Mapping with geophysical data

In the same way that topographic contours can be transformed to models of continuous elevation change using surface fitting, measurements of gravitational and magnetic field potentials, at points on the ground or along aerial survey lines, are sources of imagery.  Expressed as contours joining points with the same value, spatial distributed data are notoriously difficult to interpret, however much information they contain.  Not only do contours simplify the data by dividing them into arbitrary steps, how we interpret contour maps depends on how we perceive them.  Our eyes evolved to extract information distributed as a continuum across our field of view, and our visual cortex developed many tricks to innately interpret clues to shape, perspective and distance, to extend the limits of stereoscopic vision (we see objects in true 3-D only if they are closer than about 400 metres).  Our innate abilities “interpret” contours in terms of the spacing between them; the closer they are together the darker we perceive the area of steep gradient.  In other words we have to convert an image that is the “negative” of the first derivative to an understanding of the actual shape represented by contours!  Unsurprisingly, we have to learn to “read” maps, and that is a great deal more difficult for those showing potential-field intensity than for topographic elevation.  Cartographers long ago latched onto our use of shadows as clues to shape, and designed maps with shading as if the Sun was shining from the top of the sheet.  They also use different colours as a second clue to what is high and low.  Combining the two aids helps transform images of geographic variables – basically bland shifts from high to low – into visually stunning, and therefore more easily interpreted pictures.  Surface modelling of elevation and geophysical data, with such graphic tricks, literally throws hidden, and often unsuspected features into sharp relief.

These techniques have revitalized desktop interpretation of the world, especially using results of geophysical surveys.  However, in the same way that detail of a terrain blurs and loses information as resolving power falls, low-resolution data of other kinds obscure buried features, or give ambiguous hints to what they are and where they go.  Reducing the spacing of aerial surveys, and the height from which they are acquired, increases the resolving power of the technique.  Stunning examples of the state of this particular art appear in recent work by the US Geological Survey (Grauch, V.J.S. 2001.  High-resolution aeromagnetic data, a new tool for mapping intrabasinal faults: example from the Albuquerque basin, New Mexico.  Geology, v. 29, p. 367-370.  See also ). 

Grauch worked on an area in which superficial materials and rapid rounding of topography result in poor surface expression of all but the largest faults.  By using aeromagnetic images modelled from survey lines spaced at 100 to 150 metres, he picked out not only hidden faults, but also the magnetic signatures of pipelines, water tanks and buildings.

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