GeoExplo Ltda.
Geophysical Airborne Survey
Radiometrics -- Gamma-Ray Spectrometry

Santiago Chile Back to Home Page
Tel(56-2)326-5116
E-mail: surveys@new-sense.com Dr. W.E.S.(Ted) Urquhart

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 Principles, Compton Scattering, Cosmic Rays, Atmospheric Radiation, Instrumentation, Detectors, Analyzers, Spectrometer Calibration and Data Corrections, Calibration, Dead time Correction, Background Correction, Compton Stripping, Altitude Compensation, Radioelement Abundance Calculations, Processing of Airborne Data, Radiometric Survey Design, Counting Statistics, Line Spacing, Detector Selection and Radiometric Survey Specifications, Interpretation, Natural Radioactivity of Rocks and Effects of Weathering and Metamorphism

 
Link to more information on the GSC web page
Link to more information in IAEA TecDoc 1363(pdf file) -- "Guidelines for radioelement mapping using gamma ray spectrometry data" July 2003
 

Table of Contents

4. Airborne Radiometric (Gamma-Ray Spectrometry) Surveys
    4.1 Basic Principles
        4.1a Compton Scattering
        4.1b Cosmic Rays
        4.1c Atmospheric Radiation
    4.2 Instrumentation
        4.2a Detectors
        4.2b Analyzers
    4.3 Spectrometer Calibration and Data Corrections
        4.3a Calibration
        4.3b Deadtime Correction
        4.3c Background Correction
        4.3d Compton Stripping
        4.3e Altitude Compensation
        4.3f Radioelement Abundance Calculations
    4.4 Processing of Airborne Data
    4.5 Radiometric Survey Design
        4.5a Counting Statistics
        4.5b Line Spacing
        4.5c Detector Selection
    4.6 Radiometric Survey Specifications
    4.7 Interpretation
        4.7a Natural Radioactivity of Rocks
        4.7b Effects of Weathering and Metamorphism
    . Appendix 1: Typical Radioelement Concentrations in Earth Materials
    . SELECTED BIBLIOGRAPHY

Return to Table of Contents

4. Airborne Radiometric (Gamma Ray Spectrometry) Surveys

Radiometric surveys detect and map natural radioactive emanations, called gamma rays, from rocks 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. In parallel with the magnetic method, that is capable of detecting and mapping only magnetite (and occasionally pyrrhotite) in soils and rocks, so the radiometric method is capable of detecting only the presence of U, Th, and K at the surface of the ground.

The basic purpose of radiometric surveys is to determine either the absolute or relative amounts of U, Th., and K in the surface rocks and soils. Before considering the geologic implications of this information, we will discuss how gamma rays are affected by the natural environment and how they are measured. No 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. Meteorological conditions, the topography of the survey area, the influence of the planets cosmic environment, the height of the sensor above ground and the speed of the aircraft are just a few of the variables which affect radiometric measurements, and which can bias our analysis unless we deal with them very thoroughly. We will consider those variables that are important when designing the specifications for, and interpreting the data obtained from, airborne surveys in as non-mathematical a manner as possible.

A few of the benefits that we can expect from the interpretation of radiometric surveys include:

In appropriate areas, when used as a reconnaissance technique for mapping geology and for prospecting, the cost/benefit ratio for airborne radiometric surveying is nearly as good as that for airborne magnetometer surveying.

Return to Table of Contents

4.1 Basic Principles

Gamma rays are tiny bursts of very high frequency, hence high energy, electromagnetic waves that are spontaneously emitted by the nuclei of some isotopes of some elements. They have much shorter wavelengths than most other electromagnetic rays, including X-rays, and therefore, are less penetrating. Only a limited number of isotopes of the natural elements emit gamma rays; and among these, there are only three which are common enough within earth materials to make them geologically useful. These three are Bi214, Tl208, and K40. Bi214 comes from the decay of U238 and is, therefore, an indication of the concentration of uranium in the earth materials that lie within the range of the detector. Tl208 comes from the decay of Th232 and is an indicator of thorium content; and K40 is one of the minor natural isotopes of potassium and the only isotope of K that is radioactive. It makes up only .012% of the total potassium in rocks and soils, but because this fraction remains quite constant, even during weathering and metamorphism, the gamma radiation from it is a good indicator of changes in the amount of potassium in rocks.

Gamma rays are defined by their energies, measured in electron volts, or eV. One eV is the amount of kinetic energy that a single electron would acquire in falling through an electrical potential difference of 1 volt. The gamma rays from Tl208, the Th indicator, have an energy of 2.62 million electron volts or 2.62 MeV. We can understand the physical meaning of 2.62 MeV by noting that this amount of energy is sufficient to lift a speck of dust having a mass of one microgram a distance of 1/25 millimeter. The gamma rays from Bi214 have an energy of 1.76 MeV; while those from K40 have an energy of 1.46 MeV. All three of these energies are constant; they never change, they therefore constitute well defined peaks in the energy spectrum emanating from rocks. Figure 4.1-1 shows an example of the natural gamma ray spectrum of a typical felsic intrusive rock measured at a terrain clearance of 120 meters.

Typical Gamma-ray spectrum

Figure 4.4-1: A typical gamma ray spectrum from felsic intrusive rock measured at 120 metres terrain clearance.

In order to emphasize the smaller peaks, the spectrum in this figure is shown on a logarithmic scale. Note that there a many peaks but the three that are mentioned above are the most important ones. We also notice a sharp cut-off just beyond the Tl208 peak. This cut-off occurs because there are no radioactive isotopes in the natural environment which emit gamma rays having energies higher than 2.62 MeV. The general increase in the background level of the spectrum towards the lower energies is primarily due to Compton scattering which we will discuss shortly. The heights of the peaks are proportional to the amounts of the respective radioactive isotopes that are present in the rock. Thus, in principle, if we measure gamma ray spectra over different regions of exposed rock, and compare them, we should be able to translate changes in the heights of the 1.46, 1.76 and 2.62 MeV peaks into corresponding variations in the concentrations of potassium, uranium, and thorium within the different rock types.

Return to Table of Contents

4.1a Compton Scattering

Unfortunately, nature is not always cooperative. Not all of the gamma rays with energies of 1.76 MeV come from Bi214, nor do all of the rays with energies of 1.46 MeV come from K40. This can be seen in the general increase in the gamma ray count in the spectrum of figure 4.1-1. We need to measure, not the absolute value of the peaks, but their value relative to the general background which, unfortunately, may vary from one region to another.

The most important cause of the rise in background activity towards the lower energies is Compton scattering. Compton scattering occurs because, when a gamma ray collides with an electron it gives the electron some of its energy, somewhat like a cue ball in billiards slowing down after it strikes another ball. Extending this analogy, the gamma ray is deflected from its path, like the cue ball, and moves away in a different direction with lower energy, hence the gamma ray will have a lower frequency. Energy is invariably lost by scattering. Millions of gamma rays starting out with well defined energies will, after making numerous collisions, form a continuum of energies with a steadily rising proportion falling towards ower energy levels. Compton scattering thus accounts for the general shape of the gamma ray spectrum, whose rising background trend towards lower energies is usually called the "Compton continuum".

Return to Table of Contents

4.1b Cosmic Rays

There are other non-geologic sources of gamma radiation which contribute to the Compton continuum. Cosmic rays are the most important of these other sources. Charged particles from outer space collide with nuclei in the earth's atmosphere and produce high energy gamma rays, 3-6 MeV, which in turn are converted to lower energies by Compton scattering. Thus, gamma rays resulting from cosmic ray bombardment constitute a source of background noise in radiometric surveys that affects all parts of the spectrum. This noise is generally less sever towards the lower latitudes because of the shielding effect of the earth's magnetic field. It becomes more intense with increasing altitude, although it is discernible at normal surveying elevations. Figure 4.1-2 graphically illustrates these variations in cosmic ray intensity.

Cosmic Variation with Latitude Cosmic Variation with Altitude
Latitude Altitude

Figure 4.1-2: Variations in cosmic ray intensity with latitude (top) and with altitude (bottom).

Return to Table of Contents

4.1c Atmospheric Radiation

A further source of non-geologic gamma radiation is radon gas. In company with dust particles containing K40 and other radio-isotopes, it occurs in layers or clouds, particularly when there is little or no wind to disperse it, at heights of up to 300 meters or more above the ground. Because the radiation from these sources is indistinguishable from geologic radioactivity, special measures have to be taken to correct for this effect.

Return to Table of Contents

4.2 Instrumentation

All spectrometers used for measuring gamma ray intensity in geophysics consist of two principle parts; the detector which senses or detects the gamma rays, and the analyzer which analyzes the signal and displays the result.

Return to Table of Contents

4.2a Detectors

The most widely used detector of gamma radiation for geologic mapping is one or more crystals of thallium-activated sodium iodide. When a gamma ray enters the crystal and strikes an electron, the electron gains energy which is then emitted as a tiny flash of light when the electron returns to its original energy state. The number of flashes 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:

Because there is less chance that a gamma ray will pass through a large crystal undetected than through a small one the efficiency of the detector rises with rising crystal volume.

Solid state semiconducting detectors, like lithium-drifted germanium crystals, have superior resolving power to that of NaI (50 to 80 times). However they are difficult to grow and in order to operate effectively they must be maintained at liquid nitrogen temperatures thus presenting handling and weight problems.

Return to Table of Contents

4.2b Analyzers

There are two different types of gamma ray measuring systems; integral and differential spectrometers. The detectors are the same in both systems but the electronic analyzer is different. These systems are illustrated in Figure 3.2-1.

Single Threshold Multiple Threshold
Window Spectrometer MultiChannel Spectrometer

Figure 3.2-1: Energy discrimination characteristics of integral, and differential gamma ray spectrometers. (Hanson 1980.)

Integral spectrometers measure the total amount of incoming gamma radiation that lies above a certain energy threshold. They do not discriminate between gamma rays from thorium, uranium, or potassium sources except sometimes, very roughly, by employing three or more different thresholds and by measuring the differences in total count rates. The only integral spectrometers in current geophysical use are small-crystal, hand held instruments (sometimes called Geiger counters) used for rapid prospecting work.

Differential spectrometers measure only the gamma radiation which falls within spectral windows of fixed energy width. These windows can be centered upon the Bi214, Tl208, and K40 energy peaks at 2.62, 1.76 and 1.46 MeV respectively. Differential spectrometers require larger detector crystals because they operate within much narrower energy limits and therefore must deal with much lower light flash counting rates. It is extremely important that the "windows" are not permitted to drift, otherwise there will be significant losses in counting rates and the resulting data will be biased. Until the advent of the multichannel spectrometer in the late 70's early 80's this was the type of spectrometer used for airborne surveying.

The ultimate differential spectrometer is the multichannel spectrum analyzer, which monitors the entire gamma ray spectrum in discrete steps and is therefore immune to the problems of drift; however, a large crystal volume is needed for this type of system. The minimum crystal volume that is required to obtain adequate resolution depends on the sensor altitude and the speed of the aircraft. 1,000 cubic inches - about 16.4 liters - may be sufficient for a low-flying helicopter, but up to 2,000 cubic inches may be used for fixed-wing applications.

Return to Table of Contents

4.3 Spectrometer Calibration and Data Corrections

To convert the observed counting rates that are measured in the three or more spectral windows of the differential spectrometer into numbers of incoming gamma rays per unit of time from Bi214, Tl208, and K40, we must first calibrate the instrument and then correct the measurements for cosmic ray background effects, atmospheric noise, and Compton scattering.

Return to Table of Contents

4.3a Calibration

The systems count rate is related to the gamma ray intensity through various instrumental parameters, the most important being the sensitivity of the detector. Because this sensitivity varies with the temperature of both the crystal and the photomultiplier tubes, the temperature of the detector should be carefully controlled during operation. As well as measures taken to control the temperature, daily calibration checks, using standard isotope sources, are always a good idea.

Return to Table of Contents

4.3b Deadtime Correction

During the time that it takes the instrument to measure and analyze the scintillation from a single gamma photon, it cannot cope with other incoming gamma rays. Effectively, the instrument is "dead" during this period which lasts for only a few microseconds. If the count rate is sufficiently high, some counts may be missed during the data recovery period. The true count rate can be approximated from the measured count rate if the dead time, T, is known by the following simple correction:

NTrue = Nmeasured / (1 - T Nmeasured)

In most cases the dead time correction is insignificant for airborne survey data, but can be important for data collected during borehole logging.

Return to Table of Contents

4.3c Background Correction

The "background" refers to the general background count rate that prevails in each channel, or spectral window, that is due to non-geologic sources, primarily atmospheric radon and cosmic rays. Cosmic radiation tends to remain fairly constant over short periods of time - the time required to complete a single flight line, and sometimes a single flight. During a survey it can be monitored at the beginning and end of each line, or flight, by climbing to an altitude of 300 meters or more where the geologic contribution to the count rate is effectively zero, or by flying over a lake where the water shields the sensor from geologic radiation.

A superior method of monitoring the background radiation uses crystals that are shielded from radiation coming from below the aircraft. These "upward looking crystals" detect only the gamma rays which originate from cosmic or spatially variable atmospheric background. This provides a method of continuously monitoring the background during the survey and thus, in principle, could permit corrections to be made for it in real time.

Return to Table of Contents

4.3d Compton Stripping

The Compton scattering correction accounts for the gamma rays emitted by Tl208 that happen to fall within the Bi214 and K40 windows and for the gamma rays emitted by Bi214 which happen to fall within the K40 window, as a result of energy loss by Compton scattering. If no corrections were applied, both the uranium and potassium count rates would be over-estimated. These corrections are made by applying the following simple formulas:

For Thorium:
        NTh(corr) = NTh(obs) - bTh
For Uranium:
        NU(corr) = NU(obs) - bU - a NTh(corr)
For Potassium:
        NK (corr) = NK(obs) - bK - bNTh(corr) - g NU(corr)
where:
       (corr) stands for "background corrected count", and (obs) for "observed count" and NTh, NU, and NK are the count rates in the Tl208, Bi214 and K40 channels, respectively; bTh , bU and bK are the "Compton stripping ratios" defined as follows
 
       a = # of counts in the Bi214 channel per count in the Tl208 channel.
       b = # of counts in the K40 channel per count in the Tl208 channel.
       g = # of counts in the K40 channel per count in the Bi214 channel.
 
The values of a, b, and g are determined by measuring the systems response using artificially prepared calibration pads that are impregnated with the appropriate isotopes. For a given detector configuration, they will tend to remain constant over a fairly long period of time, but they should be checked periodically. Typically, the values for these three ratios lie between 0.5 and 1.

Return to Table of Contents

4.3e Altitude Compensation

Obviously, as the detector is moved further from the source fewer gamma rays originating in the source will be sensed. Thus, it is necessary to correct for the altitude of the sensor above the ground, and for variations in this distance. To a sufficient approximation, within the range from about 50 to 300 meters, the relationship between count rate and changes in aircraft altitude is a simple exponential one as is illustrated in figure 4.3-1.

Altitude Effect

Figure 4.3-1: The effect of altitude on the measured count rate.

Thus:

        N @ N0 e-m(h - ho)

where:

        m = the experimentally determined attenuation coefficient for air.

        N = the corrected count rate.

        N0 = the uncorrected count rate.

        h = the measured altitude above ground.

        ho = the nominal survey elevation. /p>

Because m depends somewhat upon the energy of the radiation, it has slightly different values for Tl208, Bi214 and K40 gamma rays. A typical value for m for the total count is 5.6 x 10-3 m-1.

Return to Table of Contents

4.3f Radioelement Abundance Calculations

After proper calibration of the system, the corrected count rates in each channel can be converted to the abundance's of the radioactive isotopes at the ground surface by the use of sensitivity constants. If we further assume that the daughter isotopes Tl208 and Bi214 are in equilibrium with their parent U238 and Th232 isotopes, we can write:

        eTh = CTh NTh(corr) e-mTh(h - ho)

        eU = CU NU(corr) e-mU(h - ho)

        and K = CK NK(corr) e-mK(h - ho)

where the eTh and eU signify "equivalent thorium" and "equivalent uranium", respectively, in parts per million, and K is potassium in per cent.

The three attenuation coefficients mTh, mU, and mK are the attenuation coefficients for the particular elements indicated, and the N values are the corrected count rates for indicated elements. The three sensitivity constants CTh, CU, and CK which relate the corrected count rates in the three energy windows to isotope abundance's at the ground surface, are experimentally determined. Their values depend upon crystal volume and detector altitude. To measure them, calibration pads which are made of concrete containing known amounts of U, Th, and K have been constructed by the Geological Surveys of Canada, the United States and some other countries. Test areas consisting of homogeneous granitic terrain in which the radioactive isotope content is accurately known by sampling and ground measurements are also available for periodic checking. Alternatively, if calibration pads or test areas are unavailable, comparisons can be made against pre-calibrated instruments.

Return to Table of Contents

4.4 Processing of Airborne Data

In addition to the corrections described in section 4.4, other forms of data processing are sometimes used in order to increase the accuracy and the usefulness of airborne radiometric data. The modern data compilation system includes software that permits the field geophysicist to apply all of the processes described in this section in the field during ongoing survey operations.

1. Smoothing

Radioactive decay is a random process, and the accuracy of all measurements is governed by statistical laws. The profiles of counting rates are always "noisy" as illustrated in figure 4.5-1 and usually the data cannot be contoured until they have been smoothed. Figure 4.5-1 illustrates data with no smoothing.

unfiltered stacked profile

Stacked profile with filtering

Figure 4.5-2: The profiles of figure 4.5-1 after filtering with a hanning filter operator, top, and a "boxcar" operator, bottom. Note: the phase reversal marked with the "B", caused by the inappropriate boxcar filter compared to the same point using the hanning filter marked with a "A". (Hogg, 1977)

2. Micro-Leveling of Radiometric Data

Changing background activity levels due to "pockets" of radon gas which has collected in valleys or due to variations in soil moisture content can occasionally be a serious problem. These residual leveling problems, that can remain even after applying background corrections, cause artificial lineations or corrugations in contour, or colour maps of the data. If present, this problem tends to be particularly sever for the "uranium" (Bi214) channel.

This problem can be reduced or eliminated from the data, after gridding the data, by applying a two dimensional mathematical filter that discriminates against small line-to-line base level changes. Figure 4.5-2 and 4.5-3 illustrate this process. The apparent lineations in the y direction of the map of figure 4.5-2 are caused by residual leveling errors. The application of a properly designed filter, that compensates for line to line variations produces the map shown in figure 4.5-3 immediately below.

Return to Table of Contents

Contour Map of Raw Grid Contour Map of Raw Grid Contour Map of Decorrugated Grid

Figure 4.5-3: Residual background levelling effects introduce corrugations into the uranium channel contour map, seen in the example as top-bottom trending features. The application of an appropriate decorrugation filter removes the corrugations and permits us to see underlying geological trends. Moving the mouse over the left side of the picture will reviel the decorrugated map.

3. Calculation of Ratios

The abundance ratios, U/Th, U/K and Th/K, are often more diagnostic of changes in rock types, alteration, or depositional environment than the values of the radio-isotope abundances themselves, which are subject to wide variations due to soil cover, etc. The U/Th ratio has particular value in exploration for uranium deposits because it has been found to increase locally within regions containing uranium ores. Thus profiles that include this ratio are often very useful for picking specific target anomalies for ground follow-up. The anomaly indicated by a red ball on figure 4.5-4 is an example of such a target. While stacked profile presentations are no longer standard for many radiometric surveys, when 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.

Red ball profile

Figure 4.5-4: Stacked radiometric profiles with a significant U/Th anomaly indicated by a "red-ball". In this case a "black ball".

In suitable areas, i.e., areas with reasonably low soil moisture content, maps of the ratios are useful as aids in mapping the surface geology of the area. In this connection a coloured map that effectively portrays all three ratios simultaneously as differences in colour and intensity, usually referred to as "ternary" maps, are particularly valuable.

4. Ternary Maps

A ternary map, such as the one shown in figure 4.5-2 is made by assigning one of the primary colours to each of the element abundances. For example, in the figure, Thorium is assigned red, Uranium is green and Potassium is blue. The 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. Thus, bright green areas on the map show areas where 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. Colours other than the three primary colours indicate areas with various, well defined proportions of Th, U, and K. Generally, the different colours on the map correspond closely with different rock types when compared with geological samples collected on the ground. In fact, the Ternary map has proven to be so useful that, along with contour maps of the total count and of each of the element abundances, it has become a standard method of presenting data.

Ternart colour map

Figure 4.5-2: A "Ternary" radiometric map produced by assigning three primary colours to the three radioelements (Thorium=Red, Uranium=Green and Potassium=Blue).

Many processors use cyan for uranium, magenta for potassium and yellow for thorium. While this colour scheme is different the same processing method is used and the resulting map looks similar to the one shown.

Return to Table of Contents

4.5 Radiometric Survey Design

While many of the survey design considerations for radiometric surveys are similar to those applicable to magnetic surveys, there are some significant differences. The most obvious difference is in acceptable flight elevation, i.e. while a flight elevation of 300 metres may be acceptable when flying a magnetic survey, it would probably be far too high for most radiometric surveys: A flight elevation of 100 metres or less would be more appropriate. As well as flight elevation, there are some other considerations that must be taken into account when writing the specifications for a radiometric survey.

Return to Table of Contents

4.5a Counting Statistics

Because our survey objective is to collect the best possible estimates of the abundances of uranium, thorium and potassium in the ground, we must give careful consideration to factors affecting the counting statistics. The standard error of measurement per unit distance on the ground is approximately inversely proportional to the square root of the count rate. Obviously, in order to reduce the standard error we must increase the number of counts, which means increasing the volume of the crystal detector to the maximum possible. Because there are practical limits on the size to which crystals can be grown, increasing the volume means adding more crystals. However, adding more crystals mean adding more weight and creates serious problems of detector matching and internal scattering of gamma rays; and larger and more expensive aircraft are needed to carry them. Given the load capacity of the aircraft that is to be used in the survey, there is clearly a limit on the total crystal volume that can be carried.

If we feel that the standard error is too large and we wish to reduce it by, for example, one half, we must increase the count rate by a factor of four. There are basically four things that can be done to increase the number of counts:

Clearly, the best possible vehicle for performing high-quality radiometric surveys is one which can carry a lot of weight and fly safely close to the ground at low speed. Helicopters 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 Navaho or similar fixed-wing aircraft can. Obviously, there will have to be compromises. A simple rule of thumb that defines a more or less optimum relationship between sample time, aircraft speed and survey altitude is:

t = h / 4V

where:

        t = the sampling time in seconds;

        h = the mean terrain clearance in meters;

     and V = the aircraft velocity in metres/second.

Normally, h is chosen to be not more than twice the linear dimensions of the smallest target that is considered to be of economic size. Thus, if h = 100 metres and V = 50 m/sec., then t = 0.5 second. In many cases (including this one) it may be 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.

Return to Table of Contents

4.5b Line Spacing

The "optimum" line spacing is inevitably a compromise involving the size of the area to be surveyed, the amount of detail that is required, and the total budget allocated to the project. There are, however, some rules for determining the "ideal" line spacing. The basis of this "ideal" line spacing is the concept of the "circle of investigation". This circle defines the area from which a given percentage of the total terrestrial radiation that is received by a stationary detector at a given height above ground originates. curves representing this function are shown in figure 4.6-1.

Radius of influence

Figure 4.6-1: Percentage of signal arising from the circle of investigation as a function of aircraft altitude. (Hansen 1980)

For example, at a terrain clearance of 130 metres, 60 % of the incoming gamma radiation is from within a circle having a radius of 150 m. At a clearance of 60 metres, that same circles radius shrinks to 60 m. This suggests the width of the instantaneous field of view of the gamma ray spectrometer.

As the "circle of investigation" moves with the aircraft it describes a strip or "carpet" of investigation, centered on the flight line, on the ground. The width of this strip is the diameter of the circle. To maintain good line to line coherency adjacent 60 % strips should not be separated by more than half their widths. Thus, fixed-wing surveys flown at a mean terrain clearance of 130 m. should use a line spacing that is about three times the flying height, or about 400 metres, to avoid loss of detail and aliasing of the data. Helicopter surveys flown at a height of 50 m will result in satisfactory results when flown using a line spacing of about four times that altitude, or about 200 metres.

The available budget may dictate wider line spacing than the above, however, the lack of line to line coherency that will result may yield non-contourable data.

Return to Table of Contents

4.5c Detector Selection

The choice of detector, i.e. crystal size, depends on the type of survey to be flown, aircraft speed, and altitude: a high sensitivity survey flown at a high altitude and relatively high speed may require large crystal volumes (up to 2000 in3); or a medium sensitivity reconnaissance surveys, where a volume of 1000 in3 or less may be adequate. For airborne geochemical and geological mapping, a high sensitivity system must be used. If one is only prospecting for weak, broad halos or for regions of higher than normal radioactivity, a medium sensitivity reconnaissance type survey may suffice. Because less costly aircraft may be used the difference in cost per square kilometer will usually be significantly lower with reconnaissance surveys, this type of survey permits a much larger area to be surveyed for a given total expenditure.

Using an example from Grant, 1982, I'll try to illustrate detector selection with specific examples. Suppose that it is important, in prospecting for uranium or gold, to be able to detect and locate targets that are about 100 metres in diameter, containing on the average 0.05 % U3O8. Using typical calibration values, we can calculate the expected count rate, Nu, in the Bi214 channel for a NaI(Tl) crystal detector having a volume V in3 at a height h metres above t he ground using the formula:

Nu(corr) @ (V x C(U) x A x 10-2) / (1.9 x h x e(5.6 x 10-3x h)

where:

        C(U) is the U3O8 concentration in percent

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

Using the numbers suggested for C(U) and A we find that, for a helicopter survey flown at a mean terrain clearance of 40 meters and using a crystal volume of 1000 in3 (about the maximum size that a medium size helicopter like a Bell Jetranger, can carry), Nu(corr) @ 41 cps. The total count rate obtained by integrating the spectrum over the total count window, assuming no contributions from either thorium or potassium, would be about 260 cps. We can calculate the statistical uncertainty in the background variation, Su, approximately, using the formula:

Su @ (bu(atm.) + bu(geol.))1/2

   = (36 + 520 )1/2 @ 23 cps.

The signal to noise ratio in this case being 41/23 @ 1.8, so the target should be easily detectable, assuming a line spacing of 200 metres is used. If the speed of the helicopter is 100 km/hr. (30m/sec), the optimum sampling time is 0.5 sec.

Topography (and available budget) usually controls the final selection between helicopter and fixed wing surveys. A high sensitivity spectrometer system may weigh several hundred kilograms and require the use of a twin-engine aircraft such as a Navaho, or a large and expensive helicopter like the Bell 412. The rate of climb of the fixed wing aircraft might not be sufficient to maintain satisfactory terrain clearance in very hilly or mountainous areas. The large helicopter may be too expensive for the available budget. An appreciation of the size of the crystals involved can be gained from the picture shown in figure 4.6-2. This picture shows a box containing 1,500 in3 of NaI crystal mounted on the side of a Bell 412 helicopter. In this case the complete detector consisted of two boxes identical to the one shown: one box mounted on each side of the aircraft.

Crystal pack instalation of a Bell 412

Figure 4.6-2: A 1500 cubic inch NaI crystal box mounted on a Bell 412 helicopter.

The large helicopter could be justified, for this survey, because over fifty thousand kilometers of radiometric data in Thailand was to be collected over extremely mountainous terrain. Note that in environments like Thailand, survey operations can only be conducted during the dry season. In addition, very wet areas, like the rice paddy regions, will yield marginal radiometric data.

For less ambitious projects perhaps, the optimum strategy would be to use a light fixed-wing aircraft, carrying a relatively small crystal to outline areas of high radioactivity using the total count as the primary indicator, to be followed by a high-sensitivity helicopter-borne survey to map the radioelement abundances in the areas selected for detailed study.

Return to Table of Contents

4.6 Radiometric Survey Specifications

Most of the contract specifications outlined in section 2 apply to radiometric surveys as well as to magnetometer surveys. However, in addition to those specifications, the following requirements, specific to radiometrics, are also necessary:

These last two specifications are required because heavy precipitation will act as a radiation shield and therefore significantly reduce the gamma ray count that can be measured.

Return to Table of Contents

4.8 Interpretation

Unlike the other airborne geophysical methods that we have discussed, there are no mathematical models that will allow us to calculate the theoretical radiometric response of a specific source. Interpretation of radiometric data is, therefore, more similar to interpreting the results of a conventional geological survey. It is usually necessary to correlate the results of geological and/or geochemical sampling with, for example, the colour patterns in a radiometric ternary map to achieve a full understanding of the implications of the map. However, an understanding of how radiometric surveys can be applied to exploration problems requires us to consider the geological sources of radioactivity.

Return to Table of Contents

4.8a Natural Radioactivity of Rocks

Much of the uranium and thorium in igneous rocks is concentrated in a few accessory minerals such as zircon, sphene and apitite. Other highly radioactive minerals, like monazite, allanite, uraninite, thorite, and pyrochlore, are widespread in nature but they are very minor constituents of rocks, and are distributed erratically. The minerals that carry uranium and thorium are generally associated with felsic intrusions - particularly with younger intrusions; they are found much less frequently in mafic rocks or in volcanics. The uranium and thorium content of rocks generally increases with acidity, with the highest concentrations found in pegmatites. This relationship is illustrated in appendix 1. The highest concentrations of uranium and thorium in sedimentary rocks usually occur in shales.

The potassium content of rocks also increases with acidity. In general, potassium is concentrated in micas and feldspars; rocks that are free of these minerals have very low K-activity. Thus, K-activity is very low in all mafic and ultramaific rocks. The potassium content of sedimentary rocks is highly variable but tends to be higher in shales than in carbonates or sandstones.

Return to Table of Contents

4.8b Effects of Weathering and Metamorphism

Weathering and metamorphism can modify the radioelement content of rocks profoundly. Uranium is easily oxidized to a water-soluble form; and can be readily leached from pegmatites and granites and redeposited in sediments at large distances from the source rock. Thorium has no soluble ion and therefore tends to remain with the parent rock or is transported over relatively short distances in the form of solid mineral grains. Common thorium-bearing minerals such as zircon and monazite are heavy and thus accumulate in placers and in the heavy mineral fraction of clastic sediments. Weathering, therefore, produces significant effects upon the distribution of radioelements: It decreases the U/TH ratio in weathered rock and it leads to dispersion halos, particularly in the case of uranium, that extend over a much greater area than does the parent formation.

Potassium is almost always bound up in the minerals fraction of soils, and is therefore transported in colloidal form in ground water and subsequently deposited in argillaceous sediments. It is also the radioelement that is most affected by metamorphism. A particular type of metamorphism that is often associated with felsic intrusions leads to potassium enrichment, and consequently, can be used as an exploration guide when prospecting for porphyry copper deposits or for kimberlites.

It is important to remember, when analyzing terrestrial gamma radiation, that in the case of uranium and thorium, the radiation does not emit from the parent nuclei, but from their decay products, Bi214 and Tl208. In both the U and Th decay series, there are ten relatively short lived isotopes between the parent (U238 and Th232) nuclei and the isotopes that emit the remotely detected gamma rays. Calculations of U and Th abundances derived from gamma ray measurements necessarily involve the assumption of equilibrium. Equilibrium in the chemically and biologically active uppermost few centimeters or meters of the earth is not a normal condition! The terms "equivalent uranium" and "equivalent thorium" mean the amounts of these two elements that are implied by the Bi214 and Tl208 gamma radiation if equilibrium is assumed. The real amounts could be much different.

Perhaps the commonest cause of difference between the "equivalent uranium" value and the real uranium value is the escape of radon gas, which is one of the radioactive isotopes in the U238 decay series and which immediately precedes Bi214. Diffusion of radon into the atmosphere results in a loss of Bi214 and hence an under-estimate of the uranium abundance. Radon diffusion is influenced by changes in barometric pressure, the moisture content of the ground, precipitation, and snow cover, amongst other things. All of these factors must be recognized as capable of producing false anomalies and taken into account by the interpreter.

A temperature inversion In the atmosphere and wind, or lack of it, can also produce misleading results. During periods when there is some wind to produce convective mixing of the atmosphere, radon escaping from the ground is thoroughly mixed throughout the air and forms a fairly uniform background radiation level. However, in cases when 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 where it accumulates and causes an increase in Bi214 count. From experience, we know that up to 75% of the total Bi214 count can come from inversion layers or from some deep valleys in still air. Thus, in these conditions, an error in estimating uranium abundance of up to 300% can result. When an atmospheric temperature inversion is observed, particularly if there is little or no wind, it is usually advisable to discontinue operations until conditions return to normal. In very mountainous terrain, it may be necessary to monitor radon levels in some of the most offending valleys. Figure 4.6-2 shows a sketch illustrating a few of the non-geologic causes of radiometric anomalies.

It is extremely important to remember that terrestrial gamma rays emanate from the ground surface, not from depth. A few inches of overburden, including soil, are sufficient to absorb 100% of the emissions from the rocks beneath. Therefore, unlike the aeromagnetic method, the radiometric method is capable of yielding information only on what lies at the ground surface. The value of radiometrics is as a geological mapping device that has the ability to provide chemical information on rock outcrop by remote sensing. Even though residual soils which have not been moved retain only some of the radioactive elements that were present in their parent rocks, their relative abundances tend to remain indicative of the parent, and thus the underlying parent rock 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 U/Th and the U/K ratios is its chief value.

Effect of alt. inversion and snow

Figure 4.6-2: Non-geologic causes of radiometric anomalies. (Hansen 1980)

 

 

 

Return to Table of Contents

Appendix 1: Typical Radioelement Concentrations in Earth Materials.

Rock ClassU (ppm)Th (ppm)K(%)
 MeanRangeMeanRangeMeanRange
Acid Extrusives4.10.8 - 16.411.91.1 - 41.03.11.0 - 6.2
Acid Intrusives4.50.1 - 30.025.70.1 - 253.13.40.1 - 7.6
Intermediate Extrusives1.10.2 - 2.62.40.4 - 6.41.10.01 - 2.5
Intermediate Intrusives 3.20.1 - 23.412.20.4 - 106.02.10.1 - 6.2
Basic Extrusives0.80.03 - 3.32.20.05 - 8.80.70.06 - 2.4
Basic Intrusives0.80.01 - 5.72.30.03 - 15.00.80.01 - 2.6
Ultrbasic0.30.0 - 1.61.40.0 - 7.50.30.0 - 0.8
Alkali Feldspathoidal Intermediate Extrusives29.71.9 - 62.0133.99.5 - 265.06.5 2.0 - 9.0
Alkali Feldspathoidal Intermediate Intrusives55.80.3 - 720.0132.60.4 - 880.04.21.0 - 9.9
Alkali Feldspathoidal Basic Extrusives2.40.5 - 12.08.22.1 - 60.01.90.2 - 6.9
Alkali Feldspathoidal Basic Intrusives2.30.4 - 5.48.42.8 - 19.61.80.3 - 4.8
Chemical Sedimentary Rocks*3.60.03 - 26.714.90.03 - 132.00.60.02 - 8.4
Carbonates2.00.03 - 18.01.30.03 - 0.80.30.01 - 3.5
Detrital Sedimentary Rocks4.80.1 - 80.012.40.2 - 362.01.50.01 - 9.7
Metamorphosed Igneous Rocks4.00.1 - 148.514.80.1 - 104.22.50.1 - 6.1
Metamorphosed Sedimentary Rocks3.00.1 - 53.412.00.1 - 91.42.10.01 - 5.3

 

 

Return to Table of Contents

Selected Bibliography -- Airborne Radiometric Surveys

Bristow, Q., 1979, Gamma-ray Spectrometric Methods in Uranium Exploration - Airborne Instrumentaion in Geophysics and Geochemistry in the Search for Mettalic Ores (P.J. Hood, ed.), Geol. Survey of Canada, Economic Geology Report 31, pp 135-146.

Cameron, G.W., Elliott, B.E., and Richardson, K.A., 1976, Effects of Line Spacing on Contoured Airborne Gamma-ray Spectrometry Data; in Exploration fro Uranium Ore Deposits, I.A.E.A., Vienna, pp 81-92

Darnley, A.G., 1973, Airborne Gamma-ray Survey Techniques - Present and Future; in Uranium Exploration Methods, Proc. Series, I.A.E.A., Vienna, pp 67-108.

Grant, Fraser S., 1982, Gamma Ray Spectrometry for Geological Mapping and for Prospecting, in Mining Geophysics Workshop., Paterson Grant and Watson Limited.

Grasty, R.L. 1977, A General Calibration Procedure for Gamma-ray Spectrometers - Project 720084; in Report of Activities, part C;, Geol. Survey of Canada, Paper 77-1C.

Grasty, R.L. 1979, Gamma Ray Spectrometric Methods in Uranium Exploration - theory and Operational Procedures; in Geophysics and Geochemistry in the Search for Mettalic Ores (P.J. Hood, ed.), Geol. Survey of Canada, Economic Geology Report 31, pp 147-161.

Hansen, D.A., 1980, "Radiometrics". Ch. 1 in Practical Geophysics for the Exploration Geologist, R. van Blairicom, ed., Northwest Minning Association, Spokane, Wash., U.S.A. , pp1038

Hogg, Scott R. L., 1977, Evaluation of Some Computer Compilation and Interpretation Techniques; presented at Uranium Exploration Workshop 1977, University of Toronto, Toronto Ont.

Hogg, Scott R. L., 1978, Contouring Radiometric Data: Considerations and New Developments for the Interpreter; presented at Uranium Exploration Workshop 1978, University of Toronto, Toronto, Ont.

Killeen, P.G., Carson, J.M., and Hunter, J.A., 1975, Optimizing Some Parameters for Airborne Gamma-ray Spectrometry; Geoexploration, V.13, pp 1-12.

Saunders, D.F., and Potts, M.J., 1976, Interpretation and Application of High-Sensitivity Airborne Gamma-ray Spectrometry Data; in Exploration for Uranium Ore Deposits, Proc. Series, I.A.E.A., Vienna, pp 81-92

Urquhart, W.E.S, 1988, Decorrugation of Enhanced Magnetic Field Maps . Paper presented at the 55th Annual Meeting of SEG, Anaheim, October, 1988.

Return to Table of Contents

Back to Home Page