Determination of near seafloor properties from ocean-bottom recordings

Christine Ecker and Colin Sayers

ABSTRACT

We compare two methods of determining the near seafloor parameters (density, P-wave and S-wave velocity) from data recorded by ocean-bottom seismometers (OBS) and ocean-bottom hydrophones (OBH). The first method is based on AVO from ocean-bottom pressure and vertical particle velocity, and the second on AVO from ocean-bottom vertical and radial particle velocity. Using simple synthetic seismograms, we evaluate the parameter estimation for a simple water over half-space model and explore how the results are influenced by layering and poroelastic effects. The method using two particle velocities seems to be more robust and more sensitive to parameter changes over a broader range of angles. Introducing either a 50 m or 100 m thick layer underlaying the seafloor, this method still yields P-wave velocity estimates with errors less than 3% and S-wave velocities with errors less than 7%. The use of pressure and vertical particle velocity does not result in good estimation of P-wave velocity, S-wave velocity or density. In the case of poroelasticity, the pressure-vertical particle velocity method produces reasonably good results for the P-wave velocity ($\le$ 1% error) and S-wave velocity ($\le$ 2% error), while introducing a significant error into the density (20% error). Furthermore, poroelasticity has a considerable effect on the radial particle velocity, thus causing the inversion based on the vertical and radial particle velocity components to yield errors of more than 30% for the S-wave velocity estimation.