Drape corrections of aeromagnetic data using wavelets

Abstract

Aeromagnetic surveys are commonly flown at a constant height above the terrain to minimise the magnetic effects of variable terrain clearance. This is known as drape flying. However, in mountainous regions it is often not operationally feasible to perform a drape survey. Instead, the survey is flown at a constant barometric height and the draped magnetic data are calculated numerically using a level-to-drape continuation operator. Existing techniques for this calculation include the chessboard and Taylor-series methods. An alternative method described here, based on the wavelet transform, approaches the problem by representing the continuation integral using a family of wavelet basis-functions localised in both space and frequency. This allows the generation of a set of coefficients that can be efficiently applied to the wavelet transform of the signal. The wavelet approach can be used for both 1D and 2D signals. If the drape surface is closer to the ground than the barometric survey height, a major difficulty in the drape correction is the control of noise. This is achieved in the wavelet domain by using a locally-adaptive, exponential noise-reduction filter which can be designed based on the wavelet coefficients. The method can be extended in some cases to generate draped images below the ground surface that can be used to sharpen images of magnetic basement in sedimentary basins. The wavelet method is compared with conventional techniques using data from the Edge Hills region in Canada and the Browse Basin in Western Australia. In this study, the wavelet approach combined with the exponential smoothing filter produces sharper images than either the chessboard or Taylor-series methods.