A key advantage of acquiring multicomponent data using ocean-bottom sensors, whether using cables or nodes, is the ability to separate the wavefield into up- and downgoing parts. This opens up a host of attractive possibilities such as mirror imaging using the downgoing wave, attenuating receiver-side multiples using the upgoing wave only, or combining both up- and downgoing waves to completely remove the free-surface effect using up-down deconvolution. We focus here on the latter.

Acquiring four-component seismic data with wideazimuth geometry provides an opportunity to build a very complete seismic picture for reservoir description. The recording of the full vector wavefield allows creation of both PS-wave data as well as P-wave images which contain different but complementary information. It also provides full-azimuth illumination of the subsurface. Azimuthal images improve definition of structural features, such as faults, that may only be illuminated within certain preferential shot-receiver azimuths. Differences in azimuthal images can also be very sensitive fault indicators in the case of small vertical displacements. Furthermore, and of crucial importance at the reservoir scale, wide-azimuth P-wave and PS-wave data lend themselves to the evaluation of azimuthal anisotropy. These attributes provide valuable spatial constraints in the characterization of heterogeneously distributed subseismic scale fractures.In 2003-04, Occidental Petroleum of Qatar, in partnership with Qatar Petroleum, acquired one of the first wideazimuth 4-C surveys in the Middle East, offshore Qatar which covered the Idd el Shargi "north dome" and "south dome" fields ( Figure 1). The reservoirs are within gentle anticlinal structures and have highly permeable fault zones and fracture corridors with small near-vertical displacements, which are often below the resolution of conventional narrow-azimuth streamer seismic. The reservoir now is undergoing secondary recovery with an extensive water-flooding program (Shifflet, 2003). For effective reservoir management, it is therefore of great importance to accurately detect the high-permeability zones for well placement and to increase the efficiency of injection programs.The ocean-bottom cable (OBC) data were acquired by Multiwave Geophysical using a patch geometry, with orthogonal receiver cable and shotline directions, providing full azimuthal coverage to offsets of 3000 m, equivalent to an offset/depth ratio of 2 for the main target zone. The nominal fold was 240 in the natural bin size of 9.4 ǂ 25 m. This gave sufficiently high signal/noise ratios, when used in conjunction with a more optimal bin size for imaging, to create eight azimuthlimited data sets for both the P and PS data. In total, one billion traces were acquired, making it one of the largest OBC surveys shot and processed to date. The main focus of the project was to apply an azimuth-friendly processing workflow (Gomez et al., 2004) to provide the optimum data preconditioning for a robust azimuthal anisotropy analysis following the methodology introduced by Angerer et al. (2003).In this paper, we present the main aspects of an integrated workflow comprising an adequate acquisition, specialized seismic processing, and robust attribute extraction for reservoir characterization. P-wave processing. The main objective of the study was the preservation of the azimuthal signal for subsequent anisotropy analysis. This had to be weighed against other objectives such as applying aggressive noise attenuation, or pu...

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Reliable seismic images of gas accumulations in the shallow subsurface beneath production platforms are mitigating drilling risks in the Forties oil field. Ocean-bottom nodes (OBN) were selected to record the seismic data due to their ability to operate safely and efficiently in busy and obstructed oil fields. The resulting seismic images allow extra care to be taken during drilling where gas is likely to be encountered. The resulting operations can therefore be optimized in terms of both safety and costs.

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