1. A computer-implemented method for transforming 4D seismic survey data taken over a producing reservoir to a reservoir property change data volume which provides, by location in a subsurface volume, a corresponding fourth dimension, i.e. time, change in a parameter that corresponds to a physical property of the reservoir, said method comprising: (a) obtaining at least two angle stacks of 4D seismic amplitude data, comprising a base survey seismic amplitude data volume and a later monitor survey seismic amplitude data volume; (b) for each angle stack, determining a data volume of time shifts required to time align the base survey data volume and the monitor survey data volume; (c) correcting the time shifts to zero offset; (d) obtaining for each voxel in a data volume a value of time shift representing an average of all angle stacks; and (e) using a computer, computing a derivative of the average time shifts with respect to time, or depth in the average time-shift data volume, for each voxel in the average time-shift data volume thereby generating a data volume that corresponds to a reservoir property change data volume.