Interpretation Of Airborne Geophysical Data Research Articles

From Mesoarchean drips to modern–style tectonics in the Carajás Province, Amazonian Craton

The beginning of modern–style plate tectonics on Earth is not a consensus, but many authors suggest that it happened transitionally, from 3.2 to 2.5 Ga. In the Amazonian Craton, Brazil, the contrasting crustal architecture of the Rio Maria and the Carajás domains is marked by dome–and–keel and linear belt, respectively, which may represent a shift in the tectonic styles along the Mesoarchean to Neoarchean transition. Here, we discuss this hypothesis combining existing literature and an interpretation of airborne geophysical data for the Caracol Dome. This Archean TTG (Trondhjemite–Tonalite–Granodiorite) Dome shows a concentric lithological variation, with older 2.97–2.96 Ga trondhjemites at the edges and younger 2.95–2.92 Ga tonalites within the inner parts. The Caracol Dome exhibits a similar magmatic evolution to other Archean domes worldwide (e.g., Yalgoo and Mt. Edgar Domes), commonly interpreted as the record of vertical tectonics (e.g., drip–tectonics). However, the subsequent geological record in the Carajás Province is more likely related to the onset or transition to modern–style tectonics, such as large continental shelf sedimentation (Carajás Basin), steeply dipping linear shear zones with 2.76–2.73 Ga syn–tectonic intrusions and Neoarchean iron oxide–copper–gold (IOCG) mineralization. Finally, late Neoarchean to Paleoproterozoic tectonic events with new (and/or remobilized) IOCG deposits are also recorded in the Carajás Province.

ANÁLISE GEOFÍSICA DA REGIÃO DO RIO CONCEIÇÃO, QUADRILÁTERO FERRÍFERO, ASSOCIADOS A DADOS DE CAMPO, PETROGRÁFICOS E DE IMAGENS AÉREAS

The Quadrilátero Ferrífero region, located in the southeast portion of the São Francisco Craton, is one of the main metallogenic provinces in Brazil. Fieldwork, petrography, high-resolution airborne geophysics (magnetic and gamma-ray spectrometry data), and aerial images allowed us to produce a new map at the 1:25,000 scale, with important contributions in the lithotypes detailing, understanding of the geological structures and relationship between the different stratigraphic units. Interpretation of airborne geophysical data integrated with field structural and lithological observations were successfully employed in the creation of the litho-structural framework in a poorly exposed Proterozoic and Archean terrain. Airborne gamma-ray spectrometry data aided in the mapping process in areas with regolith cover including erosional ridges. The magnetic total derivative image revealed regional and local structures. In addition, our work details the units of occurrence of Rio das Velhas and Minas Supergroup. The aerial coverage of the Mindá and Santa Quitéria formations strongly increased, as well as the area of the Cauê Formation was better defined. The new geological map provides many improvements over the pre-existing maps. New lithological facies and structures were identified and others become more visible and lithologicalboundaries are refined or confirmed.

Interpretation of airborne geophysical data of Nsukka area, southeastern Nigeria

Airborne geophysical (aeromagnetic and gravity) data of Nsukka area was interpreted qualitatively and quantitatively with the aim of determining the susceptibilities of rock types, depth/mass of the anomalous bodies, possible cause of the anomalies and type of mineralization prevalent in the area. The estimated depths from forward and inverse modeling of aeromagnetic data for profiles 1, 2, 3, 4 and 5 were 1200m, 1644m, 1972m, 2193m and 2285m respectively. The respective susceptibility values were 0.0031, 0.0073, 1.4493, 0.0069 and 0.0016. These indicate dominance of iron rich minerals like limonite, hematite, pyrrhotite, and pyrite and forms lateritic caps on sandstones. SPI depth result ranges from 151.6m minimum (shallow magnetic bodies) to 3082.7 m maximum (deep lying magnetic bodies). Euler depths for the four different structural index (SI = 0.5, 1, 2, 3) ranges from 7.99 to 128.93m which are depths of shallow magnetic sources resulting from lateritic bodies in the outcrops in the study area. From the gravity data interpretation, Euler depth estimation reveals that depth to anomalous bodies ranges from 89.13 to 2296.92m. Density of the causative body obtained from modeling results for profile 1 was 1498kg/m3, which is in the range of clay material and the depth was about 923m. From models 2, 4 and 5, the densities of the causative bodies were 3523, 4127 and 3707kg/m3, while depths to the surface were about 604, 815 and 1893m respectively. These density ranges correspond to that of ironstone. From model three, the density of causative body obtained was 2508kg/m3, located at a depth of about 268m below the surface. This work has shown that Nsukka area is underlain by thick strata of shales, sandstones and ironstones, which together are suitable for ceramic production, and sufficiently thick sediments suitable for hydrocarbon accumulation.

PREDICTED SEDIMENTARY SECTION OF SUBGLACIAL LAKE VOSTOK

In early February 2012, the drill hole at the Vostok Station encountered theLakeVostokwater. This step is important to study the lake composition including possible microbial life and to model subglacial environments however, the next ambitious target of the Vostok Drilling Project is sampling of bottom sediments, which contain the unique record of ice sheet evolution and environmental changes in centralAntarcticafor millions of years. In this connection, the forecast of sedimentary succession based on existing geophysical data, study of mineral inclusions in the accretion ice cores and tectonic models is important task. Interpretation of Airborne geophysical data suggests thatLakeVostokis the part of spacious rift system, which exists at least from Cretaceous. Reflection and refraction seismic experiments conducted in the southern part ofLakeVostokshow very thin (200–300 m) stratified sedimentary cover overlying crystalline basement with velocity of 6.0–6.2 km/s. At present, deposition in southernLakeVostokis absent and similar conditions occurred likely at least last3 m.y. when ice sheet aboveLakeVostokchanged insignificantly. It can be also inferred that from the Late Miocene the rate of deposition inLakeVostokwas extremely low and so the most of sedimentary section is older being possibly of Oligocene to early to middle Miocene age when ice sheet oscillated and deposition was more vigorous. If so, the sampling of upper few meters of this condensed section is very informative in terms of history of Antarctic glaciation. Small thickness of sedimentary cover raises a question about existence of lake (rift) depression during preglacial and early glacial times.

Locating new uranium occurrence by integrated weighted analysis in Kaladgi basin, Karnataka

Interpretation of airborne geophysical data especially gamma ray spectrometric data is important for mineral exploration, specifically radioactive elements and to identify uranium targets. Exploration was carried out in MesoNeoproterozoic Kaladgi basin for five decades to target different types of uranium mineralisation. The first sizable sub-surface uranium mineralisation was intercepted at Deshnur within Badami arenites close to unconformity contact along the southern margin of the basin (Shobhita et al., 2011). Heliborne geophysical surveys were carried out in western part of the basin from Kadgaon to Ramdurg to acquire Time Domain Electromagnetic, Magnetic and Radiometric data. The radiometric data was collected using Exploranium GR820 airborne (256-channel recording) spectrometer system mounted in a helicopter with four 4.2L downward looking crystals and one 4.2L upward looking crystal for a total effective volume of 16.8L downwards and 4.2L upwards. The raw radiometric data was processed for corrections (background, radon, stripping, height attenuation etc) and counts were converted to ground concentration of %K, eU ppm, and eTh ppm, and their grid maps were prepared (Sridhar et al, 2012) . Kaladgi basin is an irregular E-W trending area in the northern part of Dharwar craton. In the study area, Kaladgi basin sediments are exposed as unconformably overlying the basement crystallines in the south and are covered by Deccan traps in the north. The crystalline basement of the Kaladgi Supergroup is represented by NNW trending Meso to Neoarchaean peninsular gneiss, greenstone belts and Closepet granite. The sedimentary sequence in the Kaladgi Supergroup has been divided along an angular unconformity surface into two groups: the lower Bagalkot Group, and the overlying Badami Group. The sediments of Bagalkot Group comprises of arenaceous, argillaceous and carbonate sequences which are folded into a series of anticlines and synclines. The sediments of Badami Group are undeformed and rest unconformably over the Archaean basement rocks as well as the Bagalkot Group. The Badami Group comprises of horizontal to gently dipping beds of arenite, shale and limestones (Jayaprakash et al., 1987). This study aims at identifying uranium potential zones by integrated analysis of thematic layer interpreted and derived from airborne radiometric and magnetic data, satellite data along with available ground geochemical data in western part of Kaladgi basin. Integrated weighted analysis of spatial datasets which included airborne radiometric data (eU, eTh & % K conc.), litho-structural map, hydrogeochemical U conc., and geomorphological data pertaining to study area, was attempted. The weightage analysis was done in GIS environment where different spatial dataset were brought on to a single platform and were analyzed by integration (Fig.1). The processed radiometric database was taken up for quantitative analysis to identify radiometric uranium anomalies. The database was clipped by using exposed and inferred lithological contacts of Badami Group to separate out data over sediments of Badami Group only. The analysis was carried out in two methods - 1 st the whole block and 2 nd Badami Group exclusive. Statistical analysis was applied to the airborne spectrometric data for separation of uranium anomalies from background. The statistics of the databases for uranium, thorium, ratio of uranium to thorium and uranium squared by thorium were calculated. The anomalies were extracted by applying cutoff threshold values for uranium, uranium by thorium and uranium squared by thorium channels in both methods. These anomalies were plotted spatially and they were classified as cluster anomalies and point anomalies depending upon their nature. Weightages were assigned based on nature of anomalies and cluster anomalies were given more weightage as they are more significant compared to point anomalies. The anomalies extracted from the

A geophysically constrained litho-structural analysis of the Eburnean greenstone belts and associated granitoid domains, Burkina Faso, West Africa

Interpretation of airborne geophysical data integrated with field structural and lithological observation was successfully employed in the creation of the litho-structural framework in a poorly exposed Paleoproterozoic granite-greenstone terrain of the West African Craton. The geophysical data portray with sufficient detail all of the lithological units and structural features present. The results suggest that the granitoid domains are formed by numerous small to medium-sized plutons. The existence of several generations of magmatic episodes has a significant impact on the development of a regional tectonic model. The magnetic data provided a better definition of the actual pluton shapes and several highly magnetic late-orogenic plutons were reliably identified. Relic F1 fold hinges were recognized in the Houndé and Boromo greenstone belts. The D1 penetrative structures are generally affected by the D2 transcurrent shear zones, which often form as well the granite-greenstone belt boundaries. Previously unreported S2 shear zones including the Bossié shear zone, transecting the Sidéradougou granitoid domain and abundant small-scale shear zones affecting all of the lithologies represent prospective areas for gold exploration. Airborne gamma-ray spectrometry data aided in the mapping process in areas with less regolith cover including erosional ridges and parts of the Banfora greenstone belt. The magnetic data revealed at least three generations of doleritic dykes crosscutting the western Burkina Faso.

Quality control of airborne geophysical data from the EU Mining Sector Support Programme, Ghana

On 2 December 2002, EU Commissioner Poul Nielson on behalf of the European Development Fund signed a €40 million grant to the Ghana Government. The purpose of this grant was to finance a Mining Sector Support Programme (MSSP) that covered a broad spectrum of geoscientific projects and other projects aimed at an overall strengthening and modernisation of Ghana’s mining sector. One of the major components was collection and interpretation of airborne geophysical data contracted to the two commercial geophysical companies Fugro Airborne Surveys and Geotech Airborne Ltd. The Geological Survey of Denmark and Greenland (GEUS) was contracted to perform the quality control (QC) of the airborne geophysical data collection and processing in a separate MSSP project (No 8 ACP GH 027/37). The initial Provision of quality-control services to the Airborne Geophysical Survey required GEUS to be on site in Ghana for 22 man-months; an expansion of the programme and various circumstances (see below) resulted in a total of 37.25 man-months before the project was completed in January 2010. The Danish National Space Center was subcontracted by GEUS to perform part of the QC of gravity data acquisition and processing. The QC project was reported by Thorning et al. (2010).

Interpretation of airborne geophysical data for the Thor Lake area, Northwest Territories : B. W. Charbonneau & M. I. Legault, Bulletin — Geological Survey of Canada, 475, 1994, pp 17–31

Interpretation of airborne geophysical data for the Thor Lake area, Northwest Territories : B. W. Charbonneau & M. I. Legault, Bulletin — Geological Survey of Canada, 475, 1994, pp 17–31