Airborne Gamma-ray Spectrometer Research Articles

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.

Large-scale radon hazard evaluation in the Oslofjord region of Norway utilizing indoor radon concentrations, airborne gamma ray spectrometry and geological mapping

We test whether airborne gamma ray spectrometer measurements can be used to estimate levels of radon hazard in the Oslofjord region of Norway. We compile 43,000 line kilometres of gamma ray spectrometer data from 8 airborne surveys covering 10,000 km 2 and compare them with 6326 indoor radon measurements. We find a clear spatial correlation between areas with elevated concentrations of uranium daughters in the near surface of the ground and regions with high incidence of elevated radon concentrations in dwellings. This correlation permits cautious use of the airborne data in radon hazard evaluation where direct measurements of indoor radon concentrations are few or absent. In radon hazard evaluation there is a natural synergy between the mapping of radon in indoor air, bedrock and drift geology mapping and airborne gamma ray surveying. We produce radon hazard forecast maps for the Oslofjord region based on a spatial union of hazard indicators from all four of these data sources. Indication of elevated radon hazard in any one of the data sets leads to the classification of a region as having an elevated radon hazard potential. This approach is inclusive in nature and we find that the majority of actual radon hazards lie in the assumed elevated risk regions.

Pilot study of the application of Tellus airborne radiometric and soil geochemical data for radon mapping

The scope for using Tellus Project airborne gamma-ray spectrometer and soil geochemical data to predict the probability of houses in Northern Ireland having high indoor radon concentrations is evaluated, in a pilot study in the southeast of the province, by comparing these data statistically with in-house radon measurements. There is generally good agreement between radon maps modelled from the airborne radiometric and soil geochemical data using multivariate linear regression analysis and conventional radon maps which depend solely on geological and indoor radon data. The radon maps based on the Tellus Project data identify some additional areas where the radon risk appears to be relatively high compared with the conventional radon maps. One of the ways of validating radon maps modelled on the Tellus Project data will be to carry out additional indoor measurements in these areas.

Use of airborne gamma-ray spectrometry for kaolin exploration

Airborne gamma-ray spectrometry was used to define targets with kaolin potential in the Armorican Massif of Brittany, France. This exploration method is based on the principle that kaolinite, an aluminosilicate clay mineral constituting kaolin, is formed by the hydrolysis of potash feldspar with the elimination of potassium. Therefore, potassium contrast between favourable host-rock such as a leucogranite and kaolin occurrence is likely a significant pathfinder. As the relationship between the potassium-40 recorded by an airborne gamma-ray spectrometer and total potassium is constant, such data provide us a direct measurement of the potassium content of the ground flown over. Our study tested this by calculating, for each geological unit, the difference between the measured and average potassium content calculated for a given geological formation. The study was based on (i) a recent (1998) high-definition airborne geophysical survey over the Armorican Massif undertaken on behalf of the French Government, and (ii) new geological compilation maps covering the same region. Depleted zones, where the measured potassium is less than the average potassium content calculated target areas with high potential of containing kaolin, provided that the unit was originally rich in potash feldspar. By applying this method to the entire Armorican Massif, it was possible to identify 150 potassium-depleted zones, including 115 that were subjected to rapid field checks and 36 that contained kaolin (21 new discoveries). This method, which is both safe for the environment and easy to use, is therefore a good tool for rapidly defining targets with kaolin potential at a regional scale. The method may also have possibilities in exploring for other types of deposit characterised by an enrichment or depletion in U, K and/or Th.

A Calibration Range in South-Eastern Australia for Airborne Gamma-Ray Spectrometers

A calibration range is an easily navigable strip of land that is used to measure the response of an airborne spectrometer system to changes in aircraft altitude (height attenuation coefficients), and to sources of known concentration (sensitivity coefficients). This paper describes the selection and assessment of an airborne gamma-ray spectrometric calibration range for southeastern Australia. We review the selection criteria for establishing a calibration range, and present the calibration results for a new calibration range near Albury, New South Wales. Background measurements are made over Lake Hume, which is about 27 km away. The calibration range was flown during a period of particularly low atmospheric radon background and this reduced the errors associated with background estimation and removal. The calibration range has most of the desirable features expected of an ideal calibration range. We are hopeful that with the continued cooperation of the local landowners, this range will become a permanent calibration facility for southeastern Australia.

Integrated Lithostructural Mapping of the Rössing Area, Namibia using High Resolution Aeromagnetic, Radiometric, Landsat Data and Aerial Photographs

This paper presents results of an integrated appraisal of recent high-resolution aeromagnetic, radiometric and remote sensing data over the Rssing mine area and lower Khan Gorge region of Namibia. The interpretation used stereoscopic aerial photographs to establish structure around the Rssing mine and high-pass filtered aeromagnetic data for the regional structure, followed by analysis of spectral Landsat TM data, total field magnetic intensity data and imaged airborne gamma-ray spectrometer data to delineate lithology. Resulting interpretation maps clearly show an hierarchy of polyphase folding, which had been described by earlier field mapping, and a hitherto-unrecognised system of late (post-F3) sinistral strike-slip faulting and thrusting. Early Palaeozoic alaskitic intrusions of the type which host the world-class Rssing uranium deposit appear to be largely related to late sinistral transtensional ladder veins associated with the north-northeast trending post-F3 sinistral strike-slip faults.

The distribution of radioactive fallout in Nord-Trøndelag from detailed airborne and ground gamma-ray spectrometer surveying

The distribution of radioactive fallout in Nord-Trøndelag from detailed airborne and ground gamma-ray spectrometer surveying

Use of aerospace data for uranium exploration in parts of Southern Bastar, Central India

Abstract Multi-platform and multi-spectral remote sensing techniques have been widely used for geological evaluation of very large terrains. Availability of additional information such as high sensitivity Airborne Gamma-Ray Spectro-metric (AGRS), and other geophysical and geological data enable one to narrow down favourable target areas for detailed ground follow-up for uranium exploration. Late Proterozoic, intra-cratonic areas in the Indravati basin and adjoining areas of supra-crustals and basement crystalline rocks in Southern Bastar, Central India have been evaluated to determine whether a correlation exists between above average uranium enrichment zones and geological settings, in order to select areas for ground radiometric surveys. These studies have aided precise delineation of the litho-boundaries and structural trends. Of the six major lineament trends identified, NW-SE and NNE-SSW trends appear to have been the most favourable for bringing labile uranium into the system. The present location of uraniferous zones seem to have been influenced by the NE-SW and WNW-ESE trends reactivated during acidic instrusive activity and the selective removal of uranium from highly deformed areas has been concentrated along these same trends. These studies have helped to reduce the target for further investigations to 3 per cent of the total area (30000 km2) studied.

Portable Calibration Sources For Airborne Gamma-ray Spectrometers

Standard radioactive sources of known element abundances are essential for the calibration of airborne gamma-ray spectrometers. The conventional approach to calibration has been the determination of 3-channel stripping ratios and sensitivity constants from large airport calibration pads, and a well calibrated test strip, respectively. However, calibration pads are expensive to build and suitable test strips are difficult to find and expensive to calibrate. Also, both pads and test strips are subject to variations in radiation output due to variable moisture content and radon loss, and this can lead to considerable calibration errors. This paper describes an alternative approach to the calibration of airborne gamma-ray spectrometers. We have built a set of portable calibration sources, and have used these to simulate the response of an airborne spectrometer to infinite sources over a range of altitudes. The calibration has produced conventional 3-channel stripping ratios and sensitivity constants as a function of altitude, as well as good quality, pure multichannel spectra which are fundamental to any research into improved processing techniques. The sources are inexpensive, and since they are completely sealed, they do not suffer from variations in radiation output.

伊豆大島三原山の噴火と陥没 空中ガンマ線及び熱赤外線データの解析

Mihara Volcano, lzu-Ooshima Is., erupted violently on 15th November, 1986. At that time, vast erupted lava covered in craters on summit of the Volcano. After one year, the same volcano erupted again on 16th November, 1987. And then, though this eruption was on a small scale, the craters caved-in.Measurement on distribution of natural environmental gamma ray over lzu-Ooshima ls. was carried out on 4th December in 1986 and on 19th January in 1987, by the airborne gamma ray spectrometer system of AERRS on a helicopter Bell 212. Moreover, thermal imageries on the same island by the airborne thermal infrared video camera of AERMS were recorded at the following three epochs: on 28th November in 1986 after the first eruption, on 12th November in 1987 before the second eruption and on 19 th November in 1987 after the second eruption.As a result, some new facts on volcanic eruption and cave-in of the crater were obtained, that is, 1) On the occasion of volcanic eruption, spectrum composition of natural environmental gamma ray over the volcano shows some significant changes.2) Previous to cave-in of the crater, a ringlike thermal anomalous zone appears on the outskirts of crater, and it corresponds to a boundaryline of depressed area.

The construction of gamma-ray spectrometer calibration pads

Four calibration pads (8 × 8 × 0.5 m) have been constructed at Helsinki-Vantaa airport to relate the gamma-ray spectrometer counts to Th, U and K ground concentrations. The first pad was made of a typical mass of concrete to determine the background radiation. The second one was spiked with feldspar and the third and the fourth with powders containing uranium and thorium, respectively. The pads can be covered with a layer of water 3 cm thick to correct the airborne gamma-ray spectrometer calibration coefficients to a nominal flight altitude of 30 m. The concentrations of thorium, uranium and potassium in the calibration pads are: background pad 9.1 ppm Th, 4.1 ppm U and 2.54% K; potassium pad 2.4 ppm Th, 2.3 ppm U and 3.51% K; uranium pad 7.7 ppm Th, 53.2 ppm U and 2.30% K; and thorium pad 104 ppm Th, 11.3 ppm U and 2.55% K. Some preliminary measurements relevant to the calibration procedure are briefly discussed.

Some aspects of natural gamma radiation in ore search

Natural gamma radiation from the decay chains headed by 238U, 235U, 232Th and 40K can be detected and measured by a variety of instruments. Hand-held total count scintillometers indicate a portion of the gross gamma radioactivity. Portable four-channel differential spectrometers allow some discrimination between contributions from the U, Th and K series. Similar discrimination is possible with downhole and airborne gamma-ray spectrometers. Laboratory spectrometers using high-resolution germanium detectors show the contribution of many individual isotopes. The most obvious and widespread use for all of these instruments is in exploration for U deposits. Deposits which are radiometrically simple, such as Ranger 1, Northern Territory, need less complex instrumentation than deposits such as Mary Kathleen, Queensland, which has significant 232Th series activity. Radiometric complexity is also caused by disequilibrium in the U decay series. Exploration for base metal deposits can also be assisted by gamma spectrometry. The weak radiometric expression can show character attributable to recognizable events (such as K metasomatism) or can give an empirical pattern correlatable with lithology. There is increasing attention being given to airborne gamma surveys as a guide to geological mapping. A current resurgence of interest in natural gamma applications is being assisted by the development of better, more compact instrumentation.

Calibration of an Airborne Gamma Ray Spectrometer over Mary Kathleen Uranium Mine

Airborne gamma ray spectrometer test flights were made over the Mary Kathleen Uranium Ltd orebody with a large crystal assembly and modern digital recording system. The decrease in response with height is shown theoretically to be a combination of an exponential air absorption factor and a geometrical factor. The results of these tests indicate that for this specific detecting system and gamma ray source, the overall attenuation effect with height becomes exponential for the total count and thorium channels with apparent attenuation coefficients of 7.4 ´ 10?3 and 7.6 ´ 10?3 m?1 , respectively. The uranium channel count rate shows a departure from an exponential relationship; this departure can be allowed for by a height dependent component in the stripping (Compton scatter) correction factor. Ground surveys with a calibrated portable spectrometer enable abundance conversion factors to be derived for the airborne system.

Calculation of yield curves for air-borne gamma-ray spectrometers

Relative yield curves for an air-borne NaI-detector are calculated by computer simulation of radiation fields of different sizes and shapes. The detector is well shielded and remains in a horizontal position. The starting points of the fields, in favourable cases even the length of field, can be extracted from the results. The suitable altitude of flight is approximately 35 m. The calculated results are in agreement with laboratory results on a 1:100 scale.

SNOW-WATER EQUIVALENT MEASUREMENT USING NATURAL GAMMA EMISSION

The natural gamma radiation emitted by potassium, uranium and thorium is attenuated by snow. This attenuation depends on the water-equivalent of the snow layer. Air absorption coefficients were determined by flying at different altitudes over a uniform test strip and used to calculate the absorption coefficients for water. Results using the Geological Survey of Canada high sensitivity airborne gamma-ray spectrometer showed that snow water-equivalents up to 18 cm could be measured to an accuracy of 2 cm over suitable terrain. The importance of temperature and soil moisture corrections are discussed, together with statistical, instrumental and navigational errors.

Computer simulation of an air-borne gamma-ray spectrometer

A computer model is presented that simulates the data from a flight of an air-borne gamma-ray spectrometer over a surface containing gamma-ray sources of arbitrary horizontal dimension and coordinates. The mathematical formalism is given, together with a discussion of the assumptions or approximations that are made. Representative computational results are shown for simulated flights over circular sources and across linear sources.

Circle of investigation of an air-borne gamma-ray spectrometer

Calculations that will enable an experimenter to estimate the volume of material viewed by an air-borne gamma-ray spectrometer are presented. The effects of altitude, air density, source density, and gamma-ray attenuation coefficient were investigated. It is shown that the altitude is the most important parameter in the determination of the volume of material that produces a given percentage of the total observed signal.

Continuous Point Source Plume Behavior Out to 160 Miles

In order to study the behavior of the plume emitted from the air-cooled nuclear reactor at Brookhaven National Laboratory on central Long Island, radioactive argon-41 has been measured by an airborne gamma-ray spectrometer as far as 160 n mi downwind on a day with neutral stability. A vertical cross section at 144 n mi, obtained by a vertical “saw-tooth” sampling technique, shows the plume to be well organized with a nearly uniform vertical distribution to at least 2000 ft. The peak concentration, when corrected for radioactive decay, decreases by about a factor of 7–10 over a distance of 150 n mi and a travel time of 10–12 hr. The lateral standard deviation of the plume appears to fit an extension of the Pasquill-Gifford “D” curve with distance, where σy (n mi) is approximately equal to two-thirds of the travel time in hours.