Reverse Time Migration (RTM) Based On Elastic Wave Modeling And Seismic Data Interpretation

Seismic Migration is a promising data processing technique to construct subsurface images by projecting the recorded seismic data at surface back to their origins.There are numerous Migration methods.Among them,Reverse Time Migration(RTM)is considered a robust and standard imaging technology in present day exploration industry as well as in academic research field because of its superior performance compared to traditional migration methods.Although RTM is extensive computing and time consuming but it can efficiently handle the complex geology,highly dipping reflectors and strong lateral velocity variation all together.RTM takes data recorded at the surface as a boundary condition and propagates the data backwards in time until the imaging condition is met.It can use the same modeling algorithm that we use for forward modeling.The classical seismic exploration theory assumes flat surface which is almost impossible in practice for land data.So irregular surface topography has to be considered in simulation of seismic wave propagation,which is not always a straightforward undertaking but we handled surface topography properly in our forward simulation algorithm.In this study,Curved grid finite difference method(CG-FDM)is adapted to model elastic seismic wave propagation in the presence of surface topography.Boundary confirming grid has been used to avoid artificial scattering waves and grid must confirm the free surface.Non-staggered higher-order optimized DRP/opt MacCormack scheme is used to update the first order velocity-stress equations because this scheme has best trade-off between the computing efficiency and accuracy.After solving forward simulation problem we moved towards reverse time extrapolation by inversing surface records in time and finally used this inverted data for reverse time extrapolation.Imaging principle and imaging conditions are the heart of reverse time migration,source normalized cross-correlation imaging condition has been applied to cross-correlate the downgoing and upgoing wavefields at their corresponding time steps.Synthetic data experiments by using Marmousi model as the true model have been performed to explore advantages and limitations of our method,some other simple models are also tested simultaneously.Vertical finite difference imaging technique(2nd order in Z-direction)is applied for each cross-correlation results to remove the low frequency noise.This kind of approach is particularly useful to image horizontal structures efficiently.It brought significant results after suppressing the low frequency noise.We focus on elastic wave propagation rather than acoustic wave because earth actually behaves as an elastic body.Reverse time migration results are much satisfactory for all tested models and our migration algorithm is perfect to use for commercial usage.Further,real field seismic data interpretation has been performed to mark the potential geological reflectors in Nandpur gas field,located at Punjab platform area,Pakistan.For this purpose,real data of Nandpur gas field is used with the official permission of Government of Pakistan,to explain the interpretation stage more clearly and practically.Seismic and Geophysical well log data is used for the structural interpretation and finally estimate the potential of hydrocarbons in that particular area by using geophysical well logging technique.Results shows this particular area has great potential for hydrocarbon exploration.