Method and device for controlling source array geometry

1. A method for reproducing a geometry of a source array used during a previously-conducted seismic survey, the source array having plural sub-arrays, each sub-array including plural individual sources towed using a same umbilical cable, the method comprising:

towing the source array substantially along a sail-line followed during the previously-conducted seismic survey;

comparing a current value of one or more parameters related to the geometry of the source array and affecting source's signature, with a corresponding previous value of the one or more parameters during the previously-conducted seismic survey, the current value and the corresponding previous value corresponding to a first location along the sail-line,

wherein the one or more parameters include a distance perpendicular to the sail-line in a horizontal lane, between two seismic sub-arrays of the source array, a depth of a sub-array or an angle in the horizontal plane, between sub-array and the sail-line; and

adjusting a position of at least one sub-array the source array based on a result of the comparing, to make the current value to substantially match the corresponding previous value.

2. The method of claim 1 , further comprising:

comparing second current values of the one or more parameters with second previous values of corresponding one or more parameters observed during the previously-conducted seismic survey, the second current values and the second previous values respectively corresponding to each of a set of locations along the sail-line; and

adjusting the position of the components when the comparing indicates a difference between compared values larger than one or more predetermined thresholds.

3. The method of claim 2 , wherein the set of locations are determined based on one or more rules.

4. The method of claim 1 , further comprising:

comparing second current values of the one or more parameters with second previous values of the one or more parameters observed during the previously-conducted seismic survey, at predetermined time intervals while the source array is towed along the sail-line.

5. The method of claim 1 , further comprising:

retrieving the corresponding previous value of the one or more parameters from a database.

6. The method of claim 1 , further comprising:

recording the current value of the one or more parameters.

7. The method of claim 1 , wherein the adjusting includes:

changing a length of a cable connecting laterally the at least one seismic sub-array to another towed equipment.

8. The method of claim 1 , wherein

the adjusting includes actively modifying the position of the at least one of the seismic sub-arrays using a surface making a non-zero angle with a towing direction.

9. The method of claim 8 , wherein the adjusting is performed using a rudder or a flap.

10. The method of claim 1 , wherein

the adjusting is performed by changing an angle of a keel mounted on a float attached to the at least one sub-array.

11. The method of claim 1 , wherein the source array includes three seismic sub-arrays,

the one or more parameters include first and second distances in a horizontal plane, between the three seismic sub-arrays, the first and second distances being non-overlapping and substantially perpendicular to the sail-line, and

the adjusting includes sequentially

adjusting the first distance by actively modifying a first trajectory of a first one of the seismic sub-arrays, and

adjusting the second distance by actively modifying a second trajectory of a second one of the seismic sub-arrays without changing the first distance.

12. The method of claim 1 , further comprising:

acquiring monitor seismic data; and

comparing the monitor seismic data with seismic data acquired during the previously-conducted seismic survey to evaluate changes of a surveyed underground structure.

13. The method of claim 12 , wherein the monitor seismic data is acquired by firing the source array at shot locations where the source array was fired during the previously-conducted seismic survey.

14. The method of claim 12 , wherein the monitor seismic data and the seismic data acquired during the previously-conducted seismic survey are wide-azimuth seismic data.

15. The method of claim 1 , further comprising:

comparing a current three dimensional (3D) arrangement of the plural individual seismic sources with a target 3D arrangement; and

adjusting a position of at least one of the plural individual seismic sources so that an adjusted 3D arrangement of the plural individual seismic sources to substantially match the target 3D arrangement.