Basement Magnetization Estimates By Wavenumber Domain Analysis of Magnetic and Gravity Maps

Abstract

Total field magnetic anomaly maps can be reduced to the pole by wavenumber filtering operations which allow both for the orientation of the Earth's field vector and for the direction of magnetization of the bodies producing the anomalies. If the same bodies have a density contrast relative to the surrounding rock, the corresponding gravity field can be converted into a pseudo-magnetic map by another wavenumber filter. Cross-spectral analysis between this and the magnetic map at the pole yields a response function and coherence which depend on the assumptions made about the direction of magnetization. Tests with synthetic data show that the correct direction is the one which maximizes the coherence and at the same time generates zero imaginary response. the corresponding value of the real response gives the correct ratio of magnetization to density contrast for the body. the direction of the total magnetization vector can be combined with the measured directions of remanent magnetization of rocks of different ages from the region to isolate the remanent component and estimate its age, hence ‘dating’ concealed magnetized rocks in the crust. the benefits and difficulties of the method are illustrated by applying it to gravity and magnetic maps of the English Lake District.