A method of processing seismic data, comprising: - inputting seismic data (D S,G [t]) including, for each of a plurality of shots at respective source locations, seismic traces recorded at a plurality of receiver locations; - applying a depth migration process to the seismic data to obtain a first set of migrated data including, for each shot, first migrated values (M S [x,y,z]) respectively associated with a plurality of subsurface positions; - computing mid-point-modulated data (D' S,G [t]) by multiplying the seismic data in each seismic trace by a function of a common mid-point (m) location, the common mid-point (m) location being a center of the source 2. and receiver locations for said seismic trace; - applying the depth migration process to the mid-point-modulated data to obtain a second set of migrated data including, for each shot, second migrated values (M' S [x,y,z]) respectively associated with the plurality of subsurface positions; - for each position of the first and second sets of migrated data, applying a subtraction process comprising a projection of a lateral distance between said position and the common mid-point (m) location; - for each shot, estimating aperture values ( â S x y z) respectively associated with at least some of the subsurface positions, by a division process performed in a Radon domain applied to the first and second sets of migrated data; and - estimating a aperture indexed common image gather at a horizontal position, comprising respective migrated values (R x,y [z,a]) for parameter pairs each including a depth parameter and an aperture parameter, wherein the migrated value for a parameter pair in the common image gather at said horizontal position is a first migrated value of the first set of migrated data associated with a subsurface position determined by said horizontal position and the depth parameter of said parameter pair for a shot such that the estimated aperture value associated with said subsurface position is the aperture parameter of said parameter pair. - inputting seismic data (D S,G [t]) including, for each of a plurality of shots at respective source locations, seismic traces recorded at a plurality of receiver locations; - applying a depth migration process to the seismic data to obtain a first set of migrated data including, for each shot, first migrated values (M S [x,y,z]) respectively associated with a plurality of subsurface positions; - computing mid-point-modulated data (D' S,G [t]) by multiplying the seismic data in each seismic trace by a function of a common mid-point (m) location, the common mid-point (m) location being a center of the source 2. and receiver locations for said seismic trace; - applying the depth migration process to the mid-point-modulated data to obtain a second set of migrated data including, for each shot, second migrated values (M' S [x,y,z]) respectively associated with the plurality of subsurface positions; - for each position of the first and second sets of migrated data, applying a subtraction process comprising a projection of a lateral distance between said position and the common mid-point (m) location; - for each shot, estimating aperture values ( â S x y z) respectively associated with at least some of the subsurface positions, by a division process performed in a Radon domain applied to the first and second sets of migrated data; and - estimating a aperture indexed common image gather at a horizontal position, comprising respective migrated values (R x,y [z,a]) for parameter pairs each including a depth parameter and an aperture parameter, wherein the migrated value for a parameter pair in the common image gather at said horizontal position is a first migrated value of the first set of migrated data associated with a subsurface position determined by said horizontal position and the depth parameter of said parameter pair for a shot such that the estimated aperture value associated with said subsurface position is the aperture parameter of said parameter pair.