Method and apparatus for estimating source signature in shallow water

1. A seismic data processing method, comprising:

receiving near-field measurements for a plurality of shots, the near-field measurements being recorded using a sensor placed close to an air-gun of a seismic source, during a seismic survey in a shallow water surveyed area;

obtaining a far-field initial estimate from the near-field measurements corresponding to different depths stacked to have respective primary pulses time-aligned;

estimating water-bottom reflection portions corresponding to the different depths using differences between the near-field measurements and the far-field initial estimate, each one of the water-bottom reflection portions corresponding to a specific depth and being estimated using a cluster of shots among the plurality of shots, the shots in the cluster corresponding to the specific depth;

deblending a signature of the air-gun for each shot among the plurality of shots, from the near-field measurement for the shot and using the one of the water-bottom reflection portions according to a water-bottom depth for the shot;

using the deblended signatures to obtain shot-by-shot impulsive responses of a formation under the seafloor in the shallow water surveyed area, from seismic data acquired with receivers; and

generating an image of the formation based on the shot-by-shot impulsive responses, the image illustrating interfaces between layers of the formation.

2. The method of claim 1 , wherein the obtaining of the far-field initial estimate includes:

selecting a subset of the plurality of shots, shots in the selected subset being expected to yield a same signature; and

obtaining the far-field initial estimate from the time-aligned primary pulses in the near-field measurements for the shots in the subset.

3. The method of claim 2 , wherein the selecting is performed such that a bubble period of the air-gun has a same value for the shots in the subset.

4. The method of claim 2 , wherein the selecting is performed such that a pressure in the air-gun before being firing the shot has a same value for the shots in the subset.

5. The method of claim 2 , wherein the shots are selected so that shapes of the primary pulses in the near-field measurements of the selected shots to be similar.

6. The method of claim 1 , further comprising:

after estimating the water-bottom reflections, filtering out shot-by-shot far-field variations.

7. The method of claim 6 , wherein the shot-by-shot far-field variations are filtered out using predictive convolution.

8. The method of claim 1 , wherein the deblending is performed iteratively.

9. The method of claim 1 , wherein the seismic source includes plural other air-guns fired simultaneously with the air-gun when acquiring the seismic data and the receiving, the obtaining, the estimating and the deblending are performed for each of the other air-guns, the method further comprising:

calculating a source signature of the signal penetrating the seafloor in the shallow water surveyed area, using the deblended signatures of the air-gun and the other air-guns;

applying designature to the seismic data using the source signature for obtaining impulsive responses of the formation under the seafloor in the shallow water surveyed area; and

generating another image of the formation based on the impulsive responses.

10. An apparatus for processing seismic data, the apparatus comprising:

a user interface configured to receive or retrieve near-field measurements for a plurality of shots, the near-field measurements being recorded using a sensor placed close to an air-gun of a seismic source, during a seismic survey in a shallow water surveyed area; and

a data processing unit including one or more processors, the data processing unit being configured:

to obtain a far-field initial estimate from the near-field measurements corresponding to different depths stacked to have respective primary pulses time-aligned;

to estimate water-bottom reflection portions at different depths using differences between the near-field measurements and the far-field initial estimate, each one of the water-bottom reflection portions corresponding to a specific depth and being estimated using a cluster of shots among the plurality of shots, the shots in the cluster corresponding to the specific depth;

to deblend a far-field signature of the air-gun for each shot among the plurality of shots, by subtracting the one of the water-bottom reflections according to a water-bottom depth for the shot;

to use the far-field signature for obtaining shot-by-shot impulsive responses of a formation under the seafloor in the shallow water surveyed area, from the seismic data; and

to generate an image of the formation based on the shot-by-shot impulsive responses, the image illustrating interfaces between layers of the formation.

11. The apparatus of claim 10 , wherein the data processing unit is configured to obtain the far-field initial estimate by:

selecting a subset of the plurality of shots, shots in the selected subset being expected to have a same source signature; and

obtaining the far-field initial estimate from time-aligned primary pulses in the near-field measurements for the shots in the subset.

12. The apparatus of claim 11 , wherein the data processing unit is configured to select the shots such that a bubble period of the air-gun of the air-gun has a same value for the shots in the subset.

13. The apparatus of claim 11 , wherein the data processing unit is configured to select the shots such that a pressure in the air-gun shortly before firing the shot has a same value for the shots in the subset.

14. The apparatus of claim 11 , wherein the data processing unit is further configured to select the shots such that shapes of the primary pulses in the near-field measurements of the selected shots to be similar.

15. The apparatus of claim 10 , wherein the data processing unit is configured to filter out the shot-by-shot far-field variations using predictive convolution.

16. The apparatus of claim 10 , wherein the data processing unit is configured to deblend the far-field signature iteratively.

17. The apparatus of claim 10 , wherein the seismic source includes plural other air-guns fired simultaneously with the air-gun when acquiring the seismic data, the near-field measurements include measurements using sensors placed close to each of the other air-guns, and the data processing unit is configured to obtain respective far-field initial estimates, to estimate water-bottom reflections and to deblend far-field signatures for each of the other air-guns,

the data processing unit being further configured

to calculate a source signature of the signal penetrating the seafloor in the shallow water surveyed area using the far-field signatures of the air-gun and the other air-guns;

to use the source signature for obtaining the impulsive responses of a formation under the seafloor in the shallow water surveyed area; and

to generate another image of the formation based on the impulsive responses.

18. A computer readable medium non-transitorily storing executable codes which when executed on a computer make the computer perform a seismic data processing method comprising:

receiving near-field measurements for a plurality of shots, the near-field measurements being recorded using a sensor placed close to an air-gun of a seismic source, during a seismic survey in a shallow water surveyed area;

obtaining a far-field initial estimate from the near-field measurements corresponding to different depths stacked to have respective primary pulses time-aligned;

estimating water-bottom reflection portions corresponding to the different depths using differences between the near-field measurements and the far-field initial estimate, each one of the water-bottom reflection portions corresponding to a specific depth and being estimated using a cluster of shots among the plurality of shots, shots in the cluster corresponding to the specific depth; and

deblending a signature of the air-gun for each shot among the plurality of shots, from the near-field measurement for the shot and using the one of the water-bottom reflection portions according to a water-bottom depth for the shot;

using the deblended signatures to obtain shot-by-shot impulsive responses of a formation under the seafloor in the shallow water surveyed area, from seismic data acquired with receivers; and

generating an image of the formation based on the shot-by-shot impulsive responses, the image illustrating interfaces between layers of the formation.