Most seismic time-lapse studies examine the differences between a ‘baseline’ seismic image and a ‘time-lapse’ image, both obtained using the same acquisition parameters and seismic processing stream. The goal is to find a residual amplitude anomaly which can be associated with a change in the reflectivity of a boundary between two formations, where one of the formations has had its properties changed by fluid injection or extraction. Often, the amplitude anomaly is relatively small and difficult to detect. In this case, a secondary effect may prove to be more easily observed; the changed seismic transit time through the affected layer may cause a detectable time delay (or advance) of all reflections from layers beneath the target formation. We demonstrate here a method designed to detect very small time shifts, of less than one sample interval, between comparable portions of seismic images whose contributing data were recorded and processed with identical parameters. We first demonstrate the feasibility of detection with a simple image simulation; then we adapt the near-surface correction technique known as raypath interferometry to implement this difference technique. Results are first shown for a numerical model, then for a field survey for which previous anomaly analysis, based mainly on reflection amplitudes, has always been ambiguous.
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