Summary
The presence of remanent magnetization hinders the recovery of the regional or global lithospheric magnetic structures using satellite observed magnetic data. To overcome this difficulty, we utilize the amplitude of the anomalous magnetic field, which is weakly dependent on the magnetization directions, to recover the subsurface magnetic distributions in the spherical coordinates. Jointly inverting the amplitude data with satellite gravity gradiometry data further helps overcome the adverse effect from unknown directions of magnetization. Simultaneously, the amplitude data also helps the recovery of density through the structure similarity between the two physical properties. To achieve these goals, we have developed an algorithm for cross-gradient joint inversion of amplitude and gravity gradient data in spherical coordinates. We use a synthetic example to illustrate our approach in improving the inversions in the presence of strong remanent magnetization.
Introduction
Earth observation satellites are one of the most powerful tools to study the regional and global structure of lithosphere. The newly launched ESA’s SWARM satellite has delivered abundant magnetic vector field data sets to allow us to invert the lithospheric magnetic structure. For large-scale inversions the curvature of the earth surface must be taken into account. Another major challenge is due to the strong remanent magnetization of subsurface sources in some regions, especially the oceanic area. Amplitude data inversion plays an important role in exploration geophysics in dealing with the remanent magnetization issue due to their weak dependence upon the magnetization direction. We have adopted the amplitude inversion technique in Cartesian coordinates (e.g., Shearer, 2005; Li et al., 2010; Li and Li, 2014) to work in spherical coordinates.
Amplitude inversion is effective but not fully adequate because the amplitude data are not entirely independent of magnetization direction. There are also other potential field satellite missions such as GOCE, which provide data complementary to magnetic data. Comprehensive utilization of amplitude data and the GOCE gravity gradiometry data might promote the reliability of both inverted lithospheric susceptibility and density structures. Wang et al. (2015) proposed an algorithm for crossgradient joint inversion of gravity gradiometry and totalfield magnetic data in spherical coordinates. We extend this structure similarity-based joint inversion approach to the magnetic amplitude and gravity gradiometry data.
