82 Seismic Imaging 3.3.1.5 Conclusions This diffraction tomography approach produced high-resolution 2D depth elastic models from offset VSP data, collected in the North Sea. The images reveal several geological and geomorphological features that had previously been undetected or poorly mapped by other surface seismic acquisitions. The study has shown that the diffraction tomography technique is practical, efficient and particularly suitable for depth imaging of complex geological systems. 3.3.2 Cross-hole field data The second diffraction tomography example is aimed at handling acoustic and multicomponent borehole data collected at two different boreholes, located in the Paris basin. High-resolution tomograms were produced, allowing the identification of three near-surface hydrocarbon reservoirs with thicknesses of between 2–5 m. The reservoirs are separated by a set of north-south faults with east dips and throws in the order of 30-40 m, consisting of three sand levels imbedded in shales, and depths of between 575-600 m. Beydoun provides a more detailed processing and interpretation of these data (Beydoun et al., 1989). 3.3.2.1 Field parameters An oil field test site was constructed in the Paris basin, an area in which the geology is well known from previous well logs and seismic studies. The test site has several boreholes with inter-well distances ranging between small offsets (80-100 m) and large offsets (600-800 m), making it favorable for cross-hole seismic research, such as downhole source prototypes. Figure 3.16 Cross-hole test site. Source/receiver setup. Adapted from Beydoun et al. (1989). 161: Geolock recorder; 162: Downhole seismic source: sparker/ weight-drop; 163: Dynamite source; 164: Downhole streamer.
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