Gamma Ray Spectrometric Survey Research Articles

Airborne geophysical mapping of the Australian continent

Many countries have significant coverage of publicly funded airborne magnetic and gamma-ray spectrometric surveys that are available to explorers as precompetitive information to encourage exploration. However, individual surveys are generally small, and after decades of data acquisition, explorers are faced with the problem of how to effectively combine the individual surveys into coherent regional compilations. The Australian government’s response was to fly a baseline survey over the whole of Australia at approximately 75-km flight line spacing. This has enabled all of Australia’s public-domain magnetic and gamma-ray spectrometric surveys to be leveled to common datums using new techniques facilitated by the baseline control provided by the new traverses. This adds significant value to these data, as they can now be used for the interpretation of regional-scale features and for direct comparison of geophysical signatures from different parts of the continent. Also, quantitative modeling and analysis of the data can now be carried out over much larger areas.

The Radiometric Map of Australia

Geoscience Australia and the Australian State and Territory Geological Surveys have systematically surveyed most of the Australian continent over the past 40 years using airborne gamma-ray spectrometry to map potassium, uranium and thorium elemental concentrations at the Earth’s surface. However, the individual surveys that comprise the national gamma-ray spectrometric radioelement database are not all registered to the same datum. This limits the usefulness of the database as it is not possible to easily combine surveys into regional compilations or make accurate comparisons between radiometric signatures in different survey areas. To solve these problems, Geoscience Australia has undertaken an Australia-Wide Airborne Geophysical Survey (AWAGS), funded under the Australian Government’s Onshore Energy Security Program, to serve as a radioelement baseline for all current and future airborne gamma-ray spectrometric surveys in Australia.The AWAGS survey has been back-calibrated to the International Atomic Energy Agency’s (IAEA) radioelement datum. We have used the AWAGS data to level the national radioelement database by estimating survey correction factors that, once applied, minimise both the differences in radioelement estimates between surveys (where these surveys overlap) and the differences between the surveys and the AWAGS traverses. The database is thus effectively levelled to the IAEA datum. The levelled database has been used to produce the first ‘Radiometric Map of Australia’ – levelled and merged composite potassium (% K), uranium (ppm eU) and thorium (ppm eTh) grids over Australia at 100 m resolution.Interpreters can use the map to reliably compare the radiometric signatures observed over different parts of Australia. This enables the assessment of key mineralogical and geochemical properties of bedrock and regolith materials from different geological provinces and regions with contrasting landscape histories.

Levantamentos Gamaespectrométricos em Granitos Diferenciados. II: O Exemplo do Granito Joaquim Murtinho, Complexo Granítico Cunhaporanga, Paraná

Detailed mapping at the NW corner of the large Neoproterozoic Cunhaporanga Granitic Complex (CGC), Parana state, SE Brazil, redefined the Joaquim Murtinho Granite (JMG), a late intrusion in CGC with an exposed area of about 10 km², made up mainly by evolvedalaskites (alkali-feldspar leucogranites). This unit is in tectonic contact with the Neoproterozoic-Eocambrian volcano-sedimentary Castro Group, to the W, and is intrusive into other less evolved granitic units of the CGC to the E. Petrographically, JMG shows mainly mesoperthite and quartz, with subordinate amounts of altered micas and some accessory phases, mainly zircon. The equi to inequigranular granites are usually deformed with cataclastic textures, are often brecciated, and may have miarolitic structures. Formation of late albite, sericite, carbonate and hematite was caused by deuteric and hydrothermal alteration. A gamma-ray spectrometric survey at 231 stations which measured total counts (TC), Ueq K%, eU ppm and eTh ppm was used to construct several direct and derived maps. Compared to neighboring units the JMG has significant anomalies, especially in the TC, %K, eTh and eU maps, although the differences are less obvious in some derived maps. These evolved granites are enriched in these three elements. Geochemical behavior of K, Th and U is used to analyse the results observed in maps. Enhanced weathering under a subtropical climate with moderate to high average temperatures and heavy rainfall affects mainly feldspars and biotite, and may also destabilize most U and Th-bearing accessory phases. Th is most likely retained in restite minerals in soils, being relatively immobile, while part of U may migrate as uranyl ion in oxidizing media. K is especially affected by feldspar alteration to K-free clays (mainly kaolinite), and may be completely leached. Gamma-ray spectrometric methods are valid tools to study facies in granitic rocks, especially in those that are enriched in K, Th and U.

Airborne geophysics as a tool to promote mineral investment in Africa

Airborne geophysics, particularly aeromagnetic and gamma-ray spectrometer (radiometric) surveys, forms a critical component of geological mapping and mineral resource inventory programs in many African countries. In the 60?s and 70?s, regional aeromagnetic surveys were fairly widespread over much of the continent, in both sedimentary and hard rock terrains (Barritt, 1993). In the 80?s and 90?s, higher resolution surveys, incorporating radiometrics, were carried out in certain countries, particularly in southern Africa. In the last decade, a number of national initiatives (e.g. Madagascar, Mozambique, Namibia, Morocco, Mauritania, Nigeria, Ghana, etc.) have seen the high-resolution geophysical coverage greatly improve. The surveys form part of larger initiatives to improve the geological knowledge of a country or region, with the ultimate objective of increasing mineral investment and developing a sustainable mining industry. These geoscience programs are typically accompanied by reforms in the mining law to promote such investment. They contribute to tectonic reconstruction, groundwater and environmental applications, and petroleum exploration, all of which ultimately assist societal development (Reeves, 2005). International funding agencies such as the World Bank, European Community and African Development Bank have seen the value in such programs, and ensure that airborne geophysics receive a large share of project budgets. In jurisdictions throughout the world, it has been demonstrated that high-quality geophysical coverage leads directly to increased and more focused exploration. A trend in the last few years has been the inclusion of an airborne electromagnetic follow-up component to the airborne program. This paper provides current examples from two countries. In Uganda, more than 600,000 line-km of magnetic and radiometric data are being acquired over most of the country. In addition, eight blocks with high mineral potential are being flown with electromagnetic systems (Tempest and heliGeotem). In Senegal, the entire hard rock region has been covered by 130,000 line-km of magnetic and radiometric data, followed by a Tempest survey over three large blocks.

Postorogenic Origin of the Stratiform Cu Mineralization at Lufukwe, Lufilian Foreland, Democratic Republic of Congo

The Central African Copperbelt, which stretches across the border between Zambia and the Democratic Republic of Congo, is one of the largest sediment-hosted stratiform Cu-Co provinces in the world. The triangular-shaped Lufilian foreland is located to the northeast of the Copperbelt. Recently sediment-hosted stratiform copper occurrences have been explored in the foreland, including the Kinkumbi prospect in the Lufukwe anticline. Although the stratiform copper mineralization in the Copperbelt is mainly concentrated in the Roan Group sedimentary rocks, where it is commonly associated with cobalt, the Lufilian foreland copper mineralization occurs in sedimentary rock units of the Nguba and Kundelungu Groups where it is mainly associated with silver. This paper examines the metallogenesis of the stratiform copper mineralization at Lufukwe. The Lufukwe anticline is situated in the eastern part of the Lufilian foreland and is composed of Neoproterozoic sedimentary rocks belonging to the Katanga Supergroup. The Kinkumbi prospect, a sediment-hosted stratiform copper-silver occurrence, is located in the northern part of the western flank of the anticline. This part of the anticline is characterized by disseminated copper-silver mineralization hosted in the lower 10 to 15 m of the Monwezi Sandstone (Nguba Group). A comparison between the location of high copper grades in surface samples and boreholes and the location of structural lineaments visible on ASTER images indicates that the mineralization is spatially related to northeast-southwest to east-northeast‐west-southwest strike-slip faults. These faults are nearly perpendicular to the strike of the host rock and postdate both the Lufilian folding and deposition of the entire Katanga Supergroup. The Monwezi Sandstone was subjected to strong compaction and silica cementation (authigenic quartz overgrowths), followed by intense feldspar dissolution, which resulted in a well-developed secondary porosity represented by dissolution cavities. Copper sulfides are mainly concentrated in these cavities and partially replace the detrital grains. The copper mineralization is both hypogene (chalcopyrite, bornite, and chalcocite) and supergene (digenite, covellite, and minor native copper), with malachite and chrysocolla as the main oxidation products. Point counting and grain size measurements demonstrate that the sandstone horizons with high copper content (>1.25% Cu) are those with a detrital grain size larger than 175 µm, more than 35 percent altered feldspars and little or no fine-grained matrix. Microthermometry of fluid inclusions indicates that the authigenic quartz overgrowths precipitated from a moderate-temperature (80°‐130°C), high-salinity (18.8‐23.4 wt % CaCl2 equiv) H2O-NaCl-CaCl2 fluid. The hypogene copper-silver mineralization was deposited from a hot (120°‐180°C) and low- to moderate-salinity (1.9‐7.7 wt % NaCl equiv) H2O-NaCl fluid with a general trend of increasing homogenization temperatures with increasing salinities. The interpretation of the available structural, stratigraphic, petrographic, and fluid inclusion microthermometric data constrain the timing of the mineralization to a time after the Lufilian folding and deposition of the entire Katanga Supergroup. The data presented support a postorogenic fluid-mixing model in which the mineralization is related to the mixing of a copper-rich mineralizing fluid with a temperature ≥180°C and salinity ≥7.7 wt percent NaCl equiv, likely migrating upward along northeast-southwest‐ to east-northeast‐west-southwest‐oriented strike-slip faults, with a colder, low-salinity, reducing fluid in the Monwezi Sandstone. The location and distance between the northeast-southwest to east-northeast‐west-southwest faults strongly influenced the spatial distribution of the copper mineralization in the anticline, and the variability in grain size and composition of the Monwezi Sandstone caused the preferential lateral migration of the mineralizing fluid through the lower 10 to 15 m. Copper precipitation was possibly induced by reduction by preexisting noncopper sulfides (pyrite and arsenopyrite) and hydrocarbons and by the drop in fluid temperature and salinity. Based on this research the areas with dominant northeast-southwest to east-northeast‐west-southwest structural lineaments are considered favorable sites for further copper exploration in the Lufilian foreland, especially where these lineaments cut previously deformed Katanga sediments. Coarse-grained sandstones with no fine matrix that underwent intense feldspar dissolution are the most promising host rocks for the late disseminated stratiform copper mineralization in the Lufilian foreland. Airborne gamma-ray spectrometric surveys could be used as a powerful exploration tool for targeting ore-related, K-depleted zones in these sandstones.

Radioelements and Uranium Migration in Granites, El-Missikat Tunnels, Central Eastern Desert, Egypt

Fracture-filling uranium mineralization, connected mainly to black and jasperoid silica veins, occupying shear and fracture zones, was detected in 1968-1969 at the northern margin of El- Missikat post-tectonic granite, Central Eastern Desert, Egypt. Tunnels and excavations were executed, nearly at Wadi level, following these shear zones, in order to determine extensions of parts of the tunnels and evaluate their potentiality. Excavation works revealed the exis- tence of disseminated pitchblende, as well as its secondary associa- tion, in some drifts of the explored shear zones. The granite along the shear zone is intensely altered. The main alteration features are silic- ification, sericitization and kaolinization. An extensive gamma-ray spectrometric survey was carried out on some of the mineralized parts. The obtained data were statistically treated in order to outline the radiolithological features of the different rock units in the prospect area. The original uranium content and uranium migration rate were calculated in order to identify the migration trends in the granite and its alteration products. The study shows that there is a close relationship between the distribution of radioelements and lithology, in which the silica veins and silicified granites posses the highest radioelement contents. The migration of uranium took place inward in the brecciated jasperoid silica, massive silica and silicified granite. Meanwhile, the migration is outward in the case of pink (unaltered), kaolinized, and sericitized granites. The results show that, the pink granite has the highest outward uranium migration rate (-148), followed by the sericitized granite (up to -123). The kaolinized granite has the lowest outward

Towards an Australian radioelement baseline database

Approximately 75% of Australia is covered by public-domain, airborne gamma-ray spectrometric surveys. However, all the older surveys are in units of c/s and their data values depend on the survey instrumentation and acquisition parameters. Also, many of the newer surveys were inadequately calibrated with the result that data values on adjacent surveys are not necessarily comparable. This limits the usefulness of these data. Geoscience Australia and State Geological Surveys are working towards establishing a national baseline database of Australian gamma-ray spectrometric data that is consistent with the global radioelement baseline established by the International Atomic Energy Agency. This will be achieved by: (a) ensuring consistency in the calibration and processing of new gamma-ray spectrometric data through the use of standard processing procedures and calibration facilities that are tied to the global datum, and; (b) adjusting older surveys to the global datum through back-calibration and automatic grid merging. Surveys that are registered to the same datum are easily merged into regional compilations which facilitate the recognition and interpretation of broad-scale regional features, and allow lessons learnt in one area to be more easily applied to other areas.

An attempt to use aerial gamma-ray spectrometry results in petrochemical assessments of the volcanic and plutonic associations of Central Anatolia (Turkey)

SUMMARY Volcanic and magmatic rocks of Central Anatolia are fairly rich in radioelement concentrations. The aerial gamma-ray spectrometric survey data, gathered for the purpose of radioactive mineral exploration were utilized as an additional tool for the petrochemical classification of the volcanic and magmatics rocks and their environments. The survey data on acidic intrusions (e.g. granite, monzonite and syenite) have revealed radioelement concentrations to occur in wide ranges to be 2‐6 wt per cent for potassium (K), 3‐15 ppm for uranium (U) and 10‐52 ppm for thorium (Th). The chain-like high and moderately high potassium, uranium and thorium anomalies on acid intrusives of the entire area show a halo-shaped feature. Locations and lithological compositions of the volcanic rocks namely lavas, tuffs, ignimbrites and basalts, appear to be reasonably effective on their radioelement concentrations. The highest potassium, uranium and thorium concentrations of the volcanic rocks are around 4 wt per cent, 10 ppm and 35 ppm, respectively. Consequently, depending on the location and composition, volcanics show a very wide range of air absorbed dose rate. The lowest rates, which vary between 10 and 120 nGy/hr, were calculated in the ophiolitic group, thick cultivated soil covering areas, particularly at the centre of the aerial survey area, metamorphosed rocks in the north and young basalts mostly in the Kayseri district. Their average radioelement concentrations were found to be very low, that is, 1.2 wt per cent, 2.3 ppm and 10 ppm for K, U and Th, respectively. Because of the accumulation of soluble uranium isotopes, air absorbed rates stemming from

Radon Effects in Ground Gamma-ray Spectrometric Surveys

The effect of emanation radon is shown to be a significant problem for estimating uranium concentration from ground-based gamma-ray spectrometric surveys. Radon gas (a daughter product in the U238 decay series) escapes from rocks and soils near the Earth's surface into the lower atmosphere. Under early morning, still-air conditions, radon concentrates as a thin layer near the Earth's surface. If ground radiometric surveying is undertaken before this radon layer is mixed into the lower atmosphere, large errors in U concentration estimates result.This paper shows the effect of early-morning radon accumulation on a gamma-ray spectrometric survey near Boorowa, NSW. Paddocks surveyed in the early morning show much higher apparent uranium concentrations than those surveyed later in the day. We demonstrate the radon diurnal effect by monitoring the equivalent U concentration at a fixed site over several weeks. Typically, there is a build-up of radon near the Earth's surface overnight. Radon concentration reaches a maximum at about one hour after sunrise, before slowly dispersing over a period of 2-3 hours. By reporting both the Boorowa survey and the radon monitoring results in the same units of equivalent uranium concentration, we demonstrate that the anomalous effects seen in the Boorowa survey are consistent with that of a radon diurnal.The monitoring data also show the effect of rainfall on apparent U concentrations. Rain deposits radioactive daughter products of atmospheric radon onto the ground, resulting in a significant increase in apparent U concentration. These short-lived daughter products decay to insignificant concentrations within about 3 hours. Ground surveys should not be conducted during or within 3 hours of the cessation of rain, or under early-morning, still-air conditions. Also, because high moisture concentrations in soil reduces the gamma-ray fluence rate at the Earth's surface, surveys should not be conducted after prolonged or heavy rainfall where the soil profile is saturated.

Application of thorium-normalized airborne radio-spectrometric survey data of Wadi Araba area, North-eastern Desert, Egypt, as a guide to the recognition of probable subsurface petroleum accumulations

A new exploration method has been developed by Saunders et al. (Geophysics 58(10) (1993) 1417) using surface and aerial gamma-ray spectral measurements in prospecting for petroleum in stratigraphic and structural traps. Wadi Araba area, North-eastern Desert, Egypt, was selected to apply this method on its recorded aerial gamma-ray spectrometric survey data, due to its distinct stratigraphic and structural setting as well as its situation in close connection with the Gulf of Suez, which represents one of the important sites of oil production in Egypt. The three variables (eU, eTh, and K) registered for the whole study area, in the form of three contour maps, were digitized along the flight paths every 1.0 km. The DRAD arithmetic means plus three standard deviations for the data set were computed. Any single profile value greater than this quantity should have a probability of 99.87% that it represents a valid anomaly and is not caused by random variations in the background values. The use of these criteria has identified one flight line which has a valid anomaly that is not caused by random variations in the background values. This might indicate a prospective possibility for petroleum accumulation in the Wadi Araba area, North-eastern Desert, Egypt.

The Aerogeophysical Project Serra do Mar Sul in the region of the Cunhaporanga Granitic Complex, Paraná, Southern Brazil: analysis of the gamma-ray spectrometric survey

This paper analyses sprectrometric data of the Aerogeophysical Project Serra do Mar Sul (APSMS) in the Meso to Neoproterozoic basement of eastern Parana state, southern Brazil. Three main lithostratigraphic units are found, elongated N30-40E. The easternmost is the Tres Corregos Granitic Complex (TCGC), constituted by calc-alkaline granites and their mega-enclaves. The central unit is the low-grade metamorphic Itaiacoca Group. The western strip is occupied by the mostly calc-alkaline Cunhaporanga Granitic Complex (CGC), also with large enclaves. Further west, the molassic volcano-sedimentary Eopaleozoic Castro Group, deposited in a graben, overlies unconformably the CGC. The Devonian sandstone Furnas Formation, to the west, is the main unit of the Parana Basin in this region. The Cretaceous Banhadao alkaline massif (8 km2) intrudes the TCGC. The heavily weathered central-northern area of the CGC is in part covered by the alluvial deposits of the Iapo drainage basin. Data were corrected with a cosine directional filter (degree 6), and presented as cps (counts per second). U and K give answers that may be spurious, not always correlated with the geology. Most CGC intrusions present rather high Th-U, lower total (TC) and very low K counts, as opposed to the values found in the TCGC. High TC are found in some of the more evolved plutons within the CGC (e.g., the A-type Serra do Carambei Granite). The TCGC granites can be divided into two radiometric domains, one adjacent to the Itaiacoca belt and another to the E-SE (visible in TC and K counts), corresponding to the Sao Sebastiao Granite. The mega-enclaves show lower counts in all channels than the surrounding granites. The Itaiacoca belt is marked in TC and U-Th maps by low to medium counts. The Castro Group shows a complex pattern controlled by lithology. The Furnas sediments show low to medium responses (TC map). The Iapo river deposits stand out clearly by very low counts (TC, U, Th) against the CGC rocks.

Automatic merging of gridded airborne gamma-ray spectrometric surveys

Older airborne gamma-ray spectrometric survey data were often presented in units of counts per second. These data values are dependent on survey and equipment parameters such as detector volume, survey height, and the window energy limits used to measure the gamma radiation. Thus, data values on adjacent surveys may not be directly comparable. This is a critical problem for the interpretation of data over large areas, as it is difficult to reliably relate radiometric signatures in one survey area to those in another.The conventional solution to this problem is to ‘back-calibrate’ the data from older surveys to ground level concentrations of the radio-elements. This requires a linear transformation of the data that incorporates both a scaling (to accommodate detector volume, flying height and window widths) and a base-level shift (to accommodate inadequate background removal). The usual method of determining these scale and shift parameters is through field measurements using a calibrated portable spectrometer. However, because field work is expensive, an alternative solution is sought.An alternative approach is to use the differences between gridded survey data values in those areas where the surveys overlap to automatically estimate a base-level shift and scaling factor which, when applied to one of the surveys, minimises the differences in the overlap region. In this way an un-calibrated survey can be merged with a survey already calibrated to elemental concentrations of the radio-elements. Surveys can then be sequentially merged (or ‘levelled’) to produce regional compilations of the data. A similar approach has been used for many years to level aeromagnetic data. However, this sequential approach tends to propagate joining errors and introduce regional trends into the merged data.A new method overcomes this problem by considering the levelling of all of the grids in the regional compilation as a single inverse problem. Using a two-stage approach, the best base-level shift and scale for each survey grid is estimated. The first stage estimates the best relative shift and scale for each overlapping grid pair which, when applied to one of the pair, gives a least-squares fit to the grid values in the overlap area. The second stage uses the relative shift and scale parameters to estimate an absolute shift and scale for each grid that both honours the relative shift and scale parameters (in a least-squares sense) and brings each grid to the same absolute level as one or more base grids. The method works well as long as the data in the overlapping survey areas has a reasonable dynamic range.

Gamma-ray spectrometric data: Modelling to map primary lithology

During the last 10 years gamma-ray spectrometric surveys by governments and mineral companies have generated increasingly accurate and detailed data on K, Th and U surface concentrations of much of the mineral rich land currently being explored. Interpretations of these data commonly involve no more than subjective visual classification of colour images, either images of the concentrations of the individual elements, or images of the three elements combined in a RGB composite. This interpretation method does not extract the maximum information from the data.In areas of exposed and consolidated rocks, potassium, thorium and uranium contents generally correlate within each major rock type. For display and interpretation purposes there are advantages in separating the major rock types using K, Th and U values, then, for each major rock type, calculating the data in terms of components. One component measures the distance along the average geochemical variation path, and other components measure deviations of Th and U from this path. Maps of these components allow the more detailed quantification of original lithology, mapping of any sharp breaks in lithology, and mapping of later chemical modification. Compared with using principal components, the use of deviations of Th and U from the mean path as the second and third components, allows for the big difference of the three elements in fractional errors, and makes it easier to relate the components to geology.

Intergrated Airborne Gammaray Spectral and Satellite Data Analysis for U and REE Mineralisation — A Case Sudy from North Sagobandh Area, District Sonbhadra, Uttar Pradesh, India

Airborne gamma ray spectrometric (AGRS) and magnetic (AM) surveys were undertaken between 1986-1987 by Atomic Minerals Division, to locate uranium mineralisation along Son-Narmada rift zone. The imaging and interpretation of gridded AGRS data revealed many areas of anomalous radio elemental concentrations. These areas have been defined by taking thresholds as U ≥ 6 ppm, Th ≥ 24 ppm, K ≥ 2.3% and Total counts ≥ 5000 cps. The AGRS data integration with the satellite data viz., Landsat Multi spectral Scanner (MSS), Thematic Mapper (TM), and IRS LISS II data on different scales indicated the lithostructural controls of uranium mineralisation and also the predominance of the potash metasomatism in the vicinity of the southern Son rift and soda metasomatism further away in the south. p ]Systematic follow up ground checking of the target area located in the North Sagobandh area resulted in delineating the areas of K metasomatism, anatexis and grissenisation as the areas showing ≥ 2.3%K values. The depleted K, Th, and high U/Th values show zones of albitisation and oligoelasisation. The trends of radiometric breaks depicted by total counts distribution patterns defined the tectonostratigraphic boundaries. Besides these 26 radioactive anomalies with grades ranging from eU3O/0./012–0.18%, U3Ox B/r 500 meters and width of 1–2.5 meters, containing l.5%–9.04% heavy minerals.

Carborne gamma-ray spectrometric survey of an area East of Homs, Central Syria

A carborne gamma-ray spectrometric survey normally provides preliminary reconnaissance coverage of the radioelements in the topmost part of the earth's crust. However, a radiometric survey under certain conditions can be used for geologic mapping. In the present paper this approach was used to differentiate various lithological units within the Neogene formations east of Homs. The total radioactivity and the ground concentrations of potassium, uranium, and thorium were obtained over an area of 294 km 2. Despite, generally, the small variations in the contents of these variables, it was possible to categorize some rock units in the study area based on their radiometric response.

Uranium exploration in El-Missikat prospect area, central eastern desert, Egypt

A detailed ground gamma-ray spectrometric survey and a detailed geological and structural mapping were carried out in the Missikat prospect area. The spectrometric data were treated qualitatively and quantitatively. The uranium potential was evaluated through the use of the statistical method of analyses, examination of eU, eU/eTh and eU/K stacked profile and comparing the calculated content of the three radioelements with the average crustal value. The degree of the remobilization of the uranium in each rock unit was reflected in the trend variation of eU and eTh with their ratio. The main result of this study is the mapped rock unit possesses a higher concentration of the three radioelements than the equivalent crustal rocks. The silica vein, shear zone and fault show a high concentration of the radioelements. The eTh concentration is generally more than the eU concentration, which could be attributed to the fact that uranium is more leachable than thorium. The remobilization of uranium is indicated in most type of rocks, which means that post magmatic redistribution of uranium has taken place. It might have been concentrated within or near this rock type. The basic dykes and quartz diorite show the least concentration of the three radioelements.

Resolution in large rectangular NaI(Tl) detectors: a study

Experiments were carried out for studying the resolution of 4-in (10-cm)*4-in (10-cm)*16-in (40-cm) NaI(Tl) detectors used in high-sensitivity airborne gamma-ray spectrometric surveys. Change of resolution with source-detector geometry emphasized the inadequacy of existing specifications for resolution in explaining the detector response. An additional specification for resolution measurement, namely, overall resolution, is proposed. Measurement and applications of overall resolution of the detector are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Airborne gamma-ray spectrometry in uranium exploration

Abstract Uranium exploration is of paramount importance to India to maximize the indigeneous resources to meet the ever increasing demand for energy. Comprehensive regional and local zones of uranium mineralization are needed to be resolved. Such data comprise information on radioactivity in various energy ranges and on lithological knowledge. Airborne gamma-ray spectrometric techniques are particularly suited to provide spatial information. This review briefly discusses the conventional data presentation, statistically significant maps and the schemes suitable for classification of mineralized zones. A brief introduction is provided to airborne gamma-ray data, their acquisition, analysis, processing and interpretation to bring out their role in designing suitable models of uranium resources. State-of-the-art in uranium exploration covers radiometric mapping through airborne gamma-ray surveys. Through the examples of the Indian work done during this decade, the state-of-the-art and its usefulness in the country has been illustrated. It is concluded that the high sensitivity airborne gamma-ray spectrometric surveys coupled with modern methods of data analysis can further narrow down the targets.

Aeromagnetic interpretation and rock magnetism

Interpretation of the vast body of aeromagnetic survey data which now exists (and which continues to grow al a rate of same 2500 000 line-km per year worldwide) is a complex discipline, particularly where attempts are made to integrale and reconcile the interpretation of a magnetic anomaly map with other sources of map data such as gravity surveys, gamma-ray spectrometer surveys. satellite imagery. photageology. geochemical surveys and outcrop information (Reeves 1985. Paterson and Reeves 1985).

The distribution of gamma-emitting radionuclides in surface subtidal sediments near the Sellafield plant

Detailed distributions of total gamma activity, 137Cs, 106Ru and 95Zr + 95Nb in surface seabed sediments near the Sellafield plant are presented. The results are derived from a towed seabed gamma-ray spectrometer survey in September, 1982. All the distributions are similar, with contours of equal activity parallel to the coast defining a ‘ridge’ of higher activity which is displaced northwards relative to the outfall. This pattern appears to be largely a response to the transport of particle-associated radioeffluent modified in places by the type of seabed sediment present. At greater distance from Sellafield, the uptake of nuclides from solution seems to be more important. Nuclide concentrations decrease with increasing distance from Sellafield; rates of decrease being in the order Zr + Nb > Ru > Cs. This can be related to the levels of the nuclides discharged, their sorption characteristics and their half lives. The pattern of seabed activity seems to have been fairly stable over the period 1978 – 1985, but there is evidence of a small northward shift. Concentrations of 137Cs and 106Ru in 1985 were considerably lower than in 1978 or 1982. This is explicable in terms of the fall in discharge levels allied, in the case of Ru, to its short half life and, for Cs, the desorption observed in laboratory experiments. Nuclide ratios in sediment samples yield apparent transit times for the transport of nuclides in the survey area of 1·7 – 3·7 years. These times are generally greater than those obtained from sediments in the more distant Solway Firth and Ravenglass Estuary. It is suggested that they reflect fairly intense bioturbation causing mixing of relatively recent effluent with that from earlier discharges. This is supported by structures observed in X-radiographs of box cores, an abundant burrowing benthos and by interpretations of nuclide profiles and radiocarbon dating of sediment cores by other workers. A lag effect, of up to two years across the survey area, appears to be superimposed on that due to mixing.