Global High-Resolution Magnetic Field Inversion Using Spherical Harmonic Representation of Tesseroids as Individual Sources

Eldar Baykiev,Jörg Ebbing,Dilixiati Yixiati

doi:10.3390/geosciences10040147

Abstract

In this study, we present a novel approach combining the advantages of tesseroids in representing geophysical structures though their voxel-like discretization features with a spherical harmonic representation of the magnetic field. Modelling of the Earth lithospheric magnetic field is challenging since part of the spectra is hidden by the core field and the forward modeled field of a lithospheric magnetization is always biased by the spectral range used. In our approach, a spherical harmonic representation of the magnetic field of spherical prisms (tesseroids) is used for high-resolution magnetic inversion of lithospheric field models. The use of filtered spherical harmonic models of the magnetic field of each tesseroid ensures that the resulting field matches the spectral range of the input data. For the inversion, we use the projected gradient method. The projected gradient method easily allows us to assign an initial guess (i.e., a-priori assumption) for the inversion and avoids negative values of susceptibilities. The latter is providing more plausible models since induced magnetization is assumed to be dominant over the continents and, for the oceans, a remanence model can be subtracted. We show an application of the technique to a synthetic dataset and a satellite-derived lithospheric field model where the model geometry is based on seismic information. We also demonstrate a proof-of-concept for high-resolution tile-wise inversion for the Bangui anomaly in Africa.

Highlights

The Earth’s magnetic field contains a signal from various sources including the core, ionosphere, magnetosphere, and the Earth’s magnetic lithosphere [1]
The validity of the approach is demonstrated with a synthetic example and applied to the satellite-model LCS-1 in order to provide a global lithospheric susceptibility model, which, as we demonstrate at the end, can serve as a reference or background model for further studies on local scales
The loss of the long wavelength part between 1° and 15° implies that the inversion might tend to result in solutions that may not match susceptibility distribution in the true model

Summary

Introduction

The Earth’s magnetic field contains a signal from various sources including the core, ionosphere, magnetosphere, and the Earth’s magnetic lithosphere [1]. Often the lithospheric magnetic anomalies are interpreted from aeromagnetic surveys, where the contribution of the inducing core field is routinely reduced during processing. Global models of the lithospheric magnetic field, on the other hand, can provide a global heterogeneous coverage and are usually based on a spherical harmonic representation [3]. For these models, the reliable spectral range is defined by the part not dominated by the main field and within the measuring bandwidth of the data sources. The main field associated with the core dominates the spherical harmonic coefficients up to 15◦. Lithospheric models like LCS-1 [4] only define a reliable lithospheric field spectral content for more than 15◦

Methods

Results

Discussion

Conclusion