78 Seismic Imaging 3.3.1 Vertical seismic profile (VSP) field data This example describes the processing of a multicomponent offset VSP dataset, collected in the North Sea. The purpose of this survey was to detect fault blocks at the deep Brent reservoir formation, thicker than 150 m. The reservoir is located in the Middle Jurassic Brent formation, positioned under the Cimmerian unconformity (3,558 m) at the boundary between the base of the Lower Cretaceous and the top of the Upper Jurassic. An analysis of the VSP tomograms enabled the delineation of the reservoir and the identification of at least two faults. Beydoun et al. (1990), provide details of this application. 3.3.1.1 Seismic data acquisition Surface seismic data acquisition carried out previously in this area had failed to provide good quality imaging of the Brent reservoir. In particular, the strong multiples generated at the Cimmerian unconformity masked the weak primary reflections from the reservoir. To improve the quality of the seismic results and considering the surface data information, a multi-component offset VSP set up was performed with the following characteristics: Source: • 2 x 200 in2 Bolt air guns (on a supply boat); • depth 7 m; • offset: 1,200 m; • 6 shots per level (i.e., at each receiver location). Receiver: • 3 component Geolock H3 hydraulic tool (from CGG); • Geophones:15 Hz; • sampled rate: 2 ms; • station interval: 25 m; • depth range: 600 – 4,140 m; • number of depth levels: 129. A zero offset VSP was simultaneously acquired, shooting alternatively from the supply boat and from the rig, which had a 550 in2 Bolt air gun attached. The VSP survey recording time was 27 hours, while rig down time was 28 hours. 3.3.1.2 Data Processing The main focus of data preprocessing was to preserve the seismic wave amplitude. For such preprocessing, it was sufficient to only apply a few steps, which were: • the reorientation of the three-component data along Z the vertical axis, X the axis in the plane of propagation, and Y the transverse (out of plane) horizontal axis;
RkJQdWJsaXNoZXIy NjA3NzQ=