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 50-60 m 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 are absorbed in the intervening air. Because of their penetrating nature, gamma-rays recorded at survey height originate from the top 30-35 cm 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 gamma-ray photons would originate from a circle below the aircraft with diameter of about 600 m (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 (right). 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 100 m height originate from within a 100 m radius circle on the ground beneath the aircraft.
Figure 4 (left). 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).