Method and device for processing seismic data

1. A method for processing seismic data related to a subsurface of a body of water, the method comprising:

inputting data indicative of recordings made by detectors disposed on a depth-changing profile in response to an acoustic wave reflected from the subsurface;

applying a matched mirror migration procedure to the data, wherein the matched mirror migration procedure uses an up-traveling U wave and a down-traveling D wave, at least one of the up-traveling U wave or the down-traveling D wave being constructed based on (i) actual positions of the detectors disposed on the depth-changing profile and corresponding recordings, and (ii) fictitious mirror positions of the actual detectors on the depth-changing profile and corresponding recordings with a changed sign; and

generating in a processor an image of the subsurface based on the matched mirror migration procedure.

2. The method of claim 1 , wherein the up-traveling U wave travels from reflectors in the subsurface to the detectors.

3. The method of claim 1 , further comprising:

time cross-correlating the U wave with the D wave to generate the image of the subsurface,

wherein the U wave travels from reflectors in the subsurface to the detectors and the D wave travels from a sound source to the subsurface.

4. The method of claim 1 , wherein the step of calculating the down-travelling D wave does not take into consideration the fictitious mirror positions of the actual detectors and the corresponding recordings with a changed sign.

5. The method of claim 1 , wherein the U wave and the D wave are calculated regressively based on a wave equation.

6. The method of claim 1 , wherein a one-way wave equation is used to calculate the image.

7. The method of claim 1 , wherein no image from a migration step is added to the image of the matched mirror migration.

8. The method of claim 1 , wherein a step of datuming is not performed prior to calculating the final image, wherein datuming implies using data from the detectors provided at positions (x n , z n ), where n=1, . . . N and N is a natural number, and synthesizing data that would have been recorded by the detectors if they had been placed at the same horizontal positions x n but at a same constant reference depth z 0 for all the detectors.

9. The method of claim 1 , wherein the data are collected from detectors being towed by a vessel and the detectors are submerged in water and distributed on the depth-changing profile underwater.

10. The method of claim 9 , wherein the depth-changing profile is a hyperbola.

11. The method of claim 1 , further comprising:

calculating the up-travelling U wave from an initial altitude −z max , where z max is a maximum depth of the detectors;

reverse propagating the up-travelling U wave from the initial altitude by a depth recursive calculation in which the up-travelling U wave is calculated at a current depth z+Δz from an up-travelling wave for a preceding depth z; and

adding the detectors having a depth z r or a mirror depth −z r at a current z+Δz position of the up-travelling U wave when the detectors lie between z and z+Δz.

12. The method of claim 11 , further comprising:

propagating the down-travelling D wave emitted by a source of the acoustic wave, wherein the propagation is started at a depth equal to a depth of the source z s , and the propagation of the down-travelling D wave is carried out to obtain down-travelling waves for all depths greater than the depth of the source z s .

13. The method of claim 12 , further comprising:

correlating the down-travelling D wave and the up-travelling U wave for each depth greater than the depth of the source z s .

14. A method for processing seismic data related to a subsurface of a body of water, the method comprising:

inputting data indicative of recordings made by detectors disposed on a depth-changing profile in response to an acoustic wave reflected from the subsurface;

applying a matched mirror migration procedure to the data, wherein the matched mirror migration procedure uses a time migration procedure for calculating a reflected wave based on both (i) actual positions of the detectors disposed on the depth-changing profile and corresponding recordings, and (ii) fictitious mirror positions of the actual detectors on the depth-changing profile and corresponding recordings with a changed sign;

using free-surface boundary conditions for a surface of the water instead of the absorbing boundary conditions, wherein the free-surface boundary conditions assume that a wave is reflected at the surface of the water while the absorbing boundary conditions assume that the wave moves from water into air at the surface of the water; and

generating in a processor an image of the subsurface based on the matched mirror migration procedure.

15. A computer program-implemented method for processing seismic data related to a subsurface of a body of water, the method comprising:

inputting data indicative of recordings made by detectors provided on a depth-changing profile in response to an acoustic wave reflected from the subsurface;

applying a matched mirror migration procedure to the data, wherein the matched mirror migration procedure uses an up-traveling U wave and a down-traveling D wave, at least one of the up-traveling U wave or the down-traveling D wave being constructed based on (i) actual positions of the detectors and corresponding recordings, and (ii) fictitious mirror positions of the actual detectors and corresponding recordings with a changed sign; and

generating in the computer an image of the subsurface based on the matched mirror migration procedure.