High-resolution Airborne Magnetic Data Research Articles

Magnetic and Gravity Gradiometry Framework for Mesoproterozoic Iron Oxide-Apatite and Iron Oxide-Copper-Gold Deposits, Southeast Missouri

High-resolution airborne magnetic and gravity gradiometry data provide the geophysical framework for evaluating the exploration potential of hidden iron oxide deposits in Mesoproterozoic basement rocks of southeast Missouri. The data are used to calculate mineral prospectivity for iron oxide-apatite (IOA) ± rare earth element (REE) and iron oxide-copper-gold (IOCG) deposits. Results delineate the geophysical footprints of all known iron oxide deposits and reveal several previously unrecognized prospective areas. The airborne data are also inverted to three-dimensional density and magnetic susceptibility models over four concealed deposits at Pea Ridge (IOA ± REE), Boss (IOCG), Kratz Spring (IOA), and Bourbon (IOCG). The Pea Ridge susceptibility model shows a magnetic source that is vertically extensive and traceable to a depth of greater than 2 km. A smaller density source, located within the shallow Precambrian basement, is partly coincident with the magnetic source at Pea Ridge. In contrast, the Boss models show a large (625-m-wide), vertically extensive, and coincident dense and magnetic stock with shallower adjacent lobes that extend more than 2,600 m across the shallow Precambrian paleosurface. The Kratz Spring deposit appears to be a smaller volume of iron oxides and is characterized by lower density and less magnetic rock compared to the other iron deposits. A prospective area identified south of the Kratz Spring deposit shows the largest volume of coincident dense and nonmagnetic rock in the subsurface, and is interpreted as prospective for a hematite-dominant lithology that extends from the top of the Precambrian to depths exceeding 2 km. The Bourbon deposit displays a large bowl-shaped volume of coincident high density and high-magnetic susceptibility rock, and a geometry that suggests the iron mineralization is vertically restricted to the upper parts of the Precambrian basement. In order to underpin the evaluation of the prospectivity and three-dimensional models, an extensive statistical summary of density and apparent magnetic susceptibility measurements is presented that includes data on several hundred samples taken from the deposits, altered wall rocks, and unaltered country rocks.

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

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

A new tectonic model for the Palaeoproterozoic Kautokeino Greenstone Belt, northern Norway, based on high-resolution airborne magnetic data and field structural analysis and implications for mineral potential

A new tectonic model for the Palaeoproterozoic Kautokeino Greenstone Belt, northern Norway, based on high-resolution airborne magnetic data and field structural analysis and implications for mineral potential

The Paleoproterozoic Singo granite in south-central Uganda revealed as a nested igneous ring complex using geophysical data

We used high-resolution airborne magnetic and radiometric data and satellite gravity data to investigate the form of occurrence of the Paleoproterozoic Singo granite in west-central Uganda. This granitic body covers an area of ∼700 km2, intrudes Paleoproterozoic crystalline rocks and overlain by Paleoproterozoic-Mesoproterozoic sedimentary rocks, both of which belong to the Rwenzori terrane, and it is host to hydrothermally-formed economic minerals such as gold and tungsten. Our analysis provided unprecedented geometrical details of the granitic body and revealed the following: (1) the margins of the Singo granite are characterized by a higher magnetic signature compared to the interior of the granitic body as well as the surroundings. These anomalies are apparent in both the total magnetic field and horizontal derivative images and define eight overlapping ring features. (2) the depth continuation of these magnetic anomalies define outward but steeply-dipping features as indicated by the tilt images extracted from the airborne magnetic data. This is further supported by forward modeling of the magnetic and gravity data. (3) the Singo granite is characterized by relatively high and evenly-distributed equivalent concentration of Uranium (eU) and Thorium (eTh) compared to the surroundings and this is apparent in the Potassium (K)-eTh-eU radiometric ternary image. (4) the granitic body is defined by a gravity low anomaly that persisted to a depth of three km as shown by the Bouguer anomaly image and its five km upward continuation. We used these observations to identify this granitic body as a nested igneous ring complex and we refer to it as the Singo Igneous Ring Complex (SIRC). We further interpreted the eight ring structures as individual igneous ring complexes aligned in an E–W and NE–SW direction and these were developed due to repeated calderas collapse. Additionally, we interpreted the ring-shaped magnetic anomalies as due to hydrothermally-altered margins of the granite resulting from hot fluid circulation facilitated by the presence of concentric fracture zones that developed in association with the collapse of the calderas. We propose that the nested pattern of the SIRC was developed due to dominantly east and southwest migration of magma plumbing systems. Identifying the SIRC as a nested igneous ring complex has its importance in guiding future mineral exploration activities. Our study highlights the importance of high resolution geophysical data in mapping near surface igneous ring complexes, hence allowing for further examination of their magma plumbing systems.

Regional 3D geophysical investigation of the Sudbury Structure

The 3D geologic and structural setting of the Sudbury Structure was predicted by an integration of surface and subsurface geologic data with 2.5D modeling of high-resolution airborne magnetic and gravity data using 3D GeoModeller software. Unlike other CAD-based 3D software, GeoModeller uses the field interpolator method, whereby contacts of rock units are assumed to be equipotential surfaces, whereas orientation data determine the gradient and direction of the surfaces. Contacts and orientation variables are cokriged to generate 3D continuous surfaces for each geologic unit. Our 3D geologic model was qualitatively evaluated by forward computing the predicted gravity response at 1 m above topography and by comparing this response to the measured gravity field. Large-scale structures within the Onaping Formation and Archean basement, which overlie and underlie the Sudbury Igneous Complex (SIC), respectively, were not the cause of the linear gravity high in the center of the Sudbury Structure. We suggested that the deformation of the initial circular SIC may have commenced under the Sudbury Basin due to the reversal of the normal faults related to the Huronian rift system during the Penokean orogeny, therefore resulting into a north verging fold at the base of the SIC in the south range. This new interpretation was consistent with the magnetic and gravity data and honoured most of the significant seismic reflectors in the Lithoprobe seismic sections.

Extent, kinematics and tectonic origin of the Precambrian Aswa Shear Zone in eastern Africa

The Aswa Shear Zone (ASZ) is a fundamental Precambrian lithospheric structure that has been shaped by many tectonic events in eastern Africa. It separates the Saharan Metacraton in the northeast from the Northern Uganda terrane (which represents part of the Northeastern Congo block of the Congo craton) to the southwest. Nonetheless, its tectonic evolution is not fully understood. We used high-resolution airborne magnetic and radiometric data over Uganda integrated with Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) in South Sudan to assess the extent, kinematics and contribute to the understanding of the tectonic origin of the ASZ. (1) Our results showed that the ASZ extends in a NW–SE direction for ~550km in Uganda and South Sudan. (2) The airborne magnetic and radiometric data revealed a much wider (~50km) deformation belt than the 5-10km of the exposed surface expression of the ASZ. The deformation belt is defined by three NW-trending sinistral strike-slip shear zones bounding structural domains with magnetic fabric showing splays of secondary shear zones and shear-related folds. These folds are tighter close to the discrete shear zones with their axial traces becoming sub-parallel to the shear zones. A similar fold pattern is observed in South Sudan from the SRTM DEM. We interpreted these folds as due to ENE–WSW contraction associated with the sinistral strike-slip movement. (3) To the northeast, the magnetic patterns and radiometric signatures suggest the presence of a series of W-verging nappes indicative of strong E–W to NE–SW contraction deformation. (4) We relate the evolution of the ASZ to E–W to NE–SW Neoproterozoic oblique collision between East and West Gondwana. The deformation related to this collision was partitioned into E–W to NE–SW contraction resulting in W-verging thrusts in the east and a sinistral strike-slip movement along the NW-trending ASZ with the strain localized at the boundary between the Saharan Metacraton and the Northern Uganda terrane.

Connecting onshore-offshore Campos Basin structures: Interpretation of high-resolution airborne magnetic data

We interpreted northwest-trending transfer faults whose extensions are not entirely mapped in the Precambrian basement of the onshore and offshore Campos Basin. To enhance the subtle northwest–southeast lineaments not clearly seen in the total-field data, we reprocessed and merged two airborne magnetic data sets aiming at producing a single merged magnetic data set. Next, we applied a directional filter to these integrated magnetic data. Finally, we applied a multiscale edge detection method to these filtered data. This combination allowed the detection of edges and ridges that are used to produce several northwest–southeast lineations. We interpreted these northwest-trending lineations as magnetic expressions of transfer faults that cut across the onshore adjacent basement of the Campos Basin to the shallow and deep Campos Basin waters. These interpreted northwest-trending faults suggested the continuity of the known northwest-striking transfer faults in the deep Campos Basin waters toward the shallow Campos Basin waters and the adjacent continent. Moreover, our interpreted northwest-trending faults revealed the control of several known oilfields in the Campos Basin. This result supported the hypothesis of the influence of the northwest–southeast-trending transfer faults on the petroleum system of Campos Basin, which were reactivated in the Tertiary providing a pathway for the turbidite sedimentation, reworking, and redistribution of several deepwater reservoirs. In addition, it was hypothesized that this faulting system controlled the hydrocarbon migration paths from the presalt source rocks through salt windows into basal suprasalt layers.

Overview of the magnetic signatures of the Palaeoproterozoic Rustenburg Layered Suite, Bushveld Complex, South Africa

Aeromagnetic data clearly delineate the mafic rocks of the economically significant Bushveld Igneous Complex. This is mainly due to the abundance of magnetite in the Upper Zone of the Rustenburg Layered Suite of the Bushveld, but strongly remanently magnetised rocks in the Main Zone also contribute significantly in places. In addition to delineating the extent of the magnetic rocks in the complex, the magnetic anomalies also provide information about the dip and depth of these units. The presence of varying degrees of remanent magnetisation in most of the magnetic lithologies of the Rustenburg Layered Suite complicates the interpretation of the data. The combination of available regional and high resolution airborne magnetic data with published palaeomagnetic data reveals characteristic magnetic signatures associated with the different magnetic lithologies in the Rustenburg Layered Suite. As expected, the ferrogabbros of the Upper Zone cause the highest amplitude magnetic anomalies, but in places subtle features within the Main Zone can also be detected. A marker with strong remanent magnetisation located in the Main Zone close to the contact with the Upper Zone is responsible for very high amplitude negative anomalies in the southern parts of both the eastern and western lobes of the Bushveld Complex. Prominent anomalies are not necessarily related to a specific lithology, but can result from the interaction between anomalies caused by differently magnetised bodies.The magnetic data provided substantial information at different levels of detail, ranging from contacts between zones, and layering within zones, to magnetite pipes dykes and faults that can have an impact on mine planning. Finally, simple modelling of the magnetic data supports the concept of continuous mafic rocks between the western and eastern lobes.

Crisp clustering of airborne geophysical data from the Alto Ligonha pegmatite field, northeastern Mozambique, to predict zones of increased rare earth element potential

The National Geology Directorate of Mozambique (DNG) and Maputo-based Eduardo-Mondlane University (UEM) entered a joint venture with the South African Council for Geoscience (CGS) to conduct a case study over the meso-Proterozoic Alto Ligonha pegmatite field in the Zambézia Province of northeastern Mozambique to support the local exploration and mining sectors. Rare-metal minerals, i.e. tantalum and niobium, as well as rare-earth minerals have been mined in the Alto Ligonha pegmatite field since decades, but due to the civil war (1977–1992) production nearly ceased. The Government now strives to promote mining in the region as contribution to poverty alleviation. This study was undertaken to facilitate the extraction of geological information from the high resolution airborne magnetic and radiometric data sets recently acquired through a World Bank funded survey and mapping project. The aim was to generate a value-added map from the airborne geophysical data that is easier to read and use by the exploration and mining industries than mere airborne geophysical grid data or maps. As a first step towards clustering, thorium (Th) and potassium (K) concentrations were determined from the airborne geophysical data as well as apparent magnetic susceptibility and first vertical magnetic gradient data. These four datasets were projected onto a 100 m spaced regular grid to assemble 850,000 four-element (multivariate) sample vectors over the study area. Classification of the sample vectors using crisp clustering based upon the Euclidian distance between sample and class centre provided a (pseudo-) geology map or value-added map, respectively, displaying the spatial distribution of six different classes in the study area. To learn the quality of sample allocation, the degree of membership of each sample vector was determined using a-posterior discriminant analysis. Geophysical ground truth control was essential to allocate geology/geophysical attributes to the six classes. The highest probability to meet pegmatite bodies is in close vicinity to (magnetic) amphibole schist occurring in areas where depletion of potassium as indication of metasomatic processes is evident from the airborne radiometric data. Clustering has proven to be a fast and effective method to compile value-added maps from multivariate geophysical datasets. Experience made in the Alto Ligonha pegmatite field encourages adopting this new methodology for mapping other parts of the Mozambique Fold Belt.

GEOPHYSICAL MODELLING OF THE MOLOPO FARMS COMPLEX IN SOUTHERN BOTSWANA; IMPLICATIONS FOR ITS EMPLACEMENT WITHIN THE 2 GA LARGE IGNEOUS PROVINCES OF SOUTHERN AND CENTRAL AFRICA

The Molopo Farms Complex is an extremely poorly exposed, major, ultramafic-mafic layered intrusion straddling the southern border of Botswana with South Africa. It lies within the south-western part of the ~2.0 Ga large igneous province of southern and central Africa that includes the better known Bushveld Complex. Integrated interpretation of regional gravity data and new high-resolution airborne magnetic data have constrained the geometry of the Molopo Farms Complex in southern Botswana as a strongly faulted, polyphase intrusion compartmentalised by regional ductile shear zones. Previous models showing that the Complex was emplaced in at least two discrete stages are supported. Ultramafic rocks were initially emplaced as a semi-coherent lopolithic sheet up to about 4 km in thickness cutting across Transvaal Supergroup strata that had already been folded into open eastwest trending dome and basin structures with wavelengths of about 4 km. Steeply dipping, dyke-like ultramafic bodies adjacent to, and within major shears are inferred to be solidified feeders to the main lopolithic part of the MFC. It is likely that the initial ultramafic sheet was emplaced at a high crustal level (<3 km depth) into an attenuated Transvaal Supergroup sequence. This lack of a thick hanging wall sequence is thought to be significant for the emplacement of the succeeding mafic sheets. The ultramafic sheet thermally altered its wall rock and also created a complex fracture system in its hanging wall rocks. Differentiation within the ultramafic sheet produced basal harzburgites overlain by bronzites and possibly mafic sheets. Later mafic/basic sheets and dykes, again fed along shear-controlled, steeply dipping zones, spread into the fracture network created by the initial emplacement of the ultramafic lopolith to form a distinctive spider’s-web pattern on high-resolution airborne magnetic maps. It is proposed that either post-emplacement regional folding or gravitational collapse of the basal ultramafic lopolith produced a major basin with a ~40 km eastwest diameter, north of the Jwaneng-Makopong Shear Zone and smaller basin to the southeast. The newly postulated, steeply dipping ultramafic/mafic feeders, as well as the ultramafic lopolith and areas with anomalous nickel values in soils are considered to be prospective for PGE-bearing magmatic nickel-copper sulphide mineralisation. Magmatic rocks dated at about 2.0 Ga are a common feature, not only of the Kaapvaal Craton, but of all the African cratonic blocks south of the Equator. Reactivated intracratonic faults and shears appear to control emplacement of individual magmatic complexes although a sub-continental thermal anomaly unconfined by lithospheric plate boundaries is a likely driving force for the widespread magmatism.

Mapeamento aero-geofísico de pegmatitos mineralizados na Província Pegmatítica de Alto Ligonha, no norte de Moçambique

Major parts of Mozambique were recently flown to acquire high resolution airborne magnetic and radiometric data. Since then it has been the intent of the National Geology Directorate of Mozambique (DNG) to interpret these data generating value-added maps that are easier to use by the exploration and mining industries than mere airborne geophysical grid data and maps. DNG and Maputo-based Eduardo-Mondlane University (UEM) entered a joint venture with the South African Council for Geoscience (CGS) to conduct a case study over the meso-Proterozoic Alto Ligonha pegmatite province in northern Mozambique to support the regional mining sector. Integration of the airborne geophysical data was achieved using the K-means algorithm for explorative unsupervised crisp clustering of the airborne geophysical data. Discriminant analysis was then used to obtain the degree of class membership of each sample previously classified by K-means. Prior to clustering, Th- and K-surface concentrations as well as apparent magnetic susceptibility and first vertical derivative data of the total magnetic intensity anomalies were determined to project a 100 m-spaced grid comprising 850,000 multivariate sample vectors on the study area. Compilation of the classification results provided the value-added map illustrating that the probability to meet pegmatite bodies is highest where amphibole gneiss occurs. The automated integration of airborne geophysical data using the K-means algorithm is a fast, objective and efficient method to compile value-added maps. The experience made in the Alto Ligonha pegmatite province encourages the adoption of this new methodology over other parts of the Mozambique Fold Belt, becoming integral part of geological mapping ongoing in the country.

Mapeamento aero-geofísico de pegmatitos mineralizados na Província Pegmatítica de Alto Ligonha, no norte de Moçambique

Major parts of Mozambique were recently flown to acquire high resolution airborne magnetic and radiometric data. Since then it has been the intent of the National Geology Directorate of Mozambique (DNG) to interpret these data generating value-added maps that are easier to use by the exploration and mining industries than mere airborne geophysical grid data and maps. DNG and Maputo-based Eduardo-Mondlane University (UEM) entered a joint venture with the South African Council for Geoscience (CGS) to conduct a case study over the meso-Proterozoic Alto Ligonha pegmatite province in northern Mozambique to support the regional mining sector. Integration of the airborne geophysical data was achieved using the K-means algorithm for explorative unsupervised crisp clustering of the airborne geophysical data. Discriminant analysis was then used to obtain the degree of class membership of each sample previously classified by K-means. Prior to clustering, Th- and K-surface concentrations as well as apparent magnetic susceptibility and first vertical derivative data of the total magnetic intensity anomalies were determined to project a 100 m-spaced grid comprising 850,000 multivariate sample vectors on the study area. Compilation of the classification results provided the value-added map illustrating that the probability to meet pegmatite bodies is highest where amphibole gneiss occurs. The automated integration of airborne geophysical data using the K-means algorithm is a fast, objective and efficient method to compile value-added maps. The experience made in the Alto Ligonha pegmatite province encourages the adoption of this new methodology over other parts of the Mozambique Fold Belt, becoming integral part of geological mapping ongoing in the country.

The internal structure and geotectonic setting of the Xade and Tsetseng complexes in the western most part of the Kaapvaal Craton.

The Xade Complex is an unexposed Y-shaped body, approximately 100 km long and 25 km wide, located close to the western margin of the Kaapvaal craton in Botswana. The complex is characterized by large coincident magnetic and gravity anomalies. It is completely covered by varying thicknesses of Kalahari sediments as well as by Karoo strata, which means that detailed analysis of high resolution airborne magnetic data, ground gravity data and limited seismic data are essential in interpreting the internal configuration of the complex. An earlier interpretation of the first airborne magnetic survey of Botswana (Reeves, 1978) coupled with subsequent drilling discovered the Xade Complex and showed that it is made up of mafic and ultramafic rocks. However, the limited amount of drilling did not provide sufficient information to either interpret in detail its internal geology or its regional geotectonic setting (Meixner and Peart, 1984). New 2D and 3D gravity and magnetic field modelling have constrained the geometry of the complex as a syncline defined by folded mafic lavas and high-level sub-volcanic mafic sheets. The Xade Complex lies within a graben that forms the north-south arm of a triple junction with the faulted western margin of the Kaapvaal Craton. The focal point of the triple junction coincides with an inflection of the cratonic margin and is the likely site of the feeder zone to the mafic lavas of the Xade Complex. The Tsetseng Complex is shown to be an internally layered, magnetite-bearing gabbro.

As três anomalias elípticas da porção sul do Cráton São Francisco: novos alvos para a mineralização de grafita?

A porção sul do Cráton São Francisco (CSF), Minas Gerais, Brasil, inclui rochas que abrangem desde o Arqueano precoce até o Neoproterozóico tardio. O mapeamento geológico e a interpretação estrutural nessa área estão restritos a informações regionais em função dos poucos afloramentos e impossibilidade de correlações laterais. Neste artigo é apresentado o mapa geológico de três anomalias elípticas na porção meridional do CSF, com base no processamento e interpretação de dados aerogeofísicos (magnéticos e raios gama). A anomalia localizada na porção leste da área hospeda uma das maiores minas de grafite no Brasil. As outras duas anomalias, reveladas a partir desse levantamento aerogeofísico, eram até então desconhecidas. A alta resolução dos dados magnéticos e gamaespectrométricos aerotransportados permitiram a definição das estruturas geológicas, a delimitação de unidades litológicas, bem como a possível ocorrência de anomalias adicionais à noroeste da área. A fusão dos dados geofísicos com imagens de sensoriamento remoto num sistema de informações geográficas forneceu subsídios complementares para o mapeamento geológico da área. Este novo exemplo de aplicação bem sucedida de métodos aerogeofísicos demonstra sua utilidade em mapeamentos regionais, os quais são fundamentais para a indústria mineral.

Semi-automated structural analysis of high resolution magnetic and gamma-ray spectrometry airborne surveys

A user-controlled procedure was implemented for the structural analysis of geophysical maps. Local edge segments are first extracted using a suitable edge detector function, then linked into straight discontinuities and, finally, organised in complex boundary lines best delineating geophysical features. Final boundary lines may be attributed by a geologist to lithological contacts and/or structural geological features. Tests of some edge detectors, (i) horizontal gradient magnitude (HGM), (ii) various orders of the analytic signal ( A n), reduced to the pole or not, (iii) enhanced horizontal derivative (EHD), (iv) composite analytic signal (CAS), were performed on synthetic magnetic data (with and without noise). As a result of these comparisons, the horizontal gradient appears to remain the best operator for the analysis of magnetic data. Computation of gradients in the frequency domain, including filtering and upward continuation of noisy data, is well-suited to the extraction of magnetic gradients associated to deep sources, while space-domain smoothing and differentiation techniques is generally preferable in the case of shallow magnetic sources, or for gamma-ray spectrometry analysis. Algorithms for edge extraction, segment linking, and line following can be controlled by choosing adequate edge detector and processing parameters which allows adaptation to a desired scale of interpretation. Tests on synthetic and real case data demonstrate the adaptability of the procedure and its ability to produce basic layer for multi-data analysis. The method was applied to the interpretation of high-resolution airborne magnetic and gamma-ray spectrometry data collected in northern Namibia. It allowed the delineation of dyke networks concealed by superficial weathering and demonstrated the presence of lithological variations in alluvial flows. The output from the structural analysis procedure are compatible with standard GIS softwares and enable the geologist to (i) compare the structural features with various data sets in a common geo-referenced frame, and (ii) take advantage of attribute selection to highlight specific structural patterns or eliminate possible artefacts.

A moving hum filter to suppress rotor noise in high-resolution airborne magnetic data

A unique filtering approach is developed to eliminate helicopter rotor noise. It is designed to suppress harmonic noise from a rotor that varies slightly in amplitude, phase, and frequency and that contaminates aeromagnetic data. The filter provides a powerful harmonic noise-suppression tool for data acquired with modern large-dynamic-range recording systems. This three-step approach — polynomial fitting, bandpass filtering, and rotor-noise synthesis — significantly reduces rotor noise without altering the spectra of signals of interest. Two steps before hum filtering — polynomial fitting and bandpass filtering — are critical to accurately model the weak rotor noise. During rotor-noise synthesis, amplitude, phase, and frequency are determined. Data are processed segment by segment so that there is no limit on the length of data. The segment length changes dynamically along a line based on modeling results. Modeling the rotor noise is stable and efficient. Real-world data examples demonstrate that this method can suppress rotor noise by more than 95% when implemented in an aeromagnetic data-processing flow.

Geophysical characterisation of the Trident gold deposit, Western Australia

The Trident gold deposit is contained in the Plutonic Well Greenstone Belt, about 900 km northeast of Perth, Western Australia. The deposit is hosted in a highly sheared ultramfic unit overthrusted by granite. This project examined the physical properties of the important rock units to determine whether they contrast sufficiently to allow mapping of mineralisation, geology and structure. It investigated the possible occurrence at Trident of the most important mineralised lithological unit in the district, the `mine mafic'. The project also investigated the nature of the thrusted contact between granites and greenstones, one of the most important structures in the district. Drill core samples representing the major lithologies at Trident underwent a range of petrophysical testing. Gravity profiles were collected over the deposit and these were analysed along with high resolution airborne magnetic and radiometric data. The magnetic and radiometric data were assessed qualitatively with comparisons to maps of geology, regolith and known mineralisation. Gravity and magnetic data were computer modelled using drilling information and petrophysical data. The host ultramafic unit was readily mapped and modelled to depth using magnetic data. Mineralisation itself was expressed as a relative low within the magnetic ultramafic unit. The down dip extension of the mineralised ultramafic presents a good drill target. The gravity profiles are not long enough to allow accurate interpretation, but modeling shows that it is unlikely that the mine mafic is present at Trident. Radiometric data mapped broad geology and regolith trends but did not show an anomaly related to mineralisation. The results from IP and resistivity tests on core samples indicated that these techniques were not well suited for targeting this style of mineralisation.

Palaeochannels near West Wyalong, New South Wales: A case study in delineation and modelling using aeromagnetics

In November 1998, the Australian Geological Survey Organisation (AGSO) and the New South Wales Department of Mineral Resources acquired high-resolution airborne magnetic and gamma-ray spectrometric data along lines spaced 50 m apart and 60 m above ground level north of West Wyalong, New South Wales. In addition,AGSO post-processed a proprietary dataset of similarly high-resolution from an area immediately to the south, between West Wyalong and Temora, along the Gilmore Fault Zone. Anomalies interpreted to be caused by palaeochannel deposits have been identified in these datasets. Modelling of the airborne magnetic data was used to define the geometry of the palaeochannel deposits. These models were constrained using data from drill holes and downhole magnetic susceptibility measurements.Mapping palaeochannel deposits is of potential significance for mineral exploration and land and water research. Historically, alluvial gold, often palaeochannel-hosted, has accounted for a large proportion of Australia’s gold production. The study area is part of the historic Wyalong Goldfield and is part of the Bland Creek palaeovalley. Palaeochannel deposits may also be reservoirs of saline groundwater, and therefore are important in developing hydrogeological models for dryland salinity hazard assessment.Due to their depth of burial (often concealed beneath surficial sediments, to depths of 150 m), the palaeochannel deposits in the study area are mappable using either airborne magnetic or electromagnetic data, but not using gamma-ray spectrometry. The palaeochannel deposits contain detrital ferruginous maghaemitic pisoliths (which have extremely high magnetic susceptibility) concentrated in lenses in sand and clay.The resolution of the aeromagnetic datasets is compared with results obtained from surveys with different acquisition parameters. These tests suggest that an airborne geophysical survey flown with line spacing of less than 100 m can reveal important information about regolith materials, including channel-fill deposits such as in the sedimentary fill of the Bland Creek palaeovalley.