Aeromagnetic drape corrections applied to the Turner Syncline, Hamersley Basin

Aeromagnetic surveys in rugged terrain such as the Turner Syncline in the Hamersley Basin, Western Australia, are often flown as loosely draped surveys within operational safety limits. Loosely draped surveys are a compromise between retaining good spatial resolution of small magnetic sources, maintaining fairly constant terrain clearance, and operating the aircraft in a safe manner. The main problems in these surveys relate to lines flown in opposite directions causing a variable and biased terrain clearance. Over deep narrow valleys, the magnetic source depth increases, anomaly amplitudes are reduced and spatial resolution of small sources lost. Over narrow ridges, the magnetic source depth decreases, anomaly amplitudes are increased and resolution of small sources much improved. The high wavenumber content of lines flown in opposite directions may be very different, resulting in zones of marked texture differences, especially for race-track flight paths.

The ideal drape correction involves projection of the field onto a surface with constant terrain clearance, preserving the full bandwidth of the data. Several grid- and profile-based drapecorrection algorithms have been tested on the Turner Syncline with varying degrees of success. The best results were obtained with a two-pass correction approach. A first order drape correction, using a modified chessboard method was applied to the located data prior to tie-line levelling and microlevelling. The simple ‘slide-rule’ filter of Cordell (1985) was replaced by a technique of continuing different lags of the input data to avoid large downward continuation distances. A second pass of the ‘normal’ chessboard drape correction was then applied to the gridded data to further refine the correction. The results show that the drape corrections have been effective in reconstructing anomaly amplitudes, but less effective in preserving the texture of the data. Conventional approaches to drape correction appear unable to restore the missing high-wavenumber information content caused by extreme height variation.