A computer-implemented method for characterising changes within a subsurface volume between a first time and a second time, said method comprising: obtaining first seismic data (300, 400) acquired by the transmission of seismic signals into the subsurface volume and subsequent detection of some or all of the seismic signals after reflection within the subsurface, said first seismic data corresponding to the first time; processing said first seismic data by migrating (340, 440) the first seismic data to obtain a seismic image (330, 430) of the subsurface volume; reversing said processing for relevant portions of said seismic image by de-migrating (325, 425) the relevant portions of said seismic image to obtain relevant portions of first seismic data,; obtaining second seismic data acquired by the transmission of seismic signals into the subsurface volume and subsequent detection of some or all of the seismic signals after reflection within the subsurface volume, said second seismic data corresponding to the second time; characterising said changes within the subsurface volume between the first time and the second time by estimating the changes between said second seismic data and said relevant portions of first seismic data; and using the results of said method to aid hydrocarbon recovery from a reservoir; characterised in that the relevant portions of said seismic image comprise locally coherent events (315, 415) which relate to portions of the seismic image having spatial coherency in any dimension; the relevant portions of first seismic data comprising kinematic invariants (345, 445) having de-migrated positions which are independent of a velocity model used for the migration and/or de-migration, and said estimating comprising determining changes (375, 475) between the kinematic invariants of the first seismic data (345, 445) and local second seismic data (355, 455), said local second seismic data being local to said kinematic invariants of the first seismic data. obtaining first seismic data (300, 400) acquired by the transmission of seismic signals into the subsurface volume and subsequent detection of some or all of the seismic signals after reflection within the subsurface, said first seismic data corresponding to the first time; processing said first seismic data by migrating (340, 440) the first seismic data to obtain a seismic image (330, 430) of the subsurface volume; reversing said processing for relevant portions of said seismic image by de-migrating (325, 425) the relevant portions of said seismic image to obtain relevant portions of first seismic data,; obtaining second seismic data acquired by the transmission of seismic signals into the subsurface volume and subsequent detection of some or all of the seismic signals after reflection within the subsurface volume, said second seismic data corresponding to the second time; characterising said changes within the subsurface volume between the first time and the second time by estimating the changes between said second seismic data and said relevant portions of first seismic data; and using the results of said method to aid hydrocarbon recovery from a reservoir; characterised in that the relevant portions of said seismic image comprise locally coherent events (315, 415) which relate to portions of the seismic image having spatial coherency in any dimension; the relevant portions of first seismic data comprising kinematic invariants (345, 445) having de-migrated positions which are independent of a velocity model used for the migration and/or de-migration, and said estimating comprising determining changes (375, 475) between the kinematic invariants of the first seismic data (345, 445) and local second seismic data (355, 455), said local second seismic data being local to said kinematic invariants of the first seismic data.