Separation Filtering Applied to Aeromagnetic Data

Separation filtering of regional and residual gravity and magnetic fields has been the subject of extensive research. Wavenumber and spatial domain methods, variously described as matched filtering, layer filtering, depth slices etc. have been developed

Separation or layer filtering of magnetic data allows us to deconvolve the effects of causative sources occurring around a particular level. In practice, it is impossible to achieve a complete separation since the problem is non-linear. Separation filtering depends fundamentally on the concept of random distributions of sources within discrete layers and assumes that there is no statistical difference in response along each ideal layer and no correlation between the distribution in each layer.

Interpretation of the log-power spectrum provides slope and intercept parameters for filter design. It is necessary to be able to recognise distinct linear segments of the spectrum corresponding to well separated depth ensembles in the data in order to deconvolve the effects of each depth ensemble with minimum contamination from sources at different levels. Non-linear spectra cannot be interpreted. It is important to apply a finite source width correction to the raw spectrum, otherwise indicated source depths will be too deep.

Tests have been carried out on spectral matched filtering, differential upward continuation separation filtering, exponential taper filtering and a modified second vertical derivative filter. All filtering was performed in wavenumber domain. Spectral analysis of the test area suggested three depth ensembles with average depths of 1500 m, 300 m and 20 m below surface. The modified second vertical derivative technique provided the best representation of the shallow source ensemble being more stable and with a larger dynamic range than the matched filter residual filter. The spectral matched filter regional provided the best representation of the deep ensemble, although there was some contamination by shallower sources. The intermediate ensemble could not be extracted with any degree of confidence.

Separation filtering can make a useful contribution to the enhancement of magnetic data, provided the limitations in the methods are recognised. Even where spatial source separation is reasonable, the filter will not manage to perform a complete rejection of shallow sources or deeper sources and will distort intermediate sources even more. The results should be used qualitatively to assist in source discrimination rather than used as input to analytical techniques.