Wave-Field Decomposition of Ocean Bottom Seismograms

Seismograms recorded on the seafloor are affected by reverberations in the overlying ocean layer. We examine the feasibility of removing these reverberations through a 1D wave‐field decomposition based on a reflection–transmission formulation of the problem. Two decomposition schemes are presented. The first scheme involves a simple manipulation of the fundamental matrix relating stress and displacement of plane harmonic waves to upgoing and downgoing wavevector coefficients and requires measurement of both ocean bottom displacement and pressure. The second approach involves only displacement recordings and knowledge of the water column depth. This latter quantity will generally be known a priori from deployment logs, or, alternatively, it may be estimated using vertical displacement and pressure seismograms in a preprocessing step. Both approaches require prior information on seabed properties. In the case of P ‐wave incidence, the decomposition depends primarily on the seabed S velocity β . This quantity can be determined by examining trial decompositions over a range of β ’s and selecting the value that minimizes the energy of the upgoing S ‐wave component at the arrival time of the incident wave. We apply this approach to synthetic seismograms for a simple Earth structure and to recordings of two large events from the Central Oregon Locked Zone Array (COLZA) on the continental margin of Oregon. The real data indicate that the wave‐field decompositions are largely successful at lower frequencies (<0.1  Hz) but that 3D scattering, likely originating near the sediment–basement contact, is manifested at higher frequencies.