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.

Spectral depth analysis of some segments of the Bida Basin, Nigeria, using aeromagnetic data

“…The Fourier integral transform of a function that varies continuously along a profile of observation, such as magnetic field intensity, transforms the function from the space to the frequency domain, and is expressed, according to Sadek et al (1984) as:…”

“…Sadek et al (1984) further relates the energy density function E(f y ) and the Fourier transform by the relation:…”

Outline of The Ground Water Occurrences Beneath the area Around Ain Zubida by Integration of Magnetic, Electrical Resistivity Sounding, and Seismic Refraction Methods

“…According to these results, the anomaly sources beneath the area under study are classified as; regional anomalies (deep-seated) located at 121 m average depth; and residual anomalies (shallow or near-surface anomalies) located at 25 m average depth. To isolate the magnetic response of each of these anomaly groups, matched filtering (Sadek, et al, 1984) was applied using a special window designed according to the parameters determined from the frequency analysis curve of Figure (5). This window is used to calculate the regional magnetic component of each input total magnetic intensity measurement.…”

Deep gravity data interpretation using seismic reflection and well data: A case study of the West Gharib-Bakr area, Eastern Desert, Egypt

“…The isolation method depends mainly on the slopes of the spectral analysis curve, where the frequency ranges give the most efficient estimation of the gravity anomalies to fit the deep and shallow constituents. The separation using the energy spectrum of potential field data is highly advantageous over any other known conventional method (Sadek 1984). The potential anomalies in the space domain are transformed into frequency domain and the various parameters of anomaly sources are derived from the characteristic properties of the amplitude.…”