Gamma-ray Spectrometry Data Research Articles

Gravimetric, radiometric, and magnetic susceptibility study of the Paleoproterozoic Redenção and Bannach plutons, eastern Amazonian Craton, Brazil: Implications for architecture and zoning of A-type granites

The 1.88 Ga, anorogenic, A-type Redenção and Bannach granites, representative of the Jamon suite and associated dikes, are intrusive in Archean granitoids of the Rio Maria Granite–Greenstone Terrane in the eastern Amazonian Craton in northern Brazil. Petrographic and geochemical aspects associated with magnetic susceptibility and gamma-ray spectrometry data show that the Redenção and the northern part of Bannach plutons are normally zoned. They were formed by two magmatic pulses: (1) a first magma pulse was fractionated in situ after emplacement at shallow crustal level, generating a series of coarse, even-grained monzogranites with variable modal proportions of biotite and hornblende; and (2) a second, slightly younger magma pulse, located to the center of the plutons, was composed of a more evolved liquid from which even-grained leucogranites derived. Gravity modeling indicates that the Redenção and Bannach plutons are sheeted-like composite intrusions, approximately 6 and 2 km thick, respectively. These plutons follow the general power law for laccolith dimension and are similar in this respect to classical rapakivi granite plutons. Gravity data suggest that the growth of the northern part of the Bannach pluton resulted from the amalgamation of smaller sheeted-like plutons that intruded in sequence from northwest to southeast. The Jamon suite plutons were emplaced in an extensional tectonic setting, and the stress was oriented approximately NNE–SSW to ENE–WSW, as indicated by the occurrence of diabase and granite porphyry dyke swarms, orientated WNW–ESE to NNW–SSE and coeval with the Jamon suite. The 1.88 Ga A-type granite plutons and stocks of Carajás are disposed along a belt that follows the general trend defined by the dikes. The inferred tabular geometry of the studied plutons and the high contrast of viscosity between the granites and their Archean country rocks can be explained by magma transport via dikes.

Progressive drowning of carbonate platform in the Moravo-Silesian Basin (Czech Republic) before the Frasnian/Famennian event: facies, compositional variations and gamma-ray spectrometry

The Moravo-Silesian Basin (MSB; eastern Czech Republic and southern Poland) hosted an extensive shallow-water carbonate platform in the Middle Devonian to Frasnian interval. The platform drowned in a stepwise fashion from the Palmatolepis hassi to the Pa. linguiformis zone. Three types of drowning successions were revealed from conodont biostratigraphy, facies, microfacies and gamma-ray spectrometry data: (A) drowning to periplatform turbidite setting; (B) drowning to (hemi)pelagic seamount setting and (C) drowning associated with the stratigraphical gap. In the lower Pa. hassi zone, rapid subsidence caused the platform to drown locally along the N–S to NW–SE trending faults (type A drowning). In the upper Pa. rhenana to the Pa. linguiformis zone, the drowning accelerated in the western part of the MSB due to locally higher subsidence rates combined with the Late Frasnian biotic crisis (type B). In the southern part of the basin, the platform emerged shortly before the Frasnian/Famennian (F/F) boundary and drowned in the Early to Late Famennian (type C). The primary cause of drowning was differential subsidence at the Laurussian passive margin. Eustatic sea-level fluctuations, if any, contributed only to a minor extent to the Late Frasnian drowning, but were effective in type C drowning during the Famennian. The drowning boundaries are associated with increased contents of K and Th, reflecting the deceleration of carbonate production. Uranium contents display isolated peaks that roughly correlate with the drowning boundaries or the stratigraphic gaps associated with the F/F boundary. The uranium contents are considered to reflect local depositional conditions and are not suitable for stratigraphic correlation. On the other hand, from the K and Th contents, we can infer Late Frasnian sea-level fluctuations with duration on the order of 1 Myr. These cyclic variations in K and Th contents proved to be useful in platform-to-basin stratigraphic correlation.

Analysis of petrophysical properties of some granitoids in Sweden

Variations seen in the airborne magnetic, airborne spectrometry and ground gravity data are used as a base to study some granitoids with various outcrop distributions in two areas in Sweden. The ground gamma-ray spectrometry data as well as density and magnetic susceptibility of rock samples are then used to study the details of changes in the petrophysical properties of the granitoid suites. The variation of petrophysical properties is even studied within an individual suite. We have observed that in a given suite of granitoid, various rock types, namely mafic and felsic, can be discriminated using airborne and ground geophysical data. Airborne spectrometry and ground gravity data show that a more granitic composition has a higher concentration of K, U and Th and also a lower Bouguer anomaly signature. The measured concentrations on the outcrops and the densities of the rock samples support these results very well. The lower densities of the more felsic rocks with higher K concentrations are very well correlated. The geophysical data cover large areas and thus are also cost-effective to use.

Elemental and gamma-ray surveys in the volcanic soils of Ischia Island, Italy

This paper presents new data on the baseline concentrations of ten potentially harmful elements (As, Cd, Co, Cr, Cu, Pb, Sb, Tl, V and Zn) in volcanic soils collected over the whole Ischia volcanic island (46.5 km 2 ). A total of 198 residual soil samples were collected during the spring of 2001 with a sample grid of 500 × 500 m. The < 150 μm fraction was analysed (after aqua regia digestion) by ICP-AES for 30 elements. In addition to elemental analysis, gamma-ray spectrometry data were collected (for a total of 159 measurements) using a hand-held Scintrex GRS-500 spectrometer. For the compilation of baseline geochemical maps, a recently developed multifractal inverse distance weighted interpolation method and spectral analysis using a new geochemistry-dedicated GIS (GeoDAS) have been applied. The geochemical baseline includes the geogenic natural content (background) and the anthropogenic contribution in the soils. In order to obtain background values, to be used as reference values for the environmental Italian law DM 471/1999, the concentration-area fractal method has been applied. R-mode factor analysis helped in interpreting data controlled by anthropogenic sources as opposed to those controlled by geogenic sources.

Airborne gamma-ray spectrometry to quantify chemical erosion processes

Airborne gamma-ray spectrometry data (uranium, potassium and thorium contents) reveal geochemical heterogeneities within the monolithological Hyrôme watershed (ca. 150 km 2) in the Armorican massif (western France). Our observations and computations provide important constraints on the spatial distribution and the associated magnitudes of chemical erosion processes at the scale of a small watershed. Two distinct, partially preserved, weathering profiles exhibit a strong correlation between regolith evolution and airborne-derived K/Th ratios, suggesting that the variability is linked to supergene processes. Using both airborne data and laboratory measurements on rock samples, the total net export of potassium has been estimated at 422 ± 50 kg/m 2 and the chemical weathering rate of potassium at 17 ± 2 kg/km 2/a.

Use of 214Pb Photopeaks for Radon Removal: Utilising Current Airborne Gamma-Ray Spectrometer Technology and Data Processing

Removal of radiometric anomalies due to airborne radon is one of the most demanding steps in airborne gamma-ray spectrometry data processing. The contribution from airborne radon gas at the usual survey altitude is often several times that emanating from uranium ground sources. If left uncorrected, the radon background component can corrupt the whole chain of radiometric processing and preclude the production of seamless uranium and ternary data images.The concept of the use of 214Pb photopeaks for radon removal has been known for many years (Grasty, 1982), however until now it has not been introduced into regular data processing practice. The main obstacle in the implementation of this concept has been the fact that 214Pb photopeaks are found in the low energy part of the spectrum where there is a high degree of noise caused by scattered radiation. However, advances in modern instrumentation and in multichannel processing techniques allow the utilisation of these 214Pb photopeaks for radon removal in daily practice. The use of low-energy photopeaks is now possible following the release of a new generation of spectrometers, which produce improved spectrum linearity and high resolution in 512 channels for each crystal in the sensor array,The spectral ratio method suggested by Minty (1992, 1998), based on the 609 keV and 1765 keV photopeaks of 214Bi, has been to date the most accepted technique for radon removal. However, it has an inherent difficulty when processing data with a high 137Cs contribution, as is widespread in the Northern Hemisphere. In addition, this method suffers from the inability to resolve in a satisfactory way the overlap of the 208Tl photopeaks at 511 keV and 583 keV with the 214Bi 609 keV photopeak, in cases where there is strong thorium radiation.During recent surveys totalling 290 000 line kilometres, flown for the Geological Survey of Namibia, a Pico Envirotec GRS10 Spectrometer was used to collect 512 channels of raw data per second from each of eight NaI(Tl) crystal sensors. Observation of the 214Pb photopeaks at 295 keV and 352 keV, instead of the more usual 214Bi 609 keV photopeak, allowed successful radon removal.

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 Case Study Using Radiometrics as a First Pass Technique to Geological Mapping in the Musgrave Province in South Australia

The use of radiometrics as a regional geological mapping tool is widespread. Ternary, single band, ratio images, profile sections and data fusion techniques were applied to airborne gamma-ray spectrometry data of the Musgrave Province in South Australia. Chemical variations, previously unrecognised were used as a basis for subdividing and mapping key lithologies of Birksgate Giles Complex, and the Kulgera Suite Granites. Variations in emissivity were found to occur in conjunction with one or more of the following; Key tectonic structures In some cases differing magnetic susceptibility Within distinct tectonic domains Due to limited geological mapping little is known about the Musgrave Province however, the Giles Complex is considered as a unit of economic significance for nickel sulphides and PGE, so the need to map its extent becomes vital. Groundtruthing results is required to determine causes of emissivity variations which may be due to alteration associated with hydrothermal fluids or mineralogical/lithological variations within geological units.

Caracterização geológica e estrutural através da interpretação do produto integrado TM-Landsat 5 e dados aerogamaespectrométricos, região de Alta Floresta - MT

Este artigo apresenta a caracterização geológica e tectono-estrutural de uma região da Província Aurífera de Alta Floresta, Mato Grosso, através da interpretação do produto integrado TM-Landsat 5 e de dados aerogamaespectrométricos (contagem total). O produto integrado TM-contagem total possui características importantes para a interpretação geológica: apresenta num único produto, feições estruturais fornecidas pela imagem TM, representadas pelos elementos de relevo e drenagem e os valores radiométricos (RGB) que contribuem de forma bastante satisfatória na discriminação das unidades litoestruturais. A geração do mapa geológico foi baseada na interpretação dos valores radiométricos de contagem total (levando-se em consideração quais fatores, tais como topografia e atividade antrópica, poderiam influenciar a reposta radiométrica), nas feições de relevo e drenagem (que auxiliam na caracterização tectono-estrutural) fornecidas pela imagem TM e também nos dados de campo. Como a área apresenta intensa atividade antrópica e relevo muito arrasado, o que prejudica a interpretação geológica das imagens de sensoriamento remoto, a geração do produto integrado TM-contagem total foi decisiva para a obtenção de dados necessários ao mapeamento. Como resultado final foi elaborado o mapa geológico da área, em escala 1:250.000, sendo delimitadas 10 unidades litoestruturais relacionadas diretamente às unidades radiométricas e às feições estruturais. Um importante trend de direção N60-70W e altos mergulhos relacionados às zonas de cisalhamento transcorrentes sinistrais dúcteis a dúcteis rúpteis foi caracterizado através da interpretação visual de produtos integrados e dados de campo.

OLD GEOPHYSICAL DATA APPLIED TO MODERN GEOLOGICAL MAPPING PROBLEMS: A CASE-STUDY IN THE SERIDÓ BELT, NE BRAZIL

The Seridó belt, northeast Brazil, is a Brasiliano-Pan-African collisional orogen formed between the West African, São Francisco and Amazon cratons during the west Gondwana assemblage. This area is an excellent example for illustrating the usefulness of gamma-ray spectrometric data to support modern geological mapping in semi-arid terranes. Analyses of K, Th and U channels images, displayed as composite themes, correlate well with the main regional framework of the Seridó Belt. Several distinct mapped units are easily distinguishable in the gamma-ray spectrometric data, as the Paleoproterozoic basement rocks and the supracrustal metasedimentary sequences of the Neoproterozoic Seridó Group. Also, the granitic plutons and the Phanerozoic cover show different geophysical signatures. Definition of contacts among units, faults, folds and shapes of granitic intrusions are considerably enhanced by the airborne gammaray spectrometry data. The results here presented show that there is a strong relationship among geochemical, isotopic and geophysical signatures of different crustal blocks recognized in the region. This kind of approach seems fundamental for investigating regional crustal evolution in overly complex deformed Precambrian areas. It also can be of some help to better define the boundaries of the different lithostratigraphic units that could appear in similar orogenic belts within the Brazilian Shield.

Old geophysical data applied to modern geological mapping problems: a case-study in the seridó belt, ne Brazil

The Serido belt, northeast Brazil, is a Brasiliano-Pan-African collisional orogen formed between the West African, Sao Francisco and Amazon cratons during the west Gondwana assemblage. This area is an excellent example for illustrating the usefulness of gamma-ray spectrometric data to support modern geological mapping in semi-arid terranes. Analyses of K, Th and U channels images, displayed as composite themes, correlate well with the main regional framework of the Serido Belt. Several distinct mapped units are easily distinguishable in the gamma-ray spectrometric data, as the Paleoproterozoic basement rocks and the supracrustal metasedimentary sequences of the Neoproterozoic Serido Group. Also, the granitic plutons and the Phanerozoic cover show different geophysical signatures. Definition of contacts among units, faults, folds and shapes of granitic intrusions are considerably enhanced by the airborne gammaray spectrometry data. The results here presented show that there is a strong relationship among geochemical, isotopic and geophysical signatures of different crustal blocks recognized in the region. This kind of approach seems fundamental for investigating regional crustal evolution in overly complex deformed Precambrian areas. It also can be of some help to better define the boundaries of the different lithostratigraphic units that could appear in similar orogenic belts within the Brazilian Shield.

Petrogenetic interpretation from gamma-ray spectrometry and geological data: the Arch Lake zoned peraluminous granite intrusion, Western Canadian Shield

Airborne gamma-ray spectrometry data acquired over the Arch Lake granite batholith in the Western Canadian Shield display compositional patterns not previously identified by geological field mapping. An integrated analysis of gamma-ray spectrometric, geochemical and mineralogical data was conducted to map and analyse these patterns, and to obtain insight in their petrogenetic significance. Enhancement and cluster analysis of the K, eTh and eU grids reveal that the Arch Lake granite is zoned in a complex manner with a Th-rich centre and Th-rich, locally Urich outer margins. The Th-rich centre broadly corresponds to highs in interpolated patterns of Zr and TiO2, reflecting a regional mineralogical association between biotite, monazite, zircon and Fe-oxides. The negative trend of this Th-rich mineralogical association with SiO2 is not consistent with normally-zoned plutons formed by differentiation of a single magmatic pulse. Instead, two spectrally distinct K-eTh zones suggest that the Arch Lake batholith is a composite intrusion. Gamma-ray spectrometry surveys can be considered a useful tool for mapping subtle compositional variations in granites. Integrated analysis on the basis of geochemical data and mineralogical observations, however, is required to constrain the possible petrogenetic interpretations.

OARS - A New System for Mapping Surface Mineralogy Simultaneously with Airborne Geophysics

The Operational Airborne Research Spectrometer (OARS) is a novel hyperspectral profiling reflectance spectrometer developed by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and Fugro Airborne Surveys (FAS) to examine alternative means to collect and deliver exploration data, based on the principles of reflectance spectroscopy. OARS is a component of CSIRO's "Airborne Mineralogy" concept.OARS can be integrated with other airborne geophysical instruments that principally collect magnetic, gamma-ray spectrometry and digital elevation data, and can be flown simultaneously on exactly the same flight lines. OARS will measure ground reflectance spectra from contiguous 8-m pixels from an altitude of 80 m.After data processing, interpolated images of relative mineral abundance can be reconstructed using differential GPS (DGPS) positioning data to produce a variety of mineralogical maps.The OARS system consists of a downward and an upwardlooking spectrometer. Each spectrometer covers 190 spectral channels in the visible to short-wave infrared range, with an average spectral resolution of 12 nm. This wavelength range allows OARS to map most species of phyllosilicates, clays, sulphates, carbonates and iron oxides, that occur in alteration zones, regolith and host rocks. Green and dry vegetation can also be identified. A CCD camera (Charge Coupled Device – Silicon Array) allows monitoring of the flight path while flight attitude sensors and DGPS are used for geo-location.The instrument was first tested in late 1998. Subsequent test surveys in early 1999, over a more challenging geological area in NW Australia, verified its ability to deliver new mineralogical data and its capability as part of an integrated system incorporating magnetic and gamma-ray spectrometry surveying. The resulting case study shows that this line profiling spectrometer can deliver useful interpolated mineral maps with potential synergistic benefits from co-registered magnetic and gamma ray images.

Integration of Landsat TM, Gamma-Ray, Magnetic, and Field Data to Discriminate Lithological Units in Vegetated

Image classification of geological units in vegetated granite-gneiss terrain from multispectral data is hampered by vegetation cover and limited spectral contrast of its lithological variations. In this paper an alternative methodology is presented, employing airborne gamma-ray spectrometry and magnetic data. The methodology includes the selection of combinations of data channels and their transformations to enhance the discriminative power of the lithology classification. An analysis of the training set compiled from 2,795 field stations showed that the potassium, thorium, and uranium gamma-ray spectrometry and total and residual magnetic field channels provided an overall classification success rate of 65% for a total of 10 lithological units. Parametric classifications based on this training set yielded 67% coincidence with the geological map, whereas a neural network classifier provided only 23% coincidence. Exploiting the spatial autocorrelation of the geophysical signatures by adding averaged filtered channels slightly improved the coincidence percentage to 70% and enhanced the continuity of linear enclaves within larger lithological units. The contribution of magnetic data to the classification depends on the extent to which the anomaly spectrum and its processed derivatives reflect surface geology. We found that both the short and long wavelengths in the spectrum contributed to the classification performance. This is explained by the geological structure of the area, where both broad and narrow units extend downward subvertically. A comparison of our results to regional geological map patterns identified targets for map refinement and exploration.

A new technique for processing airborne gamma ray spectrometry data for mapping low level contaminations

A new technique for processing airborne gamma ray spectrometry data has been developed. It is based on the noise adjusted singular value decomposition method introduced by Hovgaard in 1997. The new technique opens for mapping of very low contamination levels. It is tested with data from Latvia where the remaining contamination from the 1986 Chernobyl accident together with fallout from the atmospheric nuclear weapon tests includes 137Cs at levels often well below 1 kBq/m 2 equivalent surface contamination. The limiting factors for obtaining reliable results are radon in the air, spectrum stability and accurate altitude measurements.

Reducing the effects of vegetation cover on airborne radiometric data using Landsat TM data

The gamma-ray spectrometry responses from bedrock in Canadian Shield areas are substantially masked by overburden and vegetation. Proper interpretation of airborne gamma-ray spectrometry data is dependent on accounting for the interference provided by surface cover. In this paper, a method is tested to correct airborne gamma-ray spectrometry, acquired over the Canadian Shield of northeastern Alberta, for vegetation screening by estimating the proportions of bedrock and vegetation cover from Landsat TM data. TM pixel values, due to the patchy network of bedrock and vegetation, result from a spectral mixture of these ground cover classes. Linear unmixing was implemented to deconvolve TM bands in abundance images to estimate proportions of bedrock and vegetation for each pixel. The outcrop abundance image, representing spatial variation in area percentage of bedrock, is used in linear regression analysis to calibrate co-registered K, eTh, and eU gamma-ray spectrometry channels to 40 per cent bedrock endmember images.

Projeto aerogeofisico Serra do Mar Sul : uma abordagem semi-quantitativa

Several airbome surveys have been promoted in Brazil by the Departamento Nacional da Producao Mineral (DNPM). Nevertheless, due to DNPM priorities, the obtained flown records are not optimized to allow more than semi-quantitative levei details. The methodology and data processing of airborne gamma-ray spectrometry in Brazil, has not reached the evolution obtained in other countries, mainly due to brazilian peculiar geological characteristics, which often limit data interpretation. Here, a re-evaluation of Serra do Mar Sul airborne gamma-ray spectrometry data is presented. Flown profiles records were re-processed, optimized and tested for potentiality analysis. Correlations between countings in different recording channels were highly improved by the adopted procedures. A terrestrial survey, using a portable gamma-ray spectrometer calibrated at the Comissao Nacional de Energia Nuclear (CNEN- RJ), was performed in order to establish a correspondence between counts and concentration for U, Th and K.

Generation and interpretation of geophysical images with examples from the Rae Province, northwestern Canada shield

Different types of images generated from gravity, magnetic, and gamma ray spectrometry data from the Rae Province of the Canadian shield were compared with each other and geologic maps to evaluate their effectiveness for displaying the geologically relevant content of the data sets. Shading methods were useful for enhancing weak directional anomalies in the aeromagnetic data. Multi‐directional, shaded‐relief images produced by overlaying three colored, shaded‐relief images are useful for analysis of anomalies associated with structure. Vertical gravity derivative images display a continuous gravity feature linking the Wager Bay and Amer Lake shear zones that is obscured on the Bouguer gravity intensity image. Detailed vertical magnetic derivative images of the shear zone clearly displayed anomalies associated with the internal structure. Composite images generated using three different geophysical parameters show correlations between the magnetic, gravity, and radiometric data which can be related to the geology. Subtle variations in uranium, thorium, and potassium concentrations determined by gamma ray spectrometry can be effectively displayed using ternary radioelement images

The relationship between uranium distribution and some major crustal features in Canada

AbstractThe availability of reconnaissance scale geochemical maps for large areas of Canada enables spatial associations between major crustal structures and surface uranium content to be identified. Maps of the distribution of uranium for an area greater than 2 million km2 compiled from airborne gamma-ray spectrometry data are supplemented by maps for uranium, based on stream and lake sediment and some bore hole sampling. These are examined in relation to gravity, aeromagnetic and geological maps.The radioelement distribution can be related in detail to exposed bedrock and surface geology, but in addition there is evidence of the control of uranium distribution by major structural features which are marked by granitoids containing elevated levels of radioelements; several of these granitoids are associated with large negative Bouguer gravity anomalies. The distribution of such granitoids appears to be related to ‘megashears’, as in the case of the South Mountain batholith in Nova Scotia, or zones of tension. A belt of uranium enrichment, the Athabasca axis which is characterized by uraniferous granitoids with negative Bouguer gravity anomalies and associated tension faulting extends 2500 km northeast-ward from Edmonton, Alberta to the Melville Peninsula. This structure passes under the Athabasca basin which contains many large uranium deposits. Recent evidence that granitoids enriched in radioelements can provide low-grade heat sources over periods of hundreds of millions of years, capable, in favourable situations of maintaining low-temperature hydrothermal circulation is considered to account for uranium and other mineralization in the basin. It is suggested that the transfer of radioelements into the crust at the end of the Lower Proterozoic, was a factor in the stabilization of North American craton.