Abstract

This paper provides detailed geological modelling and finite difference realization results of a 2D regional section from the Zagros fold-and-thrust belt, Iran. Different scale geological detail has been included in the model. The modelling approach is based on a hierarchical procedure and the model is made up of cells representing the geometry and properties (such as seismic P-wave velocity) of parts of the model. By using a hierarchical procedure for building the model, the time and space consistency of the geological model is preserved. The constructed model is 81 × 17 km and crosses several drilled and undrilled structures. The model geometry is controlled by surface, subsurface and seismic interpretation results along the model. The model building blocks were chosen based on the deformation history of the area and the modelling was carried out in two stages. First, a regional model was constructed including the major structural elements. At the second stage (fine tuning of the regional model), structural and stratigraphic details, such as onlap, truncations, pinch-outs and thickening/thinning of salt beds, were added to the regional model. The velocity and density models were extracted from well logs, check shots and seismic processing-derived velocity data. A few shots were generated using an acoustic finite difference method to see the effect of the inclusion of detailed internal geometry in the model and the seismic wave complexity around three main anticlines of the model. The seismic response shows that the detailed modelling of stratigraphy and structure gives results that simulate the real data better. It shows the existence of complex diffracted and non-hyperbolic events, suggesting the poor image quality of real data is not related entirely to acquisition problems caused by irregular topography. Advanced depth-imaging methods (e.g. reflection tomography and wave-equation-based extrapolation or migration algorithms) are required to get better images. The sharp lateral velocity changes associated with major thrust faults cause the generation of diffractions that can compromise reliable velocity analysis, especially at conventional offsets. This effect could be considered as one of the causes of the poor seismic response observed on real data. The use of larger offsets, therefore, can distinguish between diffractions and primary reflections during velocity analysis in time and depth.

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