Geophysical Maps and Images

The methods available for display of processed geophysical data (magnetic, radiometric and elevation) include traditional line profile and contour maps and more recently the pixel-image map and associated raster image file.

The line, or stacked profile map shows fine detail that is not as visible in the display of data by contour maps and images. The Australian and State Governments routinely produce pixel image maps and raster images for display of geophysical data as the utility of Geographical Information Systems (GIS) allows for the simultaneous display of geophysical data with other geospatial datasets.

For the production of contour maps and images the recorded flight-line measurements are subject to a standard process called ‘gridding’ during which the recorded geophysical data along each flight-line is converted to data samples at equally spaced locations along and between the flight lines. The gridded data sample interval, or cell size, is usually chosen to be one-quarter to one-fifth of the survey line spacing. The appropriate cell size is chosen as a balance between having a grid with too many redundant cells and having large gaps, or excessive interpolation between cells.

Geophysical pixel image maps can be shown as black and white and colour images. Magnetic images displayed with a full colour scheme use the warmer colours (reds and oranges) to represent areas of higher magnetic intensity and the cooler colours (greens and blues) to represent areas of lesser magnetic intensity.

Images can be enhanced also by the application of various filtering and transformation processes to produce images with enhanced information content. For example, magnetic data can be subject to a process called ‘Reduction to the Pole’ (RTP) in which the recorded data are converted to what they would have looked like if the magnetic field had been vertical rather than as recorded in the Earth’s inclined magnetic field. Articles which describe the vast array of additional enhancements for geophysical data are available from the Continental Geophysics Project.