Solving Shear-Wave Splitting in PS Data Processing of 3D 4C OBN Seismic Survey Offshore Nunukan, Indonesia

Abstract

A 350-km2 3D 4C ocean-bottom node (OBN) offshore survey was acquired in December 2018, in Nunukan, North Kalimantan, Indonesia (Figure 1) with the processing work completed during 2019. Indonesia as a whole and the Nunukan survey area are situated in an active tectonic area where the Pacific Plate in the east and the Australian Plate in the south are actively pressing toward the Asian Plate. The tectonic activity is the main source of regional and local stress in the survey area. The dominant stress direction is Northeast-Southwest (NE-SW) (Figure 1). This stress exhibits azimuthal anisotropy in seismic waves, defined as the dependence of seismic wave speeds on propagation azimuth. In homogenous media, once the two horizontal geophone components of OBN acquisition have been properly rotated to the source-detector direction (radial) and a direction perpendicular to it (transverse), the converted waves (PS) energy will be maximum at the radial and minimum at the transverse directions. However, in azimuthal anisotropy media, shear waves split into fast and slow velocity components and the transverse data can appear to have more energy than the radial, as we observed clearly in the Nunukan OBN PS data. Ignoring the shear-wave splitting can result in degrading the PS seismic image. Herein, we outline how we addressed the azimuthal anisotropy in the Nunukan 3D OBN data. Without a shear-wave splitting correction, the PS seismic image lost data continuity of target horizons, making any attempt to correlate it with the PP image extremely difficult. The shear-wave splitting correction provided a much better PS image with improved structural data continuity and higher vertical resolution, giving greater confidence for PP-PS event correlation.