limitations imposed by geomagnetic variations on High Quality Aeromagnetic Surveys

The accuracy and sensitivity with which aeromagnetic anomaly data may be collected — and presented as images for those who seek to maximise the representation of the geological information — is limited by the thoroughness with which temporal variations are removed from the recorded total field data. Three processes are currently used consecutively to eliminate time-variations: base station subtraction, tie-line levelling and micro-levelling. Of these, the first two involve assumptions that are not entirely justifiable and the third is essentially an arbitrary filtering process. On profile data with a noise envelope of no more than ±0.1 nT, even the third process involves adjustments that can be many times larger than other uncertainties in the data.