Radiometric Anomalies Research Articles

Role of γ-ray spectrometry in detecting potassic alteration associated with Um Ba’anib granitic gneiss and metasediments, G. Meatiq area, Central Eastern Desert, Egypt

The radiometric responses of the Gebel (G., which means mountain in Arabic) Meatiq area display the overall high radiation of the high grade metamorphic Um Ba’anib granite gneiss, metasediments, as well as Arieki adamellite rocks. Whereas, the low grade metamorphic ophiolitic nappes country reveal the lowest radiometric response. The eU, eTh, and K contents tend to increase with the youthfulness of the plutons with a maximum amounts in the more alkali varieties, e.g., Arieki adamellite (580 Ma), then the high grade metamorphic rocks of the younger Meatiqian orogeny (626 ± 2 Ma). Also, these rocks reveal that the major radiometric anomaly with exposure rates ≈139 nGy/h, more than double of the global terrestrial values. While, the low grade metamorphic ophiolitic rocks reveal the lowest average exposure rates ≈46.8 nGy/h. The areas of high gamma ray values of F-parameter of Efimov (K × U/Th), ternary composite map, K map, K/eTh, and K/eU ratios maps are related to K enrichment conditions during formation (diagenesis) or deformation of the high grade metamorphic rocks and the Arieki adamellite intrusion. From the geochemical point of view, these areas are associated with rocks that are characterized by high-K calc-alkaline, calc-alkaline affinity, and enriched in REE.

Determination of Uranium in Rocks and Soils of South Cameroon by γ-ray Spectrometry

Analytical results of uranium concentration in rocks and soils collected in the area of Lolodorf of South Cameroon where a radiometric anomaly had been found by previous investigation are reported. The analysis has been carried out by γ-ray spectrometry using a Ge detector. Radioactivity concentration of 235U was determined by comparing peak count rates of samples with those of natural uranium standard sample. Radioactivity concentration of 238U was calculated from the 235U radioactivity concentration by assuming that the isotopic composition of 235U is 0.7200% of natural abundance. Difference of γ-ray self-absorption between the samples and uranium standard was accurately corrected by using a mass attenuation coefficient of samples and standard measured by using an external γ-ray source. The radioactivity concentrations in rocks ranged from 0.92±0.02 to 228±14Bq/kg(235U) and 20±5 to 4868±307Bq/kg(238U), while radioactivity concentrations in soils ranged from 3±1 to 63±2 Bq/kg(235U) and 71±19 to 1346±38 Bq/kg(238U). The γ-ray dose rates in air at 1m above the ground surface due to the high uranium concentration in soil samples were ranged from 33±9 to 622±18 nGy/h. The highest uranium concentration of about 0.04% in rock was found in the analysis. These results show that the uranium content in the studied areas is significantly high and the studied areas have a potential for uranium mining.

Geochemistry of granites and metasediments of the urban area of Vila Real (northern Portugal) and correlative radon risk

Radon concentration was evaluated in dwellings of the urban area of Vila Real (Northern Portugal). The area is mainly composed of Hercynian granites and Cambrian metasediments, and CR-39 passive detectors (n = 112) were used for the purpose. The results obtained in winter conditions suggest that the most productive geological unit is the Hercynian granite G1 (geometric mean of 364 Bq/m3), while Cambrian metasediments of the Douro Group show the lowest average indoor radon concentration (236 Bq/m3). The geological, geochemical and radiological data obtained suggest that the most effective control on the radon concentrations of the area is related with the uranium content of the rocks; indeed, the highest contents were observed in granite G1 (21 ppm) and the lowest in the metasediments (3 ppm). This is also confirmed by the results obtained for groundwater, where granites present the highest concentrations of dissolved radon (up to 938 Bq/l), uranium (5–18 ppb) and gross α activities (0.47–0.92 Bq/l). No important radiometric anomalies were found in relation with geological structures such as faults, veins and contacts, but a moderate increase of the uranium content can occur locally in such structures. Petrographic observations and SEM studies show that uranium is mainly contained within the rock in heavy accessory minerals (apatite, zircon, monazite, xenotime), which reduces radon emanation. Notwithstanding, due to the high U contents granites show a significant potential to induce indoor radon concentrations in dwellings in excess of the recommended value of 400 Bq/m3. Overall, we can conclude that the region of Vila Real presents a moderate to high radon risk in dwellings and groundwater.

On the influence of faulting on small-scale soil-gas radon variability: a case study in the Iberian Uranium Province

In order to evaluate the influence of faulting on the variability of geogenic radon at detailed scale (1:2000), data on gamma ray fluxes, U and Th concentrations in rocks, radon in soil-gas and radon in groundwater were collected in three target areas on the Oliveira do Hospital region (Central Portugal). This region stands on the Iberian Uranium Province, and is dominantly composed of Hercynian granites and metasedimentary rocks of pre-Ordovician age, crosscut by faults with dominant strike N35°E, N55°E and N75°E. Radiometric anomalies are frequent, associated with faults of the referred systems and metasedimentary enclaves; the analytical data confirms that these anomalies are produced by local high uranium contents in rocks and fault-filling materials ( n = 34, range 13–724 ppm), while other radiogenic elements are relatively constant (e.g. Th 4–30 ppm). Radon concentration in soil can be extremely high, up to 12,850 kBq m −3 ( n = 215), with a large proportion of results above 100 kBq m −3. Unsurprisingly, groundwater also shows high radon concentrations, with observed values in the range 150–4850 Bq.L −1 ( n = 17). From the results it is concluded that metasedimentary enclaves, as well as faults, can accumulate uranium from circulating fluids, and as a consequence, strongly locally enhance geogenic radon potential. Due to this fact, for the purpose of land use planning in such uranium-enriched regions, very detailed geological mapping is needed to precisely recognize radon high risk areas. A correlation between radon concentration in soil or in groundwater and gamma ray fluxes was established pointing to the possible use of these fluxes as a first step in assessing geogenic radon potential, at least to geological setting similar to the study area.

A comparative study of uranium–thorium accumulation at the western edge of the Arabian Peninsula and mineral deposits worldwide

Jordan, located at the western edge of the Arabian Plate, stands out from the remaining part of the Arabian Peninsula by its abundance in radioactive elements, mainly uranium, in a way so far not found elsewhere on the Arabian Peninsula. Uranium (U) and thorium in Jordan are concentrated in eight different types of ore mineralization: (1) intrusive-related (intramagmatic), (2) vein-type, (3) superficial, (4) sandstone-hosted (5) limestone-hosted, (6) U-Th-REE placer-type, (7) black shales, and (8) phosphorites. The major concentration of radioactive elements are synsedimentary and diagenetic in nature, mainly in near-shore marine depositional environments where uranium contents are abnormally high in the late Cretaceous to Paleogene phosphorites and increasing towards the mobile shelf of the Tethys ocean. These uraniferous phosphorites form the source of uranium that was redeposited within terrigenous chemical residues of lacustrine-fluvial depositional systems in Central Jordan (calcretes). Faultbound radiometric anomalies are caused by hot brines being vented along with the Jordan-Dead-Sea rifting. Presumably, low-grade U accumulation in (hot) black shales and marls of Silurian age are responsible for these radiometric anomalies. In the present paper, the Jordanian uranium concentrations are compared with reference types of uranium deposits elsewhere in the world to get an idea if the geological, chemical, and mineralogical features of analogue uranium mineralization in Jordan are indicative of economic targets. The uranium concentration in Jordanian phosphorites has been tracked beyond the border into Syria, Iraq, Israel, and Saudi Arabia. The uranium potential in neighboring countries is assessed based on the current geological data available for the Mediterranean Phosphorite Belt which is poised to become a another string to the bow with respect to energy supply on the Arabian Peninsula.

Airborne electromagnetic hydrocarbon mapping in Mozambique

The Inhaminga hydrocarbon exploration licence in central Mozambique sets the location for a multi-method airborne geophysical survey. The size of the Inhaminga block, spanning some 16 500 km2 from Beira to the Zambezi, limited available data and a tight exploration schedule made an airborne survey attractive for the exploration portfolio. The aim of the survey was to map hydrocarbon seepage zones based on the evidence that seepage may create resistivity, radiometric and sometimes magnetic anomalies. The survey involved a helicopter-borne time domain electromagnetic induction system (AEM) and a fixed wing magnetic gradiometer and radiometer.Our data analysis highlights an anomaly extending some tens of kilometres through the survey area along the eastern margin of the Urema Graben. The area is imaged by AEM as a shallow resistive unit below a strong surface conductor and shows high Uranium and low Potassium concentrations (normalised to mean Thorium ratios). A seismic dimming zone on a 2D seismic line crossing the area coincides with the resistivity and radiometric anomaly. The geological exploration model expects seepage to be linked to the graben fault systems and an active seep has been sampled close to the anomaly. We thus interpret this anomaly to be associated with a gas seepage zone. Further geological ground work and seismic investigations are planned to assess this lead.Airborne data has further improved the general understanding of the regional geology allowing spatial mapping of faults and other features from 2D seismic lines crossing the survey area.

Aeromagnetic Data Interpretation of Wadi Hawashiya Area for Identifying Surface and Subsurface Structures, North Eastern Desert, Egypt

The studied area is located within the North Eastern Desert of Egypt. The lithology of this area comprises rocks related to Precambrian, Cambrian, Carboniferous, Albian to Cenomanian, Late Cenomanian to Early Turonian and Quaternary. Aeromagnetic data are fully analyzed and interpreted where the analysis and interpretation are mainly devoted towards outlining significant surface and subsurface tectonic trends and their relationship with T.C. radiometric anomalous zones. According to the qualitative interpretation of the geological and aeromagnetic, and its shaded relief maps, the lineament structural map is constructed through the signatures of the contours and shaded relief of the total aeromagnetic data. Werner de-convolution (dyke model) was applied to the aeromagnetic anomaly map. The depth estimates calculated for these major magnetic anomalies range between 0.5 and 3.9 km. lineament structural map and magnetic depth map values have been utilized to construct the interpretation of the main subsurface structures affecting the studied area, which correlate with the previous studies of aeroradiomtric anomalous zones allover the study area. The obtained results showed the most significant structural trends affecting the distribution of these radiometric anomalies in the present study area. Four main tectonic trends are outlined from the total aeromagnetic and shading relief maps. These trends were defined as N60E, N45W and E-W.

Features of acid–saline systems of Southern Australia

Abstract The discovery of layered, SO 4 -rich sediments on the Meridiani Planum on Mars has focused attention on understanding the formation of acid–saline lakes. Many salt lakes have formed in southern Australia where regional groundwaters are characterized by acidity and high salinity and show features that might be expected in the Meridiani sediments. Many (but not all) of the acid–saline Australian groundwaters are found where underlying Tertiary sediments are sulfide-rich. When waters from the formations come to the surface or interact with oxidised meteoric water, acid groundwaters result. In this paper examples of such waters around Lake Tyrrell, Victoria, and Lake Dey-Dey, South Australia, are reviewed. The acid–saline groundwaters typically have dissolved solids of 30–60 g/L and pH commonl y ρ ∼ 1.04) regional water flows out over the denser ( ρ ∼ 1.16) lake brines. In the spring zones examined, large amounts of Fe are commonly precipitated. In a few places minerals of the alunite-jarosite family are formed which can trap many other metals, including Ra. The studied groundwater systems were discovered by U exploration programs following up radiometric anomalies related to this Ra. Evaporation concentrates the lesser soluble salts (gypsum and some halite) on the surface of the lakes. The lake brines contain most of the more soluble salts and form a column within the porous sediments which is held in place by hydrostatic forces around the salt-lake. These brines are near-neutral in pH. These observations are in contrast to the jarosite-bearing aeolianites found on the Meridiani Planum, Mars. These have an almost homogeneous distribution of Fe oxides and jarosite, with little separation of salts with different solubilities (CaSO 4 and MgSO 4 ) or differential separation of elements with differing solubility (K, Na, Ti, Cr). Thus, it is considered unlikely that groundwaters or evaporative salt-lake systems, as found on earth, were involved. Instead, these features point to a water-poor system with local alteration and very little mobilization of elements.

Hydrothermal Alteration Inferred from a Radiometric Survey on Lipari (Aeolian Islands, Italy).

We carried out a radiometric survey on a sector of the Lipari island particularly affected by argillification due to hydrothermal processes. Preliminarily, on the basis of a wide set of field measurements over unaltered outcrops, we obtained a reference spatial distribution of U, Th and K. The concentration of these radioelements increases with the degree of magma differentiation; the U and K content shows maxima in the felsic materials, belonging to the youngest volcanic cycles (13.4 ppm and 3.7% on average, respectively), and minima in the mafic rocks of the oldest cycles (2.7 ppm and 2.0%). The Th/U ratio insignificantly varies with magma differentiation and it equals 3.2. Laboratory measurements on volcanic products from altered zones were then used to identify radiometric anomalies related to hydrothermal processes. On average, argillized rocks have radioactivity approximately five times lower than that measured in the unaltered volcanics. The Th/U ratio against U is rather uniform, whereas Th/K ratio is in the order of 3.7 in less altered rocks (K > 0.2%) and it ranges from 7 to 11 in kaolin deposits having the largest depletion in potassium.

Radiometric survey for exploration of hydrothermal alteration in a volcanic area

A radiometric survey on a sector of the Lipari volcanic island particularly affected by argillification due to hydrothermal processes was carried out. Preliminarily, on the basis of a wide set of field measurements over unaltered outcrops U and K distribution maps were obtained. The concentration of these radioelements increases with the degree of magma differentiation; the U and K content shows maxima (13.4 ppm and 3.7% on average, respectively) in the felsic materials, belonging to the youngest volcanic cycles, and minima (2.7 ppm and 2.0%) in the mafic rocks of the oldest cycles. The Th/U ratio insignificantly varies with magma differentiation and it equals 3.2. Laboratory measurements on volcanic products from altered zones were then used to identify radiometric anomalies related to hydrothermal processes. On the average, argillized rocks have radioactivity approximately five times lower than that measured in the unaltered volcanics. The Th/U ratio against U is rather uniform, whereas Th/K ratio is of the order of 3.6 in less altered rocks K > 0.2%) and it ranges from 7 to 11 in kaolin deposits having the largest depletion in potassium.

Aeromagnetic data analysis for the identification of concealed uranium deposits: a case history from Singhbhum uranium province, India

Abstract Aeromagnetic data over a part of the Singhbhum uranium province, India, within the framework of ‘Operation Hard Rock’ have been analyzed. The aeromagnetic anomaly map, its analytic signal amplitude, the Euler solutions and apparent susceptibility map helped in identifying the nature and depth of the magnetic sources in the study region. The Singhbhum Shear Zone is clearly delineated. The location of the mined uranium deposits coincide with the shallow magnetic sources. The present study also identified distinctive magnetic sources between the Dalma Volcanics and the Chotanagpur Granitic Gneissic Complex that suggest the presence of a sub-surface shear. The magnetic sources in this newly identified shear zone lie at a depth of 200 m and depict a similar magnetic signature and susceptibility as those of the Singhbhum Shear Zone where uranium is being mined. This shear could be the subsurface contact between high-grade metamorphic rocks (amphibolite facies) to the north and the greenschist facies low-grade metamorphics to the south. This shear zone, also characterized by radiometric anomalies, possibly indicates a zone of concealed uranium deposit that can be explored in the future. The present analysis shows the importance of aeromagnetic surveys and its utility in exploration for concealed mineral deposits.

Features and effects of some acid-saline groundwaters of southern Australia

Many salt lakes have formed in southem Australia where regional saline groundwaters form discharge zones. Some of these groundwaters are characterized by acidity and high salinity. These are commonly found where underlying Tertiary sediments are sulfide-rich. When waters from the resulting formations come to the surface or interact with oxidized meteoric water, acid groundwaters result from oxidation of the sulfide and/or the dissolved ferrous iron. In this paper examples of such waters around Lake Tyrrell, Victoria and Lake Maurice, South Australia, are reviewed. The acid-saline groundwaters typically have dissolved solids of 3060 mg/L and pH commonly <4.5. Many contain high concentrations of Fe and other metals, leached from local sediments. The combination of acidity and salinity also releases radium, a short-life daughter nuclide of uranium and thorium, by a process of cation exchange. Around salt-lakes, these acidic waters often emerge at the surface in marginal spring zones where the low density (s.g. -1.04) regional water flows out over the denser (s.g. -1.16) lake brines. In the spring zones examined, large amounts of iron are commonly precipitated. In a few places minerals of the alunite-jarosite family are formed which can trap many other metals, including radium which generates radiometric anomalies. Such anomalies may however be short-lived. The age of the present salt lakes is <40000 years but in that time very large amounts of many metals are estimated to have been deposited into the lakes but, apart from iron, this appears to be dispersed and to not have formed significant accumulations. Evaporation concentrates the least soluble salts (gypsum and some halite) on the surface of the lakes. The lake brines contain most of the soluble salts and form a colunm within the porous sediments which is held in place by hydrostatic forces around the salt lakes. These brines are near neutral in pH.

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.

A spatially selective approach to Doppler estimation for frame-based satellite SAR processing

When Doppler centroid estimators are applied to satellite synthetic aperture radar (SAR) data, biased estimates are often obtained because of anomalies in the received data. Typical anomalies include areas of low SNR, strong discrete targets, and radiometric discontinuities. In this paper, a new method of Doppler centroid estimation is presented that takes advantage of principles such as spatial diversity, estimator quality checks, geometric models, and the fitting of a "global" estimate over a wide area of a SAR scene. In the proposed scheme, Doppler estimates are made over small blocks of data covering a whole frame, so that all parts of the scene are potentially represented. The quality of each block estimate is examined using data statistics or estimator quality measures. Poor estimates are rejected, and the remaining estimates are used to fit a surface model of the Doppler centroid versus the range and azimuth extent of the scene. A physical model that relates the satellite's orbit, attitude, and beam-pointing-direction to the Doppler centroid is used to get realistic surface fits and to reduce the complexity (dimensionality) of the estimation problem. The method is tested with RADARSAT-1 and Shuttle Radar Topography Mission X-band SAR (SRTM/X-SAR) spaceborne data and is found to work well with scenes that do have radiometric anomalies, and in scenes where attitude adjustments cause the Doppler to change rapidly.

Sandstone-type uranium deposits in South Australia and North America: A comparison of their geophysical characteristics

The sandstone-type uranium deposits in the Frome Embayment and Eucla Basin, South Australia, are characterised by uranium oxide mineralisation in the form of roll-fronts at redox boundaries. The host rocks are Tertiary clastic sediments deposited in palaeochannels, but now covered by up to 150 m of cover. None of the deposits are associated with surface radiometric anomalies. These deposits were discovered in the 1970s, but Government policy prevented exploitation at that time. With mining now permitted, there is renewed interest in these deposits and exploitation is underway, or imminent, and new resources are being sought. The comparatively low cost of drilling restricted the use of geophysical techniques during initial exploration, with downhole logging being the most extensively used method. Nevertheless, reconnaissance geophysical surveys were used to locate the prospective palaeochannels, with resistivity soundings proving most useful. Recent TEMPEST airborne EM surveys over the Billeroo/Curnamona and Yarramba palaeochannels, located in the Frome Embayment, have demonstrated that this method allows detailed mapping of the palaeochannels, despite the thickness of the cover. The electromagnetic response is attributed to the conductivity of contained saline groundwater. Once the palaeochannel sequence had been located, alteration zoning within the host sandstones, associated with characteristic responses on downhole logs, was used to locate the redox interface. An understanding of the channel stratigraphy and sedimentary environment, specifically the likely location of prospective permeable formations, was also largely based on downhole logs. Attempts to use electrical methods to detect areas of increased pyrite content, associated with the redox boundary, and to determine the redox state of the host sandstones, met with mixed success. A comparison of the use of geophysics in South Australia with exploration for similar deposits in the United States shows some aspects in common, in particular the use of downhole logging to map alteration. However, care must be taken when making comparisons, since variations in the type of deposit can significantly affect their geophysical signatures.

NOVAS OCORRÊNCIAS DE URÂNIO NA REGIÃO DE LAGOA REAL, A PARTIR DA SUPERPOSIÇÃO DE DADOS GEOFÍSICOS, GEOLÓGICOS E DE SENSORIAMENTO REMOTO

The origin and control of Uranium occurrences and deposits at the Uraniferous District of Lagoa Real, State of Bahia (Brazil), has been largely studied over the last decades. Currently, the deposit controls are bounded to albitites in the Granite-Gneissic Complex at the center-south area of Lagoa Real. Data from the Sao Timoteo Airborne Geophysical Project have been analyzed and superimposed to geological, geophysical and remote sensing data in order to identify new occurrences of Uranium for prospection. Reappraisal of the gammaspectrometric data (K, U e Th) shows that distinctive litotypes indicate some radiometric anomalies in the northeast area. The magnetometric data suggest the existence of depth-rooted magnetic blocks separated by shear zones recognized by the Landsat-5TM’s image. The combination of these products with the geological information shows a structural evolution not previously described in literature. The kinematics of the shear zones with NS, NE-SW and NE-SW directions was established based on bodies´ displacement and magnetic features. The ductile-ruptile, ruptileductile and ruptile structures are compatible with the geological framework of the Sao Francisco Craton during the Brazilian Cicle: East-West convergence with West mass transport. The continuing deformation produced dextral and sinistral shear zones that resulted in the inflection of the pre-existent foliation. The well-known Uranium deposits are located in the N-S shear zones; the new occurrences, located in the northeastern area, are associated with the NW-SE shear zones and they introduce new targets for prospection.

Gamma-ray analysis of the sediments overlying the Helez and Kochav oil fields, Israel

A very sensitive, low-background gamma-ray spectrometer was used to show that a negative total gamma-ray intensity anomaly over the Helez and Kochav oil fields is due to the depletion in all three radionuclides ( 40K, 232Th, and 238U). The location of the radiometric anomalies at the surface above petroleum reservoirs at depth, suggests a genetic connection. It is proposed that the soil radioactivity is not the result of vertical radionuclide transport from the reservoir, rather that migrating hydrocarbons interact and degrade clays and other soil minerals which then release these large cations that substitute for each other diadochously.

Surface radiometric anomalies in mineral deposits covered by lateritic weathered mantles. Example of the uranium deposit at Laranjeiras, Lagoa Real, B: E. B. Rocha, J. M. Schmitt & A. J. Melfi, Revista Brasileir de Geofisica, 11(2), 1993, pp 187–204

Surface radiometric anomalies in mineral deposits covered by lateritic weathered mantles. Example of the uranium deposit at Laranjeiras, Lagoa Real, B: E. B. Rocha, J. M. Schmitt & A. J. Melfi, Revista Brasileir de Geofisica, 11(2), 1993, pp 187–204

Integrated radiometric prospecting for petroleum in the Kailu Basin, Inner Mongolia, China

The Kailu Basin of Inner Mongolia contains anticlinal petroleum traps in the Jurassic Jiufutang Formation. Characteristic radiometric anomaly patterns were established over a known hydrocarbon pool and were used as a model to predict the potential of two unexplored traps. Some environmental variables, including the occurrence of sand dunes, interfere with the signal associated with the petroleum reservoirs making it difficult to pinpoint their location. Thermoluminescence dosimetry (TLD), which measures an accumulated radiation flux signal rather than short-term radiation obtained by gamma ray spectrometry, is less affected by these factors and is better able to predict the location of oil reservoirs. The predictions made in the present study have been tested by drilling 7 wells. The results of these tests show that our interpretation was essentially accurate.

OVERVIEWS: Geophysical signatures of Western Australian mineral deposits: an overview

Geophysical exploration in Western Australia is hindered by a mantle of conductive and magnetic weathered rocks that covers much of the State. This has required the adaptation of most geophysical methods for successful application in Western Australian conditions, and has led to the development and widespread use of, for instance, high-resolution aeromagnetics and time-domain electromagnetic methods. However, these difficulties have not prevented geophysics from being an integral part of exploration for base metal, diamond, gold, iron ore, manganese, nickel and uranium deposits in Western Australia. Mississippi Valley-type base-metal deposits are difficult geophysical targets and direct detection of the ore is not usually possible. However, gravity and magnetic data can be used to locate basement highs associated with such deposits and, on a semi-regional scale, induced polarisation surveys have been used to locate marcasite halos associated with the orebodies. Volcanic-hosted massive sulphide base-metal deposits have variable geophysical responses. Physical property contrasts with their host are highly variable and thus methods such as magnetics, induced polarisation and electromagnetics may fail to generate recognisable responses. Mise-a-lamasse surveys have proved highly successful for mapping such mineralisation on a prospect scale, once it has been intersected by drilling. The only example of a sedimentary exhalative deposit in the State for which data are available has distinct gravity, magnetic and time-domain electromagnetic anomalies. Diamonds in Western Australia mainly occur in lamproite pipes. These pipes have variable magnetisations but can usually be detected using high-resolution aeromagnetic surveys. The pipes can also be conductive and mapped using electromagnetic techniques if the host rocks are suitably resistive. The major geophysical method utilised in gold exploration is high-resolution aeromagnetics which is used to map favourable structures and rock types. Electrical and electromagnetic methods can also be used where gold is associated with sulphides. Geophysics has been comparatively little used in exploration for iron ore. Exploration for supergene-enriched deposits mainly uses aeromagnetics, to map favourable structures and to detect magnetite destruction and replacement associated with mineralisation, and gamma-ray logging for stratigraphic correlation purposes. The major technique used in manganese exploration is the gravity method, taking advantage of the positive density contrast between ore and host rocks. The mineral sands industry uses aeromagnetic data to map placer deposits containing ilmenite, but the relatively low cost of drilling limits the use of geophysical exploration methods. Nickel sulphide mineralisation can be directly detected using induced polarisation and electromagnetic techniques. Gravity and magnetic surveys are also used, but mainly in a mapping role. Carbonatitic intrusions associated with rare-earth-element mineralisation give rise to large magnetic anomalies. Radiometric and gravity anomalies can also occur. Uranium mineralisation has been directly detected using radiometric data, but some deposits are concealed below cover. Magnetic, electromagnetic, electrical and gravity surveys can be used to locate the rocks and structures which host mineralisation.