bida Basin, North Central Nigeria

ABSTRACTHigh‐resolution aeromagnetic data over the Bida Basin, North Central Nigeria has been analysed to investigate the possible continuity of Ifewara fault zone, through the Bida Basin, to Zungeru fault zone. Analytic signal magnitude, horizontal gradient magnitude, and Euler deconvolution methods were applied to the aeromagnetic data to delineate the subsurface structures. The results showed that a prominent NNE–SSW trending fault associated with the Ifewara fault zone extends through the study area. Other faults trending in the ENE–WSW, NE–SW, NW–SE, E–W, and WNW–ESE directions were also mapped. Interpreted models revealed the presence of intrusives and a possible mineralised zone within the study area. We therefore concluded that the inferred fault zones within the basin have affinity with the trend of the Ifewara fault zone, which is an indication of possible extension and linkage with Zungeru fault zone through the Bida Basin.

Application of aeromagnetics as a basin depth discriminating tool prior to seismic exploration: A case study of Central Niger Delta, Nigeria

The importance of aeromagnetic as a basin depth discriminating tool is the emphasis in this study. Eight sheets of aeromagnetic total field intensity maps on a scale of 1:100,000 were analysed and the depth to magnetic basement compared with available drilled well data. The data was processed, filtered and transformed to other grids using either Oasis Montaj and USGS potential field software with their associated extension packages. Anomaly Separation, Spectral Analysis, Euler Deconvolution, Horizontal Gradient Magnitude were done using Oasis montaj and U.S.G.S softwares .The results of spectral analysis, Euler Deconvolution and horizontal gradient depth maps generated clearly indicates that the well bottom (well B) depth of 3.234km compared with the different thematic magnetic basement depth from spectral 4.2km, Euler 6.421km, and HGM 4.0kmis effective in basement depth discrimination. The result show a unique closeness when compared between already drilled depth with interpreted magnetic depth results. This indicate that the method can be reliable as a basin depth determining tool in virgin sedimentary basins prior to seismic explorations.

Combined high-resolution aeromagnetic and radiometric mapping of uranium mineralization and tectonic settings in Northeastern Nigeria

Geological lineaments, depths to the basement, uranium concentrations, and remobilization in parts of the Upper Benue Trough, covering about 55 × 55 km2 (longitudes 11°30′–12°00′E and 10°30′–10°30′N), Northeastern Nigeria were investigated using integrated High-Resolution Aeromagnetic Data (HRAD) and radiometric data. This was with a view to identifying the potential zones of uranium occurrence in the area. The HRAD was processed to accentuate anomalies of interest and depths estimate of 150–1941 m were obtained from source parameter imaging technique. The results from the superposition of the horizontal gradient magnitude, analytical signal amplitude, first vertical derivative, and 3D Euler solutions of the HRAD revealed that the study area was dissected by linear structures that trend ENE–WSW, NE–SW, E–W, NNE–SSW, WNW–ESE, and NW–SE; among which the ENE–WSW and NE–SW trends dominated. Analyses of radiometric data showed that uranium ores in the study area were possibly remobilized epigenetically from the granitic rocks, and were later deposited into sedimentary rocks (Bima formation). Burashika group (Bongna hills) and Wawa area of the study area showed vein-type deposits, while the anatectic migmatite in the northeastern region and the uranium rich Bima formation showed both fault/fracture and contact types of deposition. It was also observed the northwesterly and southeasterly, dominant dip direction, dipping faults dip in the same direction as the paleocurrent direction (direction of depositions of sediments), and trend in a direction perpendicular to the hypothetical direction of uranium deposition. The study concluded that the studied area is dissected by several linear structures and the studied area possibly contains deposits of uranium ore, which are likely to be found in: the Bima Sandstones of Wade, Shinga, Bima hill, Wuyo, Teli, Bryel, Dali, Barkan, Gasi, Kunkun, Boragara, Deba, and Gberundi localities; the anatectic migmatite at Kubuku, Whada, and Hyama; and the Bongna hills and agglomerates around Burashika, Kawaba, and Galu.

Evidence of basement controlled faulting of cretaceous strata in the Middle Benue Trough, Nigeria from lineament analysis of gravity data

The Bouguer anomaly map of part of the Middle Benue Trough, Nigeria, was used to investigate the genetic relationship between the basement and intra-sedimentary structures in the study area. This was with a view to establishing the evidence of possible basement controlled faulting of the Cretaceous strata. Regional-residual separation of the anomalies was carried out to produce the gravity maps associated with crustal masses at deep and intermediate depth layers. The derived maps were further processed to produce Horizontal Gradient Magnitude (HGM) maps that were used to infer deep seated structures and structures existing at intermediate depth layer of the study area. HGM maxima of the anomaly at various altitudes of upward continuation were used to highlight the locations and dip directions of subsurface structures as controlled from deep to shallow zones. The results of this study showed that the structures at different depth layers dominantly trend in the same direction (N-S, NNE-SSW and NE-SW) and that the deep seated faults reflect structurally in the overlying sedimentary column. It was therefore concluded that basement faulting might have influenced the structures in the overlying sedimentary rocks.Keywords: Cretaceous Sediments, Lineaments, Azimuth-frequency, Fault Propagation

Peripheral Design of Progressive Addition Lenses and the Lag of Accommodation in Myopes.

Insufficient accommodative response is assumed to result in myopia progression. We have investigated if the accommodative lag in myopes is different between a single vision lens (SVL) and the progressive addition lens PAL 2, clinically trialled for its ability to reduce progression of myopia, and if there exist differences in accommodative lag between PAL 2 and other PALs with the same addition power (+1.50 D). The influence of spherical SVL and four different designs of PALs that differ in the near zone width (PAL 1) or that have different signs and magnitude of horizontal gradients of mean power adjacent to their near vision zones (PAL 3 and PAL 4) on the accommodative response was investigated for different near viewing distances (40, 33, and 25 cm) in 31 subjects, aged 18 to 25 years. The SVL correction resulted in insufficient accommodative response for the near object viewing distances tested. PAL 2 did significantly reduce accommodative lag for all near object distances tested. The PAL design with a more negative horizontal mean power gradient (PAL 4) provided a lower lag of accommodation when compared with PAL 2 at the shortest object distance of 25 cm (P = 0.03) and was able to reduce the lag of accommodation to a level below the depth of focus for the higher near working distances tested. Designs of PAL with more negative horizontal mean power gradients are the most effective in lowering the lag of accommodation in myopes. This could make them good test candidates for myopia control applications.

Practical considerations in the use of edge detectors for geologic mapping using magnetic data

Locating the edges of magnetized sources provides a fundamental tool in the geologic interpretation of magnetic field data. Much recent effort has been expended on developing improvements to existing edge-detection methods, resulting in purported increases in accuracy and continuity along edges, reduction of noise effects, and limiting the influences of variable depth to source, magnetization direction, and source dip. These endeavors are valuable and provide interpreters with a wider range of tools to carry out geologic interpretations of aeromagnetic data. Nevertheless, survey parameters such as flight height and line spacing impose limits on the quality of edge locations that can be achieved. Using model studies, we quantify the effects that source size, depth, and interference between sources have on calculated edge locations. Based on the known behavior of established edge detectors, we found that many of the newer approaches offer limited advantages over older methods. Consequently, we studied an example of field mapping of geologic contacts in the Canadian Shield, supported by aeromagnetic data, using calculation of a standard edge detector: the horizontal gradient magnitude of the total magnetic field or TF-hgm. Calculated edge locations estimated from this method appear sufficiently accurate and continuous to provide a solid basis on which the mapping campaign was based and executed successfully.

Evidence of Structural Facts Inferred from Aeromagnetic Data Analysis over the Guider-Maroua Area (Northern Cameroon)

In this study, we performed some filters to highlight geological structures and/or features which are found in the igneous rocks, between the latitudes 9°45'N to 10°45'N and longitudes 13°15'E to 14°30'E. The application of the first vertical derivative (FVD) and the horizontal gradient magnitude (HGM) on a total magnetic data over the study area has led to put in evidence: 1) geological features as geological boundaries, faults, dykes, folds on the FVD map; 2) abundant aeromagnetic lineaments probably fractures, dykes and contacts, ex-hibit a conjugate relationship suggesting a near NE and NW tectonic trends; 3) existence of a possible prominent near E-W compression, characterized by a possible dextral displacement of geological formations by the shear movements; 4) and the magnetic signature of the country rocks.

Geophysical investigation of the possible extension of Ifewara fault zone beyond Ilesa area, southwestern Nigeria

High-resolution aeromagnetic data (HRAD) of parts of the southwestern and north-central Nigeria between 7° 30′ N – 8° 30′ N and 4° 30′ E – 5° 30′ E have been analyzed with a view to investigate the possible extension of Ifewara fault zone beyond Ilesa area. Analyses of the filtered HRAD by analytic signal amplitude (ASA), horizontal gradient magnitude (HGM), first vertical derivative (FVD), and Euler deconvolution (ED) methods were combined to delineate the locations, strikes, and dip directions of the subsurface structures in the area. The inferred structural map and the associated rose diagram indicate that the area is bisected by a number of major and minor faults that trend NE-SW, ENE-WSW, E-W, NW-SE, WNW-ESE, NNE-SSW, and NNW-SSE, among which the NE-SW trending faults predominate. The Ifewara and other major fault zones were clearly delineated. The mapped Ifewara faults trend between NNE-SSW and NE-SW directions and extend from the southwestern part (Ilesa area) to the extreme of north-eastern part of the study area. The study concluded that the Ifewara fault zones exist and do not terminate in the southwestern part of Nigeria but further extends to the northern part, possibly to Nupe basin and Zungeru area.

Enhancing the resolution of airborne gamma-ray data using horizontal gradients

The spatial resolution characteristics of airborne gamma-ray data are largely controlled by survey elevation and line separation. In the UK, although low nominal surveys altitudes may be permitted, regulatory zones with elevations in excess of 180m are required above conurbations. Since the data, typically in the form of grids, are evaluated alongside many other detailed geoscientific spatial datasets their absolute resolution limits, together with their spatial characteristics, become relevant. Here, using published software, we study the theoretical resolution characteristics of this form of survey data obtained with a line separation of 200m. Of particular interest is the airborne response behaviour when non-uniform distributions of radioactivity are encountered. Although ultimately a function of the radioelement-concentration contrast encountered, the calculations reveal that such zones are most difficult to identify when their scale length decreases below the scale of the line separation. This limited resolution then further decreases with elevation. In order to increase our ability to resolve the edges of non-uniform source regions we calculate the horizontal gradient magnitude (HGM) of the observed data. While the data used can be the estimated radioelement concentrations (potassium, thorium and uranium) or their ratios, we demonstrate that the total count is particularly suited to this type of analysis. The theoretical calculations are supported by an examination of survey data across a series of isolated bodies (offshore islands). This empirical study indicates the practical limits to resolution when using the horizontal gradient and these are governed by the survey line separation. The HGM response provides an enhanced mapping of the edges of zones associated with a contrast in flux behaviour. The edges are detected using the maxima in the response and these can be additionally examined using grid curvature analysis. The technique is assessed using recent survey data containing geological, soil and environmental influences. The results demonstrate the spatially pervasive nature of flux contrasts associated with soil and environmental contributions which potentially mask, or perturb, the underlying bedrock geological response.

Investigation of basement fault propagation in Chad Basin of Nigeria using high resolution aeromagnetic data

This study was conducted to investigate the basement fault propagation into the overlying sedimentary cover in parts of the Nigerian sector of Chad Basin. The Total Magnetic Intensity (TMI) map was compiled from the digital aeromagnetic data and was reduced to the equator to produce the Reduced-to-Equator (RTE) map. Residual Magnetic Intensity (RMI) map of the study area was obtained after the removal of regional trend from the RTE data. Regional-residual separation of the RMI map was carried out using upward continuation filtering technique adopting the depths obtained from spectral analysis to produce magnetic anomaly maps associated with the basement and intra-sedimentary magnetic sources. The maxima of the Horizontal Gradient Magnitude (HGM) of the basement and intra-sedimentary magnetic anomaly maps were computed and used to delineate faults that produced the structural maps of the basement and the intra-sedimentary column, respectively. Upward continuation of the RMI map at various altitudes and the maxima of their HGM were used to highlight faults from shallow to deep depths, as well as their strikes and dips. Both major and minor faults dominated the study area. The faults strike in the directions NE-SW (Pan-African trend), ENE-WSW, NW-SE, and E-W. Two profiles were drawn on the basement and intra-sedimentary maps, respectively, to model the subsurface structures. The results of this study revealed that the sedimentary section was affected by the tectonics of the underlying basement, with faults propagating from the basement upwards into the sedimentary cover. These faults constitute potential structural traps for oil accumulation or conduit for oil migration.

Determination of the Structural Lineaments in the Kribi-Campo-Ma’an Area from a Multi-Scale Analysis of Gravity Data Using the HGM and Euler 3D Deconvolution Approaches

With the aim to produce a detailed structural map and then enhance the geological information of the south-western Cameroon, a field gravity study is carried out in the Kribi-Campo-Ma’an area (SW Cameroon). This study area is assumed to undergo pending active tectonics as witnessed by several earthquakes [1]. Besides, the area has been affected by some shears that gave rise to many major faults including the Kribi-Campo Fault (KCF). This N-S lineament is an active fault corresponding to the NW margin of the CC [2] [3] and related to the development of the Kribi-Campo basin. The gravity data analysis enables to highlight many deeply-seated structural features trending in the NE, NNE, N-S, NNW, NW and E-W directions in this region. The NW-SE trend is more strongly developed than the other identified trends and could have deeply affected the major NNE structure. The NNE-SSW to N-S fractures and faults which are local relays of the Kribi-Campo fault are related to the Kribi shear zone. The fairly good clustering observed on local maxima of the horizontal gradient magnitude enables to suggest that the contacts outlined have vertical to subvertical dips. However, a probable interpretation of these features is that they are vertical to subvertical faults which follow the aforementioned directions. The location of the study area in the gulf of Guinea suggests that it sustains tectonics related both to the equatorial and the south Atlantic oceans opening.

Structural and geodynamic study in central Tunisia using field and geophysical data: new structural interpretation of the N–S axis and associated Atlassic structures

A structural and geodynamic evolution of central Tunisia is constructed from field studies and geophysical data within the Tunisian Atlassic domain. Bouguer gravity and horizontal gradient magnitude (HGM) maps indicate that computed anomalies are related to subsurface structures and are in accordance with the general structural trends of central Tunisia. Near Cherichira fault, the strike is ENE–WSW, whereas it is N–S along the N–S axis, a major discontinuity between the Pelagian shelf to the east and Central Atlas Mountains to the west. E–W-oriented HGM profiles on the north side of the Cherichira fault show moderate HGM variations, which correspond to the NE–SW fault system of the Ousselet–Bou Dabbous. This trend is also expressed by the second-order vertical derivative map of this area. However, on the south side of the fault, a dominant peak is indicated rather than the anomalies characteristic of the north side of the fault. This anomaly is associated with the N–S axis discontinuity. Seismic reflection data are also used to elucidate the structural configuration and style north and south of the Cherichira fault. The seismic lines demonstrate that faulting and associate halokinesis in Ousselet–Bou Dabbous area are not comparable to the paleogeographic fault system of the N–S axis southward. To the south of Cherichira fault, the seismic lines show massive Triassic evaporates piercing along the major paleogeographic faults associated with thickness variations of Mesozoic and Cenozoic stratigraphic units and a reverse slip of western structural elements. It seems that the N–S axis corresponds to a steeply dipping major zone of weakness in the Tunisian Atlassic Mountains and indicates evidence of several stages of activity, including the rise of Triassic evaporates rise up which outcrop at several localities such as Rheouis and Mezzouna. The loading and upward rise of Triassic evaporites along this weak zone are a key factor in the thinning of the Mesozoic–Cenozoic sedimentary cover along this paleogeographic discontinuity. These new data indicate the northeast extrapolation of fold and grabens of the Central Atlas of Tunisia to the northeast of Kairouan, whereas the northern extent of the N–S axis is limited to south of the Cherichira fault. The Cherichira fault is NE–SW oriented and parallels the Ballouta–Zaghouan fault system, which reflects the regional strike of the Tellian orogeny and the associated nappes to the north. The geometry and kinematics of these Tellian features are linked to the tectonic evolution of the northern African margin, including Mesozoic extension separating the African and European plates to the inversion of these structures since the Late Cretaceous. The N–S axis is correlated in its direction and tectonic framework to the strike-slip fault system of the African craton, which is present to the south in the Algerian Saharan domain of Hoggar.

Evidence for an east–west regional gravity trend in northern Tunisia: Insight into the structural evolution of northern Tunisian Atlas

The Atlas orogeny in northern Algeria and Tunisia led to the destruction of Tethys oceanic lithosphere and cumulated in a collision of microplates rifted off the European margin with the North African continental margin. The location of the boundary between African plate and Kabylian microplate is expressed in northern Algeria by a crustal wedge with double vergence of thrust sheets, whereas in northern Tunisia the geologic environment is more complex and the location of the plate boundary is ambiguous. In this study, we analyzed gravity data to constrain the crustal structure along the northern margin of Tunisia. The analysis includes a separation of regional and residual gravity anomalies and the application of gradient operators to locate density contrast boundaries. The horizontal gradient magnitude and directional gradient highlight a prominent regional E–W gravity gradient in the northern Tunisian Atlas interpreted as a deep fault (active since at least the Early Mesozoic) having a variable kinematic activity depending on the tectonic regime in the region. The main E–W gravity gradient separates two blocks having different gravitational and seismic responses. The southern block has numerous gravity lineaments trending in different directions implying several density variations within the crust, whereas the northern block shows a long-wavelength negative gravity anomaly with a few lineaments. Taking into account the geologic context of the Western Mediterranean region, we consider the E–W prominent feature as the boundary between African plate and Kabylian microplate in northern Tunisia that rifted off Europe. This hypothesis fits most previous geological and geophysical studies and has an important impact on the petroleum and mineral resource prospection as these two blocks were separated by an ocean and they did not belong to the same margin.

Applications of aeromagnetic data to detect the Basement Tectonics of Eastern Yemen region

The present study aims to throw light on the tectonic implications concerned with the distribution of the sedimentary sequence belts and the related basement complex zones, as well as to differentiate between the causative sources (contacts, dykes and faults) of Eastern Yemen region. The total intensity aeromagnetic map of the study area was first corrected by the application of the Reduction To the magnetic pole (for low latitude areas). The visual inspection of the RTP magnetic map defines a rapid change in the subsurface geologic conditions in the form of lithologic characters and tectonic inferences. On the other hand, this map showed different anomalies of varying frequencies and amplitudes that revealed various causative sources, as well as varying compositions and depths. At the interpretation stage, various techniques and software tools are available for extracting the geologic information from the data concerned. The magnetic fields of shallow sources can be separated from those of deeper causatives, using two processes known as power spectrum transformation and matched band pass filtering. Three methods for locating magnetic sources (Magnitude of Horizontal Gradients (HGM), the analytical signals (AS) and the local wavenumbers (LW)) in three dimensions and identifying the properties of their sources indicated that, the area was affected by some intrusions at various depths in sill or dyke forms, almost oriented in the NW–SE, NE–SW, E–W and N–S trends. Tectonically, the area is highly affected by the tectonics related to the Arabian Sea, Gulf of Aden and Red Sea. It is affecting both the basement and sedimentary rocks, dividing the study area into several faulted blocks.