Accurate display of fine detail in aeromagnetic data

Abstract

A contour map of total magnetic intensity often either distorts or fails to depict those components of aeromagnetic data which have high spatial frequency and (or) small amplitude. These components may be just as significant geologically as any other. Because of the broad range of amplitudes and wavelengths in much magnetic data, total intensity profiles along flight lines also often fail to depict these components clearly. Second horizontal derivative profiles along flight lines clearly show high frequency components in the data without any assumptions being made about magnetic field values in areas of no data between flight lines. If the dynamic range of such a second derivative profile is halved bv taking the square root of amplitudes along the profile, all the anomalies down to the smallest can be clearly displayed, without larger anomalies overprinting adjacent profiles. Profiles of these high spatial-frequency components can be drawn along a curved baseline which follows the flight path of the aircraft: hence these components are shown in their true locations. The practical advantages of this suggested data display are: (1) computation is simple; (2) the visual display is clear; (3) interpolation between flight lines is in the hands of the interpreter; (4) areas of insufficient data density can be readily recognised; (5) in areas of steep dip and magnetic inclination, the display forms a map of near-surface magnetised rock units. It is suggested that three basic display formats are appropriate for most aeromagnetic data. These are (1) total intensity contours to display major features of the magnetic field, (2) total intensity profiles so that true anomaly shapes and amplitudes are available for modelling; (3) second derivative or similarly filtered stacked profiles, to display clearly significant features which may otherwise be overlooked.