Three-component OBS-data processing for lithology and fluid prediction in the mid-Norway margin, NE Atlantic

In 1992, a comprehensive three-component ocean bottom seismic survey was pe rformed in the central and northern area of the Voring Basin, offshore mid- Norway, NE Atlantic. An important part of the data acquisition program cons isted of a local survey with 20 Ocean Bottom Seismographs (OBS) dropped at approximately 200 m interval in 1300 m water depth. The main purpose of the local survey was to acquire densely sampled P- and S-wave reflection data above a seismic flatspot anomaly observed earlier, in order to more accurat ely predict if hydrocarbons could be related to it. The conventional reflec tion data processing methods applied to the vertical components included pr edictive deconvolution in order to attenuate low frequency ringing, near of fset mute and a series of constant velocity stacks in order to obtain the o ptimal velocity function. The final result is a "trouser" shaped, high reso lution V-z stacked section with minor influence of water multiples. The inl ine (V-x) component contains no strong multiples, and extensive near trace muting was hence not necessary to apply for this component. Velocity analys is together with ray-tracing modelling indicate that P-S-converted shear wa ves (reflections) represent the dominant mode. The results of the interpret ation and modelling indicated a V-p/V-s-ratio of approximately 2.6 in the o verburden, which suggests domination of partly unconsolidated shale, while the V-p/V-s-ratio in the assumed reservoir was approximately 1.8, which ind icates a more sand dominated facies. Outside the flatspot area a higher V-p /V-s-ratio ratio (approximately 2.0) was estimated, indicating that hydroca rbons could be present in the assumed reservoir.