Wavelet analysis of detailed drillhole magnetic susceptibility data, Brockman Iron Formation, Hamersley Basin, Western Australia

Abstract

Closely spaced drill-core susceptibility measurements for the complete Dales Gorge Member of the Brockman Iron Formation have been analysed using different techniques to try to understand the periodicity of layering in banded iron-formations (BIFs) in the Hamersley Basin, Western Australia. Bulk magnetic susceptibility varies considerably throughout the BIF units due to variations in thickness and composition of oxide/silicate mesobands, variations in the proportion of microbands and changes in the hematite/magnetite ratio. This local scale variability and the presence of layering effects at several scales means that the drill-hole susceptibility data are non-stationary and conventional tools for analysis such as autocorrelation, spectral analysis and semivariogram techniques have limited value. Sliding window spectral analysis is used for non-stationary spatial series and the results show significant changes in amplitudes and spectral shape down the drill hole but were difficult to interpret. Both continuous wavelet (CWT) and discrete wavelet (DWT) techniques proved to be very powerful for analysis of this non-stationary data. CWT analysis using a Morlet wavelet helped to quantify the periodicity of the layering and DWT analysis using a Haar wavelet provided an effective means of ?blocking? or segmenting the log so that susceptibilities of individual BIF macrobands can be compared. The results suggest that wavelet analysis can provide definitive information on scaling properties of magnetic susceptibility needed to determine ?formation response? or facies variation within the major BIF units.