Essential Corrections

Gamma-ray spectra are typically recorded once per second, along with ancillary data including positional information (GPS navigation) and 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 Rn 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.

Figure 6The 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).

Figure 6 (right). Measuring the response of an airborne gamma-ray spectrometer to sources with known concentrations of the radioelements. The concrete calibration slab (the black box on the pallet) under the aircraft has been doped with a known concentration of one of the radioelements.

Figure 7Gamma-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). Once this has occurred, data may then have a noise reduction correction applied (Figure 8).


Figure 7 (above). 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 (Left Image) Figure 8 (Right Image)

Figure 8 (above). Modern processing methods include the removal of statistical noise from the raw spectra. The left image shows
estimated U concentrations derived from spectra without noise reduction. The right 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).