Oceanic loading monitored by ground-based tiltmeters at Cherbourg (France)

We installed two orthogonal Blum-Esnoult silica tiltmeters in an underground military facility close to the shore in Cherbourg (France). They have recorded the oceanic loading effects from March 2004 to July 2005. The signal to noise ratio is such that, within a period range from a few minutes to a few days, the main non-linear oceanic tides up to the M10 group can be observed. The modelling of the tidal tilt deformation has been carried out using oceanic models of the FES2004 family, with a stepwise refinement of the grid size based on the unstructured grid T-UGAm model leading to the NEA-2004 tidal solution. This improvement permits to reduce the discrepancy between the model and the data with respect to the use of FES2004 alone, and show that, although the misfit remains significant, one progresses toward an independent mean to validate the oceanic models and finally the whole modelling process. We also show that tiltmeters open new opportunities to explore loading of non-linear tides on a larger spectrum than gravimeters and GPS do.