Radiometric Data Processing

All processing steps as recommended e. g. in the publication of the IAEA Airborne Gamma Ray Spectrometer Surveying Technical Reports Series No. 323 (dead time, cosmic, radon, background, attenuation) can be performed together with the required calibrations.
Ratio and ternary maps can be produced.

  1. Radiometric Spectra
    Spectra as gained by the acquisition system will first be loaded into our line data base which consists of files called cubes (one cube for each major processing stream like magnetics, radiometrics etc., though all streams of the complete project could go into one single cube). The spectra are stored like any other channel, only that they consist of all energy channels of which the first and last usually have a special meaning, i.e. system up-time and cosmic count. Additionally there is a channel for live time added to the always given channel of the fiducial giving the point in time of the sample.
  2. Spectral Components NASVD
    The program spc_pc can be used to improve the signal/noise ratio by about the factor 3 using the method as developed by Jens Hoovgaard. This usually provides the channels for the elements of interest already dead-time corrected.
  3. Line List
    A line list is added to the cube giving the line numbers or names with the start and end times. This way distinguishing between on-line and off-line times. The cube can contain any number of line lists, there can be a list with extensions for certain steps of processing to avoid edge effects at the line ends and another line list for final data delivery or gridding. Though most processing is done on whole flights.
  4. Windowing
    Window counts, i.e. combinations of energy channel counts can be put into normal cube channels representing counts for potassium, uranium and thorium etc.
  5. Adding Position and other Data
    The add_channel program will then add positional data from another cube to the cube of radiometric data. Other channels like temperature and pressure are added in the same way.
  6. Filtering
    Some channels are required in a filtered form and fourier1d is used for this purpose.
  7. Corrections
    There are a number of corrections necessary. There might be a special program sometime to apply corrections from a given set of coefficients. Right now, there is a very versatile program called function_line for almost any formula to be carried out on records. It is using polish reverse notation and can have any number of factors with temporary items and also create any number of channels in a single calculation. The full stripping takes for 20thousand lkm only about two seconds.
    1. cosmic and background correction
    2. radon correction
    3. effective height determination
    4. stripping
    5. attenuation correction
    6. concentrations
  8. Height Adaptive Filtering
    In more rugged terrain the flying height can not always be maintained as low as desired for having reliable counts in each energy window. The usual sampling rate is at one second. A method for height adaptive filtering in a similar fashion as filtering seismic traces depending on reflection depth (the author worked once for Prakla-Seismos) can be used for an optimal trade-off between spacial resolution and signal/noise ratio. A sampling time over 9 seconds should improve this factor by square root of 9 or 3 which is about the effect of such filter at higher altitudes, eventually as high as 500m effectively, where the spectrometer footprint is anyway large.
  9. Gridding and Maps
    The Generic Mapping Tools provide different options for gridding and all sorts of maps can be produced. Examples will soon be provided.
  10. Ternary Map
    To produce a good looking ternary map seems to be a somewhat tricky procedure: The current procedure consists of a number of steps carried out on grids of concentrations which are converted to three grids of RGB (Red, Green, Blue) which can directly be used to create an image using the Generic Mapping Tools. It involves the use of certain scaling factors and a number of grid histogram equalizations. The colour is exclusively determined by the ratio of the three elements while the colour intensity is then adjusted according to air absorbed dose rate or similar formula for the total intensity. The colour schema can also be based on CMY instead of RGB and conversions between CMY, RGB or HSV can be done as needed.