Some isotopes are unstable and change to a more stable state by the emission of ionising radiation (e.g. gamma-rays). These are the so-called radioactive isotopes or "radioisotopes". All rocks and soil contain radioactive isotopes, and the decay of these isotopes gives rise to a natural gamma-ray flux at the earth's surface. Almost all gamma radiation detected near or at the earth's surface derives from the natural radioactive decay of just 3 elements - potassium (K), thorium (Th) and uranium (U). Gamma-ray surveys map the distribution of these elements at the earth's surface.
Gamma rays emitted from the natural decay of K, Th and U can penetrate some 35cm of rock and several hundred metres of air. Gamma-rays can thus be used for the remote sensing of terrestrial radioelement concentrations. Airborne gamma-ray spectrometry was originally developed as a uranium exploration tool. However, the method is now widely used for geological and environmental mapping.
Gamma-ray photons have an associated energy, and the energy is diagnostic of the source isotope. Some of the gamma rays from radioisotope disintegrations near the earth's surface penetrate through the earth and lower atmosphere and can be recorded in a survey aircraft. Most airborne gamma-ray detectors used today consist of crystals of thallium-activated sodium iodide (NaI). Gamma-rays absorbed in the crystals result in a scintillation of light being emitted - the intensity of which is proportional to the energy of the absorbed photon. Airborne detectors measure a gamma-ray spectrum (Figure 1) - i.e. both the number of gamma-rays recorded during a specific sample period, and the energy of each photon. The number of gamma-rays recorded is proportional to the concentration of the radioelements in the source, and the energies of the gamma-rays can be used to determine the composition of the source isotopes.
Potassium abundance is measured using the 1.46 MeV gamma-ray photons emitted when 40K decays to Argon. Uranium and Th abundances are measured from daughter nuclides in their respective decay chains (Figure 2). Distinct emission peaks associated with 214Bi (a daughter product in the 238U decay series) and 208Tl (a daughter product in the 232Th decay series) at 1.76 MeV and 2.61 MeV (Figure 1) are used to estimate the concentrations of U and Th, respectively.
The estimation of U and Th using daughter isotopes in their respective decay series is based on the assumption that their respective radioactive decay series are in equilibrium. However, disequilibrium is common in the 238U decay series, and this should be taken into consideration when interpreting estimated U abundances. For example, U anomalies can be caused by the accumulation of radium (226Ra) in ground waters (Giblin and Dickson, 1984). U and Th concentrations derived from gamma-ray spectrometry are normally expressed in units of "equivalent" parts per million (eU and eTh), as a reminder that these estimates are based on the assumption of equilibrium in their respective decay series.
Figure 1. An airborne gamma-ray spectrum (averaged over a long period of time) showing the diagnostic photopeaks and the positions of the K, U and Th windows used in airborne gamma-ray spectrometry.
Figure 2. Radioactive decay series for K, Th and U.
Survey design for airborne gamma-ray spectrometry is governed mainly by the precision required of the estimates of K, Th and U concentrations, and by the spatial resolution required of these estimates. Spatial resolution is governed mainly by the flight line spacing, but also by the sample interval and speed of the aircraft. The line spacing is a trade-off between spatial resolution and the cost of the survey. For geological mapping applications, line spacings between 100 and 400 m are typically used. Sample spacing along the lines is governed by the speed of the aircraft - typically 50-60m for a fixed-wing aircraft and a 1 second sample interval.
The greater the number of gamma rays recorded during each sampling interval, the better the precision of the data. Count rates can be increased by increasing the size of the detector, by flying closer to the ground, or by increasing the sample time. However, because the sample time also affects the spatial resolution of the survey, this is never increased beyond 1-second. The detector size is limited by the weight penalty any extra volume of detector imposes. Detectors of 33-50 litres are commonly used. For greater detector volumes, more expensive aircraft types would need to be considered in order to cope with the extra payload.
Gamma-ray surveys are typically flown at less than 100 m above ground level on a regular grid of parallel flight lines. Above 500 m height, most of the gamma rays emitted from the ground would be absorbed in the intervening air. Because of their penetrating nature, gamma rays recorded at survey height originate from the top 30-35cm of the earth's surface and from an area below the aircraft several hundred metres in diameter. The size of this "circle of investigation" depends on the survey height. At 100 m height, about 80% of recorded photons would originate from a circle below the aircraft with diameter of about 600m (Figure 3 and 4). So a single airborne estimate of radioelement concentrations is representative of the average concentrations over a fairly large area.
Figure 3. Airborne gamma-ray measurements reflect the concentration of radioelements near the earth's surface over a considerable area - less than half the gamma rays detected at 100m height originate from within a 100m radius circle on the ground beneath the aircraft.
Figure 4. Percent of the total signal originating from a circle below the detector as a function of aircraft height (for Th gamma-rays at 2.61 MeV and a detector height of 100 m).
Portable, hand-held gamma-ray spectrometers are widely used in mineral exploration and environmental studies. However, for mapping applications vehicle-borne surveys are more commonly employed. The gamma-ray detectors are mounted on a motor vehicle or quad-bike for continuous recording as the survey area is traversed (Figure 5). The gamma-ray detectors are usually airborne detectors modified for use from a vehicle. Otherwise the acquisition, navigation and processing of the data is essentially the same.
A big advantage of ground-based surveys is that the background component of radiation is much smaller (as a fraction of the signal) than for airborne surveys. Consequently a much simpler background correction can be applied to ground-based data. The greatest disadvantage of ground-based surveys is that the measurements are very much affected by the topography - even in moderate terrain. Any variations from a "flat-earth" affect the accuracy of the final measurements. Road cuttings, boulders, buildings and undulating terrain all affect the accuracy of ground-based measurements.
Figure 5. Quad bike fitted with a 4 litre crystal pack (red box between handle bars), differential GPS and EM31 meter (white pole). The system is used to acquire high resolution gamma-ray imagery and near surface conductivity (see figure 24).
Gamma-ray spectra are typically recorded every second. Ancillary data would include positional information (GPS navigation), as well as temperature, pressure, and the height above ground in the case of airborne surveys. The recorded data require substantial processing before accurate estimates of the ground concentrations of K, Th and U can be made. The main corrections, explained below, are for equipment livetime, energy drift, background radiation, channel interactions (stripping correction), the height of the aircraft above the ground, and the sensitivity of the detector.
Gamma-ray spectrometers take a finite time to process each gamma ray recorded. During this time the spectrometer is not available for counting and any gamma-rays interacting with the detector during these periods (dead time) are rejected. Modern spectrometers record the total livetime of the instrument for each sampling period. The livetime correction corrects for this equipment "dead time" by scaling the raw spectra.
Measured spectra all suffer from energy drift to some degree - i.e. photopeaks are displaced from their nominal channel positions in the recorded spectra. Raw spectra are energy calibrated by numerically "shifting" and "stretching" the spectra so that all the photopeaks align correctly in the relevant spectral channels.
The background correction is the largest correction applied to airborne gamma-ray spectrometric data. One of the daughter products in the 238U decay series is 222Rn (radon gas). Radon can escape from rocks and soils and find its way into the lower atmosphere where its daughter products (which are the major gamma-ray emitters in the 238U decay chain) attach to dust particles and aerosols and form the major component of the gamma-ray background. The other components are cosmic radiation, and the radioactivity of the aircraft and its equipment. For airborne surveys, the radon background correction is estimated by either using a second detector (an "upward-looking detector") mounted on top of the main detector package, or by using a full spectrum method that uses the relative size of 238U decay series photopeaks to estimate the atmospheric radon contribution. For ground-based surveys, the background is usually measured over a body of water, and a constant correction applied to all survey measurements.
The stripping correction (or channel interaction correction) is used to correct each of the K, U and Th window count rates for gamma rays not originating from the radioelement or decay series being monitored by that window. For example, Th series radioelements contribute to the measured count rates in the U and K windows, and U series gamma-ray contribute to the measured count rates in the Th and K windows. The correction requires prior knowledge of the pure spectra due to each of K, Th and U. These are obtained by measuring the response of the detector to sources of known K, Th and U concentrations (Figure 6).
Gamma rays attenuate with distance from their source. Stripped window count rates are corrected for deviations in the height of the detector from the nominal survey height using the height correction. This scales the window count rates to those that would have been observed had the data been acquired at a fixed height above the ground. The height correction is not necessary for ground-based surveys.
Finally, the window data are transformed to equivalent elemental concentrations on the ground using the sensitivity correction. This scales the processed window count rates using scaling factors derived from simultaneous airborne and ground gamma-ray measurements over a calibration range (Figure 7).
Figure 6. Measuring the response of an airborne gamma-ray spectrometer to sources with known concentrations of the radioelements. The concrete calibration slabs have been doped with known concentrations of the radioelements.
Figure 7. Simultaneous airborne and ground gamma-ray measurements over a calibration range are used to estimate the sensitivity of the airborne spectrometer to K, Th and U concentrations on the ground.
Figure 8. Modern processing methods include the removal of statistical noise from the raw spectra. The top image shows estimated U concentrations derived from spectra without noise reduction. The bottom image shows the same data derived from spectra that had noise removed using the NASVD (Noise Adjusted Singular Value Decomposition) method (see calibration and data processing references).