METHODS AND SYSTEMS THAT DETERMINE A VELOCITY WAVEFIELD FROM A MEASURED PRESSURE WAVEFIELD

• US 20160061973A1
• Filed: 05/18/2015
• Published: 03/03/2016
• Est. Priority Date: 08/29/2014
• Status: Active Grant

• Current Assignee: PGS Geophysical As (PGS ASA)
• Original Assignee: PGS Geophysical As (PGS ASA)
• Inventors: Orji, Okwudili Chuks, Asgedom, Endrias Getachew, Sllner, Walter
• Granted Patent: US 10,444,386 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G01V 1/28 Processing seismic data, e.g. for interpretation or for event detection G01V1/48 takes precedence
• CPC Class Codes: G01V 1/282 Application of seismic models, synthetic seismograms
• CPC Class Codes: G01V 1/30 Analysis G01V1/50 takes precedence
• CPC Class Codes: G01V 1/301 for determining seismic cross-sections or geostructures
• CPC Class Codes: G01V 1/32 Transforming one recording into another or one representation into another
• CPC Class Codes: G01V 1/36 Effecting static or dynamic corrections on records, e.g. correcting spread; Correlating seismic signals; Eliminating effects of unwanted energy
• CPC Class Codes: G01V 1/38 specially adapted for water-covered areas G01V1/28 takes precedence
• CPC Class Codes: G01V 2210/40 Transforming data representation
• CPC Class Codes: G01V 2210/44 F-k domain
• CPC Class Codes: G01V 2210/51 Migration
• CPC Class Codes: G01V 2210/56 De-ghosting; Reverberation compensation
• CPC Class Codes: G01V 2210/64 Geostructures, e.g. in 3D data cubes
• CPC Class Codes: G01V 2210/675 Wave equation; Green's functions

Abstract

Methods and systems that compute an approximate vertical-velocity wavefield based on a measured pressure wavefield and knowledge of the free-surface shape when the pressure wavefield was measured are described. The measured pressure wavefield is used to compute an approximate frozen free-surface profile of the free surface. The approximate frozen free-surface profile and the measured pressure wavefield are then used to compute an approximate vertical-velocity wavefield that does not include low-frequency streamer vibrational noise. The approximate vertical-velocity wavefield and measured pressure wavefield may be used to separate the pressure wavefield into up-going and down-going pressure wavefields.

Claims

1. A method that determines a velocity wavefield from a measured pressure wavefield indicative of properties of a subterranean formation, the method comprising:

computing an approximate frozen free-surface profile of a free surface above a streamer based on pressure data generated by pressure sensors located at receiver locations along the streamer, the pressure data represents the measured pressure wavefield;

computing reflectivity and normal derivative of the reflectivity at the receiver locations;

computing a normal derivative of the measured pressure wavefield at the receiver locations based on the reflectivity and the normal derivative of the reflectivity at the receiver locations; and

computing an approximate normal-velocity wavefield at the receiver locations along the streamer based on the normal derivative of the measured pressure wavefield at the receiver locations, the approximate normal-velocity wavefield does not include low-frequency, streamer vibrational noise.

2. The method of claim 1, wherein computing the approximate frozen free-surface profile further comprises:

transforming pressure data from space-time domain to frequency-wavenumber domain;

for each trial depth of a series of trial depths,

computing forward and backward extrapolated gathers based on the pressure data,

transforming forward and backward extrapolated gathers from the frequency-wavenumber domain to the space-time domain;

computing a difference gather from forward and backward extrapolated gathers;

computing a set of maximum energy differences for a series of time windows applied to the difference gather; and

computing a profile of the approximate frozen free surface.

3. The method of claim 2, wherein computing the set of maximum energy differences for the series of time windows further comprises:

for each time window,

determining amplitude differences in the time window;

computing energy difference for each amplitude difference in the time window; and

determining a maximum energy differences from the energy differences in the time window.

4. The method of claim 2, wherein computing the profile of the approximate frozen free surface further comprises:

for each time window and for each trial depth, determining peak coordinates associated maximum energy difference in each of set of maximum energy differences;

determining receiver in-line and cross-line coordinates of receiver locations associated with the peak coordinates; and

computing interpolated frozen free-surface profile based on peak coordinates and associated in-line and cross-line coordinates, the interpolated frozen free-surface profile is the approximate frozen free-surface profile.

5. The method of claim 4, further comprising extending the approximate frozen free-surface profile beyond first and last receiver locations of the streamer.

6. The method of claim 1, wherein computing the reflectivity and normal derivative of reflectivity further comprises:

computing a unit normal vector of the approximate frozen free-surface profile;

computing reflectivity of the approximate frozen free-surface profile based on the unit normal vector at points along the approximate frozen free-surface profile;

computing reflectivity at the receiver coordinates based on the reflectivity of the approximate frozen free-surface profile; and

computing normal derivative of the reflectivity at the receiver coordinates.

7. The method of claim 1, wherein computing a normal derivative of the pressure wavefield at receiver locations further comprises:

compute elements of a monopole matrix based on the reflectivity;

compute elements of a dipole matrix based on the normal derivative of the reflectivity; and

compute derivative of the pressure wavefield at the receiver locations based on the monopole matrix and the dipole matrix.

8. The method of claim 1, further comprising computing an approximate vertical-velocity wavefield at receiver locations based on the normal-velocity wavefield at receiver locations and orientation of the receivers.

9. The method of claim 8, further comprising computing a combined vertical-velocity wavefield composed of the approximate vertical-velocity wavefield over a low-frequency range and a measured vertical-velocity wavefield for frequencies outside the low-frequency range.

10. The method of claim 8, further comprising computing an up-going pressure wavefield based on the approximate vertical-velocity wavefield and the measured pressure wavefield.

11. The method of claim 1, wherein the approximate normal-velocity wavefield and measured pressure wavefield are geophysical data products, further comprising recording the geophysical data product on a physical, non-volatile computer-readable medium suitable for importing onshore.

12. The method of claim 10, further comprising performing geophysical analysis onshore on the geophysical data product.

13. The method of claim 1, further comprising the measured pressure wavefield generated by one or more receivers located along the streamer.

14. A computer system that computes a velocity wavefield from a measured pressure wavefield, the system comprising:

one or more processors;

one or more data-storage devices; and

machine-readable instructions stored in one or more of data-storage devices and executed by the one or more processors, the machine-readable instructions directed to

computing an approximate frozen free-surface profile of a free surface above a streamer based on pressure data generated by pressure sensors located at receiver locations along the streamer, the pressure data represents the measured pressure wavefield;

computing reflectivity and normal derivative of the reflectivity at the receiver locations;

computing a normal derivative of the measured pressure wavefield at the receiver locations based on the reflectivity and the normal derivative of the reflectivity at the receiver locations; and

computing an approximate normal-velocity wavefield at the receiver locations along the streamer based on the normal derivative of the measured pressure wavefield at the receiver locations, the approximate normal-velocity wavefield does not include low-frequency, streamer vibrational noise.

15. The system of claim 14, wherein computing the approximate frozen free-surface profile further comprises:

transforming pressure data from space-time domain to frequency-wavenumber domain;

for each trial depth of a series of trial depths,

computing forward and backward extrapolated gathers based on the pressure data,

transforming forward and backward extrapolated gathers from the frequency-wavenumber domain to the space-time domain;

computing a difference gather from forward and backward extrapolated gathers;

computing a set of maximum energy differences for a series of time windows applied to the difference gather; and

computing a profile of the approximate frozen free surface.

16. The system of claim 15, wherein computing the set of maximum energy differences for the series of time windows further comprises:

for each time window,

determining amplitude differences in the time window;

computing energy difference for each amplitude difference in the time window; and

determining a maximum energy differences from the energy differences in the time window.

17. The system of claim 15, wherein computing the profile of the approximate frozen free surface further comprises:

for each time window and for each trial depth, determining peak coordinates associated maximum energy difference in each of set of maximum energy differences;

determining receiver in-line and cross-line coordinates of receiver locations associated with the peak coordinates; and

computing interpolated frozen free-surface profile based on peak coordinates and associated in-line and cross-line coordinates, the interpolated frozen free-surface profile is the approximate frozen free-surface profile.

18. The system of claim 17, further comprising extending the approximate frozen free-surface profile beyond first and last receiver locations of the streamer.

19. The system of claim 14, wherein computing the reflectivity and normal derivative of reflectivity further comprises:

computing a unit normal vector of the approximate frozen free-surface profile;

computing reflectivity of the approximate frozen free-surface profile based on the unit normal vector at points along the approximate frozen free-surface profile;

computing reflectivity at the receiver coordinates based on the reflectivity of the approximate frozen free-surface profile; and

computing normal derivative of the reflectivity at the receiver coordinates.

20. The system of claim 14, wherein computing a normal derivative of the pressure wavefield at receiver locations further comprises:

compute elements of a monopole matrix based on the reflectivity;

compute elements of a dipole matrix based on the normal derivative of the reflectivity; and

compute derivative of the pressure wavefield at the receiver locations based on the monopole matrix and the dipole matrix.

21. The system of claim 14, further comprising computing an approximate vertical-velocity wavefield at receiver locations based on the normal-velocity wavefield at receiver locations and orientation of the receivers.

28. The medium of claim 21, wherein computing a normal derivative of the pressure wavefield at receiver locations further comprises:

compute elements of a monopole matrix based on the reflectivity;

compute elements of a dipole matrix based on the normal derivative of the reflectivity; and

compute derivative of the pressure wavefield at the receiver locations based on the monopole matrix and the dipole matrix.

22. A non-transitory 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 an approximate frozen free-surface profile of a free surface above a streamer based on pressure data generated by pressure sensors located at receiver locations along the streamer, the pressure data represents the measured pressure wavefield;

computing reflectivity and normal derivative of the reflectivity at the receiver locations;

computing a normal derivative of the measured pressure wavefield at the receiver locations based on the reflectivity and the normal derivative of the reflectivity at the receiver locations; and

computing an approximate normal-velocity wavefield at the receiver locations along the streamer based on the normal derivative of the measured pressure wavefield at the receiver locations, the approximate normal-velocity wavefield does not include low-frequency, streamer vibrational noise.

23. The medium of claim 22, wherein computing the approximate frozen free-surface profile further comprises:

transforming pressure data from space-time domain to frequency-wavenumber domain;

for each trial depth of a series of trial depths,

computing forward and backward extrapolated gathers based on the pressure data,

transforming forward and backward extrapolated gathers from the frequency-wavenumber domain to the space-time domain;

computing a difference gather from forward and backward extrapolated gathers;

computing a set of maximum energy differences for a series of time windows applied to the difference gather; and

computing a profile of the approximate frozen free surface.

24. The medium of claim 23, wherein computing the set of maximum energy differences for the series of time windows further comprises:

for each time window,

determining amplitude differences in the time window;

computing energy difference for each amplitude difference in the time window; and

determining a maximum energy differences from the energy differences in the time window.

25. The medium of claim 23, wherein computing the profile of the approximate frozen free surface further comprises:

for each time window and for each trial depth, determining peak coordinates associated maximum energy difference in each of set of maximum energy differences;

determining receiver in-line and cross-line coordinates of receiver locations associated with the peak coordinates; and

compute interpolated frozen free-surface profile based on peak coordinates and associated in-line and cross-line coordinates, the interpolated frozen free-surface profile is the approximate frozen free-surface profile.

26. The medium of claim 25, further comprising extending the approximate frozen free-surface profile beyond first and last receiver locations of the streamer.

27. The medium of claim 22, wherein computing the reflectivity and normal derivative of the reflectivity further comprises:

computing a unit normal vector of the approximate frozen free-surface profile;

computing reflectivity of the approximate frozen free-surface profile based on the unit normal vector at points along the approximate frozen free-surface profile;

computing reflectivity at the receiver coordinates based on the reflectivity of the approximate frozen free-surface profile; and

computing normal derivative of the reflectivity at the receiver coordinates.

29. The medium of claim 22, further comprising computing an approximate vertical-velocity wavefield at receiver locations based on the normal-velocity wavefield at receiver locations and orientation of the receivers.