Seismic Modelling for the Sub-Basalt Imaging Problem Including an Analysis and Development of the Boundary Element Method

摘要

The north-east Atlantic margin (NEAM) is important for hydrocarbon explorationudbecause of the growing evidence of hydrocarbon reserves in the region.udHowever, seismic exploration of the sub-surface is hampered by large deposits ofudflood basalts, which cover possible hydrocarbon-bearing reservoirs underneath.udThere are several hypotheses as to why imaging beneath basalt is a problem.udThese include: the high impedance contrast between the basalt and the layersudabove; the thin-layering of the basalt due to the many flows which make up audbasalt succession; and the rough interfaces on the top-basalt interface caused byudweathering and emplacement mechanisms.udI perform forward modelling to assess the relative importance of these factorsudfor imaging of sub-basalt reflections. The boundary element method (BEM) isudused for the rough-interface modelling. The method was selected because onlyudthe interfaces between layers need to be discretized, in contrast to grid methodsudsuch as finite difference for which the whole model needs to be discretized, and soudshould lead to fast generation of shot gathers for models which have only a fewudhomogeneous layers.udI have had to develop criteria for accurate modelling with the boundaryudelement method and have considered the following: source near an interface,udtwo interfaces close together, removal of model edge effects and precise modellingudof a transparent interface. I have improved efficiency of my code by: resamplingudthe model so that fewer discretization elements are required at low frequencies,udand suppressing wrap-around so that the time window length can be reduced.udI introduce a new scheme which combines domain decomposition and a far-fieldudapproximation to improve the efficiency of the boundary element code further. Iudcompare performance with a standard finite difference code. I show that the BEMudis well suited to seismic modelling in an exploration environment when there are only a few layers in the model and when a seismic profile containing many shotudgathers for one model is required. For many other cases the finite difference codeudis still the best option.udThe input models for the forward modelling are based on real seismic dataudwhich were acquired in the Faeroe-Shetland Channel in 2001. The modellingudshows that roughness on the surface of the basalt has little effect on the imagingudin this particular area of the NEAM. The thin layers in the basalt act as audlow-pass filter to the seismic wave. For the real-data acquisition, even the topbasaltudreflection is a low frequency event. This is most likely to be due to highudattenuation in the layers above the basalt. I show that sea-surface multiple energyudis considerable and that it could mask possible sub-basalt events on a seismicudshot gather, but any shallow sub-basalt events should still be visible even withudthe presence of multiple energy. This leaves the possibility that there is onlyudone major stratigraphic unit between the base of the basalt and the crystallineudbasement.udThe implication of the forward modelling and real data analysis for acquisitionudis that the acquisition parameters must emphasize the low frequencies, since theudhigh frequencies are attenuated before they even reach the top-basalt interface.udThe implication for processing is that multiple removal is of prime importance.