Estimation of the mean magnetic crustal depth and magnetization over Southern Africa by means of a regional spatial power spectrum
Abstract
The spatial power spectrum, or degree variance, of a signal decomposed in spherical harmonics is defined as the energy carried by each degree averaged over a reference sphere. It has been widely used for global scale studies for comparisons between magnetic field models and source field separation since its first expression in the framework of geomagnetism. At regional and local scales, a wide variety of spectral tools (e.g. based on Fourier or wavelet filters) are available for gridded data. They allow a first order depth and shape estimation of the magnetic sources. However, there exists a lack of spectral methods for processing vector and multilevel data, particularly in spherical coordinates. We present the formulation of a magnetic spatial power spectrum defined in a portion of a spherical shell by use of the Revised Spherical Cap Harmonic Analysis (R-SCHA). We discuss its theoretical framework, its properties, its similarities and differences from the spherical harmonics power spectrum. These comparisons show that it is possible with minor modifications to exploit the body of statistical and deterministic power spectra developed in spherical harmonics for regional magnetic field purposes. We present an application for the lithospheric magnetic field over Southern Africa and compare its R-SCHA power spectrum to various theoretical expressions. In particular, we show that the regional spectrum is best fitted with a new expression proposed in spherical harmonics by Thébault and Vervelidou for the power spectrum of the magnetic field of the Earth's lithosphere ("Is the Earth's lithospheric magnetic field globally self-similar?", see abstract ID 1468666). This spectral analysis suggests that the magnetic crust over Southern Africa is 24.5±0.5 km thick with a mean magnetization of 0.8 ±0.01 A/m. We finally discuss the relevance of such estimates by considering independent geophysical measurements.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMGP13A1114V
- Keywords:
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- 1517 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic anomalies: modeling and interpretation;
- 1541 GEOMAGNETISM AND PALEOMAGNETISM / Satellite magnetics: main field;
- crustal field;
- external field;
- 1545 GEOMAGNETISM AND PALEOMAGNETISM / Spatial variations: all harmonics and anomalies