He, Y.-X., Angus, D.A., Yuan, S.-Y., Blanchard, T.D., Clark, R.A. and Hildyard, M.W., 2015. Time-lapse seismic interpretation in 7-p space using pre-stack data. Journal of Seismic Exploration, 24: 475-496. We present a new algorithm to measure time-lapse vertical traveltime shifts in seismic pre-stack shot and CMP gathers by tracking traces having constant horizontal slowness in 7-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 traveltime 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 a hydro-mechanical simulation model. We compare our estimates of layer interval vertical time-lapse traveltime shifts and velocity and thickness changes with those of the input model. The results indicate that the new T-p time-lapse method produces sufficiently accurate results compared to conventional methods.