Method for processing seismic images

1. A method for processing a reference seismic image of a medium and a control seismic image of said medium, the reference seismic image comprising at least one seismic trace expressed in a depth scale and the control seismic image comprising at least one seismic trace expressed in the depth scale, the method comprising:

a) receiving the reference seismic image and the control seismic image;

b) defining at least one first planar capture level situated on one trace from among the at least one seismic trace of the reference seismic image and the at least one seismic trace of the control seismic image, and at least one second planar capture level on the at least one seismic trace of the reference seismic image, said first planar capture level being defined above a first disturbance of said medium;

c) transforming a continuous portion of the at least one seismic trace of the control seismic image by demigration on the at least one first planar capture level by means of an original velocity model, into a transformed control seismic trace described in a time scale;

d) transforming a continuous portion of the at least one seismic trace of the reference seismic image comprising the at least one second planar capture level by demigration and datuming operations on the at least one second planar capture level by means of a current velocity model, into a transformed reference seismic trace portion described in the time scale;

e) correcting the portion of the transformed control seismic trace by calibration on the at least one second planar capture level by means of the current velocity model in a new transformed control seismic trace portion described in the time scale said calibration being a transformation by datuming operation;

f) determining an optimised current velocity model by the addition, to the current velocity model, of a correction factor for the current velocity model determined from an offset in time between the portion of the transformed reference seismic trace and the new transformed control seismic trace portion; and

g) monitoring a change over time in the imaged medium of oil/gas reservoirs in a subsoil based on said optimized current velocity model.

2. The method of claim 1 , wherein f) further comprises:

calculating a function dv(t) representing change over time in instantaneous velocity making it possible to transform the transformed reference seismic trace portion into the new transformed control seismic trace portion; and

transforming the function dv(t) into a function dv(z) representing instantaneous velocity on the depth scale, by means of the current velocity model, the function dv(z) forming a correction factor for the current velocity model.

3. The method of claim 1 , wherein f) further comprises:

calculating a function dv(t) representing change over time in instantaneous velocity making it possible to transform the transformed reference seismic trace portion into the new transformed control seismic trace portion;

multiplying the function dv(t) by any one of the at least one seismic traces described in the time scale, said any one of the at least one seismic traces described in the time scale forming a support trace;

transforming, using the current velocity model, the support trace and the support trace multiplied by the function dv(t) into second and third transformed traces described in the depth scale; and

determining a correction factor dv(z) representing value of instantaneous velocity on the depth scale, of the current velocity model, said correction factor being a ratio between second and third transformed traces described in the depth scale.

4. The method of claim 1 , wherein the at least one seismic trace of the control seismic image and the at least one seismic trace of the reference seismic image are traces corresponding to zero-offset seismic profiles.

5. The method of claim 1 , wherein the current velocity model is chosen as being the original velocity model.

6. The method of claim 1 , wherein transformation by demigration and datuming operations at d) consists of demigration on the at least one first planar capture level by means of the current velocity model followed by a correction by calibration on the at least one second planar capture level by means of the current velocity model.

7. The method of claim 1 , wherein steps d) to f) are repeated iteratively a chosen number of times, the optimised current velocity model determined at the end of an iteration serving as a current velocity model for a consecutive iteration.

8. The method according to claim 1 , wherein the at least one first planar capture level and the at least one second planar capture level are redefined at the start of each iteration of the method.

9. The method according to claim 1 , further comprising:

defining a plurality of capture levels comprising the at least one first planar capture level, the at least one second planar capture level, and at least one third planar capture level, said plurality of capture levels ranging successively, by increasing depth according to the depth scale, from the at least one first planar capture level to an at least one last planar capture level, between b) and c); and at the end of f), the method further comprises:

g) naming the at least one second planar capture level as the at least one first planar capture level and naming the at least one third planar capture level as the at least one second planar capture level, and

steps c) to g) being implemented iteratively until the at least one last planar capture level is named the at least one first planar capture level.

10. A non transitory computer readable storage medium having stored thereon a computer program comprising program instructions, the computer program being loadable into a data-processing unit and adapted to cause the data-processing unit to carry out the method of claim 1 when the computer program is executed by the data-processing device.