Analytic Signal and Reduction-to-the-Pole in the Interpretation of Total Magnetic Field Data at Low Magnetic Latitudes

Abstract

The problem of interpreting total magnetic field data collected at low magnetic latitudes is addressed through the use of 3-D analytic signal amplitude (Nabighian, 1972. Roest, 1992) applied to the total magnetic field and the vertical integral of the magnetic field. This technique is contrasted with conventional reduction to the pole as implemented by Grant and Dodds in 1972. The amplitude of the analytic signal of the total magnetic field produces maxima over magnetic contacts regardless of the direction of magnetization. When applied to the vertical integral of. the magnetic field. results are more similar to pseudo-gravity (Shonov. 1957). in that the strength of magnetization of underlying rocks is related to the amplitude of the analytic signal. The direction of magnetization. which may vary depending on the level of induced magnetization. permanent (remanent) magnetization and magnetic anisotropy. is removed by the process of calculating the analytic signal. Where rocks are magnetized predominantly by induction of the earth's field. the amplitude of the analytic signal is still dependent on the geologic strike. but this dependency is very much easier to deal with in the interpretation of analytic signal amplitude as compared to the original or pole reduced magnetic field. Results are shown for the application of this method to synthetic data and total magnetic field data collected over the Crixas greenstone belt in Goias, central Brazil.