Seafloor geomagnetic observatory program in France

The accurate determination of the main geomagnetic field and its secular variation at the global scale remains limited by the number of observatories and their poor distribution over the Earth´s surface, particularly over the ocean areas. This biases significantly the determination of the source field geometry even at very long wavelength. In France, a collaborative action is undertaken to build a semi-permanent seafloor observatory that matches the INTERMAGNET measurement requirements. This observatory comprises a triaxial variometer and an Overhauser magnetometer measuring absolute total field intensity. The main difficulty remains the measurement of the geomagnetic declination, which needs a known geographical reference within a few tens of arc-seconds. The declination may be obtained using a precise mechanical gyro. A full working absolute observatory has been built and tested. The observatory is going to be deployed in the North Pacific Ocean at the site H2O in September 2003 along with a US observatory for intercomparison and qualification of the absolute measurement.