Fields of view for airborne detectors
geophysical consulting
One of the characteristics of gamma radiation is that it is quite penetrating—gamma-rays from the radioactive decay of K, U and Th
in the ground can penetrate through hundreds of metres of air before being completely absorbed in the atmosphere. This raises the
question of what the “footprint” of an airborne gamma-ray measurement is. Or, put another way, what is the “field of view” of an
airborne detector at a given height. To get an insight into this, we can calculate what percentage of the detected gamma radiation
originates from within a circle of specified diameter beneath the detector.
The equation above is for a spherical detector. Tewari and Raghuwanshi (1987) give the expression for the rectangular detectors (40.6×10.2×10.2
cm) in common use today. The figures below shows the field of view for K for 2 different heights. These show that the “field
of view” represented by a typical airborne sample is much larger than the 60-70 m interval over which 1-s airborne gamma-ray samples
are typically acquired.
A technical note pdf showing the fields of view for K, U and Th for a range of detector heights can be
downloaded from the Downloads page.
Consider a 2-layer model with the earth as an infinite half-space with uniform density and radioelement concentrations overlain by
non-radioactive air. Grasty (1987) and others have shown that the radiation due to a thick circular source expressed as a percentage
of the radiation due to an infinite source is given by
where h is the detector height above ground level, µ is the linear attenuation coefficient of the gamma rays due to either K, U or Th in air, E2 is the exponential integral of the second kind, and the circular source subtends an angle of 2-phi at the detector.
References
Grasty, R.L., 1987, Gamma-ray spectrometric methods in uranium exploration—theory and operational procedures. Geological Survey Canada Economic Geology Report 31, 147-161.
Tewari, S.G., Raghuwanshi, S.S., 1987, Some problems on the range of investigation
in airborne gamma-ray spectrometry. Uranium, 4(1), 67-82.