airborne gamma-ray spectrometry surveys

8/17/2019 Airborne Gamma-Ray Spectrometry Surveys

1/24

GeoExploLtda.

Geophysical Airborne Survey

Radiometrics -- Gamma-RaySpectrometry

Santiago Chile

Airborne Gamma-Ray Spectrometry Surveys

Abstract

Airborne Spectrometer

surveys are an important

exploration technique. A

number of topics on

radiometric surveying

are discussed in this

section, these

Gamma-ray topics

include: Basic rinciples,

!ompton Scattering,

!osmic "ays,

Atmospheric "adiation,

#nstrumentation,

$etectors, Analy%ers,

Spectrometer !alibration

and $ata !orrections,

!alibration, $ead time!orrection, Bac&ground

!orrection, !ompton

Stripping, Altitude

!ompensation,

"adioelement Abundance

!alculations, rocessing

of Airborne $ata,

"adiometric Survey

Table of Contents'. Airborne "adiometric (Gamma-"aySpectrometry) Surveys '.* Basic rinciples '.*a !ompton Scattering '.*b !osmic "ays '.*c Atmospheric "adiation '.+ #nstrumentation '.+a $etectors '.+b Analy%ers

'. Spectrometer !alibration and $ata!orrections '.a !alibration '.b $eadtime !orrection '.c Bac&ground !orrection '.d !ompton Stripping '.e Altitude !ompensation '.f "adioelement Abundance !alculations

$esign, !ounting

Statistics, ine Spacing,$etector Selection and

"adiometric Survey

Specications,

#nterpretation, /atural

"adioactivity of "oc&s

and 01ects of

2eathering and

3etamorphism

'.' rocessing of Airborne $ata '.4 "adiometric Survey $esign '.4a !ounting Statistics '.4b ine Spacing

'.4c $etector Selection '.5 "adiometric Survey Specications '.6 #nterpretation '.6a /atural "adioactivity of "oc&s '.6b 01ects of 2eathering and 3etamorphism

. Appendix *: 7ypical "adioelement

!oncentrations in 0arth 3aterials . Selected Bibliography


2/24

4. Airborne Radiometric (Gamma Ray Spectrometry)Surveys

"adiometric surveys detect and map natural radioactive emanations, called

gamma rays, from roc&s and soils. All detectable gamma radiation from earth

materials come from the natural decay products of only three elements, i.e.

uranium, thorium, and potassium. #n parallel 8ith the magnetic method, that is

capable of detecting and mapping only magnetite (and occasionally pyrrhotite) in

soils and roc&s, so the radiometric method is capable of detecting only the

presence of 9, 7h, and at the surface of the ground.

7he basic purpose of radiometric surveys is to determine either the absolute or

relative amounts of 9, 7h., and in the surface roc&s and soils. Before

considering the geologic implications of this information, 8e 8ill discuss ho8

gamma rays are a1ected by the natural environment and ho8 they are

measured. /o other geophysical method, and probably no other remote sensing

method, requires us to consider so many variables in order to reduce the

observational data to a form that is useful for geological interpretation.3eteorological conditions, the topography of the survey area, the in;uence of

the planets cosmic environment, the height of the sensor above ground and the

speed of the aircraft are


3/24

4.1 asic !rinciples

Gamma rays are tiny bursts of very high frequency, hence high energy,

electromagnetic 8aves that are spontaneously emitted by the nuclei of some

isotopes of some elements. 7hey have much shorter 8avelengths than most

other electromagnetic rays, including ?-rays, and therefore, are less penetrating.

@nly a limited number of isotopes of the natural elements emit gamma rays and

among these, there are only three 8hich are common enough 8ithin earthmaterials to ma&e them geologically useful. 7hese three are Bi+*', 7l+C, and

'. Bi+*' comes from the decay of 9+C and is, therefore, an indication of the

concentration of uranium in the earth materials that lie 8ithin the range of the

detector. 7l+C comes from the decay of 7h++ and is an indicator of thorium

content and ' is one of the minor natural isotopes of potassium and the only

isotope of that is radioactive. #t ma&es up only .*+D of the total potassium in

roc&s and soils, but because this fraction remains quite constant, even during8eathering and metamorphism, the gamma radiation from it is a good indicator

of changes in the amount of potassium in roc&s.

Gamma rays are dened by their energies, measured in electron volts, or e=. @nee= is the amount of &inetic energy that a single electron 8ould acquire in falling

through an electrical potential di1erence of * volt. 7he gamma rays from 7l+C,

the 7h indicator, have an energy of +.5+ million electron volts or +.5+ 3e=. 2e

can understand the physical meaning of +.5+ 3e= by noting that this amount of

energy is suEcient to lift a spec& of dust having a mass of one microgram a

distance of *>+4 millimeter. 7he gamma rays from Bi+*' have an energy of *.65

3e= 8hile those from ' have an energy of *.'5 3e=. All three of these

energies are constant they never change, they therefore constitute 8ell dened

pea&s in the energy spectrum emanating from roc&s. Figure '.*-* sho8s an

example of the natural gamma ray spectrum of a typical felsic intrusive roc& measured at a terrain clearance of *+ meters.

Figure '.'-*: A typical gamma ray spectrum from

at *+ metres terrain clearance felsic intrusive roc& measured


4/24

#n order to emphasi%e the smaller pea&s, the spectrum in this gure is sho8n on

a logarithmic scale. /ote that there a many pea&s but the three that arementioned above are the most important ones. 2e also notice a sharp cut-o1


5/24

atitude Altitude

Figure '.*-+: =ariations in cosmic ray intensity 8ith latitude (top) and 8ith

altitude (bottom).

'.*c Atmospheric "adiation

A further source of non-geologic gamma radiation is radon gas. #n company 8ith

dust particles containing ' and other radio-isotopes, it occurs in layers orclouds, particularly 8hen there is little or no 8ind to disperse it, at heights of up

to meters or more above the ground. Because the radiation from these

sources is indistinguishable from geologic radioactivity, special measures have to

be ta&en to correct for this e1ect.

4.# $nstrumentation

All spectrometers used for measuring gamma ray intensity in geophysics consist

of t8o principle parts the detector 8hich senses or detects the gamma rays, and

the analy%er 8hich analy%es the signal and displays the result.4.#a %etectors

7he most 8idely used detector of gamma radiation for geologic mapping is one

or more crystals of thallium-activated sodium iodide. 2hen a gamma ray enters

the crystal and stri&es an electron, the electron gains energy 8hich is then

emitted as a tiny ;ash of light 8hen the electron returns to its original energy

state. 7he number of ;ashes is proportional to the gamma ray energy , so that

the total light intensity is a measure of the energy of the incoming gamma ray.

An array of photomultiplier tubes converts the light into an electrical signal.

Sodium iodide crystals are preferred to other detector types for three principle

reasons:

7hey have good resolution of the energies in the . to 3e= range.

7hey have a high transparency and thus, even 8ea& ;ashes of light can be

detected.

#t is relatively easy to gro8 large crystals of /a#, and therefore they are a

relatively economical detector.


6/24

4.#b Analy&ers

7here are t8o di1erent types of gamma ray measuring systems integral and

di1erential spectrometers. 7he detectors are the same in both systems but the

electronic analy%er is di1erent. 7hese systems are illustrated in Figure .+-*.

Because there is less chance that a gamma ray 8ill pass through a large crystal

undetected than through a small one the eEciency of the detector rises 8ith

rising crystal volume.

Solid state semiconducting detectors, li&e lithium-drifted germanium crystals,

have superior resolving po8er to that of /a# (4 to C times). Io8ever they are

diEcult to gro8 and in order to operate e1ectively they must be maintained at

liquid nitrogen temperatures thus presenting handling and 8eight problems.

Figure .+-*: 0nergy discrimination characteristics of integral, and di1erential

gamma ray spectrometers. (Ianson *JC.)


7/24

#ntegral spectrometers measure the total amount of incoming gamma radiation

that lies above a certain energy threshold. 7hey do not discriminate bet8een

gamma rays from thorium, uranium, or potassium sources except sometimes,

very roughly, by employing three or more di1erent thresholds and by measuring

the di1erences in total count rates. 7he only integral spectrometers in current

geophysical use are small-crystal, hand held instruments (sometimes called

Geiger counters) used for rapid prospecting 8or&.

$i1erential spectrometers measure only the gamma radiation 8hich falls 8ithinspectral 8indo8s of xed energy 8idth. 7hese 8indo8s can be centered upon

the Bi+*', 7l+C, and ' energy pea&s at +.5+, *.65 and *.'5 3e= respectively.

$i1erential spectrometers require larger detector crystals because they operate

8ithin much narro8er energy limits and therefore must deal 8ith much lo8er

light ;ash counting rates. #t is extremely important that the 8indo8s are not

permitted to drift, other8ise there 8ill be signicant losses in counting rates and

the resulting data 8ill be biased. 9ntil the advent of the multichannel

spectrometer in the late 6Hs early CHs this 8as the type of spectrometer used

for airborne surveying.

7he ultimate di1erential spectrometer is the multichannel spectrum analy%er,

8hich monitors the entire gamma ray spectrum in discrete steps and is therefore

immune to the problems of drift ho8ever, a large crystal volume is needed for

this type of system. 7he minimum crystal volume that is required to obtain

adequate resolution depends on the sensor altitude and the speed of the aircraft.

*, cubic inches - about *5.' liters - may be suEcient for a lo8-;ying

helicopter, but up to +, cubic inches may be used for xed-8ing applications.

4.' Spectrometer Calibration and %ata

Corrections7o convert the observed counting rates that are measured in the three or more

spectral 8indo8s of the di1erential spectrometer into numbers of incoming

gamma rays per unit of time from Bi+*', 7l+C, and ', 8e must rst calibrate

the instrument and then correct the measurements for cosmic ray bac&ground

e1ects, atmospheric noise, and !ompton scattering.

4.'a Calibration

7he systems count rate is related to the gamma ray intensity through various

instrumental parameters, the most important being the sensitivity of thedetector. Because this sensitivity varies 8ith the temperature of both the crystal

and the photomultiplier tubes, the temperature of the detector should be

carefully controlled during operation. As 8ell as measures ta&en to control the

temperature, daily calibration chec&s, using standard isotope sources, are

al8ays a good idea.


8/24

$uring the time that it ta&es the instrument to measure and analy%e the

scintillation from a single gamma photon, it cannot cope 8ith other incoming

gamma rays. 01ectively, the instrument is dead during this period 8hich lasts

for only a fe8 microseconds. #f the count rate is suEciently high, some counts

may be missed during the data recovery period. 7he true count rate can be

approximated from the measured count rate if the dead time, 7, is &no8n by the

follo8ing simple correction:

/7rue K /measured > (* - 7 /measured)

#n most cases the dead time correction is insignicant for airborne survey data,

but can be important for data collected during borehole logging.

4.'c ac"round Correction

7he bac&ground refers to the general bac&ground count rate that prevails in

each channel, or spectral 8indo8, that is due to non-geologic sources, primarilyatmospheric radon and cosmic rays. !osmic radiation tends to remain fairly

constant over short periods of time - the time required to complete a single ;ight

line, and sometimes a single ;ight. $uring a survey it can be monitored at the

beginning and end of each line, or ;ight, by climbing to an altitude of meters

or more 8here the geologic contribution to the count rate is e1ectively %ero, or

by ;ying over a la&e 8here the 8ater shields the sensor from geologic radiation.

A superior method of monitoring the bac&ground radiation uses crystals that are

shielded from radiation coming from belo8 the aircraft. 7hese up8ard loo&ing

crystals detect only the gamma rays 8hich originate from cosmic or spatially

variable atmospheric bac&ground. 7his provides a method of continuouslymonitoring the bac&ground during the survey and thus, in principle, could

permit corrections to be made for it in real time.

4.'d Compton Strippin"

7he !ompton scattering correction accounts for the gamma rays emitted by

7l+C that happen to fall 8ithin the Bi+*' and ' 8indo8s and for the gamma

rays emitted by Bi+*' 8hich happen to fall 8ithin the ' 8indo8, as a result of

energy loss by !ompton scattering. #f no corrections 8ere applied, both the

uranium and potassium count rates 8ould be over-estimated. 7hese correctionsare made by applying the follo8ing simple formulas:

For 7horium:

4.'b %eadtime Correction

/7h(corr) K /7h(obs) - b7h

For 9ranium:

/9(corr) K /9(obs) - b9 - a /7h(corr)

For otassium:

/ (corr) K / (obs) - b - b/7h(corr) - g /9(corr)


9/24

8here:

(corr) stands for bac&ground corrected count, and (obs) for

observed count and /7h, /9, and / are the count rates in the

7l+C, Bi+*' and ' channels, respectively b7h , b9 and b are the

!ompton stripping ratios dened as follo8s

a K L of counts in the Bi+*' channel per count in the 7l+C channel.

b K L of counts in the '

channel per count in the 7l+C

channel. g K L of counts in the ' channel per count in the Bi+*' channel.

7he values of a, b, and g are determined by measuring the systems

response using articially prepared calibration pads that are

impregnated 8ith the appropriate isotopes. For a given detector

conguration, they 8ill tend to remain constant over a fairly long

period of time, but they should be chec&ed periodically. 7ypically, the

values for these three ratios lie bet8een .4 and *.

4.'e Altitude Compensation

@bviously, as the detector is moved further from the source fe8er gamma rays

originating in the source 8ill be sensed. 7hus, it is necessary to correct for the

altitude of the sensor above the ground, and for variations in this distance. 7o a

suEcient approximation, 8ithin the range from about 4 to meters, the

relationship bet8een count rate and changes in aircraft altitude is a simple

exponential one as is illustrated in gure '.-*.

Figure '.-*: 7he e1ect of altitude on the measured count rate.

7hus:

/ M / e-m(h - ho)

8here:

m K the experimentally determined

/ K the corrected count rate.

/ K the uncorrected count rate.

h K the measured altitude above ground.

ho K the nominal survey elevation. >pN

attenuation coeEcient for air.

Because m depends some8hat upon the energy of the radiation, it has slightly

di1erent values for 7l+C, Bi+*' and ' gamma rays. A typical value for m for

the total count is 4.5 x *- m-*.


10/24

After proper calibration of the system, the corrected count rates in each channel

can be converted to the abundanceHs of the radioactive isotopes at the ground

surface by the use of sensitivity constants. #f 8e further assume that the

daughter isotopes 7l+C and Bi+*' are in equilibrium 8ith their parent 9+C and

7h++ isotopes, 8e can 8rite:

e7h K !7h /7h(corr) e-m7h(h - ho)

e9 K !9 /9(corr) e-m9(h - ho)

and K ! / (corr) e-m(h - ho)

8here the e7h and e9 signify equivalent thorium and equivalent uranium,

respectively, in parts per million, and is potassium in per cent.

7he three attenuation coeEcients m7h, m9, and m are the attenuation

coeEcients for the particular elements indicated, and the / values are thecorrected count rates for indicated elements. 7he three sensitivity constants !7h,

!9, and ! 8hich relate the corrected count rates in the three energy 8indo8s

to isotope abundanceHs at the ground surface, are experimentally determined.

7heir values depend upon crystal volume and detector altitude. 7o measure

them, calibration pads 8hich are made of concrete containing &no8n amounts of

9, 7h, and have been constructed by the Geological Surveys of !anada, the

9nited States and some other countries. 7est areas consisting of homogeneous

granitic terrain in 8hich the radioactive isotope content is accurately &no8n by

sampling and ground measurements are also available for periodic chec&ing.

Alternatively, if calibration pads or test areas are unavailable, comparisons canbe made against pre-calibrated instruments.

4.4 !rocessin" of Airborne %ata

#n addition to the corrections described in section '.', other forms of data

processing are sometimes used in order to increase the accuracy and the

usefulness of airborne radiometric data. 7he modern data compilation system

includes soft8are that permits the eld geophysicist to apply all of the processes

described in this section in the eld during ongoing survey operations.

1. Smootin"

"adioactive decay is a random process, and the accuracy of all measurements is

governed by statistical la8s. 7he proles of counting rates are al8ays noisy as

illustrated in gure '.4-* and usually the data cannot be contoured until they

4.'f Radioelement Abundance Calculations

have been smoothed. Figure '.4-* illustrates data 8ith no smoothing.


11/24

Figure '.4-+: 7he proles of gure '.4-* after ltering 8ith a hanning lter

operator, top, and a boxcar operator, bottom. /ote: the phase reversal mar&ed

8ith the B, caused by the inappropriate boxcar lter compared to the same

point using the hanning lter mar&ed 8ith a A. (Iogg, *J66)


12/24

#. *icro-+evelin" of Radiometric %ata

!hanging bac&ground activity levels due to poc&ets of radon gas 8hich has

collected in valleys or due to variations in soil moisture content can occasionally

be a serious problem. 7hese residual leveling problems, that can remain evenafter applying bac&ground corrections, cause articial lineations or corrugations

in contour, or colour maps of the data. #f present, this problem tends to be

particularly sever for the uranium (Bi+*') channel.

7his problem can be reduced or eliminated from the data, after gridding the

data, by applying a t8o dimensional mathematical lter that discriminates

against small line-to-line base level changes. Figure '.4-+ and '.4- illustrate this

process. 7he apparent lineations in the y direction of the map of gure '.4-+ are

caused by residual leveling errors. 7he application of a properly designed lter,

that compensates for line to line variations produces the map sho8n in gure

'.4- immediately belo8.


13/24

Figure '.4-: "esidual bac&ground levelling e1ects introduce corrugations into

the uranium channel contour map, seen in the example as top-bottom trending

features. 7he application of an appropriate decorrugation lter removes the

corrugations and permits us to see underlying geological trends.

'. Calculation of Ratios

7he abundance ratios, 9>7h, 9> and 7h>, are often more diagnostic of changes

in roc& types, alteration, or depositional environment than the values of the

radio-isotope abundances themselves, 8hich are sub7h ratio has particular value in exploration for uranium

deposits because it has been found to increase locally 8ithin regions containing

uranium ores. 7hus proles that include this ratio are often very useful for

pic&ing specic target anomalies for ground follo8-up. 7he anomaly indicated by

a red ball on gure '.4-' is an example of such a target. 2hile stac&ed prole

presentations are no longer standard for many radiometric surveys, 8hen using

this method for the direct detection of uranium deposits this data presentation

technique, either on the computer screen or in hard copy, is invaluable.

Figure '.4-': Stac&ed radiometric proles 8ith a signicant 9>7h anomalyindicated by a red-ball. #n this case a blac& ball.

#n suitable areas, i.e., areas 8ith reasonably lo8 soil moisture content, maps of

the ratios are useful as aids in mapping the surface geology of the area. #n this

connection a coloured map that e1ectively portrays all three ratios

simultaneously as di1erences in colour and intensity, usually referred to as

ternary maps, are particularly valuable.


14/24

4. Ternary *aps

A ternary map, such as the one sho8n in gure '.4-+ is made by assigning one of

the primary colours to each of the element abundances. For example, in thegure, 7horium is assigned red, 9ranium is green and otassium is blue. 7he

total count rate is used to assign an intensity scale to each of the elements and

the resulting colours are then combined to produce a coloured map. 7hus, bright

green areas on the map sho8 areas 8here the uranium count is very high

relative to both of the other element count rates bright blue indicates areas of

high potassium count rate, etc. !olours other than the three primary colours

indicate areas 8ith various, 8ell dened proportions of 7h, 9, and . Generally,

the di1erent colours on the map correspond closely 8ith di1erent roc& types

8hen compared 8ith geological samples collected on the ground. #n fact, the

7ernary map has proven to be so useful that, along 8ith contour maps of the

total count and of each of the element abundances, it has become a standard

method of presenting data.

Figure '.4-+: A 7ernary radiometric map produced by assigning three primary

colours to the three radioelements (7horiumK"ed, 9raniumKGreen and

otassiumKBlue).


15/24

3any processors use cyan for uranium, magenta for potassium and yello8 for

thorium. 2hile this colour scheme is di1erent the same processing method is

used and the resulting map loo&s similar to the one sho8n.

4., Radiometric Survey %esi"n

2hile many of the survey design considerations for radiometric surveys aresimilar to those applicable to magnetic surveys, there are some signicant

di1erences. 7he most obvious di1erence is in acceptable ;ight elevation, i.e.

8hile a ;ight elevation of metres may be acceptable 8hen ;ying a magnetic

survey, it 8ould probably be far too high for most radiometric surveys: A ;ight

elevation of * metres or less 8ould be more appropriate. As 8ell as ;ightelevation, there are some other considerations that must be ta&en into account

8hen 8riting the specications for a radiometric survey.

4.,a Countin" Statistics

Because our survey ob


16/24

!learly, the best possible vehicle for performing high-quality radiometric surveys

is one 8hich can carry a lot of 8eight and ;y safely close to the ground at lo8

speed. Ielicopters give the best performance in terms of ground clearance and

speed, but all but the largest, can carry only about one third the number of

crystals that a /avaho or similar xed-8ing aircraft can. @bviously, there 8ill

have to be compromises. A simple rule of thumb that denes a more or less

optimum relationship bet8een sample time, aircraft speed and survey altitude is:

t K h > '=

8here:

t K the sampling time in seconds

h K the mean terrain clearance in meters

and = K the aircraft velocity in metres>second.

/ormally, h is chosen to be not more than t8ice the linear dimensions of the

smallest target that is considered to be of economic si%e. 7hus, if h K * metres

and = K 4 m>sec., then t K .4 second. #n many cases (including this one) it maybe impractical to adhere to the optimum rule because the sampling time may

turn out to be too small to give statistically meaningful rates nonetheless, it is

useful as a general guide.

4.,b +ine Spacin"

7he optimum line spacing is inevitably a compromise involving the si%e of the

area to be surveyed, the amount of detail that is required, and the total budget

allocated to the pro


17/24

Figure '.5-*: ercentage of signal arising from the circle of investigation as a

function of aircraft altitude. (Iansen *JC)

4.,c %etector Selection7he choice of detector, i.e. crystal si%e, depends on the type of survey to be

;o8n, aircraft speed, and altitude: a high sensitivity survey ;o8n at a high

altitude and relatively high speed may require large crystal volumes (up to +

in) or a medium sensitivity reconnaissance surveys, 8here a volume of * in

or less may be adequate. For airborne geochemical and geological mapping, a

high sensitivity system must be used. #f one is only prospecting for 8ea&, broad

halos or for regions of higher than normal radioactivity, a medium sensitivity

reconnaissance type survey may suEce. Because less costly aircraft may be used

the di1erence in cost per square &ilometer 8ill usually be signicantly lo8er

8ith reconnaissance surveys, this type of survey permits a much larger area to

be surveyed for a given total expenditure.

9sing an example from Grant, *JC+, #Hll try to illustrate detector selection 8ith

specic examples. Suppose that it is important, in prospecting for uranium or

gold, to be able to detect and locate targets that are about * metres in

diameter, containing on the average .4 D 9@C. 9sing typical calibration

values, 8e can calculate the expected count rate, /u, in the Bi+*' channel for a

/a#(7l) crystal detector having a volume = in at a height h metres above t he

ground using the formula:


18/24

9sing the numbers suggested for !(9) and A 8e nd that, for a helicoptersurvey ;o8n at a mean terrain clearance of ' meters and using a crystal

volume of * in (about the maximum si%e that a medium si%e helicopter li&e a

Bell Oetranger, can carry), /u(corr) M '* cps. 7he total count rate obtained by

integrating the spectrum over the total count 8indo8, assuming no contributions

from either thorium or potassium, 8ould be about +5 cps. 2e can calculate the

statistical uncertainty in the bac&ground variation, Su, approximately, using the

formula:

/u(corr) M (= x !(9) x A x *-+) > (*.J x h x e(4.5 x *

-x h)

8here:

!(9) is the 9@C concentration in percent

and A is the area of the outcrop in square metres.

Su M (bu(atm.) P bu(geol.))*>+

K (5 P 4+ )*>+ M + cps.7he signal to noise ratio in this case being '*>+ M *.C, so the target should be

easily detectable, assuming a line spacing of + metres is used. #f the speed of

the helicopter is * &m>hr. (m>sec), the optimum sampling time is .4 sec.

7opography (and available budget) usually controls the nal selection bet8een

helicopter and xed 8ing surveys. A high sensitivity spectrometer system may

8eigh several hundred &ilograms and require the use of a t8in-engine aircraft

such as a /avaho, or a large and expensive helicopter li&e the Bell '*+. 7he rate

of climb of the xed 8ing aircraft might not be suEcient to maintain satisfactoryterrain clearance in very hilly or mountainous areas. 7he large helicopter may be

too expensive for the available budget. An appreciation of the si%e of the crystals

involved can be gained from the picture sho8n in gure '.5-+. 7his picture

sho8s a box containing *,4 in of /a# crystal mounted on the side of a Bell

'*+ helicopter. #n this case the complete detector consisted of t8o boxes

identical to the one sho8n: one box mounted on each side of the aircraft.

Figure '.5-+: A *4 cubic inch /a#

helicopter.

crystal box mounted on a Bell '*+


19/24

For less ambitious proor full energy

spectrum specications.

+. Survey Flying Specications.

7he maximum ;ying speed permitted.

7he maximum precipitation allo8ed during survey operations.

7he delay of operations required follo8ing precipitation exceeding the

maximum allo8ed.

7hese last t8o specications are required because heavy precipitation 8ill act as

a radiation shield and therefore signicantly reduce the gamma ray count that

can be measured.

. $ata !ompilation and #nterpretation

3ap and chart scales

7he corrections that are to be applied to the data

7he maximum smoothing to be applied to the data.

7he data presentation products required: e.g., contour maps of 8hich

elements, ternary maps, stac&ed prole charts, etc.

4./ $nterpretation

7he main applications of airborne radiometric surveys are:

Geological>geochemical mapping as an indirect aid in exploration for

economic minerals.

0xploration for uranium deposits.

7he large helicopter could be


20/24

0xploration for porphyry copper deposits 8here potassic alteration gives

rise to a radiometric signature.

0xploration for gold using the Au-9 association.

0xploration for radioactive haloes over hydrocarbon deposits.

9nli&e the other airborne geophysical methods that 8e have discussed, there are

no mathematical models that 8ill allo8 us to calculate the theoretical

radiometric response of a specic source. #nterpretation of radiometric data is,

therefore, more similar to interpreting the results of a conventional geologicalsurvey. #t is usually necessary to correlate the results of geological and>or

geochemical sampling 8ith, for example, the colour patterns in a radiometric

ternary map to achieve a full understanding of the implications of the map.

Io8ever, an understanding of ho8 radiometric surveys can be applied to

exploration problems requires us to consider the geological sources of

radioactivity.

4./a 0atural Radioactivity of Rocs

3uch of the uranium and thorium in igneous roc&s is concentrated in a fe8

accessory minerals such as %ircon, sphene and apitite. @ther highly radioactive

minerals, li&e mona%ite, allanite, uraninite, thorite, and pyrochlore, are

8idespread in nature but they are very minor constituents of roc&s, and are

distributed erratically. 7he minerals that carry uranium and thorium are

generally associated 8ith felsic intrusions - particularly 8ith younger intrusions

they are found much less frequently in mac roc&s or in volcanics. 7he uranium

and thorium content of roc&s generally increases 8ith acidity, 8ith the highest

concentrations found in pegmatites. 7his relationship is illustrated in appendix *.

7he highest concentrations of uranium and thorium in sedimentary roc&s usually

occur in shales.

7he potassium content of roc&s also increases 8ith acidity. #n general, potassium

is concentrated in micas and feldspars roc&s that are free of these minerals

have very lo8 -activity. 7hus, -activity is very lo8 in all mac and ultramaic

roc&s. 7he potassium content of sedimentary roc&s is highly variable but tends to

be higher in shales than in carbonates or sandstones.

4./b 2ects of 3eaterin" and *etamorpism

2eathering and metamorphism can modify the radioelement content of roc&sprofoundly. 9ranium is easily oxidi%ed to a 8ater-soluble form and can be

readily leached from pegmatites and granites and redeposited in sediments atlarge distances from the source roc&. 7horium has no soluble ion and therefore

tends to remain 8ith the parent roc& or is transported over relatively short

distances in the form of solid mineral grains. !ommon thorium-bearing minerals

such as %ircon and mona%ite are heavy and thus accumulate in placers and in the

heavy mineral fraction of clastic sediments. 2eathering, therefore, produces

signicant e1ects upon the distribution of radioelements: #t decreases the 9>7I

ratio in 8eathered roc& and it leads to dispersion halos, particularly in the case

of uranium, that extend over a much greater area than does the parent

formation.


21/24

#t is important to remember, 8hen analy%ing terrestrial gamma radiation, that inthe case of uranium and thorium, the radiation does not emit from the parent

nuclei, but from their decay products, Bi+*' and 7l+C. #n both the 9 and 7h

decay series, there are ten relatively short lived isotopes bet8een the parent

(9+C and 7h++) nuclei and the isotopes that emit the remotely detected gamma

rays. !alculations of 9 and 7h abundances derived from gamma ray

measurements necessarily involve the assumption of equilibrium. 0quilibrium in

the chemically and biologically active uppermost fe8 centimeters or meters of

the earth is not a normal conditionQ 7he terms equivalent uranium and

equivalent thorium mean the amounts of these t8o elements that are implied

by the Bi+*' and 7l+C gamma radiation if equilibrium is assumed. 7he realamounts could be much di1erent.

erhaps the commonest cause of di1erence bet8een the equivalent uranium

value and the real uranium value is the escape of radon gas, 8hich is one of the

radioactive isotopes in the 9+C decay series and 8hich immediately precedes

Bi+*'. $i1usion of radon into the atmosphere results in a loss of Bi+*' and hence

an under-estimate of the uranium abundance. "adon di1usion is in;uenced by

changes in barometric pressure, the moisture content of the ground,

precipitation, and sno8 cover, amongst other things. All of these factors must be

recogni%ed as capable of producing false anomalies and ta&en into account bythe interpreter.

A temperature inversion #n the atmosphere and 8ind, or lac& of it, can also

otassium is almost al8ays bound up in the minerals fraction of soils, and is

therefore transported in colloidal form in ground 8ater and subsequently

deposited in argillaceous sediments. #t is also the radioelement that is most

a1ected by metamorphism. A particular type of metamorphism that is often

associated 8ith felsic intrusions leads to potassium enrichment, and

consequently, can be used as an exploration guide 8hen prospecting for

porphyry copper deposits or for &imberlites.

produce misleading results. $uring periods 8hen there is some 8ind to produce

convective mixing of the atmosphere, radon escaping from the ground is

thoroughly mixed throughout the air and forms a fairly uniform bac&ground

radiation level. Io8ever, in cases 8hen a temperature inversion occurs, or in

cases of still air, particularly in deep valleys in hilly terrain, the escaping radon is

trapped near the ground 8here it accumulates and causes an increase in Bi+*'

count. From experience, 8e &no8 that up to 64D of the total Bi+*'

count cancome from inversion layers or from some deep valleys in still air. 7hus, in these

conditions, an error in estimating uranium abundance of up to D can result.

2hen an atmospheric temperature inversion is observed, particularly if there is

little or no 8ind, it is usually advisable to discontinue operations until conditions

return to normal. #n very mountainous terrain, it may be necessary to monitor

radon levels in some of the most o1ending valleys. Figure '.5-+ sho8s a s&etch

illustrating a fe8 of the non-geologic causes of radiometric anomalies.


22/24

#t is extremely important to remember that terrestrial gamma rays emanate from

the ground surface, not from depth. A fe8 inches of overburden, including soil,

are suEcient to absorb *D of the emissions from the roc&s beneath.

7herefore, unli&e the aeromagnetic method, the radiometric method is capable

of yielding information only on 8hat lies at the ground surface. 7he value of

radiometrics is as a geological mapping device that has the ability to provide

chemical information on roc& outcrop by remote sensing. 0ven though residual

soils 8hich have not been moved retain only some of the radioactive elements

that 8ere present in their parent roc&s, their relative abundances tend to remain

indicative of the parent, and thus the underlying parent roc& can sometimes be

mapped through a thin layer of residual soil. As a prospecting tool, the ability of

radiometrics to map uranium dispersion halos and to indicate local anomalies in

the 9>7h and the 9> ratios is its chief value.

Figure '.5-+: /on-geologic causes of radiometric anomalies. (Iansen *JC)


23/24

Appendi 15 Typical RadioelementConcentrations in art *aterials.

"oc& !lass 9 (ppm) 7h (ppm) (D)

3ean "ange 3ean "ange 3ean "ange

Acid 0xtrusives '.*

.C -

*5.' **.J

*.* -

'*. .*

*. -

5.+

Acid #ntrusives '.4.* -

.+4.6

.* -

+4.*.'

.* -

6.5

#ntermediate 0xtrusives *.* .+ - +.5 +.' .' - 5.' *.*.* -

+.4

#ntermediate #ntrusives .+.* -

+.'*+.+

.' -

*5.+.*

.* -

5.+

Basic 0xtrusives .C. -

.+.+

.4 -

C.C.6

.5 -

+.'

Basic #ntrusives .C.* -

4.6+.

. -

*4..C

.* -

+.5

9ltrbasic . . - *.5 *.' . - 6.4 .. -

.C

Al&ali Feldspathoidal

#ntermediate 0xtrusives+J.6

*.J -

5+.*.J

J.4 -

+54.5.4

+. -

J.

Al&ali Feldspathoidal

#ntermediate #ntrusives44.C

. -

6+.*+.5

.' -

CC.'.+

*. -

J.J

Al&ali Feldspathoidal Basic0xtrusives +.' .4 -*+. C.+ +.* -5. *.J .+ -5.J

Al&ali Feldspathoidal Basic

#ntrusives+. .' - 4.' C.'

+.C -

*J.5*.C

. -

'.C

!hemical Sedimentary "oc&sR .5. -

+5.6*'.J

. -

*+..5

.+ -

C.'

!arbonates +.. -

*C.*.

. -

.C.

.* -

.4

$etrital Sedimentary "oc&s '.C.* -

C.

*+.'.+ -

5+.

*.4.* -

J.6

3etamorphosed #gneous "oc&s '..* -

*'C.4*'.C

.* -

*'.++.4

.* -

5.*

3etamorphosed Sedimentary

"oc&s.

.* -

4.'*+.

.* -

J*.'+.*

.* -

4.


24/24

Selected iblio"rapy -- Airborne RadiometricSurveys

risto67 8.7 19:9, Gamma-ray Spectrometric 3ethods in 9ranium 0xploration -

Airborne #nstrumentaion in Geophysics and Geochemistry in the Search for

3ettalic @res (.O. Iood, ed.), Geol. Survey of !anada, 0conomic Geology "eport*, pp *4-*'5.

Cameron7 G.3.7 lliott7 ..7 and Ricardson7 ;.A.7 19:, 01ects of ine

Spacing on !ontoured Airborne Gamma-ray Spectrometry $ata in 0xploration

fro 9ranium @re $eposits, #.A.0.A., =ienna, pp C*-J+

%arnley7 A.G.7 19:', Airborne Gamma-ray Survey 7echniques - resent andFuture in 9ranium 0xploration 3ethods, roc. Series, #.A.0.A., =ienna, pp

56-*C.

Grant7

airborne gamma-ray spectrometry surveys

Documents