METHOD AND APPARATUS FOR MEASURING SEISMIC DATA

Abstract

The present invention relates to a method of processing seismic data. The method may include calculating a number of calculated structure tensors for each of a number of seismic data lines, the seismic data lines being spatially distributed about an area of the surface of the Earth. The method also may include interpolating the calculated structure tensors to find interpolated structure tensors in a region of the area between the lines of the seismic data lines, and calculating calculated seismic data from the interpolated structure tensors.

Claims

1. A method of processing seismic data, comprising: (a) calculating a plurality of calculated structure tensors for each of a plurality of seismic data lines, the seismic data lines being spatially distributed about an area of the Earth's surface; (b) interpolating the calculated structure tensors to find interpolated structure tensors in a region of the area between the lines of the seismic data lines; and (c) calculating calculated seismic data from the interpolated structure tensors.

2. A method as claimed in claim 1, wherein the seismic data used in step (a) is 2D seismic data.

3. A method as claimed in claim 2, wherein the step (a) comprises: estimating a gradient vector by estimating components of the gradient of the 2D seismic data within the 2D plane of the 2D seismic data; and calculating the outer product of the estimated gradient vector.

4. A method as claimed in claim 3, wherein step (a) further comprises applying smoothing.

5. A method as claimed in claim 1, wherein the seismic data is wavefield data.

6. A method as claimed in claim 1, wherein step (b) comprises: calculating eigenvalues and eigenvectors of each of the calculated structure tensors; stepping the interpolation using the eigenvalues and eigenvectors to calculate the interpolated structure tensors adjacent the calculated structure tensors; and repeating the eigenvalue and eigenvector-calculating step and the stepping step for the interpolated structure tensors until interpolation throughout the region(s) is complete.

7. A method as claimed in claim 6, wherein the seismic data is wavefield data, and wherein the eigenvector, and corresponding eigenvalue, used for the stepping step is tangential to the wavefield.

8. A method as claimed in claim wherein the seismic data lines comprise 2D zero-offset stacks.

9. A method as claimed in claim 1, wherein the lines are sparsely distributed over the area of the Earth's surface.

10. A method as claimed in claim 1, wherein the lines are irregularly distributed over the areas of the Earth's surface.

11. A method as claimed in claim 1, wherein the seismic data lines comprise non-migrated or de-migrated seismic data.

12. A method as claimed in claim 1, further comprising, prior to step (a), performing de-migration to migrated seismic data to produce the seismic data lines.

13. A method as claimed in claim 1, further comprising performing migration, or re-migration, to the seismic data after step (c).

14. A method as claimed in claim 1, wherein the seismic data lines comprise measured seismic data.

15. A computer program product comprising instructions that when executed will cause a processor to perform a method as claimed in claim 1.

16. A computer program product as claimed in claim 15 stored on a storage medium.

17. Transmission of a program as claimed in claim 15 across a communications network.

18. A computer programmed to perform a method as claimed in claim 1.

19. An apparatus for processing data representing a physical system, the apparatus being arranged to perform a method as claimed in claim 1.

Description

[0054] Certain preferred embodiments will now be described by way of example only and with reference to the accompanying drawings, in which:

[0055] FIG. 1 illustrates an area of the Earth over which seismic data has been acquired;

[0056] FIG. 2 illustrates a method according to an embodiment of the present invention;

[0057] FIG. 3 illustrates a block diagram of an apparatus according to an embodiment of the present invention;

[0058] FIGS. 4(a) to (f) show the results of a comparison of the data output from this method with a data from a known dataset.

[0059] With reference to FIG. 1, a spatially irregular grid 2 of 2D seismic wavefield data lines 3 has been recorded over an area 1 of the Earth. This grid 2 may be made by a collection of data sets measured at different times. These may even have been recorded by different parties. The 2D seismic wavefield data is non-migrated, or de-migrated, 2D zero-offset seismic data. Between the distributed 2D seismic wavefield data lines is a plurality of regions 4 where no seismic wavefield data has been measured. The 2D seismic wavefield data lines 3 are non-migrated and/or de-migrated seismic wavefield data lines that tie at intersections 5.

[0060] As shown in FIG. 2, an embodiment of the method of the present invention comprises the step of calculating 10 a plurality of calculated structure tensors for each of the 2D wavefield seismic data lines 3. The step of calculating 10 includes estimating a gradient vector by estimating components of the gradient of the 2D seismic wavefield data within the 2D plane of the 2D seismic wavefield data. Once the gradient vector has been estimated the calculated structure tensors are calculated by calculating the outer product of the estimated gradient vector with itself. Gaussian smoothing is then applied.

[0061] Thus, the structure tensors are formed in the 2D plane of the seismic wavefield data by estimating the in-plane components of the gradients, forming the outer-product and applying Gaussian smoothing. The calculated structure tensors are therefore calculated along the seismic data lines 3.

[0062] The next step of the embodiment is to interpolate 11 the calculated structure tensors to find interpolated structure tensors in the regions 4 of the area 1 between the 2D seismic wavefield data lines 3. The interpolating step 11 initially comprises calculating eigenvalues and eigenvectors of each of the calculated structure tensors. The interpolation is then stepped using the eigenvalues and eigenvectors to calculate interpolated structure tensors adjacent the calculated structure tensors. The interpolation then continues by calculating the eigenvectors and eigenvalues of the interpolated structure tensors to step the interpolation to produce subsequent interpolated structure tensors. The interpolation process continues until interpolation throughout the regions is complete.

[0063] The eigenvector, and corresponding eigenvalue, used for the stepping step is the eigenvector tangential to the wavefield. The eigenvalue of the tangential eigenvector is approximately the singular eigenvalue of the structure tensor. It is this dominant eigenvalue and its eigenvector which are used to interpolate the structure tensors.

[0064] Thus, having calculated the structure tensors for the 2D seismic wavefield data lines at step 10, the calculated structure tensors can be interpolated throughout the area 1 at step 11.

[0065] The next step of the embodiment is to use the interpolated structure tensors to reconstruct an approximation of the wavefield. This is done by calculating 12 calculated seismic data from the interpolated structure tensors.

[0066] In summary, a technique has been disclosed of interpolating structure tensors for the construction of spatially regular 3D data sets from spatially irregular 2D data sets. A technique has also been disclosed of using 2D de-migration followed by 3D re-migration to undo errors made using 2D migration, thus allowing for the transfer of large scale interpretations from 2D to 3D.

[0067] An advantage of using an embodiment of the present invention is to allow for direct transfer of 2D interpretations to a 3D setting. This will aid in the evaluation of areas in frontier exploration.

[0068] An advantage of using an embodiment of the present invention is to allow construction of a consistent 3D depth cube that ties in at intersections, due to proper 3D migration afterwards. The methods described above may be embodied purely in hardware or may be embodied at least in part in a program for controlling a computer to perform at least some (perhaps all) of the steps. The program may be stored on a computer-readable storage medium, for example hard or floppy discs, CD or DVD-recordable media or flash memory storage products. The program may also be embodied in a signal such as a downloadable data signal transmitted across a computer network, for example the Internet or a group of computers connected together in a LAN. Any appended claims now or in future are to be interpreted as covering a computer program by itself, or as a record on a carrier, or as a signal, or in any other form.

[0069] The schematic diagram of FIG. 3 illustrates a central processing unit (CPU) 21 connected to a read-only memory (ROM) 22 and a random access memory (RAM) 23. The CPU is provided with measured seismic data 24 and any other input data 25 via an input/output mechanism 26. The CPU then performs the method steps described above on the provided data in accordance with program steps or instructions provided by the program storage 27 (which may be a part of the ROM 22) and provides output data 28 via the input/output mechanism 26. The program itself, or any of the inputs and/or outputs to the system may be provided or transmitted to/from a communications network 29, which may be, for example, the Internet.

[0070] The appended schematic workflow diagram can be considered not only to depict a series of method steps, but also to depict apparatus for performing those method steps. In this respect, a functional block depicted in a workflow diagram can be considered to represent a component such as a processor or processing unit which is adapted to or at least operable to perform the depicted function. Operation of one or more of these components can be controlled or provided at least in part by a program operating on the device or apparatus. The function of several depicted components may in fact be performed by a single component, such as the CPU 21, under control of the program.

[0071] One or more of the components may be provided as dedicated hardware.

[0072] FIGS. 4(a) to (c) show 2D samples of real 3D seismic data. These 2D seismic data lines have been taken by slicing through a real 3D seismic data set at lines of constant easting (i.e. constant x, FIG. 4(a)), constant northing (i.e. constant y, FIG. 4(b)) and constant time (i.e. constant z, FIG. 4(c)). The dotted lines in each Figure denote the locations of the other slices plotted in the other Figures.

[0073] FIG. 4(d) to (f) show 2D samples of calculated 3D seismic data calculated using the present method. To generate the calculated 3D seismic data, a set of 2D samples of the real 3D seismic data were taken at intervals of approximately 130 m throughout the real 3D seismic data. This set of 2D samples hence formed a grid of 2D seismic data lines. The present method was used to generate the calculated 3D seismic data from the grid. 2D samples of this calculated 3D seismic data set corresponding to the locations and orientations of the 2D samples of the real 3D seismic data set of FIGS. 4(a) to 4(c) are shown in FIGS. 4(d) to 4(f). Thus, FIG. 4(d) shows a sample of the calculated 3D seismic data corresponding to the sample of the real 3D seismic data shown in FIG. 4(a). Likewise, FIG. 4(e) corresponds to FIG. 4(b) and FIG. 4(f) corresponds to FIG. 4(c).

[0074] Thus FIGS. 4(a) and 4(c), 4(b) and 4(d), and 4(c) and 4(f) can be compared as a quality control test of the present method, since 2D samples of a real 3D dataset (i.e. a measured 3D dataset) can be compared to 2D samples of calculated 3D dataset corresponding to the real 3D dataset.

[0075] Whilst the interval between the samples in the set of 2D samples of the real 3D seismic data was approximately 130 m, similar analysis could have been done for intervals of 16 m, 33 m, 65 m and 260 m, for example. These intervals may vary somewhat over the length of the sample lines because the geometry may be imperfect.

[0076] As can be seen by comparing the relevant Figures, the samples of the calculated 3D seismic data closely correlate to the corresponding samples of the real 3D seismic data. Thus, these results show the effectiveness of the claimed invention. It should be noted that issues, such as aliasing issues, have been reduced.

[0077] It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention, which is defined by the claims.