A New Method of Surface Wave Analysis and OBC Signal Enhancement in Shallow Water Environment of the Arabian Gulf using Time-Frequency-Wavenumber Polarization Analysis

Abstract

In the shallow water environment of the Arabian Gulf, a combination of strong lateral seafloor heterogeneities, shallow water depths (~10m), and hard sea bottom results in highly dispersive and aliased surface waves with complex wave-field polarization in all four components of the OBC seismic data. Compared to body waves, surface waves are characterized by low velocity, low frequency and high polarization and their polarization states are time, frequency and mode dependent. Those properties challenge the existing surface waves filtering techniques because they take into account velocity, frequency or polarization only. Consequently, the residual surface wave leads to inaccurate directional analysis and has severe adverse effect on the quality of subsequent seismic data processing algorithms.Considering the joint relationship among wavenumber, time, frequency and polarization of surface waves, we propose a two-step rotation method to separate the surface waves from the reflected waves and estimate wave polarizations. First, we use a non-stationary FK filter to eliminate the effect of the surface wave from OBC record in the time-frequency-wavenumber (TFK) domain, where we can define the surface wave rejection zone by its low frequency, low velocity and high amplitude characteristics. Second, considering the pattern of particle motion and propagation directions, we estimate wave-field polarization features by extending the traditional single domain polarization analysis into the decomposed TFK domain. Spherically polarization noises (surface waves) can be distinguished from rectilinearly polarized waves (body waves) through covariance matrix calculation for each TFK point. Therefore, the residual surface wave which overlaps with body waves in TFK domain are eliminated based on its polarization characteristic. Analysis on field data shows that this proposed new method is effective in both attenuating the surface waves and providing reliable information on orientation, which are critical for the further processing on the shear wave and converted wave in horizontal components.