Orthogonal Source and Receiver Encoding

• US 20130238246A1
• Filed: 01/23/2013
• Published: 09/12/2013
• Est. Priority Date: 03/08/2012
• Status: Active Grant

• Current Assignee: Exxonmobil Upstream Research Company (Exxon Mobil Corporation)
• Original Assignee: Exxonmobil Upstream Research Company (Exxon Mobil Corporation)
• Inventors: Krebs, Jerome R., Cha, Young Ho, Lee, Sunwoong, Dimitrov, Pavel, Mullur, Anoop A., Routh, Partha S., Downey, Nathan J.
• Granted Patent: US 10,012,745 B2
• Time in Patent Office: Days
• US Class Current: 702/14
• CPC Class Codes: G01V 1/003 Seismic data acquisition in general, e.g. survey design G01V1/3808, G01V1/42 take precedence
• CPC Class Codes: G01V 1/005 with exploration systems emitting special signals, e.g. frequency swept signals, pulse sequences or slip sweep arrangements
• 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 2210/66 Subsurface modeling
• CPC Class Codes: G01V 9/00 Prospecting or detecting by methods not provided for in groups G01V1/00 - G01V8/00
• CPC Class Codes: G06F 17/00 Digital computing or data processing equipment or methods, specially adapted for specific functions information retrieval, database structures or file system structures therefor G06F16/00

Abstract

Method for performing simultaneous encoded-source inversion of geophysical data to estimate parameters of a physical property model (41), especially adapted for surveys without fixed-receiver acquisition geometry, such as marine seismic surveys with moving source and receivers. The encoding functions (32) used on the sources to generate one or more simultaneous encoded-source gathers of data (35), as well as to simulate the same (34), are orthogonal or pseudo-orthogonal with respect to cross-correlation. In addition, receivers are also encoded, with the receiver encoding being designed to make a given receiver less sensitive to sources to which it was not listening during the survey (38). The encoding functions may be temporal bandpass filters differing one from another by central frequency, phase, or both. Efficiency of the method may be further improved by grouping several sources into a super-source, grouping the corresponding gathers into a super-gather, and then applying the above encoding strategy.

Claims

1. A computer implemented method for iterative inversion of measured geophysical data to determine a physical properties model for a subsurface region, comprising using a computer to sum a plurality of encoded gathers of the measured geophysical data, each gather being associated with a single source or group of sources and encoded with a different encoding function selected from a set of encoding functions that are orthogonal or pseudo-orthogonal with respect to cross-correlation, thereby forming a simultaneous encoded gather of measured geophysical data representing a plurality of sources, then using an assumed physical properties model or an updated physical properties model from a prior iteration to simulate the simultaneous encoded gather of measured geophysical data, then computing an objective function measuring misfit between the simultaneous encoded gather of measured geophysical data and the simulated simultaneous encoded gather, then optimizing the objective function to determine a model update, wherein receivers are encoded to make computation of the objective function less sensitive to one or more of the plurality of sources for a given receiver.

2. The method of claim 1, wherein the encoding functions for sources are a set of bandpass filters, differing from each other in central frequency, phase, or both.

3. The method of claim 2, wherein the phase differences include polarity switches.

4. The method of claim 1, wherein the encoding of the receivers is performed using a different encoding function for each receiver, selected to be orthogonal or pseudo-orthogonal with respect to the encoding functions for sources to which the receiver did not listen when the measured geophysical data were measured.

5. The method of claim 4, wherein the encoding functions for receivers are a set of notch filters, differing from each other in central frequency, phase, or both, and wherein each receiver filter is designed to notch out bandpass frequencies of the encoding functions corresponding to sources for which that receiver was not active.

6. The method of claim 1, wherein the simulated simultaneous encoded gather is simulated in a single simulation operation, wherein source signatures in the simulation are encoded using the same encoding used to encode corresponding gathers of the measured geophysical data.

7. The method of claim 1, wherein the misfit is measured as a norm of a difference between the simultaneous encoded gather of measured geophysical data and the simulated simultaneous encoded gather or as a cross-correlation between the simultaneous encoded gather of measured geophysical data and the simulated simultaneous encoded gather.

8. The method of claim 1, wherein the measured geophysical data are acquired under conditions where the fixed-receiver assumption of simultaneous encoded-source inversion is not valid.

9. The method of claim 1, wherein the measured geophysical data are full wavefield data from a seismic survey.

10. The method of claim 1, wherein the sources to which the computation is made less sensitive are the sources that the given receiver did not listen to.

11. The method of claim 10, wherein the receiver encoding is chosen such that the selected encoding function for a given source is orthogonal or pseudo-orthogonal with respect to encoding functions of receivers that did not listen to that source.

12. The method of claim 1, wherein in some or all of the iterations, different encoding functions are used compared to a preceding iteration.

13. The method of claim 1, further comprising forming one or more additional simultaneous encoded gathers of measured geophysical data, wherein the model update is determined based on a sum of the objective functions corresponding to each simultaneous encoded gather.

14. The method of claim 1, wherein computing the objective function comprises computing differences, called residuals, by receiver between the simultaneous encoded gather of measured geophysical data and the simulated simultaneous encoded gather, and applying the receiver encoding to each receiver residual, said receiver encoding being selected to attenuate contributions from sources for which the receiver was inactive, then computing the objective function from the receiver-encoded residuals.

22. A method for producing hydrocarbons, comprising:

conducting a seismic survey have a subsurface region;

inverting measured seismic data from the seismic survey by a method of claim 1 to determine a physical properties model of the subsurface region;

using the physical properties model to process the measured seismic data;

using the processed seismic data to assess hydrocarbon potential of the subsurface region; and

upon identifying hydrocarbon potential from the processed seismic data, drilling a well into the subsurface region and producing hydrocarbons.

15. A computer-implemented method for inversion of measured geophysical data to determine a physical properties model for a subsurface region, comprising:

(a) obtaining a group of two or more gathers of the measured geophysical data, wherein each gather is associated with a single source or group of sources;

(b) encoding each gather with a different encoding function wherein the encoding is orthogonal or pseudo-orthogonal with respect to cross-correlation;

(c) summing the encoded gathers in the group by summing all data records in each gather that correspond to a single receiver and repeating for each different receiver, resulting in a simultaneous encoded-source gather;

(d) assuming a physical properties model of the subsurface region, said model providing values of at least one physical property at locations throughout the subsurface region;

(e) using the assumed physical properties model, simulating the simultaneous encoded-source gather, encoding source signatures in the simulation using the same encoding functions used to encode corresponding gathers of measured data, wherein an entire simultaneous encoded-source gather is simulated in a single simulation operation;

(f) calculating a difference for each receiver between the simultaneous encoded-source gather made up of measured geophysical data and the simulated simultaneous encoded-source gather, said difference being referred to as the residual for that receiver;

(g) applying receiver encoding to each residual, said receiver encoding being selected to attenuate contributions from sources for which the receiver was inactive;

(h) computing an objective function from the receiver-encoded residuals, and updating the assumed physical properties model based on the objective function computation;

(i) repeating (b)-(h) at least one more iteration, using the updated physical properties model from the previous iteration as the assumed physical properties model, to produce a further updated physical properties model of the subsurface region; and

(j) downloading the further updated physical properties model or saving it to computer storage;

wherein, at least one of (a)-(j) are performed using a computer.

16. The method of claim 15, wherein each gather in (a) is a multi-shot gather, each multi-shot gather consisting of a plurality of individual-shot gathers having a common set of illuminated receivers.

17. The method of claim 16, wherein the receiver encoding is a set of frequency filters, differing from each other in central frequency, phase, or both.

18. The method of claim 17, wherein both the encoding in (b) and the encoding in (e) are accompanied by a second encoding, making the gathers in (b) and the source signatures in (e) doubly encoded.

19. The method of claim 18, wherein the second encoding is +1/−

1 encoding.

20. A non-transitory computer usable medium having a computer readable program code embodied therein, said computer readable program code adapted to be executed to implement a method for iterative inversion of measured geophysical data to determine a physical properties model for a subsurface region, said method comprising:

summing a plurality of encoded gathers of the measured geophysical data, each gather being associated with a single source or group of sources and encoded with a different encoding function selected from a set of encoding functions that are orthogonal or pseudo-orthogonal with respect to cross-correlation, thereby forming a simultaneous encoded gather of measured geophysical data representing a plurality of sources, then using an assumed physical properties model or an updated physical properties model from a prior iteration to simulate the simultaneous encoded gather of measured geophysical data, then computing an objective function measuring misfit between the simultaneous encoded gather of measured geophysical data and the simulated simultaneous encoded gather, then optimizing the objective function to determine a model update, wherein receivers are encoded to make computation of the objective function less sensitive to one or more of the plurality of sources for a given receiver.

21. The method of claim 20, wherein the encoding of the receivers is performed using a different encoding function for each receiver, selected to be orthogonal or pseudo-orthogonal with respect to the encoding functions for sources to which the receiver did not listen when the measured geophysical data were measured.

23. A computer-implemented method for inversion of measured geophysical data to determine a physical properties model for a subsurface region, comprising:

(a) obtaining a group of two or more multi-shot gathers of the measured geophysical data, each multi-shot gather consisting of a plurality of individual-shot gathers having a common set of illuminated receivers;

(b) encoding each multi-shot gather with a different encoding function selected from a set of encoding functions that are orthogonal or pseudo-orthogonal with respect to cross-correlation;

(c) summing the encoded multi-shot gathers by summing all data records in each gather that correspond to a single receiver and repeating for each different receiver, resulting in a simultaneous encoded-source gather;

(d) assuming a physical properties model of the subsurface region, said model providing values of at least one physical property at locations throughout the subsurface region;

(e) using the assumed physical properties model, simulating the simultaneous encoded-source gather, encoding source signatures in the simulation using the same encoding functions used to encode corresponding gathers of the measured data, wherein an entire simultaneous encoded-source gather is simulated, using a programmed computer, in a single simulation operation;

(f) calculating a difference for each receiver between the simultaneous encoded-source gather and the simulated simultaneous encoded-source gather, said difference being referred to as the residual for that receiver;

(g) applying receiver encoding to each residual, said receiver encoding being selected to attenuate contributions from sources for which the receiver was inactive;

(h) computing an objective function from the receiver-encoded residuals, and updating the assumed physical properties model based on the objective function computation;

(i) repeating (b)-(h) at least one more iteration, using the updated physical properties model from the previous iteration as the assumed physical properties model, to produce a further updated physical properties model of the subsurface region; and

(j) downloading the further updated physical properties model or saving it to computer storage.

24. A computer-implemented method for inversion of measured data from a geophysical survey to determine a physical properties model for a subsurface region, comprising:

(a) simulating, using an assumed physical properties model, all source gathers in the measured geophysical data in a single simulation, using encoding, thereby generating a simulated data set;

(b) determining a grouping strategy that helps distinguish between data records corresponding to a receiver and a source where the receiver did not listen to the source during the geophysical survey;

(c) for each group, encoding the sources, each source being encoded with the same encoding used in (a), and performing steps comprising;

(i) simulating, using the assumed physical properties model, all source gathers in the group in a single simulation, thereby generating a simulated group data set;

(ii) determining receiver locations in the simulated group data set that were not illuminated by at least one of the sources in the group;

(iii) for each of the determined receiver locations, retrieving corresponding data from the simulated group data set and subtracting it from the same receiver location in the simulated data set;

(d) adjusting the measured data to include only data corresponding to that remaining in the simulated data set after (iii), and computing a data residual, being a difference between the adjusted measured data and the simulated data set;

(e) adjusting the assumed physical properties model to reduce the data residual; and

(f) repeating (a)-(e) until a predetermined convergence criterion is satisfied or other stopping condition is met.

25. The method of claim 24, wherein the encoding in (a) and (c) is encoding each source with a different encoding function wherein the encoding is orthogonal or pseudo-orthogonal with respect to cross-correlation, and simulating some or all groups simultaneously in (c)(i).

26. The method of claim 25, wherein the orthogonal or pseudo-orthogonal encoding comprises a set of bandpass filters, differing from each other in central frequency, phase, or both.

27. The method of claim 24, wherein both the encoding in (a) and the encoding in (c) are accompanied by a second encoding, making the source gathers in (a) and the sources in (c) doubly encoded.

28. The method of claim 27, wherein the second encoding is +1/−

1 encoding.

29. The method of claim 24, wherein the grouping strategy is to group a plurality of individual-source gathers having a common set of illuminated receivers.