Airborne Geophysical Surveys in the North-Central Region of Goias (Brazil): Implications for Radiometric Characterization of Subtropical Soils
11 Airborne Geophysical Surveys in the North-Central Region of Goias (Brazil): Implications for Radiometric Characterization of Subtropical Soils
Guimarães S.N.P., Hamza V.M. and Justo J. S.
National Observatory (ON/MCT), 20921-400, RJ, Brazil.
Abstract
In this work we present progress obtained in analysis airborne geophysical survey data for the north-central region of the state of Goias (Brazil). The data base includes results of both gamma spectrometric and aeromagnetic surveys. Analysis of radiometric data has allowed determination of relative abundances of natural radioactive elements (Uranium, Thorium and Potassium) in the main soil types. The results obtained indicate that most of the subtropical soil types are characterized by Uranium contents of greater than one parts per million (ppm). Only ultisol and oxisol soils are found to have Uranium contents lower than one ppm. Thorium and Potassium abundances also display trends similar to those of Uranium. The K/U ratios fall in the expected range of values for common soils while the Th/U ratios are higher than normal. This latter observation may indicate a characteristic feature of subtropical soils. Alternatively it may be considered as indicative of disequilibrium conditions in radioactive series and consequent underestimation of Uranium in soil layers of the study area. A detailed examination of the radiometric data reveals that the currently adopted system for classification of soils is rather insensitive to variations in radioelement abundances. In this context we point out the possibility of using results of radiometric surveys as a convenient complementary tool in identifying geochemical zoning of soils in subtropical environments. Analysis of aeromagnetic data reveals the presence of a large number of magnetic lineaments, indicative of distinct structural features in subsoil layers. The analytic signal values point to the existence of substantial small scale variations related to lithologic changes. There are indications that deposition of soil types are controlled to a large extent by the system of northeast – southwest trending faults and fracture systems. According to results of radiogenic heat production calculations the cambisol soil in this region are found to have a mean heat production of 3.32 ± 5.9 µWm -3 while that of ultisol soil is only 0.36 ± 0.3 µWm -3 . The mean heat production of soil layers at the surface is 0.68 ± 0.4 µWm -3 . Heat production values of basement rocks are estimated to be more than 1.3 µWm -3 , after corrections for density changes and non-equilibrium conditions of Uranium series. Keywords: airborne radiometry; aeromagnetic data; north central Goias; soils types.
1. Introduction
The intensities of nuclear radiations in any particular terrestrial environment are in general related to the abundances of natural radioactive elements in rocks and soils of that locality (Adams, 1961; Roser and Cullen, 1964; Paul et al, 1982; Wilford et al., 1997). Gamma ray spectrometric methods have been widely used in measurements of radioactive minerals in soils and basement rocks (see for example: Adams and Gasparini, 1970; Iyengar et al, 1980; Mohanty et al, 2004). It has also been used for mapping area extent of geologic rock formations associated with such radioactiveal. (2003), Meradioactiveof airbornerocks beneweatheringgeologic tithe geosphwidely useBrazil incluNevradiometricfacilities itpresent woout in the sis to mapradiogenic
2. Geology
Accstructural pduring thegranulitic Uruaçu andthe volcanNeoprotero TheRADAM pcarried oute elements. Sapucaia eteasurementse elements ie measuremeath the sog of basemime periodshere – biosped as suppleude Vasconvertheless c surveys wt has been ork we repostate of Goip the radioheat produ y and Soil C cording to province ofe Brasilianoblocks (Med Brasília) no sedimenozoic age. Ae surface feproject of tht for determExamples t al. (2005).s of gamma in the top sments are cooil cover. Tment rocks , between sphere interaementary toncelos et al. very few with soil typossible toort progress ias, an impoometric chauction of sub
Cover in th earlier geolf Tocantinso orogenic edian Massof early Prontary sequeA simplified
Figure ( features andhe 1980, by mining the pof such stu. radiation atsoil layers wonsidered reThe reasonand in addsoil and subactions. Henools in geol(1994) andstudies hasypes in tropo analyze reobtained inortant sub traracteristics bsoil layers he Study Ar logic studie is situatedcycle. Thesif of Goiasoterozoic agences (belid geologic m (1) Simplifie d topographthe Ministrphysical anddies carriedt ground levwith thickneepresentativning is thatdition somebsoil layers. nce results ologic mappid Maas et als been carpical regionesults of lan analysis oropical regioof subtroon regional rea es (see for d in the rege main strus) of Achaege, the magieved to bmap of the a d geologic m hy of the stry of Minesd chemical cd out in Bravel provide esses of lessve of radiomt soil covee degrees oAlso, soil lof gamma sing. Examp. (2003). rried out fons. With thearge scale aof airborne gon in the cepical soilsl scales. example, Agion affecteuctural unitean age, mma belt of Gbe associatearea is given map of the stu tudy area hs and Energycharacteristazil include informations than 40cmmetric charaers are in of mixing tlayers play spectrometrples of suchor relating e advent ofair-borne ragamma specentral parts and its reAlmeida, 19d by cratonts are the meta sedimenGoias of mied with isn in Figure ( udy area. has also bey. A numbetics of soil cthe works n on the abum. Nevertheacteristics ogeneral detake place,key roles inric methodsh studies carresults off computer adiometric dctrometric dof Brazil. Televance fo968; Wingenic collisiongranitic – ntary belts id Proterozosland arc r(1). een mappeder of studiescover in the of Blum etundances ofeless resultsof basementrived from over longn several ofs have beenrried out inf air borneprocessingdata. In thedata carriedThe purposeor estimate, 1984) then processesgneissic –(Araguaia,oic age andregions) of d under thes have beene study area t f s t m g f n n e g e d e e e s – , d f e n a (EMBRAPresults of pthe study aoxisol and Accfeldspar anvalues of a(x10 kg/mThe mean v
3. Databas
Thesurveys of and releaseGoias (SGFigure (3)ArenópolisMara RosaThethe radioaclocal geomon flight ahas spacindata acquis PA, 2009). previous stuarea. Most pultisol soilscording to Knd colloidaabout 2,65xm ). This is value of bu se e data sets f the state ofed since 201M-GO). Th). The survs arc (inclua Arc, Southe data sets active elememagnetic fiealtitude and g of 500m sition, but aMore recenudies. Repropredominans. Kiehl (1979al aluminumx10 kg/m a consequenulk density i Figure used in thef Goias. The10 for acadehe locations vey work uding the Juh Brasilia Bacquired incents of Uraneld. These datmospherand altitudealso during antly, Kerr (oduced in thnt among th9) the domm silicates. the bulk dnce of the rs 1,5x10 kg e (2) Map of e present we data acquiemic researand area exwas carrieduscelândia Selt and nortclude measunium, Thoridata sets areic conditione of 100m. and after the(2001) has he map of Fhese in the sinant constEven thoudensities of relatively hig/m . Soil types in work has beisition was cch by Secrextent covered out in fSequence), th-eastern purements ofium and Poe recorded ins. The fligQuality coe surveys. carried outFigure (2) arstudy area wituents of sugh individumost of theigh porositie n the state of en acquiredcarried out etariat for Ged by the sufive stages,Mara Rosaparts of Goiaf the intensiotassium as in separate ght lines wentrol tests wt a more dere the eight with large spsoils in the ual minerale soils varyes which va f Goias. d as part ofduring the pGeology andurveys are i, covering a Magmaticas. ity of gammwell as thechannels, aere set in nowere carriedetailed analsoil types ipatial coverstudy area l densities y between 1ary between f airborne gperiod of 20d Mining of ndicated in regions dec Arc, westma radiatione characterialong with iorth – southd out not on lysis of thedentified inrage are theare quartz,have mean1.1 and 1.6n 40 a 60%.geophysical004 to 2005the state ofthe map ofescribed astern part ofn emitted byistics of theinformationh direction,nly prior to e n e , n 6 . l 5 f f s f y e n , o Figure (3) Areas covered in airborne geophysical data acquisitions in the state of Goias.
4. Methodology Used in Data Processing
Most of the data processing in the present work has been carried out using the computational package Geosoft ® , Oasis Montaj. Initially the raw data were corrected for the perturbing effects of technical survey operations (LAG and Heading effects and altitude variations). In addition, procedures were adopted for filtering, leveling and micro leveling operations, as per standard data processing techniques (see for example, Hood and Ward, 1969; Guimarães and Hamza, 2009). Following this stage a suitable grid system was set up and homogenized data sets derived from the raw data sets for the chosen grid system, using suitable interpolations schemes. The grid size used for interpolations is 125m, which is in accordance with Nyquist criteria. The procedures employed included the method of Minimum Curvature – RANGRID for the geomagnetic data sets and the technique of bi-cubic splines – BIGRID for radiometric data sets. The unification of data sets was achieved using the standard techniques of suture (Geosoft ® ). The aeromagnetic data were further corrected for the effects of diurnal variations and of the reference magnetic field. The values of primary radiometric data of air borne surveys are in units of counts per second (cps). These were transformed into values of relative abundances of U, Th and K using conversion factors specific to instrumentation and sensor systems employed and characteristics of each survey operation. In general, these factors depend on the sensitivity and geometry of detectors used and the survey altitude. Such factors are known as sensitivity coefficients (IAEA, 1991) and are specific for the measuring equipment used in each data collection. The values of sensitivity coefficients relevant for data employed in this work are listed in Table 1. Table (1) – Sensitivity coefficients used in this work.
Area
Equipment
Sensitivity Coefficients
K (cps to %) U (cps to ppm) Th (cps to ppm) 1
PT-FZN 79,01 9,71 5,05 PT-WOT Upto flight 26 77,11 11,23 5,21 PT-WOT After flight 26 78,05 13,05 5,10 2
PR-FAS 72,10 12,87 4,41 PT-WQT 76,25 11,34 4,45 3
PT-FZN 78,76 8,89 4,94 4
PT-FZN 78,76 8,89 4,94 PT-WOT 78,05 13,05 5,10 5
PT-FZN 80,28 12,59 4,99 PT-WOT 78,50 12,15 4,57
5. Results Obtained
In presenting results of the radiometric survey we focus first on large scale regional features present in the study area. The geographic distribution of Uranium and Thorium abundances, illustrated in the maps of Figures (4) and (5), provide the basis for this analysis. Referring to Figure (4) we note that Uranium abundances are in the range of 0 to 2.5 ppm but there are several localized zones of relatively high values (in excess of 1.5 ppm) distributed all over the study area. Though no clear patters are evident at first sight, careful comparative examination of the maps in figures (2) and (4) reveal that lower values of Uranium abundances are roughly related to the presence of ultisol soil and oxisol soil in the study area. Similar patterns can also be seen in the maps of Thorium and Potassium abundances, illustrated respectively in Figures (5) and (6). The Thorium abundances are in the range of 0 to 20 ppm, while that of Potassium is in the range of 0 to 2.5%. The overall pattern of abundances of radioactive elements Th and K seem to be related to the characteristics of soils types present in near surface layers. The ternary diagram presented in Figure (7) illustrates the distribution of relative abundances of the three radioactive elements, U, Th and K. The patterns in this figure are quite complex, but it is possible to identify some general trends. Foremost among these are the zones with relative enrichment of Potassium in the central parts of the study area. There are indications of relative enrichment of Uranium along the western border of the study area, while Thorium enrichment occurs along the eastern border.
Figure (4) Geographic distribution of Uranium in the study area.
Figure (5) Geographic distribution of Thorium in the study area.
Figure (6) Geographic distribution of Potassium in the study area.
Figure (7) Ternary diagram of the radioactive elements (U, Th and K) in the study area.
In an attempt to examine such relations in detail, grid averaged values of radioactive elements and overall mean values were computed. The mean value of Uranium in the study area is 1.28 ± 1.06 ppm while that of Thorium is 11.4 ± 8.5 ppm. The large values of standard deviations of the estimates obtained are indications of substantial variations in the lithologic characteristics of soil types in near surface layers. These results also reveal that Th/U ratios in the study area are high (> 8). It is possible this is a characteristic feature of radioelement abundances of the study area. The alternate possibility is that disequilibrium conditions of radioactive series exist in soil layers and have contributed to underestimation of Uranium in surface layers. The mean value of Potassium is 1.2 ± 0.8 % which imply that K/U is of the order of 1x10 , a value typical of common crustal rocks. In presenting results for the aeromagnetic survey the focus has been on identification of structural features present at shallow depths. In such a context the standard practice is to outline the magnetic lineaments (deduced from the first derivative of the crustal field). The geographic distribution of the main lineaments identified in the study area is illustrated in the map of Figure (8). The outstanding features in this map are the two systems of magnetic lineaments with distinctly different directions. In the southern and north eastern parts of the study area the orientations of the magnetic lineaments are predominantly southeast to northwest, whereas these are southwest to northeast in the northern and western parts. According to results of geologic studies the preferred directions of structural features of Precambrian age in the study area is predominantly south west to northeast. Hence we deduce that the presence of lineaments of SE – NW direction in the southern parts is a product of tectonic activities of relatively more recent times.
Figure (8) Magnetic lineaments identified in the study area.
The spatial distribution of the analytic signal, which is calculated as the modulus of the second derivative in the three directions of the magnetic field provided additional information on the characteristics of structural features in the study area. The technique is often considered as the best for outlining the borders of subsurface bodies with contrasts in magnetic properties. The map of Figure (9) illustrates the geographic distribution of the analytic signal.
Figure (9) Amplitudes of analytic signal in the study area.
As can be seen in this map the magnitudes of analytic signal are in the range of -0.9 to 45 nT/m with an average value of 0.03 ± 0.2 nT/m. The wide range of values is an indication that contrasts in magnetic properties occur on local scales in the subsurface layers beneath the soil cover in the study area. Note that the amplitude analytic signal is large in the western and central parts compared to that in the north eastern parts.
6. Discussion
In this item we discuss the results relevant to the study of soil types in the study area. In doing so we compare regional scale features present in the maps of abundances of radioactive elements with those of soil types. Superposition of the relevant shape files has allowed determination mean values of U, Th and K in the main soil types. The results obtained are presented in Table (2). Note that plintosol, cambisol, entisol, gleisol and nitisol soils are characterized by relatively high values of Uranium and Thorium, when compared with those of oxisol and ultisol soils. The Th/U ratios are in the range of 5 to 10 for all of the soil types. The Potassium abundances also follow a similar trend, the only exception being low mean value for plintosol soils. The possibility that widespread use of potash rich fertilizers have interfered with the results airborne radiometric surveys cannot entirely be ruled out.
Table (2) – Mean and standard deviation of U, Th and K in soil types of the study area . Soil Type eU (ppm) eTh (ppm) K (%) Plintosol
Cambisol
Entisol
Gleisol
Nitisol
Oxisol
Ultisol
Thoproblem inclassificatias a tracerpresent in the study ain the radio1.4 ± 1.3 pgreater thaSucrelatively abundancethat resultsunderstand
Aburadiometricstudy areaHamza (19where H isof the radThorium innumbers iindividual values (A) source matin units of ough the van mapping ion of soil tr and also aFigure (10)area. It is cloelement abppm it is faan 2.5 ppm. ch variationshort distaes are not res of airbornding the fine genic Heat undances oc surveys ha. The proce973). The re H s the rate ofdioactive eln units of pin equationisotopes, dewere calcuterial. For m μ W/m , is: ariations in radiometrictypes. For eas a means ) Uranium vlear that cambundances. fairly simplens are indicances. It iseadily identne radiomee structure o Figure (1 of Uraniumhave also aledures adopelevant equa ( H = − f heat produements. It parts per mn (1) are cescribed in ulated as themean soil dethe abundac characteriexample, theof identifyvariations inmbisol soil Even thouge to note thative of raps clear thatifiable by setric surveyof geochemi
10) Variation m, Thoriumllowed detepted are deation used fo .251. U C + uction in unis commonmillion (ppmconversion the earlier e product of ensity of 1.ances of radstics of base variationsing sub clan a segmentin this locagh the overahe presencepid changesat geochemstandard prs can be uical zoning ns in the U c m and Potaermination oerived fromfor this purp .356 Th C + nits of W/kgn practice m) and thatfactors dework of Haf heat produ5 x 10 -3 kgdioactive elesement rocks in the abunasses of soilt of cambisolity is charaall mean vale of severals in compomical zoninocedures insed as a coin subtropi ontents of C ssium calcof radiogenm the work pose is: ) K C g, and C U , Cto express t of Potassierived fromamza (1973)ction per un/m , the relements in nks it has pondances of ls. As an ilol soil in thacterized bylue of Uranil localities wsitional strung associaten soil classionvenient ccal soils. ambisoil . culated fromnic heat proof Hamza C Th and C K concentratium in unitm radioactiv). The volumnit mass (H)lation volumnear surfaceotential appUranium mllustrative ehe north easy large scaleium in cambwhere abunucture of thed with raification. Wcomplementm results ooduction valand Beck )1( are the contions of Urs of percenve decay smetric heat ) and the demetric heat p e layers is alications inmay be usedexample wetern part ofe variationsbisol soil isndances arehe soil overadioelementWe concludetary tool in of airbornelues for the(1972) andncentrationsranium andnt (%). Theschemes ofproductionensity ( ρ ) ofproduction, a n d e f s s e r t e n e e d s d e f n f , )2(0522.00383.01428.0 KThU
CCCA ++=
In using equation (2) it has been assumed that local soil types in general have a density of 1.5 kg/m . The heat production values calculated for the main soil types and basement rocks are given in Table (3). Table (3) Heat production values of the main soil types in the study area.
Soil Type Heat Production (µW m -3 ) Cambisol Plintosol
Nitisol
Gleisol
Entisol
Ultisol
Oxisol . The only exception is cambisoils which have a relatively high value of 3.32 µW/m . The weighted mean value of heat production of soils the study area is 0.68 ± 0.4 µW/m . This is in reasonably good agreement with the value obtained by Roque and Ribeiro (1997) for core samples of carbonate sequences in shallow boreholes. However these are relatively less than the mean values calculated for a large number of basement rock samples collected from outcrops in the study area by Iyer et al (1984) and also values inferred from crustal seismic velocities by Alexandrino and Hamza (2008) and Hamza et al. (2010). The geographic distribution of heat production values obtained is illustrated in the map of Figure (11). There are a number crustal segments in the eastern parts of the study area where heat production is higher than 1µW/m , the common range of variation being in the interval of 0.2 to 1.2 µW/m . There are indications that much of the small scale variations are related to variations in lithologic types mapped in geologic studies. Figure (11) Distribution of radiogenic heat production in the study area.
7. Conclusions
In this work progress obtained in analysis airborne geophysical survey data for the north-central region of the state of Goiás (Brazil) has been examined. Analysis of aeromagnetic data has revealed the presence of two distinct sets of magnetic lineaments in the study area: one associated with post metamorphic consolidation of basement rocks during Precambrian times and a second one associated with tectonic activities of later times. The analytic signal values of the magnetic field point to the existence of substantial small scale variations related to lithologic changes. The results of radiometric survey indicate that most of the common soil types are characterized by Uranium contents of greater than one ppm. Only ultisol soil and oxisol soil are found to have Uranium contents lower than one ppm. Thorium abundances are in the range of 5 to 30 ppm and have trends similar to those of Uranium. However, the Th/U ratios are higher than normal, indicating a specific radiometric characteristic of the study area. Nevertheless the possibility of such high ratios arising from non-equilibrium conditions of the radioactive series in soft sedimentary layers cannot entirely be ruled out. Another important result of the present study has been identification of sharp changes in radioelement concentrations within individual soil types. Results of air-borne radiometric surveys may be used as a convenient complementary tool in identifying such variations. The results have also allowed determination of radiogenic heat of the main soil types. Thus cambisol soil in this region is found to have a mean heat production of 3.32 ± 5.9 µWm -3 while that for ultisol soil is 0.36 ± 0.3 µWm -3 . The mean heat production of soil layers at the surface is 0.68 ± 0.4 µWm -3 .
8. Acknowledgments:
We thank Companhia Pesquisa de Recursos Minerais - CPRM, Secretaria de Geologia Mineração - SGM-GO, Empresa Brasileira de Agricultura e Pecuária - EMBRAPA and Observatório Nacional - ON for providing data sets used in this work. The first author of this work is recipient of a scholarship granted by Coordenadoria de Apoio á Pesquisa e Ensino Superior - CAPES.
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