Why no anomaly is visible over most of the continent-ocean boundary in the global crustal magnetic field

It is generally believed that an anomaly should be expected over the continent-ocean (C-O) boundary in the global magnetic anomaly maps. However, such maps prepared at satellite altitude do not show an anomaly over most of the C-O boundary. We address this issue by a forward modelling technique. We use a Geographic Information System (GIS) technique to integrate information of the various geological units of continents and oceans, the seismic crustal structure, and standard susceptibility values of the rock types and compute a global vertically integrated susceptibility (VIS) model. In addition, a remanent magnetisation model is used for the oceanic crust. Combining the induced and remanent magnetisation models, the crustal field anomaly is predicted at a satellite altitude of 400 km for spherical harmonic degrees 16-80. The results show that an anomaly is indeed not predicted over the C-O boundaries where the oceanic crust is flanked by continental regions Phanerozoic in age. We demonstrate that, largely, a C-O anomaly over these regions is not even expected in the global magnetic anomaly map for degrees 1-80, confirming that it is not a long-wavelength feature masked by the core field. An anomaly, however, is predicted over some parts of the C-O boundary where the flanking geological provinces over the continents are Precambrian in age. We argue that the VIS values for the continents and oceans are largely comparable and therefore no anomaly is expected over most of the C-O boundary.