New method for discriminating 4D time shifts in the overburden and reservoirr

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Understanding seismic changes in the subsurface is important for reservoir management and health, safety and environmental (HSE) issues. Typically the changes are interpreted based on the time shifts in seismic time-lapse (4D) data, where sources are at the surface and receivers are either at the surface or in a borehole. With these types of acquisition geometry, it is more straightforward to detect and interpret changes in the overburden, close to the source and receivers, than changes in the deeper part close to the reservoir, because the time shift is accumulative along its ray path from source to receiver. We propose a new method for reconstructing the reflection responses of the overburden and the reservoir, separately, for 4D time shift analysis. This method virtually moves sources and receivers to a horizontal borehole level, which enables a more direct interpretation of the time shifts to the changes close to the borehole, instead of to the surface. A realistic field model is used to demonstrate the method, and we observe a clear discrimination of the different time shifts in the overburden and reservoir, which is not obvious in the original datasets.