METHODS AND SYSTEMS FOR RECONSTRUCTION OF LOW FREQUENCY PARTICLE VELOCITY WAVEFIELDS AND DEGHOSTING OF SEISMIC STREAMER DATA

• US 20140016436A1
• Filed: 07/10/2012
• Published: 01/16/2014
• Est. Priority Date: 07/10/2012
• Status: Active Grant

• Current Assignee: PGS Geophysical As (PGS ASA)
• Original Assignee: PGS Geophysical As (PGS ASA)
• Inventors: SLLNER, WALTER, ORJI, OKWUDILI, HEGNA, STIAN
• Granted Patent: US 9,442,209 B2
• Time in Patent Office: Days
• US Class Current: 367/24
• CPC Class Codes: G01V 1/364 Seismic filtering G01V1/37 takes precedence
• CPC Class Codes: G01V 2210/56 De-ghosting; Reverberation compensation

Abstract

Computational methods and systems for deghosting marine seismic streamer data are described. In particular, an exploration-seismology vessel tows a number of streamers that form a data acquisition surface located beneath a free surface. The methods computationally deghost or substantially remove receiver ghost signals from seismic data recorded by steamer receivers. The deghosting methods include low frequency compensation to recover vertical velocity wavefield information that is typically lost due to a low signal-to-noise ratio over a low frequency range independent of the free surface conditions or the shape of the data acquisition surface.

Claims

1. A method for deghosting marine seismic data to be carried out by a computer system that includes one or more processors and one or more data-storage devices, the method comprising machine-readable instruction directed to:

computing vertical velocity wavefields associated with marine seismic receivers for a time-varying, arbitrarily-rough free surface and for frequencies below a threshold frequency using pressure wavefields measured at the receivers; and

computing receiver deghosted wavefields based on the measured pressure wavefields, vertical velocity wavefields measured at the receivers above the threshold frequency, and the computed vertical velocity wavefields.

2. The method of claim 1, wherein computing the vertical velocity wavefields further comprises:

decomposing the measured wavefields into up-going and down-going wavefields;

computing a free-surface profile image of the free-surface as a function of the up-going and down-going wavefields;

computing a free-surface profile extension of the free-surface profile assuming a frozen free surface and using a spatial roughness spectrum of the free surface; and

computing free-surface wavefield reflectivity for the free-surface profile and the free-surface profile extension.

3. The method of claim 2, further comprising:

computing a gradient of the pressure wavefield at receiver coordinate locations below the threshold frequency using the free-surface wavefield reflectivity; and

computing the vertical velocity wavefield below the threshold frequency as a function of the gradient of the pressure wavefield at the receiver coordinate locations.

4. The method of claim 2, wherein decomposing the measured wavefields into up-going and down-going wavefields further comprises decomposing the measured pressure wavefields into up-going and down-going pressure wavefields and decomposing the measured vertical velocity wavefields into up-going and down-going vertical velocity wavefields.

5. The method of claim 1, wherein computing the receiver deghosted wavefields further comprises computing receiver deghosted pressure wavefields as a function of the measured vertical velocity wavefields and the computed vertical velocity wavefields below the threshold frequency, and computing receiver deghosted vertical velocity wavefields as a function of the deghosted pressure wavefields.

6. The method of claim 1, wherein marine seismic receivers form an arbitrarily-shaped data acquisition surface.

7. The method of claim 1, wherein the threshold frequency is a highest frequency of a frequency range over which measured vertical velocity wavefields have a low signal-to-noise ratio.

8. A computer system to deghost marine seismic data for a time-varying, arbitrarily-rough free surface, the computer system comprising:

one or more processors;

one or more data-storage devices; and

a deghosting routine stored in one or more of the one or more data-storage devices and executed by the one or more processors, the routine directed to

retrieving from the one or more data-storage devices pressure wavefield data measured at marine seismic receivers over a frequency range and vertical velocity wavefields measured at the receivers above a threshold frequency of the frequency range;

computing vertical velocity wavefields associated with marine seismic receivers for frequencies below the threshold frequency using the measured pressure wavefields; and

computing receiver deghosted wavefields over the frequency range based on the measured pressure wavefields, the measured vertical velocity wavefields above the threshold frequency, and the computed vertical velocity wavefields.

9. The computer system of claim 8, wherein computing the vertical velocity wavefields for frequencies below the threshold frequency further comprises:

decomposing the measured wavefields into up-going and down-going wavefields;

computing a free-surface profile image of the free-surface as a function of the up-going and down-going wavefields;

computing a free-surface profile extension of the free-surface profile assuming a frozen free surface and using a spatial roughness spectrum of the free surface; and

computing free-surface wavefield reflectivity for the free-surface profile and the free-surface profile extension.

10. The computer system of claim 9, further comprising:

computing a gradient of the pressure wavefield at receiver coordinate locations below the threshold frequency using the free-surface wavefield reflectivity; and

computing the vertical velocity wavefield associated with the receivers for frequencies below the threshold frequency as a function of the gradient of the pressure wavefield at the receiver coordinate locations.

11. The computer system of claim 9, wherein decomposing the measured wavefields into up-going and down-going wavefields further comprises decomposing the measured pressure wavefields into up-going and down-going pressure wavefields and decomposing the measured vertical velocity wavefields into up-going and down-going vertical velocity wavefields.

12. The computer system of claim 8, wherein computing the receiver deghosted wavefields further comprises computing receiver deghosted pressure wavefields as a function of the measured vertical velocity wavefields and the computed vertical velocity wavefields below the threshold frequency, and computing receiver deghosted vertical velocity wavefields as a function of the deghosted pressure wavefields.

13. The computer system of claim 8, wherein marine seismic receivers form an arbitrarily-shaped data acquisition surface.

14. The computer system of claim 8, wherein the threshold frequency is a highest frequency of a frequency range over which measured vertical velocity wavefields have a low signal-to-noise ratio.

15. A computer-readable medium having machine-readable instructions encoded thereon for enabling one or more processors of a computer system to perform the operations of

computing vertical velocity wavefields associated with marine seismic receivers for frequencies below a threshold frequency using pressure wavefields measured at the receivers;

and

computing receiver deghosted wavefields based on the measured pressure wavefields, vertical velocity wavefields measured at the receivers above the threshold frequency, and the computed vertical velocity wavefields associated with the receivers for a time-varying, arbitrarily-rough free surface.

16. The medium of claim 15, wherein computing the vertical velocity wavefields further comprises:

decomposing the measured wavefields into up-going and down-going wavefields;

computing a free-surface profile image of the free-surface as a function of the up-going and down-going wavefields;

computing a free-surface profile extension of the free-surface profile assuming a frozen free surface and using a spatial roughness spectrum of the free surface; and

computing free-surface wavefield reflectivity for the free-surface profile and the free-surface profile extension.

17. The medium of claim 16, further comprising:

computing a gradient of the pressure wavefield at receiver coordinate locations below the threshold frequency using the free-surface wavefield reflectivity; and

computing the vertical velocity wavefield below the threshold frequency as a function of the gradient of the pressure wavefield at the receiver coordinate locations.

18. The medium of claim 15, wherein computing the receiver deghosted wavefields further comprises computing receiver deghosted pressure wavefields as a function of the measured vertical velocity wavefields and the computed vertical velocity wavefields below the threshold frequency, and computing receiver deghosted vertical velocity wavefields as a function of the deghosted pressure wavefields.

19. The medium of claim 15, wherein marine seismic receivers form an arbitrarily-shaped data acquisition surface.

20. The medium of claim 15, wherein the threshold frequency is a highest frequency of a frequency range over which measured vertical velocity wavefields have a low signal-to-noise ratio.