TY  - JOUR
A1  - Geese, Anne
A1  - Mandea, Mioara
A1  - Lesur, Vincent
A1  - Hayn, Michael
T1  - Regional modelling of the Southern African geomagnetic field using harmonic splines
N2  - Over the southern African region the geomagnetic field is weak and changes rapidly. For this area series of geomagnetic field measurements exist since the 1950s. We take advantage of the existing repeat station surveys and observatory annual means, and clean these data sets by eliminating jumps and minimizing external field contributions in the original time-series. This unique data set allows us to obtain a detailed view of the geomagnetic field behaviour in space and time by computing a regional model. For this, we use a system of representation similar to harmonic splines. Initially, the technique is systematically tested on synthetic data. After systematically testing the method on synthetic data, we derive a model for 1961-2001 that gives a detailed view of the fast changes of the geomagnetic field in this region.
Y1  - 2010
UR  - http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1365-246X
U6  - https://doi.org/10.1111/j.1365-246X.2010.04575.x
SN  - 0956-540X
ER  - 
TY  - JOUR
A1  - Hayn, Michael
A1  - Panet, I.
A1  - Diament, M.
A1  - Holschneider, Matthias
A1  - Mandea, Mioara
A1  - Davaille, A.
T1  - Wavelet-based directional analysis of the gravity field  evidence for large-scale undulations
JF  - Geophysical journal international
N2  - In the eighties, the analysis of satellite altimetry data leads to the major discovery of gravity lineations in the oceans, with wavelengths between 200 and 1400 km. While the existence of the 200 km scale undulations is widely accepted, undulations at scales larger than 400 km are still a matter of debate. In this paper, we revisit the topic of the large-scale geoid undulations over the oceans in the light of the satellite gravity data provided by the GRACE mission, considerably more precise than the altimetry data at wavelengths larger than 400 km. First, we develop a dedicated method of directional Poisson wavelet analysis on the sphere with significance testing, in order to detect and characterize directional structures in geophysical data on the sphere at different spatial scales. This method is particularly well suited for potential field analysis. We validate it on a series of synthetic tests, and then apply it to analyze recent gravity models, as well as a bathymetry data set independent from gravity. Our analysis confirms the existence of gravity undulations at large scale in the oceans, with characteristic scales between 600 and 2000 km. Their direction correlates well with present-day plate motion over the Pacific ocean, where they are particularly clear, and associated with a conjugate direction at 1500 km scale. A major finding is that the 2000 km scale geoid undulations dominate and had never been so clearly observed previously. This is due to the great precision of GRACE data at those wavelengths. Given the large scale of these undulations, they are most likely related to mantle processes. Taking into account observations and models from other geophysical information, as seismological tomography, convection and geochemical models and electrical conductivity in the mantle, we conceive that all these inputs indicate a directional fabric of the mantle flows at depth, reflecting how the history of subduction influences the organization of lower mantle upwellings.
KW  - Wavelet transform
KW  - Satellite geodesy
KW  - Gravity anomalies and Earth structure
KW  - Pacific Ocean
Y1  - 2012
U6  - https://doi.org/10.1111/j.1365-246X.2012.05455.x
SN  - 0956-540X
SN  - 1365-246X
VL  - 189
IS  - 3
SP  - 1430
EP  - 1456
PB  - Oxford Univ. Press
CY  - Oxford
ER  -