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 (time) change in a parameter that corresponds to a physical property of the reservoir, said method comprising: (a) obtaining 4D seismic amplitude data, comprising a base survey seismic amplitude data volume and a later monitor survey seismic amplitude data volume; (b) determining a data volume of time shifts required to time align the two survey data volumes; (c) fitting time shifts within a running data window to a straight line function of time (depth in the time-shift data volume), wherein the window includes at least three time samples or the seismic data volumes include at least two angle stacks or both; and (d) determining slope of each fitted straight line, thereby generating a data volume of slope values, which corresponds to a reservoir property change data volume.