Magnetic Basement Research Articles

Philippine Island Arc System Tectonic Features Inferred from Magnetic Data Analysis

Running along the middle of the Philippine archipelago from south to north, the Philippine fault zone is one of the world’s major strike-slip faults. Intense volcanism in the archipelago is attributed to the ongoing subduction along the trench systems surrounding it. This study interprets the magnetic data covering the Philippine fault zone and the bounding archipelago subduction systems to understand the structural characteristics of the study area. Magnetic data analysis suggests that the Philippine fault is roughly distributed along the boundary of high/low magnetization and separates the different amplitude features of the first order analytic signal. Visayas province is a specific area bounded by the other parts of the Philippine archipelago. Further differentiating the tectonic units, the proto-Southeast Bohol Trench should be the main tectonic boundary between Visayas and Mindanao. A clear NE - SW boundary separates Luzon from Visayas as shown by the variant depths to the top of the magnetic basement. This boundary could suggest the different tectonic characteristics of the two regions.

Comparison of Magnetic Basement Depth Values from Spectral Technique, SPITH and Slope Techniques using HRAM

The quantitative results obtained from the use of spectral analysis technique, Source parameter Imaging technique and empirical depth rule method (maximum and half slope techniques) in interpreting High Resolution AeroMagnetic of middle Benue trough data have been presented and compared. Before employing each technique, the regional residual seperation was done with least square method using polifit program to get the residual data for onward processing. The slope technique usually require that the residual values be contoured into map and then profiled along the most prominent anomaly to obtain the depth curves from where depth to magnetic sources could be calculated. The Spectral requires that the residual values be sub-divided into spectral blocks and radial energy spectrum be generated from where depth would be calculated from the slopes of the straight line plots. The SPI requires the griding of the residual values and the conversion of this to image map from where depth could be read off easily. The deep magnetic anomally source gotten ranges from 2.00 km to 6.29 km with an average depth of 3.25 km for SPI technique; 2.33 km to less than 5.66 km with an average depth of 3.65 km for Spectral analysis technique; and average of 3.74 km and 3.66 km for Half-slope and maximum slope technique respectively. These average depth could be taken as the magnetic basement depth and has the significant that if other conditions are met, this study area of marine sedimentary layer of the Albian Age is favourable for hydrocation accumulation. The shallow source depth gotten ranges from 0.02 km to 2.00 km with an average depth of 1.08 km for SPI; 0.05 km to less than 0.42 km with an average depth of 0.21 km for spectral analysis; and an average depth 0.80 km for Half slope technique. This can be viewed as magmatic intrussion into the sediment and could be responsible for the lead-Zinc mineralization found in the area. These methods thus compares favourably well and moreso the slope technique though empirical has compared favourably with more modern automatic source depth methods.

Estimation of depth to top of magnetic sources using the local-wavenumber approach in an area of shallow Moho and Curie depth — The Red Sea

The local-wavenumber method estimates the depth to a magnetic source based on the spectral content of a single anomaly assuming that the base of the magnetic body is at infinite depth. However, the “infinite-depth” assumption can lead to significant underestimation of the depth to the top of magnetic bodies, especially in areas where the depth to the bottom of the magnetic layer is not large compared to the depth to the top, as would occur in high heat-flow regions and thinned continental margins. Such underestimation of depths has been demonstrated in model studies and using real data with seismic and well control. We evaluated a modification to the local-wavenumber approach to estimate the depth to the top of magnetic sources assuming that the depth to the bottom of the magnetic sources is controlled by the Curie temperature or crustal thickness. We applied this new method to a simple model of a continental margin and to magnetic survey data over the central Red Sea where the Curie isotherm is shallow. The effective structural index of this finite depth extent model is found to increase continuously from the continent to the ocean as the depth to the magnetic basement increases and the depth to the bottom of the magnetic layer decreases. We have also discovered in this study that the local-wavenumber maxima correlate well with major seafloor spreading magnetic reversal epochs in the central Red Sea segment.

A geophysical view of the Southeastern Brazilian margin at Santos Basin: Insights into rifting evolution

This study investigates the rifting structures of Santos Basin at the Southeastern Brazilian margin, based on an integrated geophysical approach. Our aim is to constrain the crustal basement topography of central and northern Santos basin, the presence of magmatism and the role of inherited structures in space and time through the rifting processes. We present a new high resolution aeromagnetic dataset, which in correlation with gravity anomalies enables us to interpret the tectonic trends and crustal basement structures. We calculated the magnetic basement depth for the central and northern Santos Basin using power spectrum analysis. The obtained depths range between 2 and 9 kms, and are comparable with results from previous works. From our integrated study, three margin domains could be identified, which display distinct rifting structures and are characterized by important lateral variation along the margin. The proximal domain displays trends and magnetic basement blocks NE–SW oriented, i.e., parallel to inherited onshore crustal basement with an inflexion to E–W oriented trends; the necking domain is characterized by oblique magnetic basement highs and lows (E–W and NW–SE) and a structural trend change. The trends and magnetic basement highs are bounded by NW–SE negative anomalies, interpreted as transfer zones. Oceanwards at the distal domain, the lineaments and transfer zones show a progressive structural inflexion to ENE and E–W, sub-parallel to adjacent South Atlantic Fracture Zones. The observed crustal basement architecture and segmentation suggest the reactivation of pre-rift structures at the proximal margin and the obliquity of rifting relative to them. From the proximal domain oceanwards the structural pattern may reflect the passage from a “continental type” domain, where lithospheric inheritance controls the deformation, to a distal margin where this influence diminishes and “new” structural trends are formed. We propose that northern Santos Basin show evidences of an intensely deformed zone, where rift evolved under oblique extension, similar to that observed at transform margin segments.

Mapping the depth to magnetic basement using inversion of pseudogravity: Application to the Bishop model and the Stord Basin, northern North Sea

Magnetic depth estimation methods are routinely used to map the depth of sedimentary basins by assuming that the sediments are nonmagnetic and underlain by magnetic basement rocks. Most of these methods generate basement depth estimates at discrete points. Converting these depth estimates into a grid or map form often requires the application of qualitative methods. The reason for this is twofold: first, in deeper parts of basins, there is generally a scarcity of depth estimates and those that have been determined tend to be biased toward the shallower basement structures close to the basin edge; and second, depth estimates intrinsically relate to magnetic anomalies that emanate from the top edges of basement faults/contacts resulting in a shallow depth bias. Thus, simple grid interpolation of these depth estimates often forms a shallower and structurally unrepresentative map when evaluated in detail. To overcome these problems of qualitative and/or simple grid interpolation of these point-depth estimates into a regular grid, we use the pseudogravity field transform response of the magnetic field to constrain this interpolation using inversion methods together with the relationship between the point-depth estimates and their pseudogravity values. The pseudogravity transformation converts a grid of magnetic data such that the resulting grid has the same simple relationship to magnetic susceptibility that a gravity grid has to density. The pseudogravity map is thus straightforward to visualize in terms of basement structure, but it only maps the magnetic properties of the subsurface and is not related to the gravity anomaly or the density. We describe a practical approach to invert pseudogravity grids using gravity inversion software to produce a 3D basin model assuming a constant susceptibility basement. The approach is initially tested on the Bishop 3D model and then applied to an example from the northern North Sea. This approach can be considered complementary to 3D gravity inversion and has the advantage that the pseudogravity response is not affected by structure within the sediments or effects such as sediment compaction, inversion, or isostatic compensation, all of which often complicate the gravity response of sedimentary basins.

Crustal and basin evolution of the southwestern Barents Sea: From Caledonian orogeny to continental breakup

AbstractA new generation of aeromagnetic data documents the post‐Caledonide rift evolution of the southwestern Barents Sea (SWBS) from the Norwegian mainland up to the continent‐ocean transition. We propose a geological and tectonic scenario of the SWBS in which the Caledonian nappes and thrust sheets, well‐constrained onshore, swing from a NE‐SW trend onshore Norway to NW‐SE/NNW‐SSE across the SWBS platform area. On the Finnmark and Bjarmeland platforms, the dominant inherited magnetic basement pattern may also reflect the regional and post‐Caledonian development of the late Paleozoic basins. Farther west, the pre‐breakup rift system is characterized by the Loppa and Stappen Highs, which are interpreted as a series of rigid continental blocks (ribbons) poorly thinned as compared to the adjacent grabens and sag basins. As part of the complex western rift system, the Bjørnøya Basin is interpreted as a propagating system of highly thinned crust, which aborted in late Mesozoic time. This thick Cretaceous sag basin is underlain by a deep‐seated high‐density body, interpreted as exhumed high‐grade metamorphic lower crust. The abortion of this propagating basin coincides with a migration and complete reorganization of the crustal extension toward a second necking zone defined at the level of the western volcanic sheared margin and proto‐breakup axis. The abortion of the Bjørnøya Basin may be partly explained by its trend oblique to the regional, inherited, structural grain, revealed by the new aeromagnetic compilation, and by the onset of further weakening later sustained by the onset of magmatism to the west.

Source Depth Determination from Aeromagnetic Data of Ilesha, Southwest Nigeria, Using the Peters’ Half Slope Method

The slope method is one of the earliest magnetic depth estimate techniques which can determine the depth to the top of magnetic basement. Reduction-to-pole was applied to the aeromagnetic data of Ilesha southwest, Nigeria with sheet number 243. The data which was originally partitioned into different profiles was later processed to generate reduction-to-pole graph. The contour map of the study area was partitioned into four quadrants, with the highest magnetic anomaly occurring at the third quadrant (southwest region), while the northwest region has the lowest magnetic anomaly. The depth to top for very thin body, intermediate body and very thick body was determined using the Peter’s half slope method. These values range between 2.40 km and 10.60 km for the very thin body, 1.8 km to 7.93 km for the intermediate body and 1.41 km to 6.35 km for the very thick body. These results revealed the extent of the depth of the source of the magnetic anomaly that was produced by the aeromagnetic data of the study area.

Basin architecture from high-resolution gravity gradient, magnetic, and seismic data, King Sound, Canning Basin, Western Australia

An interpretation of geologic structure at King Sound in the Canning Basin was completed using airborne gravity gradient, magnetic, and seismic data. During the Late Devonian and Mississippian periods, the elevated part of the basement in the north was rimmed by carbonate reefs and redeposited carbonate debris, whereas in the south, siliciclastic submarine fans and turbidites were deposited along the margin of the basement in a deep-marine environment. Three principal lithologic units were identified from the vertical gravity gradient (GDD) in the basin: (1) the Fairfield Group carbonates of high density are interpreted to be the source of prominent positive gravity anomalies; (2) forereef debris and carbonate clastics reworked from carbonates higher up the slope or from the carbonate platform are interpreted to be the source of medium-density responses; and (3) turbidites, debris flows, and associated clastic basinal sequences of low density are interpreted to be the source of prominent negative gravity anomalies. Depth slices of GDD indicate the channelized nature of turbidite flows. In the lower section of the basin, intrasedimentary intrusives were identified from magnetic, GDD, seismic, and well data. Depth to magnetic basement calculation indicates that the surface of the Archean to Paleoproterozoic basement ranges from 3200 to 130 m (10,499–427 ft) below sea level. The northwest- and northeast-oriented south-dipping faults cut the basement and propagate upward into the sediments. A three-dimensional geologic model constructed for King Sound satisfies all known geologic constraints and is consistent with the gravity, magnetic, seismic, and well data.

Study of Residual Basin and Tectonolayering Based on Airborne Gravity and Magnetic Data

Abstract:This paper takes the South Yellow Sea as an example to show a new method for comprehensive geological‐geophysical research such as residual basin and tectonolayering using airborne gravity and magnetic data in China. Based on airborne gravity and magnetic data, by measuring and analyzing stratigraphic density and susceptibility, the depths to the pre‐Sinian magnetic basement top, the pre‐Jurassic top and the Cenozoic bottom, are obtained by forward and inverse methods constrained by seismic and drilling data; and furthermore, the residual thicknesses of the Sinian–Triassic, the Jurassic–Cretaceous and the Cenozoic are calculated. Based on airborne gravity and magnetic anomalies, the faults in the pre‐Sinian magnetic basement, the Jurassic‐Cretaceous and the Cenozoic are respectively interpreted by the qualitative and quantitative methods. On the basis of the above study, and combining regional important tectonic events, four tectonolayers are divided in the vertical succession in South Yellow Sea, namely the pre‐Sinian magnetic basement, the Sinian‐Triassic, the Jurassic‐Cretaceous and the Cenozoic. The result shows that there are thick Cenozoic and Jurassic‐Cretaceous strata and thin residual Sinian‐Triassic strata in the Suzhong‐Huangnan depression area, but there are thin or only sporadic Mesozoic‐Cenozoic terrestrial strata and thick Sinian‐Triassic marine strata reserved in Subei‐Huangzhong uplift area and Sunan‐Wunansha uplift area. The four tectonolayers are very different in structures as well as distributions in plane.

Localization of temperature anomalies in the Upper Rhine Graben: insights from geophysics and neotectonic activity

The European Cenozoic Rift System hosts major temperature anomalies in Central Europe. In its central segment, the Upper Rhine Graben (URG), temperatures range from 75°C to nearly 150°C at a depth of 2000 m. Different hypotheses have been suggested to explain the localization of these anomalies. Our review and comprehensive interpretation of gravimetric and magnetic data, as well as neotectonic activity patterns, suggests that low-density, mostly magnetic and fractured granitic basement is systematically associated with major temperature anomalies. Further analyses provide insight into different heat transport processes contributing to the localization of these anomalies. Magnetic and gravity anomalies are known to represent lithological variations associated with the pre-Permian. We show their spatial relationship with positive temperature anomalies in the URG. Correlation between magnetics and temperature reveal a mean contribution of heat production to the temperature anomaly of about 10–15°C. A slightly higher mean value is obtained from correlation between gravity and temperature, which may be attributed to effects resulting from fracture porosity. The spatial relationship between temperature anomalies and neotectonic patterns indicates compressional shear and uplift regime for the major anomalies of the central segment of the URG. This is in agreement with different numerical models indicating free convection on fracture zones linked to faults. Our findings show that about 15–25% of the temperature anomaly can be attributed to variation in heat production. Hydrothermal circulation convection along faults, activated by the tectonic context, may explain the remaining 75–85% of the temperature anomalies.

Fast mapping of magnetic basement depth, structure and nature using aeromagnetic and gravity data: combined methods and their application in the Paris Basin

ABSTRACTAssessment of deep buried basin/basement relationships using geophysical data is a challenge for the energy and mining industries as well as for geothermal or CO2storage purposes. In deep environments, few methods can provide geological information; magnetic and gravity data remain among the most informative and cost‐effective methods. Here, in order to derive fast first‐order information on the basement/basin interface, we propose a combination of existing and original approaches devoted to potential field data analysis. Namely, we investigate the geometry (i.e., depth and structure) and the nature of a deep buried basement through a case study SW of the Paris Basin. Joint processing of new high‐resolution magnetic data and up‐to‐date gravity data provides an updated overview of the deep basin.First, the main structures of the magnetic basement are highlighted using Euler deconvolution and are interpreted in a structural sketch map. The new high‐resolution aeromagnetic map actually offers a continuous view of regional basement structures and reveals poorly known and complex deformation at the junction between major domains of the Variscan collision belt.Second, Werner deconvolution and an ad hoc post‐processing analysis allow the extraction of a set of magnetic sources at (or close to) the basin/basement interface. Interpolation of these sources together with the magnetic structural sketch provides a Werner magnetic basement map displaying realistic 3D patterns and basement depths consistent with data available in deep petroleum boreholes.The last step of processing was designed as a way to quickly combine gravity and magnetic information and to simply visualize first‐order petrophysical patterns of the basement lithology. This is achieved through unsupervised classification of suitably selected gravity and magnetic maps and, as compared to previous work, provides a realistic and updated overview of the cartographic distribution of density/magnetization of basement rocks.Altogether, the three steps of processing proposed in this paper quickly provide relevant information on a deep buried basement in terms of structure, geometry and nature (through petrophysics). Notwithstanding, limitations of the proposed procedure are raised: in the case of the Paris Basin for instance, this study does not provide proper information on Pre‐Mesozoic basins, some of which have been sampled in deep boreholes.

Intraplate Transtensional Tectonics in the East Antarctic Craton: Insight from Buried Subglacial Bedrock in the Lake Vostok—Dome C Region

This study presents the results of forward numerical models of a series of sections of the Aurora Trench (East Antarctica) derived from radio echo-sounding data that allowed to reconstruct the 3D shape of the Aurora Fault, a crustal listric normal fault characterized by a length exceeding 100 km. A similar extensional fault setting allows to replicate the asymmetric buried morphology of the magnetic basement at the Lake Vostok depression derived by the available gravity and magnetic profiles. Both the Aurora and Vostok listric fault reach their basal decollment at 34 km depth, possibly the base of the crust in this intracratonic environment. Integration of these results with the existing geologic interpretations of the tectonic origin of the Concordia Trench by normal faulting allowed to frame the Concordia, Aurora and Vostok normal faults within an intraplate transtensional corridor with a left-lateral movement component. The westward projection of the proposed strike-slip deformation belt may develop in correspondence of an older tectonic lineament stretching from the Eastern flanks of the Gamburtsev Subglacial Mts to the Lambert rift and characterized by a poly-phased complex tectonic history. The possible Cenozoic reactivation of these structures is discussed in the paper.

An iterative approach to optimising depth to magnetic source using the spectral method

Knowledge of the depth of cover is poor across large areas of Australia. The spectral method is an efficient method of producing reliable depth to magnetic basement estimates across large regions of the continent. A semi-automated work-flow has been created that enables the generation of depth to magnetic source estimates from windowed magnetic data using the Spector and Grant method. The work-flow allows for the correction of the power spectra, prior to the picking of straight-line segments, to account for the fractal distribution of magnetic sources. The fractal parameter (s) varies with depth and was determined by picking multiple depth estimates in regions of outcropping magnetic basement which have been upward continued to different levels in order to simulate different amounts of burial beneath non-magnetic sediments. A power law function best approximates the decay of s with depth. An iterative schema used to determine the optimum s where the depths of magnetic sources are unknown, has been incorporated into the workflow. Preliminary testing in a region of known magnetic basement depth has produced encouraging results, although further testing is required. The decrease of s with increasing depth suggests that the fractal distribution of magnetisation becomes less correlated, or fractal, over larger volumes of observation.

Mapping basement relief of Abu Gharadig Basin, Western Desert of Egypt using 3D inversion of pseudo-gravity data

There are a number of magnetic inversion methods that have been developed to map the structure and depth of sedimentary basins, assuming that sediments are nonmagnetic and underlain by magnetic basement. Gridding/mapping the basement depth estimates from such methods has two significant problems: 1) the magnetic results are dominated by signals coming from the top edges of basement faults such that the depth information from the down thrown sides of the faults is not captured and 2) within the centres of large basins there is often little variation in the magnetic anomaly to provide depth estimates such that when gridding the depth data, grid interpolation has to rely on a sparse distribution of estimated depths. In this study we convert the magnetic data into pseudogravity which is then inverted to produce a 3D basin model with a constant susceptibility basement. This overcomes the interpolation problem since the 3D model now uses the complete pseudo-gravity field as well as the magnetic depth estimates to constrain the depth model. The method is applied to the Abu Gharadig basin, Western Desert, Egypt and generates results which match well controls on basement depth. The advantages of this method over 3D gravity inversion are that the pseudogravity response is not affected by structure within the sediments and is not compensated isostati

Aeromagnetics assists Oil & Gas exploration in the Bedout Sub-basin, offshore Canning Basin

Finder Exploration and its joint venturer Carnarvon Petroleum commissioned Fugro Airborne Surveys to acquire and interpret 21,622 line kilometres of airborne magnetic data over Petroleum Exploration Permits WA-436-P and WA-438-P in the Bedout Sub-basin, offshore Canning Basin within the Northwest Shelf, Western Australia. These data were acquired to augment the existing sparse 2D seismic data in the study area. The newly acquired aeromagnetic data were integrated with selected seismic profiles, regional structural and geological data; tied to the well data to provide an interpretation of the tectono-structural evolution of the Bedout Sub-basin, a depth to magnetic basement and total sediment thickness estimation in the study area. The interpretation revealed a complex basin architecture reflecting multiple structural and depositional features which contributed to the development of the Bedout Sub-basin. The estimated depths to magnetic basement are consistent with previous assessments of sediment thickness derived from the interpretation of the existing 2D seismic dataset, with the exception of previously unrecognised thick sedimentary deposits interpreted in the eastern part of the study area. The interpretation of the aeromagnetic data shows variations in sediment thicknesses of up to 5kms over relatively short lateral distances in the vicinity of the Bedout High in the northern part of the study area. A significant shallowing of the basement towards the Lambert Shelf in the south is also identified. As part of the ongoing exploration effort, this new aeromagnetic data and interpretation has added to the understanding of the study area by identifying a possible additional hydrocarbon source kitchen and improving the tectono-structural understanding of the Bedout Sub-basin.

Geological Interpretation of Heliborne Geophysical Data from the Kaladgi Basin; Southern India

Geological interpretation from the Kaladgi Basin is based on integration of heliborne time domain electromagnetic (HELITEM), magnetic and gamma-ray spectrometry data with available geological mapping, ground exploration data, various published papers and spectral remote sensing data such as Landsat 7ETM+ and ASTER GDEM imagery. Electromagnetic (EM) data, magnetic, radiometric, Landsat TM and Aster datasets were used for delineation of surface and subsurface extent of geological units and structural elements and for better understanding of the overall structural pattern of the Kaladgi Basin, including distribution of its local depocentres, the resistive basement surface and the magnetic basement topography. The interpretation of the available data details the localisation of intrusive bodies, basaltic lava flows, mafic dykes, basement faults and major intrabasinal fault systems. Three dimensional grids (voxel models) of EM data were used for visualization in three dimensions.

Depth to magnetic sources in the offshore northern Perth Basin

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

Geological Milieu of the Bijawar Basin based on interpretation of geophysical data; Central India

SummaryIntegrated interpretation through physical properties of rocks provides better approach towards understanding of geological set-up. High resolution heliborne time domain electromagnetic, magnetic and radiometric dataset acquired over Palaeo Proterozoic Bijawar Basin has been interpreted to enhance the geological understanding. The process involved knowledge driven analysis through correlation with geological units that outcrop and data driven analysis for extrapolation of information. Radiometric data in conjunction with spectral remote sensing data was used for generation of surface geology map. Spatial distribution of rock magnetization properties defines the subsurface extension of litho-structural elements and magnetic basement configuration. Electromagnetic data aided demarcation of resistive basement topography and various conductive layers. Radio-elemental distribution refines the unconformable contact shared by Bijawar Group with overlying and underlying group of rocks and also outlined the formational boundaries of arenaceous-argillaceouscarbonate-volcano-sedimentary sequence. Subsurface extent of basic volcanics is much more than its surface manifestation and appears to extend well below the overlying Vindhyan Supergroup of rocks. The unconformable contact of Bijawar Group with underlying basement and overlying Vindhyan Supergroup is marked by distinct change in conductivity parameter and has been outlined in depth to resistive basement topography. Fault system and its manifestation over different litho-units indicate multiple tectonic episodes.

Petroleum potential of the interpreted Paleozoic geoseismic sequences in the South Diyur Block, Western Desert of Egypt

ABSTRACT The South Diyur exploration block of nearly 38,000 sq km is located in the Farafra Oasis region in the Western Desert of Egypt. It is a frontier exploration area, the nearest well being Ammonite-1, a dry hole drilled by Conoco in 1979 immediately outside the southwestern corner of the block. The South Diyur Block is located on the probable northeast extension of the Kufra Basin in southeast Libya. Although prolific reserves of oil and gas occur in Paleozoic basins in North Africa and throughout the Middle East, to date, the targets for petroleum exploration in the northern Western Desert have been in Jurassic and Cretaceous rocks. The regional structural surface features in the South Diyur Block are the NE-trending Bahariya and Farafra anticlines interpreted as a deeply eroded and inverted Late Cretaceous structure on the southern extension of the Syrian Arc system. The oldest exposed rocks are a Cretaceous sequence of sublittoral sediments (the Campanian Wadi Hennis Formation) in the core of the anticline. The interpretation of the subsurface is based on 1,175 line-km of reprocessed 1970s-vintage 2-D seismic. Four sequence boundaries have been identified from the seismic data. SB-1 correlates with the Jurassic/Cretaceous boundary in Ammonite-1. SB-2 is regionally correlated with the Late Devonian to Early Carboniferous Hercynian unconformity that overlies deeply eroded and truncated Paleozoic sequences and possibly marks the regionally extensive Late Paleozoic basin inversion. SB-3 near the base of the interpreted Silurian sequence coincides with the ‘hot shale’ petroleum source rock that is present throughout North Africa and the Middle East. SB-4 is interpreted as a major unconformity at the top of an Upper Proterozoic sedimentary section that was misinterpreted as the Precambrian acoustic basement in Ammonite-1. Five seismic sequences relate to the seismic boundaries. SS-1, from the surface to SB-1 is characterized by subparallel seismic stratification and is composed mainly of sandstone with shale interbeds in Ammonite-1. SS-2, bounded by SB-1 and SB-2, is distinguished by parallel to subparallel seismic stratification. In Ammonite-1, the sequence of interbedded sandstone and shale is fresh-water bearing and lacking in top seals, thus reducing its prospectivity. The underlying SS-3 (SB-2 to SB-3) directly underlies the Hercynian unconformity and is characterized by semi-transparent seismic facies that may correspond to a thick Silurian shale sequence. SS-4 (SB-3 to SB-4) of probable Cambrian–Ordovician age has parallel seismic stratification. Deep channels are interpreted as evidence of a Late Ordovician–Early Silurian glacial phase that is present throughout North Africa and the Middle East. SS-5 (below SB-4) is marked by partial subparallel seismic stratification and block faulting. It probably belongs to the Late Proterozoic (Pan-African) phase of block faulting and pull-apart basins. Similar seismic geometries and facies occur in the Kufra Basin in southeast Libya and in many parts of the Arabian Plate, including the prolific petroleum systems of Oman. Exploration plays in the South Diyur Block are a combination of Paleozoic structural and stratigraphic traps associated with prospective fairways, and possible stratigraphic traps in the Late Ordovician–Early Silurian glacial channels. In addition, the interpreted Late Proterozoic sequences (SS-5) warrant further evaluation. In order to identify future exploration plays and drill targets, additional 2-D seismic (4,490 line-km), aeromagnetic and airborne gravity surveys will be integrated with the present seismic data and drilling results from Ammonite-1. This will allow a proper assessment of the magnetic basement, basin configuration and prospective fairways.

MATLAB code for estimating magnetic basement depth using prisms

There is a need, within both geophysical exploration and deep geophysical research, to estimate magnetic basement depth. Forward and inverse modeling studies to map the basement depth are commonly used within petroleum geophysics. To obtain the basement topography, modeling studies are made of the 2D profile data or 3D map data.In this study, a different algorithm was introduced to estimate the magnetic basement depth from map data. The algorithm is based on the analytical solution of exponential equations obtained from Fourier transformation of magnetic data. This algorithm has been tested on synthetic magnetic anomalies originated from multi-prisms. Following encouraging test results, the proposed algorithm was also tested on field data. The depths obtained from the proposed approach were satisfactory in comparison with the depths obtained from seismic survey cross-sections and boreholes. Basic MATLAB code is included in the Appendix.