Specializing in the processing and enhancement of airborne geophysical data
GAMMA_Grid is a new method for the rigorous inversion of airborne gamma-ray spectrometric data to a regular grid of radioelement concentrations on the ground. The 3D topography within the field of view of the detector is incorporated into the inversion.
We use a source model comprising vertical rectangular prisms of uniform radioactivity and with the same horizontal dimensions as the required grid cell size. The top of each prism is a plane surface derived from a best-fit plane to the digital elevation model of the earth’s surface within each grid cell area. The directional sensitivity of the detector and the velocity of the detector are incorporated into the reponse. The estimated errors in the line data are used to weight the data inversely as their variances during the inversion.
The method inverts the line data directly to a regular grid of concentrations - so there is no need for gridding. It gives superior interpolation between flight lines and better anomaly definition. It also eliminates terrain effects.
The incorporation of the topography into the inversion can be clearly seen at A, and elsewhere, in the figure below. In the deeply-weathered Australian environment the tops of mountains and ridges, which are actively eroding and thus exposing fresh rocks and soils, often show higher K concentrations than other more deeply-weathered parts of the landscape. As the aircraft passes over a ridge, many of the sources within the field of view are further from the detector than is the case for a flat earth. In these circumstances conventional methods underestimate radioelement concentrations as the data are corrected for the height of the ground directly beneath the aircraft assuming flat-earth geometry. This effect is evident along the ridges at A – the 3D inversion is correctly identifying the increase in K concentration whereas the conventional processing is not.
(data courtesy Geological Survey of NSW)
Source-detector response for a slab detector at 60 m height above a rectangular vertical prism
Parameterization of the topography using prism sources with slanted top surfaces relative to the detector
3D inversion of airborne gamma-ray spectrometric data