Method of processing seismic data by providing surface offset common image gathers

1. A method of seismic inspection of a subsoil region, comprising:

firing a plurality of shots at respective source locations to propagate seismic waves in the subsoil region;

recording seismic traces at a plurality of receiver locations in response to the plurality of shots to generate seismic data including, for each of the plurality of shots at the respective source locations, seismic traces recorded at the 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 respectively associated with a plurality of subsurface positions;

computing offset-modulated data by multiplying the seismic data in each seismic trace by a horizontal offset between the source and receiver locations for said seismic trace;

applying the depth migration process to the offset-modulated data to obtain a second set of migrated data including, for each shot, second migrated values respectively associated with the plurality of subsurface positions;

for each shot, estimating offset values respectively associated with at least some of the subsurface positions, by a division process applied to the first and second sets of migrated data; and

estimating a common image gather at a horizontal position, comprising respective migrated values for parameter pairs each including a depth parameter and an offset 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 offset value associated with said subsurface position is the offset parameter of said parameter pair.

2. The method as claimed in claim 1 , wherein the division process used for estimating the offset values associated with a subsurface position comprises minimizing a cost function defined by an offset variable and local values of the first and second migrated values in a neighborhood of said subsurface position.

3. The method as claimed in claim 1 , wherein the depth migration process is a reverse-time migration process.

4. The method as claimed in claim 1 , wherein the depth migration process is a wave equation pre-stack depth migration process.

5. A system for inspecting a subsoil region, comprising:

at least one seismic source for firing a plurality of shots at respective source locations to propagate seismic waves in the subsoil region;

at least one receiver for recording seismic traces at a plurality of receiver locations in response to the plurality of shots to generate seismic data including, for each of the plurality of shots at the respective source locations, seismic traces recorded at the plurality of receiver locations; and

computer resources for processing the seismic data

wherein the computer resources are configured for:

applying a depth migration process to the seismic data to obtain a first set of migrated data including, for each shot, first migrated values respectively associated with a plurality of subsurface positions;

computing offset-modulated data by multiplying the seismic data in each seismic trace by a horizontal offset between the source and receiver locations for said seismic trace;

applying the depth migration process to the offset-modulated data to obtain a second set of migrated data including, for each shot, second migrated values respectively associated with the plurality of subsurface positions;

for each shot, estimating offset values respectively associated with at least some of the subsurface positions, by a division process applied to the first and second sets of migrated data; and

estimating a common image gather at a horizontal position, comprising respective migrated values for parameter pairs each including a depth parameter and an offset 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 offset value associated with said subsurface position is the offset parameter of said parameter pair.

6. The system as claimed in claim 5 , wherein the division process used for estimating the offset values associated with a subsurface position comprises minimizing a cost function defined by an offset variable and local values of the first and second migrated values in a neighborhood of said subsurface position.

7. The system as claimed in claim 5 , wherein the depth migration process is a reverse-time migration process.

8. The system as claimed in claim 5 , wherein the depth migration process is a wave equation pre-stack depth migration process.

9. A non-transitory computer-readable medium having program instructions stored thereon for running in a system for inspecting a subsoil region, wherein said system comprises:

at least one seismic source for firing a plurality of shots at respective source locations to propagate seismic waves in the subsoil region;

at least one receiver for recording seismic traces at a plurality of receiver locations in response to the plurality of shots to generate seismic data including, for each of the plurality of shots at the respective source locations, seismic traces recorded at the plurality of receiver locations; and

a computer processing unit having an input for receiving the generated seismic data,

wherein the program instructions comprises instructions to carry out the following steps when run in said computer processing unit:

applying a depth migration process to the seismic data to obtain a first set of migrated data including, for each shot, first migrated values respectively associated with a plurality of subsurface positions;

computing offset-modulated data by multiplying the seismic data in each seismic trace by a horizontal offset between the source and receiver locations for said seismic trace;

applying the depth migration process to the offset-modulated data to obtain a second set of migrated data including, for each shot, second migrated values respectively associated with the plurality of subsurface positions;

for each shot, estimating offset values respectively associated with at least some of the subsurface positions, by a division process applied to the first and second sets of migrated data; and

estimating a common image gather at a horizontal position, comprising respective migrated values for parameter pairs each including a depth parameter and an offset parameter,

and 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 offset value associated with said subsurface position is the offset parameter of said parameter pair.

10. The non-transitory computer-readable medium as claimed in claim 9 , wherein the division process used for estimating the offset values associated with a subsurface position comprises minimizing a cost function defined by an offset variable and local values of the first and second migrated values in a neighborhood of said subsurface position.

11. The non-transitory computer-readable medium as claimed in claim 9 , wherein the depth migration process is a reverse-time migration process.

12. The non-transitory computer-readable medium as claimed in claim 9 , wherein the depth migration process is a wave equation pre-stack depth migration process.