Analyzing 4-C marine seismic data from the Valhall field, Norway

Carlos Rodriguez-Suarez, Robert R. Stewart, Xinxiang Li, John C. Bancroft

A 2-D seismic line using four-component (4-C) receivers - a 3-C geophoneand hydrophone - laid on the sea bottom was acquired in 1996 by PGS. The survey was undertaken by Amoco and its partners over the Valhall Field, offshore Norway. The main objective of the survey was to provide a better image of a chalk reservoir. Converted (P-S) waves are used, as P-P waves are strongly attenuated and scattereddue to the presence of gas in the layers over the reservoir.

The vertical component of the geophone and hydrophone showed a largely reflection-free zone for the target at the middle of the section. Four processing flows were applied to the radial receiver component: conventional CDP processing (forpossible P-S-S events, i.e., a conversion at the sea bottom), common conversion point (CCP) asymptotic binning, P-S DMO, and equivalent offset migration (EOM). We did not find convincing evidence of a P-S-S event. The final result for the radial component processed for P-S events was good, as a continuous image for the target was obtained. A very good overall section was generated using the asymptotic binning method. The EOM method gave better results than converted-wave DMO at practically the same CPU time. The transverse component has reflections for the same events mapped by the radial component, but with much lower continuity.

To investigate the presence of P-wave energy on the radial and transverse components, the processing flow for the P-P wave was applied to the radial and transverse data. In addition, the asymptotic binning P-S flow was applied to the vertical channel data. The results show that: 1) little P-S energy is present on the vertical component, and 2) only small P-P energy occurs in radial and transverse components.

Severe periodic notches in the amplitude spectra, probably due to reverberation inthe water layer, were observed in all components. An algorithm is being developed to attenuate the reverberation in the geophone (mainly for the radial component, where the signal bandwidth is strongly affected) using the hydrophone measurements.