Processed gamma-ray data can be presented in several ways. Line data can be viewed as profiles at the resolution at which the data were acquired. The K, Th and U elemental concentrations can also be interpolated onto regular grids, from which a series of presentations can be derived. Gridding generally leads to a loss of spatial resolution of the original data, but offers a spatially coherent view of the data not readily apparent on profiles. Contour maps are the traditional derivatives of gridded data. While these have their advantages, data are best visualized using coloured or greyscale images in combination with other datasets created using specialised image processing software. Examples of gridded spectrometry data are shown throughout this training module.
Potassium, Th and U grids can be rendered as grey-scale images by scaling the values of the grids to the dynamic range of the display device, typically with values between 0 (black) and 255 (white). An analysis of a histogram of the image values can be used to enhance the image such that the range in grey scale contrast covers the full dynamic range of the image values or over an area of specific interest.
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Pseudo-colour images can be used as an alternative to grey-scale images for displaying gridded gamma-ray data. Here a "lookup table" is used to assign a hue to grid values. Typically, red is assigned to high values and blue to low values, with a continuous spectrum of colours (through yellow and green) in-between (Figure 9A). Both grey-scale and pseudo-colour images can be enhanced using shaded relief (or gradient enhancement) to incorporate an artificial illumination of the image from a certain direction (Figures 9B and C). This tends to enhance features perpendicular to the illumination direction. The ratios between radioelement point data or grids are also often used to enhance particular features of gamma-ray data (Figure 10). These can be represented as profiles or grey-scale/pseudo-colour images. Ternary images (Figures 11A and B) are the most common way to display gamma-ray spectrometric imagery. The primary colours red, green and blue are assigned to K, Th and U respectively. By modulating the intensity of each of these primary colours by the normalised grid values of the individual K, U and Th grids, a single image can be used to represent all 3 of the radioelement distributions. Here dark colours represent areas low in all 3 radioelements. Near-white colours represent areas high in all 3 radioelements. Red, green and blue colours represent areas high in each of K, Th and U, respectively to the exclusion of the other radioelements, and so on (see colour cube on this page). Ternary images are also often gradient-enhanced using the total-count as the relief. Colour modulation techniques can be further used to enhance the colour variation of three band composite images (Milligan and Gunn 1997) by removing dark and saturated areas in the image. Figure 9 (left). A (top) - Pseudo-colour image, B (middle) - Gradient image of the total count image and C (bottom) - Combined K grid with intensity of the colour from the gradient image. Adding the gradient component into the original image highlights major regolith, lithological and structural contacts / boundaries. |
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Figure 10 (right). K/Th ratio image. Low values (blue in image) correlate to mostly highly weathered soils. High values (red hues) are mostly associated with areas of exposed and only partly weathered bedrock.
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The standard method of displaying a ternary image is with K in red, Th in green and U in blue. Blue is used to display the U channel because it is the noisiest channel and the human eye is least sensitive to subtle changes of blue. Scatter plots are also a useful way of analysing the distribution and inter-relationships between radioelements. Radioelement trends and clusters can often be identified by this type of presentation. Figure 11. A (top) - Ternary image with K in red, Th in green and U in blue. B (bottom) - Total count gradient image added as an intensity component of the ternary image. |
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Gridded gamma-ray data can be combined with a variety of other datasets to enhance specific features or relationships in the data. For example combining gamma-ray images with SPOT or Landsat TM using additive or HIS colour space transformations (Wilford, 1997; Figure 12 and 13) is particularly useful integrating landform information with surface geochemistry reflected in the gamma-ray imagery. One of the most useful presentations are 3D perspective views where either a single element pseudo-colour image or a ternary image is draped over a digital elevation model (Figure 14). This type of representation is a powerful interpretation aid by helping to understand the relationships between landforms, geomorphic processes and gamma-ray responses.
Figure 12 (above). A (left) - Ternary gamma-ray image. B (center) - Edge-enhanced Landsat TM band 5. C (right) - Combined Landsat TM and ternary image. Approximate foreground scale 25 km across each image.
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Figure 14 (above). A ternary gamma-ray image image draped over topography. Perspective drapes are an excellent way of understanding geomorphic relationships and separating bedrock and regolith gamma-ray responses. Potassium is in red, Th in green and U in blue. Scale: Approximate foreground distance accross image is 80 km.