Spectral analysis of aeromagnetic profiles for depth estimation principles, software, and practical application

Abstract

Fourier spectral analysis in recent years has become a widely utilized tool for the processing and interpretation of potential field data. It is particularly well suited to analysis of aeromagnetic maps and profiles, where coverage commonly is of broad scope and statistical treatment is appropriate. The techniques developed by earlier workers for map data are readily adapted for depth estimates using aeromagnetic profiles. Three subroutines are presented: "FRQAN", which employs the complete Fourier transform to convert field intensity to the frequency domain and then computes the logarithmic energy spectrum; "CSIZE", which refines the spectrum to correct for the finite horizontal dimensions of magnetic sources, and "ENSMTH", which smooths the spectrum to clarify its decay characteristics. The average depths to sources of ensembles are obtained by manually fitting a straight line to each linear interval of the logarithmic energy-decay curve. If proper account is taken of the constraints of the method, it is capable of providing depth estimates to within an accuracy of about 10 percent under suitable circumstances. The estimates are unaffected by source magnetization and are relatively insensitive to assumptions as to source shape or distribution. The validity of the method is demonstrated by analyses of synthetic profiles and profiles recorded over Harrat Rahat, Saudi Arabia, and Diyur, Egypt, where source depths have been proved by drilling.