Determining the shear velocity structure of the oceanic crust from measurements of seafloor compliance

The ocean covers two-thirds of the Earth's surface, making it difficult to study the structure of the Earth in these areas.  The ocean, however, provides a pressure signal that can be used to study the oceanic crust, by measuring the seafloor deformation under this pressure signal.  We use continuous signals of seismic ground velocity combined with differential pressure at the sea floor recorded by ocean bottom stations (OBS).  By combining pressure and displacement data of the OBS, we can calculate the compliance function from the pressure signal of infra-gravity waves in the frequency band (0.003-0.03 Hz) . The displacement depends on the seismic properties of the earth's crustal layers, primarily its shear modulus.
The compliance function is very sensitive to the regions with a low shear velocity which makes it an excellent tool for investigating the earth crust for these regions.

To calculate the compliance function, the data must be preprocessed in several steps including glitch and earthquake removal, tilt correction, etc.
To remove earthquakes we used earthquake catalogs for large events and a recursive STA/LTA algorithm for local events. We use a comb
filter to remove hourly glitches  produced by the internal OBS system . 
Tilt noise is caused by deep ocean currents,  we minimize it on the vertical record by rotating the 3-component seismic data to an angle that minimize the variance of the vertical record, then by removing noise coherent with the horizontal components.

In this study, we used the French OBS of the Rhum-Rum experiment (Barruol and Sigloch, EOS, 2013) near La Reunion Island for 13 months in 2012, then calculated the compliance function. The crustal shear velocity structure below each station is then recovered by depth inversion of compliance.