New developments in resolving detail in aeromagnetic data

Abstract

Stacked profiles of aeromagnetic data often fail to resolve and display detail as this can be obliterated by larger amplitude features on adjacent flight lines. Contours and images of interpolated data can also obscure detail because they are sensitive to survey levelling errors and can show erroneous distortions caused by the interpolation method. Mclntyre (1981) showed that shaded stacked profiles of the second horizontal differences of flight-line data are effective in detecting and displaying detail in aeromagnetic data. Mclntyre used a square-root function to control the dynamic range of the computed difference so that both small and large amplitude features could be displayed without being obliterated by larger amplitude features on adjacent flight-lines. The shaded profile preferentially enhances positive values. Improvements on Mclntyre's shaded stacked profiles of horizontal differences form the basis of a new method of presenting horizontal differences of aeromagnetic data: the bipole map. This is a series of stacked flight-line profiles of bar-graphs of the horizontal difference values. The heights of the bar-graphs are controlled by the amplitude of the difference values whilst polarity is depicted with colour: red for positive values and blue for negative values. The bar-graphs are centred and located precisely on the curved flight-line; the flight-line forming the centre-line of the profile. Wavelength dependent automatic gain control (AGC) filters are used to modify the dynamic range of the horizontal differences so that both small and large amplitude anomalies can be displayed. The bipole map visually enhances both negative and positive values, more precisely positions the data and does not depend upon dubious magnetic values interpolated between the flight-lines. Bipole maps of the first, third and fourth horizontal differences are effective in resolving detail in other parts of the frequency spectrum. They supplement contours and images of longer wavelength features. Magnetic models show the responses of the horizontal differences for various inclinations of the Earth's magnetic field and different widths of the magnetic source. In areas of steep inclination, the horizontal differences delineate the strike axis of thin bodies and the edges of thick bodies. Bipole maps of horizontal differences may find useful application in displaying other types of geophysical data such as VLF-EM and aeromagnetic gradiometer data. Bipole maps of higher order differences are an effective means of resolving high frequency (instrument) noise in geophysical data. For aeromagnetic data from the Mt Magnet goldfield, located in the Archean greenstone belt of Western Australia, bipole maps of the second horizontal differences revealed that an anomaly from a banded iron formation might be caused by several close-spaced banded iron bodies rather than a single body, as might first be interpreted from the contours and images of the gridded data. Bipole maps of the fourth horizontal differences revealed a series of NNE striking magnetic lineaments which are not apparent in the contours and images of the gridded data.