Time-lapse seismic interpretation in tau-p space using pre-stack gather data

We present a new algorithm to measure time-lapse vertical travel-time shifts in seismic pre-stack shot and CMP gathers by tracking traces having constant horizontal slowness in τ-p space. Unlike other methods for measuring these attributes from stacked volumes, our use of pre-stack data avoids errors and uncertainties inevitably introduced in conventional time-lapse processing, such as choosing a suitable migration velocity model and cross-correlation time-window size. Results are localised to a given interval and thus free from overburden effects. This approach is used to estimate layer vertical travel-time shifts, a reservoir compaction-dilation coefficient, and hence calculate both velocity and thickness changes within a reservoir and the overburden. We demonstrate the method using synthetic reflection data generated using both a ray-based and a finite-difference full-waveform algorithms on two suites of models: a simple four-layer reservoir model; and two hydro-mechanical simulation models. We compare our estimates of layer interval vertical time-lapse travel-time shifts and velocity and thickness changes with those of the input model. The results indicate that the new τ-p time-lapse method produces sufficiently accurate results compared to conventional methods.