Deep Magnetic Sources Research Articles

Improved Mapping of the Structural Disposition of Some Younger Granite Ring Complexes of Nigeria Using High Resolution Aeromagnetic Data

Surface and subsurface structural interpretation over a part of the Younger Granite province of Northern Nigeria using Aeromagnetic data was carried out to demonstrate the use of recently available high resolution aeromagnetic data in mapping precisely the Younger Granite. State of the art software packages were used in data reduction, processing/analysis and interpretation. Results of the aeromagnetic study show that the distinct magnetic anomalies clearly revealed in the area coincide spatially with outcropping ring complexes. Apart from outcropping ring complexes, some ring complexes without surface exposures (unexposed ring complexes) were also delineated. One of such ring complexes was studied in greater details and its lateral dimensions with that of the adjoining ring complexes were better resolved. The unexposed ring complex was found to have a surface area extent of approximately 169.5 km2 which is similar to that of Ririwai (180 km2), Banke (128 km2), Kudaru (174 km2) and Zuku (121 km2). The results of the 3D Euler solutions indicate that the depths of the ring complexes range from outcrop to 1800 m. For the inferred unexposed ring complex, the depth ranges from 5 to 1200 m. The Younger Granites show low Source Parameter Imaging (SPI) depth values of about 4 km below the sea level indicating deeper magnetic sources. Three (3) profiles drawn along the NESW, NNW-SSE and E-W directions on the Analytic Signal map of the study area gave an overburden thickness of less than 20 m that masked the unexposed ring complex with its mappable root situated at approximately 200 m below the sea level. The Ririwai, Tibchi, Banke, Amo, Kudaru and Zuku ring complexes have outcrops of approximately 1300 m, 950 m, 920 m, 1400 m, 1150 m and 1220 m respectively above mean sea level with their mappable roots situated at approximately 400 m, 180 m, 400 m, 800 m, 10 m and 300 m respectively below the sea level. It delineated numerous lineaments as structural features whose major trend is along the NE-SW direction. It was equally observed that the ring complexes of Northern Nigeria display diverse surface plans/shapes and exhibit both inward and outward dipping structural orientations with depth. The likely reason for the unexposed nature of the newly proposed ring complex could be as a result of partial subsidence of the enclosed block on which the extrusive edifice of the central volcano originally rested probably occasioned by excess space (much subsurface accommodation) for continuously rising magma within the country rock. Its geometry is supportive of this assertion as it revealed majorly outward dipping trend with depth along different axis. Another reason could be due to incomplete fluidization along the already formed ring fracture and poor assimilation of the country rock by the fluidizing agent amidst a slow/near quiescent piece meal stopping process during the central granite intrusion phase which usually marks the end of ring complex formation. Finally, this research has demonstrated the efficacy of aeromagnetic data in delineating both outcropping ring complexes as well as those without surface exposures (outcrops).

Estimation of depth to magnetic source bodies of Nsukka and Udi areas using spectral analysis approach

The study was aimed at estimating the depth to magnetic source bodies in Nsukka and Udi areas which falls within the Anambra basin. Aeromagnetic data were used and Spectral Analysis approach was employed in the quantitative interpretation. The total magnetic intensity (TMI) contour map which ranges from -53.1 to 133.7 nanoTesla (nT) was separated into regional and residual contour maps to produce a residual aeromagnetic intensity contour map. The residual intensity varies from -51.2 to 59.6 nT while the regional intensity varies from -8.7 to 85.1 nT. Depth results obtained from spectral analysis revealed two depth sources; the shallower magnetic source bodies and the deeper magnetic source bodies. The depth of shallower magnetic sources ranges from 314.12 to 1061.05 m with an average depth value of 632.32 m, whereas the depth of deeper magnetic sources vary from 1465.90 to 5888.73 m with an average depth value of 3260.41 m. Key words: Magnetic source bodies, depth estimation, spectral analysis, Nsukka and Udi areas, aeromagnetic data

Magnetic anomalies and the features of geodynamic development of the Antarctic Peninsula continental margin structures

New geophysical models of the Earth's crust were used to study the possible nature of the Pacific Margin Anomaly (PMA) near the Antarctic Peninsula (AP). The data have been presented, which permitted analyzing spatial distribution of PMA and its deep sources on the continental margin of AP. Western and Eastern PMA-branches may correspond to different crustal blocks associated with predominantly Cretaceous magmatic intrusions of basic rocks. These branches of PMA were formed in the Early Cretaceous in structures, separated by a fault zone. Spatial heterogeneity of different segments of РМА may also be connected with different depth, thickness and magnetic susceptibility of separate bodies, which form the sources of regional anomalies. Active tectonic processes in the Mesozoic — Cenozoic resulted in the complex evolution of the structures of the region and appearance of vast deep magnetic sources of PMA along the AP margin formed during the phases of magmatic activity. Formation of separate segments of PMA can be connected with processes of tectonic movements near the border of the plates (Antarctica and Scotia) as well as in the areas of paleorifts formation. New geophysical results for different PMA-segments (from the Palmer Land to Powell Basin) replenish current ideas on the sources and origin of this big positive magnetic anomaly of the Pacific coast and show the necessity of searching new mechanisms of forming the structures of continental margin of AP

Magnetic investigation and 2½ D gravity profile modelling across the Beattie magnetic anomaly in the southeastern Karoo Basin, South Africa

The southeastern Karoo Basin is considered to be one of the most prospective areas for shale gas exploration in South Africa. An interesting magnetic anomaly, the Beattie magnetic anomaly (BMA), and geologic intrusions are seen on the magnetic map. To date, the source of the BMA and interconnectivity of the igneous intrusions are not well understood. In this study, we investigate the interconnectivity of the igneous intrusions and possible location of the source of the BMA using gravity and magnetic methods. The gravity model results showed that igneous intrusions are interconnected at depth, which probably pose threat by increasing the risk of fracking the Karoo for shale gas exploration. The magnetic results revealed that the BMA becomes stronger with depth. The average depths to the top of the shallow and deep magnetic sources were estimated to be approximately 0.6 and 15 km, respectively.

The deep crust beneath the Trans‐European Suture Zone from a multiscale magnetic model

AbstractWe present a 3‐D interpretation of the deep magnetic sources beneath the main geological structure of central‐eastern Europe, the “Trans‐European Suture Zone” (TESZ). We used a multiscale analysis of aeromagnetic data, based on a multiscale data set generated by upward continuation of the European and Mediterranean Magnetic Project data set from 5 to 100 km altitude. We also computed the multiscale total gradient |∇T| of the multiscale field. Both of the multiscale data sets allow us to discriminate the main crustal contributions to the field at various scales, and we showed that at large altitudes the field is dominated by the sole effect of the TESZ. The multiridge geometric method was very useful in studying the complex features of the main crustal interfaces, either shallow or deep. The interpreted interfaces are largely agreement with geological models based on seismic surveys and, in some cases, complete the models out of the analyzed region. In order to estimate the deepest source depths in the TESZ region, we applied the multiridge method to the large scales (50–100 km altitude), obtaining a set of singular points at depths ranging between 35 and 40 km. Considering the heat flow trend and the geological models around the TESZ area, we found a meaningful correspondence among the location of the estimated singular points and the most abrupt variations and complex morphology features of the Moho boundary. The multiridge estimates are consistent with known structural information and can be used for a 3‐D representation of the Moho depth along the TESZ.

Ground Magnetic, GPR, and Dipole-Dipole Resistivity for Landfill Investigation

Ground magnetic, ground penetrating radar (GPR), and dipole-dipole resistivity were carried out to environmentally investigate a landfill. In this context, these geophysical techniques were conducted to identify the subsurface contents of the landfill, furthermore, specify any possible leakage and/or contamination in the study area. The ground-magnetic survey carried out in the study area comprised 31 profiles each 120 m in length. Different wavelength filters were applied to the measured data. Vertical derivative, downward continuation, apparent susceptibility, band-pass, and analytical signal filters separated successfully the shallow sources. Whereas, upward continuation and low-pass Gaussian filters isolated significantly the deep magnetic sources. 3D Euler deconvolution (SI = 3) remarkably estimated the depths of the shallow sources (0 - 10 m) of the landfill contents. The conducted GPR and dipole-dipole resistivity allocated tangibly the locations and depths of the near surface anomalies. Both techniques didn’t reveal any possible leakage and/or contamination. Noteworthy, integration among magnetic, GPR, and dipole-dipole resistivity confirmed positively the results of each method. Nevertheless, some anomalies were recognized successfully by one technique and not by the others.

2-D Spectral Analysis of Aeromagnetic Anomalies over Parts of Monguno and Environs, Northeastern Nigeria

2-D Spectral Analysis of aeromagnetic anomalies over parts of Monguno and environs, northeastern Nigeria was carried out to determine the depth to basement, establish the basement topography and determine the temperature at depths in the area. Data enhancement was carried out to delineate residual features and the more intense anomalies relative to the strong regional gradients. Typical techniques applied were computations of reduction to pole, trend surface analysis, and a number of filtering processes. Results of the 2-D spectral analysis of the aeromagnetic data revealed a two depth source model. The depth to the deeper magnetic source bodies ranges from 1.22 to 3.14 km, with an average depth of 2.341 km. This layer may be attributed to magnetic rocks of the basement, lateral variations in basement susceptibilities and intra- basement features like faults and fractures. The shallower magnetic sources range in depth from 0.12 to 0.93 km, with an average depth of 0.572 km and could be attributed to near surface magnetic sources, which are magnetic rocks that intruded into the sedimentary formations. Based on the computed average sediment thickness obtained in this study together with the temperature at depths in the area, some parts of the study area have been demarcated for detail hydrocarbon exploration in-line with the renewed interest in hydrocarbon exploration in the basin.

The tectonic evolution of the Transbrasiliano Lineament in northern Paraná Basin, Brazil, as inferred from aeromagnetic data

AbstractData from six airborne magnetic surveys were compiled and analyzed to develop a structural interpretation for the Transbrasiliano Lineament in northern Paraná Basin, Brazil. Magnetic lineaments, interpreted to reflect geologic contacts and structures at different depths, were illuminated using the matched‐filter technique applied to aeromagnetic anomalies. Field‐based structural measurements generally support our magnetic analysis. We estimated three primary magnetic zones: (i) a zone of deep magnetic sources at 20 km depth, (ii) an intermediate basement zone at 6 km depth, and (iii) a shallow zone of near‐surface geological features at 1.5 km depth. The deepest zone exhibits three major NE trending crustal discontinuities related to the Transbrasiliano Lineament, dividing the region into four geotectonic blocks. Anomalies associated with the intermediate zone indicate directional divergence of subsidiary structures away from the main Transbrasiliano Fault, which strikes N30°E. The shallow magnetic zone includes near‐surface post‐Brasiliano orogenic granites. Our analysis identified NE trending sigmoidal lineaments around these intrusions, indicating intense zones of deformation associated with probable shear structures. At the shallow depth zone, magnetic anomalies caused by Cretaceous alkaline intrusive bodies and basalts of the Serra Geral Formation are enhanced by the matched‐filter method. These igneous bodies are related to extensional NW striking lineaments and seismicity aligned along these lineaments suggests that they currently are reactivated. Prior to flexural subsidence of the Paraná Basin, reactivation processes along transcurrent elements of the Transbrasiliano Lineament promoted extensional processes and formed initial Paraná Basin depocenters. Cretaceous and more recent sedimentation also correlate with reactivations of NE striking structures.

Gravity and magnetic data analysis of block-35, Jiza' basin, Eastern Yemen

The Jiza' basin is located in the eastern part of Yemen, trending generally in the E–W direction. It is filled with Middle Jurassic to recent sediments, which increase in thickness approximately from 3,000 m to more than 9,000 m. In this study, block-35 of this sedimentary basin is selected to detect the major subsurface geological and structural features characterizing this basin and controlling its hydrocarbon potentials. To achieve these goals, the available detailed gravity and magnetic data, scale 1:100,000, were intensively subjected to different kinds of processing and interpretation steps. Also, the available seismic reflection sections and deep wells data were used to confirm the interpretation. The results indicated three average depth levels; 12.5, 2.4, and 0.65 km for the deep, intermediate, and shallow gravity sources and 5.1 and 0.65 km for the deep and shallow magnetic sources. Accordingly, the residual and regional anomaly maps were constructed. These maps revealed a number of high and low structures (horsts and grabens and half grabens), ranging in depth from 0.5 km to less than 4.5 km and trending mainly in the ENE, NW, and NE directions. However, the analytical signal for both gravity and magnetic data also showed locations, dimensions, and approximate depths of the shallow and near surface anomaly sources. The interpretation of the gravity and magnetic anomalies in the area indicated that the NW, NNW, ENE, and NE trends characterize the shallow to deep gravity anomaly sources; however, the NE, NW, and NNE trends characterize the magnetic anomaly sources, mainly the basement. Two-dimensional geologic models were also constructed for three long gravity anomaly profiles that confirmed and tied with the available deep wells data and previously interpreted seismic sections. These models show the basement surface and the overlying sedimentary section as well as the associated faults.

Potential-field interpretation of the Kars Belt, western NSW

The Cambrian Ponto Group of the Koonenberry Belt is a suite of fore-arc rocks of the Mt Wright Arc, and includes basalts that are associated with copper mineralisation. Magnetic derivative maps allow the Ponto Group to be followed southwest under shallow cover, where they define the magnetic texture of the Kars Zone that flanks the southeastern margin of the Curnamona Craton. Shallow-sourced positive magnetic anomalies in the Kars one are superimposed on a longer-wavelength magnetic low; this contrasts with their equivalents along the Koonenberry Belt, which sit on broad magnetic highs. Gravity images of the Koonenberry Belt suggest that shallow dense sources associated with the Ponto Group overlie deeper mass excesses, while equivalent shallow and deep mass excesses appear to be uncoupled in the Kars Zone. Joint magnetic and gravity modelling of a profile across the Kars Zone suggests that high susceptibility, dense bodies representing the Ponto Group have been displaced to the northwest, over-riding in their path deep magnetic sources below the Menindee Trough. Similar deep magnetic sources below the Bancannia Trough have been interpreted as very large intrusive features forming the base of the Mt Wright Arc. Together, the potential field imagery and modelling suggests very large scale, low-angle thrusting of the Mt Wright fore-arc over a rigid basement of the Mt Wright Arc.

GEOPHYSICAL INTERPRETATION OF THE NATURE AND EXTENT OF THE XADE MAFIC COMPLEX, BOTSWANA

The Xade Mafic Complex, situated in central Botswana, was identified during the first regional aeromagnetic survey of the country in 1975–77. It is covered by sedimentary rocks of the Kalahari Group and Karoo Supergroup, including Karoo lavas, with a combined thickness varying from 220 m to 1000 m, extending to greater depths in the north beneath the Meso-proterozoic Passarge Basin. What has generally been viewed as the Xade Complex is evidenced by a high amplitude kidney-shaped zoned magnetic anomaly with two semi-linear anomalies extending to the northwest and northeast in a Y-shaped form. The complex is also marked by a coincident Bouguer gravity anomaly. Historical work has been limited, with only three diamond drill holes having been drilled, separately intersecting gabbroic rocks, weathered basalt grading into dolerite, and amygdaloidal lavas followed by Waterberg Group shales. An U-Pb zircon age of 1109.0 ± 1.3 Ma, which is coeval with the Umkondo Igneous episode, has been published for the gabbroic unit. This study has shown, for the first time, that the Xade Complex comprises two lobes of potentially different magmatic character, i.e. a Southern Lobe, which is the historically identified kidney-shaped zoned magnetic anomaly, as well as a hitherto unrecognized large Northern Lobe. The latter is mostly deeply buried in the north and northwest beneath the Passarge Basin, as evidenced by deep magnetic and gravity sources, but its southern and eastern margins partially suboutcrop beneath Karoo sediments, forming the Y-shaped anomalies north of the Southern Lobe. Forward modelling, of both magnetic and gravity data, indicates that the Southern Lobe is a lopolith with a depth extent of approximately 4 km, whereas the southeastern margin of the Northern Lobe has westerly dips consistent with deepening to the northwest. In addition to the historic boreholes, a recent borehole drilled into the Southern Lobe shows that it comprises a mafic volcanic sequence with subordinate gabbro. A further two boreholes drilled into the Northern Lobe margins indicate that it comprises a texturally heterogeneous and magmatically differentiated sequence of gabbroic rocks, with minor dioritic and monzonitic rocks. The present interpretation has also identified a dyke system associated with the Northern Lobe, which may represent either feeder or exit magmatic conduits. The interpretation further shows that the complex is located in a craton margin setting. The combined extent of both lobes is approximately one-third the size of the Bushveld Complex, making the Xade Mafic Complex the largest Meso-Proterozoic magmatic system in southern Africa. The craton margin setting, size, Meso-Proterozoic age and variability of plutonic and volcanic rocks hallmark the Xade Mafic Complex as being a very large dynamic magmatic system, with potential for Ni-Cu-PGE mineralization.

Upward Continuation and Reduction to Pole Process on Aeromagnetic Data of Ibadan Area, South-Western Nigeria

A magnetic survey of Ibadan area in the South-Western part of Nigeria was performed as part of the largest airborne geophysical survey blocks flown between 2003 and 2010 by the Nigeria Geological Survey Agency. Two magnetic data enhancement procedures were employed to determine their effectiveness in characterizing the study area. The upward continuation together with reduction to Pole technique significantly improve the interpretation of magnetic data in terms of discriminating between shallow and deep magnetic sources within the study area.

Magnetic recording of the Cenozoic oceanic crustal accretion and evolution of the South China Sea basin

We review and discuss some of the recent scientific findings made on magnetic data in the South China Sea (SCS). Magnetic anomalies bear extremely rich information on Mesozoic and Cenozoic tectonic evolution. 3D analytical signal amplitudes computed from magnetic anomalies reveal very precisely relict distributions of Mesozoic sedimentary sequences on the two conjugate continental margins, and they are also found very effective in depicting later-stage magmatism and tectonic transitions and zonation within the SCS oceanic crust. Through integrated analyses of magnetic, gravity and reflection seismic data, we define the continent-ocean boundary (COB) around the South China Sea continental margin, and find that the COB coincides very well with a transition zone from mostly positive to negative free-air gravity anomalies. This accurate outlining of the COB is critical for better tracing magnetic anomalies induced by the oceanic crust. The geometrically complex COB and inner magnetic zonation require the introduction of an episodic opening model, as well as a transform fault (here coined as Zhongnan Fault) between the East and Southwest Sub-basins, while within the East and Southwest Sub-basins, magnetic anomalies are rather continuous laterally, indicating nonexistence of large transform faults within these sub-basins. We enhance magnetic anomalies caused by the shallow basaltic layer via a band-pass filter, and recognize that the likely oldest magnetic anomaly near the northern continental margin is C12 according to the magnetic time scale CK95. Near the southern continental margin, magnetic anomalies are less recognizable and the anomaly C12 appears to be missing. These differences show an asymmetrical opening style with respect to the relict spreading center, and the northern part appears to have slightly faster spreading rates than to the south. The magnetic anomalies C8 (M1 and M2, ∼26 Ma) represent important magnetic boundaries within the oceanic basin, and are possibly related to changes in spreading rates and magmatic intensities. The magnetic evidence for a previously proposed ridge jump after the anomaly C7 is not clear. The age of the Southwest Sub-basin has yet to be further examined, most favorably with deep-tow magnetic surveys and ocean drilling. Our magnetic spectral study shows that the shallowest Curie points are located around the eastern part of the Southwestern Sub-basin, whereas within the East Sub-basin Curie depths are smaller to the north of the relict spreading center than to the south. This pattern of Curie depths is consistent to regional heat flow measurements and later-stage volcanic seamount distributions, and we therefore reason that Curie-depth variations are closely associated with later-stage magmatism, rather than with crustal ages. Although magnetic anomalies located around the northern continent-ocean transition zone (COT) are relatively quiet, this area is not a typical magnetic quiet zone since conceptually it differs markedly from an oceanic magnetic quiet zone. The relatively quiet magnetic anomalies are seemingly associated with a shallowing in Curie isotherm and thinning in magnetic layer, but our comprehensive observations suggest that the well-preserved thick Mesozoic sedimentary rocks are major causes for the magnetically quiet zone. The high similarities between various low-pass filtered marine and air-borne magnetic anomalies and satellite magnetic anomalies clearly confirm that deeper magnetic sources (in the lower crust and the uppermost mantle) have contributions to long-wavelength surface magnetic anomalies in the area, as already inferred from magnetically inversed Curie depths. The offshore south China magnetic anomaly (SCMA) becomes more prominent on low-pass filtered marine and air-borne magnetic anomalies and satellite magnetic anomalies, indicating very deeply-buried magnetic sources beneath it.

Curie Point Depth Estimates and Correlation with Subduction in Mexico

We investigate the regional thermal structure of the crust in Mexico using Curie Point Depth (CPD) estimates. The top and bottom of the magnetized crust were calculated using the power-density spectra of the total magnetic field from the freely available “Magnetic Anomaly Map of North America”. We applied this method to estimate the regional crustal thermal structure in overlapping square windows of 2° × 2°. The CPD estimates range between 10 and 40 km and show several regions of relatively shallow and deep magnetic sources, with a general inverse correlation with measured heat flow. A deep CPD region (20–30 km) is located in the fore-arc area where the subducting Cocos plate has a flat-slab geometry. This deep region is bound to the NW and SE by shallow CPD areas beneath the states of Michoacan (CPD = 12–16 km) and Oaxaca (CPD = ~16 km), respectively. There is a good spatial correlation between this deep CPD area and two main fracture zones located on the incoming Cocos plate (Orozco and O’Gorman fracture zones), suggesting that subduction plays an important role in setting apart different CPD provinces along the Mexican coast. Another deep CPD (16–32 km) area corresponds to the region where the Rivera plate subducts beneath Jalisco block. The Trans-Mexican Volcanic Belt is characterized by a decrease in Curie depths from west (16–20 km) to east (10–12 km). Finally, several deep CPD areas are situated in the back-arc region where old Mesozoic terrains are present. Our results suggest that the main control on the crust’s regional thermal structure in the fore-arc and volcanic arc regions is due to the subduction of the Cocos and Rivera plates beneath Mexico.

Interactions between volcanism and tectonics in the western Aeolian sector, southern Tyrrhenian Sea

SUMMARY New high-resolution bathymetric and magnetic data from the western Aeolian sector, southern Tyrrhenian Sea, provide insights into structural and volcanic development of the area, suggesting a strong interaction between volcanism and tectonics. The analysis of these data combined with relocated earthquake distribution, focal plane solutions and strain rate evaluation indicates that the dextral strike-slip Sisifo-Alicudi shear zone is a complex and wide area of active deformation, representing the superficial expression of the deep seated lithospheric tear fault separating the subduction slab below Sicily and Calabria. Most of the observed volcanic features are aligned along a NW–SE trend, such as the Filicudi island-Alicudi North Seamount and Eolo-Enarete alignments, and are dissected by hundred-metre-high scarps along conjugate NNE–SSW trending fault systems. The magnetic field pattern matches the main trends of volcanic features. Spectral analysis and Euler deconvolution of magnetic anomalies show the existence of both deep and shallow sources. High-amplitude, high-frequency anomalies due to shallow sources are dominant close to the volcanic edifices of Alicudi and Filicudi, while the main contribution on the surrounding Eolo, Enarete, Alicudi North and Filicudi North seamounts is given by low-amplitude anomalies and/or deeper magnetic sources. This is probably related to different ages of the volcanic rocks, although hydrothermal processes may have played an important role in blanketing magnetic anomalies, in particular at Enarete and Eolo seamounts. Relative chronology of the eruptive centres and the inferred deformation pattern outline the Quaternary evolution of the western Aeolian Arc: Sisifo, Alicudi North and Filicudi North seamounts might have developed in an early stage, following the Late Pliocene–Early Pleistocene SE-ward migration of arc-related volcanism due to the Ionian subduction hinge retreat; Eolo, Enarete and Filicudi represent later manifestations that led volcanoes to develop during Mid-Late Pleistocene, when the stress regime in the area changed, due to the SSE-ward propagation of the subduction slab tear fault and the consequent reorientation and decrease of trench migration velocity. Finally, volcanic activity occurred in a very short time span at Alicudi, where an almost conical volcanic edifice emerged, suggesting negligible interactions with regional fault systems.

An improved pseudo-gravity magnetic transform technique for investigation of deep magnetic source rocks

The pseudo-gravity transform is one of many possible FFT techniques that can be applied to aeromagnetic data. It enhances the anomalies associated with deep magnetic sources at the expense of the dominating shallow magnetic sources. This transform is an excellent interpretation tool for the detection of deep, magnetic igneous plutons and volcanic piles and the transformed data can be modelled using conventional gravity modelling tools. It is a suitable tool for interpreting deep-seated mineral plumbing systems associated with known, shallow mineral occurrences.The pseudo-gravity transform is derived by an integration of the total magnetic intensity grid data using conventional FFT tools. Padding of the grid around the region covered by the survey introduces long wavelength artefacts into the transformed grids. These long wavelength artefacts can obscure the targets that are the object of investigation. A variety of regional residual separation procedures is applied to the transformed grid to minimise the impact of the long wavelength artifacts.The improved pseudo-gravity transform is applied to the Goulburn 1:250 000 survey in the Lachlan Fold Belt of New South Wales to demonstrate the clear separation of deep sources that are difficult to detect or understand in the context of conventional magnetic image analysis. The results are contrasted with other filter techniques. Interpretive modelling of both the magnetic and the gravity transform data show how to derive more relevant geological information from magnetic surveys. By comparing the pseudo-gravity transform results with lower resolution ground gravity data, it is possible to obtain additional geological information by analysing the correlations.

New enhancement filters for geological mapping

Two types of filters have been developed for the purpose of enhancing weak magnetic anomalies from near-surface sources while simultaneously enhancing low-amplitude, long-wavelength magnetic anomalies from deep-seated or regional sources. The Edge filter group highlights edges surrounding both shallow and deeper magnetic sources. The results are used to infer the location of the boundaries of magnetised lithologies. The Block filter group has the effect of transforming the data into ``zones' which, similar to image classification systems, segregate anomalous zones into apparent lithological categories. Both filter groups change the textural character of a dataset and thereby facilitate interpretation of geological structures. The effect of each filter is demonstrated using theoretical model studies. The models include both shallow and deep sources with a range of magnetisations. Comparative studies are made with traditional filters using the same theoretical models. In order to simulate real conditions, Gaussian noise has been added to the model response. Techniques for noise reduction and geological signature enhancement are discussed in the paper. The new approaches are applied to actual magnetic survey data covering part of the Goulburn 1:100 000 scale map sheet area, New South Wales. Some new geological inferences revealed by this process are discussed.

Magnetic telechemistry and magmatic segmentation on the Southern East Pacific Rise

Results from axial dredges and a profile inversion of magnetic anomaly data along the axis of the East Pacific Rise (EPR) at 13–23°S provide an estimate of the average degree of fractionation for the extrusive layer at this ultrafast-spreading (∼145 mm/yr full rate) ridge. We find a high correlation ( R=0.81) between dredge mean FeO * (total iron as FeO) and natural remanence for 34 axial dredges with multiple samples having coincident geochemical and magnetic data. We attribute this good correlation to detailed sampling spanning the full range of cooling-related magnetization changes within a flow and to the young age (0–6 ka) of these axial samples, which effectively minimizes time-dependent magnetization changes due to geomagnetic intensity or alteration. A composite axial magnetic anomaly profile shows large amplitude (up to 400 nT) fluctuations with wavelengths of 50–200 km, which theoretical considerations suggest can reliably be related to the magnetization directly beneath the ship. For much of the southern EPR, seismic data provide independent limits on the axial thickness (259 ± 55 m) and the pattern of off-axis thickening of the extrusive magnetic source layer. These data also provide evidence for an axial magma lens that effectively eliminates anomaly contributions from deeper magnetic sources. Inversion of the axial magnetic anomaly data utilizing these geophysical constraints yields a magnetization solution which, through use of the regression relating FeO * and natural remanence, may be related to the average degree of differentiation of the extrusive source layer. The magnetic data reveal a pattern of magmatic segmentation that closely parallels the tectonic segmentation of the ridge, suggesting that magma supply may be an important control on the average degree of differentiation of the extrusive layer.