Patent ID: 6906982

Claim:
A method of marine time-lapse seismic surveying the subsurface, using a seismic source and a seismic receiver system comprising at least one seismic receiver, the steps comprising: (a) positioning the seismic source at a source location having a predetermined position, and positioning the at least one seismic receiver of the seismic receiver system at a receiver location having predetermined position; (b) recording for these source and receiver locations for each seismic receiver a set of at least two repeat signals obtained from the seismic receiver in response to a set of at least two sound waves emitted by the seismic source, wherein the at least two sound waves are emitted at different sea states but so closely in time that differences in the repeat signals due to changes in the subsurface can be ignored, and measuring the two-way water travel times when recording the repeat signals; (c) positioning after a predetermined period of time the seismic source and the at least one seismic receiver at the source and receiver locations of step (a), recording for each seismic receiver a monitor signal from the seismic receiver in response to a sound wave emitted by the seismic source, and measuring the two-way water travel time when recording the monitor signal; (d) assuming a model for the signal that includes the subsurface signal, differences in the subsurface signals due to changes in the subsurface, and the contributions of surface multiple reflections; (e) determining a set of filter functions that enable eliminating the surface multiple reflections, which filter function(s) are functions of the measured two-way water travel times at the position of the seismic source of step (a); (f) calculating for each receiver the estimated differences in the subsurface signals from the repeat signals, the monitor signal and the filter function(s), which estimated difference is a measure of changes in the subsurface signal caused by changes in the target layer in the time elapsed between recording the repeat signals and recording the monitor signal; and (g) outputting the estimated difference.