A method, apparatus, and electronic device for stratigraphic imaging

Abstract

This application provides a stratigraphic imaging method, apparatus, and electronic device. The stratigraphic imaging method includes: determining the locations within the target stratigraphic layer where azimuth anisotropy exists based on wide-azimuth seismic data of the target stratigraphic layer; correcting the seismic data at each location with azimuth anisotropy using azimuth anisotropy correction technology to obtain corrected seismic data; performing grid tomography on the corrected seismic data to eliminate velocity differences between different azimuths, obtaining a velocity field with an accuracy greater than a preset threshold; performing orthogonal crystal system migration based on the anisotropy parameters and velocity field of the wide-azimuth seismic data of the target stratigraphic layer to obtain pre-stack preparation data; and stacking the azimuth data of the pre-stack preparation data to obtain a cross-sectional view of the target stratigraphic layer.

Description

Technical Field

[0001] This application relates to the field of geophysical exploration, and in particular to a stratigraphic imaging method, apparatus, and electronic equipment. Background Technology

[0002] With the development of seismic exploration technology, wide-azimuth seismic acquisition technology has become the mainstream acquisition method. Wide-azimuth seismic data contains rich geological information, which helps to identify small faults, microstructures, and high-angle fractures. However, as the acquisition azimuth increases, while increasing the acquisition illumination, it also leads to the emergence of azimuth-related velocity anisotropy problems.

[0003] Current methods for addressing the above problems are mainly based on time-domain OVT spiral gather azimuth time difference correction. However, this method is only limited to improving the superimposed imaging effect of seismic data and does not fully consider the velocity variation characteristics of seismic waves in different azimuths.

[0004] Therefore, how to correct the azimuth anisotropy problem in seismic data and improve the imaging quality of complex tectonic strata is an urgent technical problem to be solved. Summary of the Invention

[0005] This application provides a stratigraphic imaging method, apparatus, electronic device, and storage medium for correcting azimuth anisotropy problems in seismic data and improving the imaging quality of complex tectonic strata.

[0006] One embodiment of this application provides a stratigraphic imaging method, the method comprising: determining, based on wide-azimuth seismic data of a target stratigraphy, the locations in the target stratigraphy where azimuth anisotropy exists; using azimuth anisotropy correction technology to correct the seismic data of the locations in the wide-azimuth seismic data where azimuth anisotropy exists, to obtain corrected seismic data; performing grid tomography on the corrected seismic data to eliminate velocity differences in different azimuths in the corrected seismic data, to obtain a velocity field with an accuracy greater than a preset threshold; performing orthogonal crystal system migration based on the anisotropy parameters of the wide-azimuth seismic data of the target stratigraphy and the velocity field, to obtain pre-stack preparation data; and performing stacking processing on the azimuth data of the pre-stack preparation data to obtain a profile of the target stratigraphy.

[0007] In some embodiments, determining the locations in which azimuth anisotropy exists in the target stratum based on wide-azimuth seismic data of the target stratum includes: determining whether azimuth anisotropy exists in the target stratum based on spiral gathers and azimuth-split-shot-receiver-offset gathers; wherein the spiral gathers and azimuth-split-shot-receiver-offset gathers are obtained based on wide-azimuth seismic data of the target stratum; and, in the case where azimuth anisotropy exists in the target stratum, determining the locations in which azimuth anisotropy exists in the target stratum based on the morphology of butterfly gathers; wherein the butterfly gathers are obtained based on wide-azimuth seismic data of the target stratum.

[0008] In some embodiments, determining whether the target formation exhibits azimuth anisotropy based on the spiral gather and the azimuth-specific shot-receiver distance gather includes: performing an OVT (Out-of-Venture) migration operation on the spiral gather; obtaining the wavy characteristics of the same-direction axis of the spiral gather after OVT migration; superimposing shot-receiver distance gathers of different azimuths in the azimuth-specific shot-receiver distance gather; obtaining the profile feature differences of the superimposed shot-receiver distance gathers of different azimuths; and determining whether the target formation exhibits azimuth anisotropy based on the wavy characteristics of the same-direction axis and the profile feature differences.

[0009] In some embodiments, the anisotropy parameters of the wide-azimuth seismic data of the target stratum are obtained by: selecting multiple seismic data from different azimuths in the wide-azimuth seismic data of the target stratum; updating the anisotropy parameter volume for the seismic data in each azimuth; and performing elliptic linear fitting on the seismic data after the anisotropy parameter volume update to obtain the anisotropy parameters.

[0010] One embodiment of this application provides a stratigraphic imaging device, the device comprising: a determining module, configured to determine, based on wide-azimuth seismic data of a target stratigraphy, various azimuths in the target stratigraphy where azimuth anisotropy exists; a first acquiring module, configured to correct the seismic data of the wide-azimuth seismic data at the various azimuths where azimuth anisotropy exists using azimuth anisotropy correction technology, to obtain corrected seismic data; a second acquiring module, configured to perform grid tomography processing on the corrected seismic data to eliminate velocity differences in different azimuths in the corrected seismic data, to obtain a velocity field with an accuracy greater than a preset threshold; a third acquiring module, configured to perform orthogonal crystal system migration based on the anisotropy parameters of the wide-azimuth seismic data of the target stratigraphy and the velocity field, to obtain pre-stack preparation data; and an imaging module, configured to perform superposition processing on the various azimuth data of the pre-stack preparation data to obtain a cross-sectional view of the target stratigraphy.

[0011] This application provides an electronic device, which includes a memory and a processor. The memory stores a computer program, and the processor executes the method described above when running the program.

[0012] This application provides a storage medium for storing a computer-readable program, which, when run, performs the method described above.

[0013] The technical solutions provided in this application have at least the following advantages compared with the prior art:

[0014] In the embodiments provided in this application, based on the wide-azimuth seismic data of the target stratum, the azimuth anisotropy problems in each azimuth of the target stratum are determined; azimuth anisotropy correction technology is used to correct the seismic data in each azimuth of the wide-azimuth seismic data where azimuth anisotropy exists, resulting in corrected seismic data; grid tomography is performed on the corrected seismic data to eliminate velocity differences in different azimuths, obtaining a velocity field with an accuracy greater than a preset threshold; based on the anisotropy parameters and velocity field of the wide-azimuth seismic data of the target stratum, orthogonal crystal system migration is performed to obtain pre-stack preparation data; the azimuth data of each azimuth of the pre-stack preparation data are stacked to obtain a profile of the target stratum. This process can correct the azimuth anisotropy problems in the seismic data and improve the imaging quality of complex structural strata. Attached Figure Description

[0015] This application will be further described by way of exemplary embodiments, which will be described in detail with reference to the accompanying drawings. These embodiments are not limiting; in these embodiments, the same reference numerals denote the same structures, wherein:

[0016] Figure 1 This is an exemplary flowchart of a formation imaging method according to some embodiments of this application;

[0017] Figure 2 This is an exemplary schematic diagram of a target stratum image obtained according to an embodiment of this application;

[0018] Figure 3 This is an exemplary schematic diagram of a target stratum image obtained based on relevant technologies;

[0019] Figure 4 These are exemplary schematic diagrams of a formation imaging apparatus according to some embodiments of this application;

[0020] Figure 5 This is an exemplary structural diagram of an electronic device according to some embodiments of this application. Detailed Implementation

[0021] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are merely some examples or embodiments of this application. For those skilled in the art, these drawings can be applied to other similar scenarios without creative effort. Unless obvious from the context or otherwise specified, the same reference numerals in the drawings represent the same structures or operations.

[0022] It should be understood that the terms “system,” “device,” “unit,” and / or “module” used herein are one method of distinguishing different components, elements, parts, sections, or assemblies at different levels. However, if other words can achieve the same purpose, they may be replaced by other expressions.

[0023] As indicated in this application and claims, unless the context clearly indicates otherwise, the words "a," "an," "an," and / or "the" do not specifically refer to the singular and may also include the plural. Generally speaking, the terms "comprising" and "including" only indicate the inclusion of explicitly identified steps and elements, which do not constitute an exclusive list, and the method or apparatus may also include other steps or elements.

[0024] Flowcharts are used in this application to illustrate the operations performed by the system according to embodiments of this application. It should be understood that the preceding or following operations are not necessarily performed precisely in sequence. Instead, the steps can be processed in reverse order or simultaneously. Furthermore, other operations can be added to these processes, or one or more steps can be removed from them.

[0025] Figure 1 This is an exemplary flowchart of a formation imaging method according to some embodiments of this application. Figure 1 As shown, the stratigraphic imaging method includes the following steps:

[0026] In step S110, based on the wide-azimuth seismic data of the target stratum, the azimuths in the target stratum where azimuth anisotropy exists are determined.

[0027] In some embodiments, the presence of azimuth anisotropy in the target formation can be determined based on spiral gathers and azimuth-split-shot-receiver-offset gathers. The spiral gathers and azimuth-split-shot-receiver-offset gathers are obtained from wide-azimuth seismic data of the target formation.

[0028] In the specific implementation process, the wide-azimuth seismic data of the target strata can be processed according to preset rules to obtain spiral gathers and azimuth shot-receiver distance gathers.

[0029] In some embodiments, the presence of azimuth anisotropy in the target stratum can be determined by the following methods:

[0030] An OVT offset operation is performed on the spiral gather; the wavy characteristics of the same-direction axis of the spiral gather after OVT offset are obtained; the shot-receiver distance gathers of different azimuths in the azimuth-separated shot-receiver distance gathers are superimposed; the profile feature differences of the superimposed shot-receiver distance gathers of different azimuths are obtained; based on the wavy characteristics of the same-direction axis and the profile feature differences, it is determined whether the target stratum has an azimuth anisotropy problem.

[0031] When azimuth anisotropy exists in the target stratum, the azimuth anisotropy is determined based on the morphology of the butterfly gather; the butterfly gather is obtained from the wide azimuth seismic data of the target stratum.

[0032] In the specific implementation process, data from adjacent groups in the wide-azimuth seismic data of the target stratum can be combined according to a certain angle increment to form a butterfly gather; then, through the shape of the butterfly gather, the azimuth of the target stratum with azimuth anisotropy problem can be clearly determined.

[0033] In step S120, azimuth anisotropy correction technology is used to correct the seismic data in each azimuth of the wide-azimuth seismic data that have azimuth anisotropy problems, so as to obtain corrected seismic data.

[0034] In practice, various azimuth anisotropy correction techniques can be used to correct seismic data in different azimuth directions where azimuth anisotropy exists. These techniques may include, but are not limited to, velocity model iteration techniques and coherence spectrum acquisition techniques.

[0035] Azimuth anisotropy correction techniques can eliminate various effects of anisotropy on wide-azimuth seismic data. For example, it can flatten the wavy characteristics of spiral gathers caused by the anisotropy of the formation medium. Azimuth anisotropy correction techniques can also improve the imaging quality of wide-azimuth seismic data, providing experimental support for subsequent data processing steps.

[0036] In practice, spiral gathers can be extracted from wide-azimuth seismic data according to preset rules, and anisotropic correction techniques can be used to correct the data in the spiral gathers.

[0037] In step S130, the corrected seismic data is subjected to grid tomography to eliminate velocity differences in different orientations in the corrected seismic data, and a velocity field with an accuracy greater than a preset threshold is obtained.

[0038] By performing grid tomography on the corrected seismic data from different azimuths, the velocity differences between seismic data from different azimuths in wide-azimuth seismic data caused by anisotropy can be eliminated, forming a high-precision velocity field with an accuracy greater than a preset threshold.

[0039] In the specific implementation process, the corrected spiral gather in step S120 can be subjected to mesh tomography to obtain a high-precision velocity field.

[0040] In step S140, based on the anisotropy parameters and velocity field of the wide-azimuth seismic data of the target strata, orthogonal crystal system migration is performed to obtain pre-stack preparation data.

[0041] In the specific implementation process, the anisotropy parameters of the wide-azimuth seismic data of the target strata can be obtained through the following methods:

[0042] Multiple seismic data points from different azimuths are selected from the wide-azimuth seismic data of the target stratum; anisotropic parameter volume is updated for each azimuth seismic data point; and elliptic linear fitting is performed on the seismic data after the anisotropic parameter volume update to obtain the anisotropic parameters.

[0043] In step S150, the azimuth data of the pre-stack preparation data are overlaid to obtain a cross-sectional view of the target stratum.

[0044] contrast Figure 2 and Figure 3 It can be seen that the imaging quality of the cross-sectional image of the target stratum obtained according to the embodiments of this application is significantly higher than that of the cross-sectional image of the target stratum obtained by related technologies, and can more clearly reflect the structure of the target stratum.

[0045] Figure 4 This is an exemplary schematic diagram of a formation imaging apparatus according to some embodiments of this application.

[0046] like Figure 4 As shown, the formation imaging device includes: a determination module 410, a first acquisition module 420, a second acquisition module 430, a third acquisition module 440, and an imaging module 450.

[0047] The determination module 410 is used to determine, based on the wide-azimuth seismic data of the target stratum, the various azimuths in the target stratum where azimuth anisotropy exists.

[0048] The first acquisition module 420 is used to correct the seismic data in each azimuth of the wide-azimuth seismic data that have azimuth anisotropy problems by using azimuth anisotropy correction technology, so as to obtain corrected seismic data.

[0049] The second acquisition module 430 is used to perform grid tomography on the corrected seismic data to eliminate velocity differences in different orientations in the corrected seismic data and obtain a velocity field with an accuracy greater than a preset threshold.

[0050] The third acquisition module 440 is used to perform orthogonal crystal system migration based on the anisotropy parameters of the wide azimuth seismic data of the target stratum and the velocity field to obtain pre-stack preparation data.

[0051] The imaging module 450 is used to overlay the pre-stack preparation data in various orientations to obtain a cross-sectional view of the target stratum.

[0052] In some embodiments, determining the locations in which azimuth anisotropy exists in the target stratum based on wide-azimuth seismic data of the target stratum includes: determining whether azimuth anisotropy exists in the target stratum based on spiral gathers and azimuth-split-shot-receiver-offset gathers; wherein the spiral gathers and azimuth-split-shot-receiver-offset gathers are obtained based on wide-azimuth seismic data of the target stratum; and, in the case where azimuth anisotropy exists in the target stratum, determining the locations in which azimuth anisotropy exists in the target stratum based on the morphology of butterfly gathers; wherein the butterfly gathers are obtained based on wide-azimuth seismic data of the target stratum.

[0053] In some embodiments, determining whether the target formation exhibits azimuth anisotropy based on the spiral gather and the azimuth-specific shot-receiver distance gather includes: performing an OVT (Out-of-Venture) migration operation on the spiral gather; obtaining the wavy characteristics of the same-direction axis of the spiral gather after OVT migration; superimposing shot-receiver distance gathers of different azimuths in the azimuth-specific shot-receiver distance gather; obtaining the profile feature differences of the superimposed shot-receiver distance gathers of different azimuths; and determining whether the target formation exhibits azimuth anisotropy based on the wavy characteristics of the same-direction axis and the profile feature differences.

[0054] In some embodiments, the anisotropy parameters of the wide-azimuth seismic data of the target stratum are obtained by: selecting multiple seismic data from different azimuths in the wide-azimuth seismic data of the target stratum; updating the anisotropy parameter volume for the seismic data in each azimuth; and performing elliptic linear fitting on the seismic data after the anisotropy parameter volume update to obtain the anisotropy parameters.

[0055] In the embodiments of the above-mentioned stratigraphic imaging device, the specific processing of each module and the resulting technical effects can be referred to the relevant descriptions in the corresponding method embodiments, and will not be repeated here.

[0056] Figure 5 This is an exemplary structural diagram of an electronic device according to some embodiments of this application.

[0057] like Figure 5 As shown, the electronic device includes: processors 501, communication interfaces 502, memories 503, and communication buses 504. Optionally, the communication interface 502 can be an interface for a communication module, such as a GSM module interface. The processor 501 may be a CPU, an Application Specific Integrated Circuit (ASIC), or one or more integrated circuits configured to implement embodiments of the present invention. The memory 503 may include high-speed RAM and may also include non-volatile memory, such as disk drives. The memory 503 stores programs, and the processor 501 calls the programs stored in the memory 503 to execute some or all of the above-described method embodiments.

[0058] This application relates to a storage medium for storing a computer-readable program, which, when run, performs some or all of the above-described method embodiments.

[0059] Optionally, the storage medium may be a non-transitory computer-readable storage medium, such as a ROM, random access memory (RAM), CD-ROM, magnetic tape, floppy disk, and optical data storage device.

[0060] Based on the same inventive concept, this application also provides a computer program product, including a computer program that, when executed by a processor, implements some or all of the above-described method embodiments.

[0061] The basic concepts have been described above. Obviously, for those skilled in the art, the detailed disclosure above is merely illustrative and does not constitute a limitation of this application. Although not explicitly stated herein, those skilled in the art may make various modifications, improvements, and corrections to this application. Such modifications, improvements, and corrections are suggested in this application, and therefore remain within the spirit and scope of the exemplary embodiments of this application.

[0062] Furthermore, this application uses specific terms to describe its embodiments. For example, "an embodiment," "one embodiment," and / or "some embodiments" refer to a particular feature, structure, or characteristic related to various embodiments of this application. Therefore, it should be emphasized and noted that "an embodiment," "one embodiment," or "an alternative embodiment" mentioned twice or more in different locations in this application do not necessarily refer to the same embodiment. In addition, certain features, structures, or characteristics in one or more embodiments of this application can be appropriately combined.

[0063] Furthermore, unless expressly stated in the claims, the order of processing elements and sequences, the use of numbers and letters, or other names described in this application are not intended to limit the order of the processes and methods of this application. Although the foregoing disclosure has discussed some currently considered useful embodiments of the invention through various examples, it should be understood that such details are for illustrative purposes only, and the appended claims are not limited to the disclosed embodiments; rather, the claims are intended to cover all modifications and equivalent combinations that conform to the substance and scope of the embodiments of this application. For example, while the system components described above can be implemented using hardware devices, they can also be implemented solely through software solutions, such as installing the described system on existing servers or mobile devices.

[0064] Similarly, it should be noted that, in order to simplify the description of the present application and thus aid in the understanding of one or more embodiments of the invention, the foregoing description of the embodiments of the present application sometimes combines multiple features into a single embodiment, drawing, or description thereof. However, this disclosure method does not imply that the subject matter of the application requires more features than those mentioned in the claims. In fact, the embodiments contain fewer features than all the features of the single embodiments disclosed above.

[0065] In some embodiments, numbers describing the quantity of components and attributes are used. It should be understood that such numbers used in the description of embodiments are modified in some examples with the terms "approximately," "approximately," or "generally." Unless otherwise stated, "approximately," "approximately," or "generally" indicates that the numbers are allowed to vary by ±20%. Accordingly, in some embodiments, the numerical parameters used in the specification and claims are approximate values, which may be changed depending on the characteristics required by individual embodiments. In some embodiments, numerical parameters should take into account specified significant digits and employ a general method of digit reservation. Although the numerical ranges and parameters used to confirm their breadth of scope in some embodiments of this application are approximate values, in specific embodiments, such values ​​are set as precisely as feasible.

[0066] For each patent, patent application, patent application publication, and other material such as articles, books, specifications, publications, and documents referenced in this application, the entire contents of that patent are incorporated herein by reference. This excludes historical application documents that are inconsistent with or conflict with the content of this application, as well as documents that limit the broadest scope of the claims in this application (currently or subsequently appended to this application). It should be noted that if there are any inconsistencies or conflicts between the descriptions, definitions, and / or terminology used in the supplementary materials of this application and the content of this application, the descriptions, definitions, and / or terminology used in this application shall prevail.

[0067] Finally, it should be understood that the embodiments described in this application are merely illustrative of the principles of the embodiments of this application. Other modifications may also fall within the scope of this application. Therefore, alternative configurations of the embodiments of this application are considered as examples and not limitations, and are regarded as consistent with the teachings of this application. Accordingly, the embodiments of this application are not limited to the embodiments explicitly described and illustrated in this application.

Claims

1. A stratigraphic imaging method, characterized in that, The method comprises: Based on the wide-azimuth seismic data of the target stratum, determine the azimuth anisotropy problem in the target stratum. Using azimuth anisotropy correction technology, the seismic data in each azimuth of the wide azimuth seismic data that have azimuth anisotropy problems are corrected to obtain corrected seismic data. The corrected seismic data is subjected to grid tomography to eliminate velocity differences in different orientations and obtain a velocity field with an accuracy greater than a preset threshold. Based on the anisotropic parameters and velocity field of the wide-azimuth seismic data of the target stratum, orthogonal crystal system migration is performed to obtain pre-stack preparation data; The azimuth data of the pre-stack preparation data are overlaid to obtain a cross-sectional view of the target stratum. The step of determining the locations within the target stratum where azimuth anisotropy exists based on wide-azimuth seismic data includes: Based on the spiral gather and the azimuth-based shot-receiver distance gather, it is determined whether the target stratum has an azimuth anisotropy problem; wherein, the spiral gather and the azimuth-based shot-receiver distance gather are obtained from the wide azimuth seismic data of the target stratum; In the case of azimuth anisotropy in the target stratum, the azimuth anisotropy in the target stratum is determined according to the morphology of the butterfly gather; wherein the butterfly gather is obtained from the wide azimuth seismic data of the target stratum. The determination of whether the target strata exhibit azimuth anisotropy based on spiral gathers and azimuth-separated shot-receiver distance gathers includes: Perform an OVT offset operation on the spiral gather; Obtain the wavy characteristics of the same-direction axis of the spiral gather after OVT offset; The shot-receiver distance collections of different azimuths in the azimuth-divided shot-receiver distance collections are superimposed. To obtain the differences in profile features of shot-receiver distance gathers from different azimuths after superposition; Based on the wavy characteristics of the co-directional axis and the differences in the profile characteristics, it is determined whether the target stratum has an azimuth anisotropy problem.

2. The method according to claim 1, characterized in that, The anisotropy parameters of the wide-azimuth seismic data of the target stratum were obtained in the following manner: Select seismic data from multiple different azimuths from the wide-azimuth seismic data of the target stratum; Anisotropic parameters are updated for seismic data in each azimuth. The anisotropic parameters are obtained by performing elliptic linear fitting on the updated seismic data with anisotropic parameters.

3. A stratigraphic imaging device, characterized in that, The device comprises: The determination module is used to determine the azimuth anisotropy problem in the target stratum based on the wide-azimuth seismic data of the target stratum. The first acquisition module is used to correct the seismic data in the wide azimuth seismic data that have azimuth anisotropy problems by using azimuth anisotropy correction technology, so as to obtain corrected seismic data. The second acquisition module is used to perform grid tomography on the corrected seismic data to eliminate velocity differences in different orientations in the corrected seismic data and obtain a velocity field with an accuracy greater than a preset threshold. The third acquisition module is used to perform orthogonal crystal system migration based on the anisotropy parameters of the wide azimuth seismic data of the target stratum and the velocity field to obtain pre-stack preparation data. The imaging module is used to overlay the pre-stack preparation data in various orientations to obtain a cross-sectional view of the target stratum. The step of determining the locations within the target stratum where azimuth anisotropy exists based on wide-azimuth seismic data includes: Based on the spiral gather and the azimuth-based shot-receiver distance gather, it is determined whether the target stratum has an azimuth anisotropy problem; wherein, the spiral gather and the azimuth-based shot-receiver distance gather are obtained from the wide azimuth seismic data of the target stratum; In the case of azimuth anisotropy in the target stratum, the azimuth anisotropy in the target stratum is determined according to the morphology of the butterfly gather; wherein the butterfly gather is obtained from the wide azimuth seismic data of the target stratum. The determination of whether the target strata exhibit azimuth anisotropy based on spiral gathers and azimuth-separated shot-receiver distance gathers includes: Perform an OVT offset operation on the spiral gather; Obtain the wavy characteristics of the same-direction axis of the spiral gather after OVT offset; The shot-receiver distance collections of different azimuths in the azimuth-divided shot-receiver distance collections are superimposed. To obtain the differences in profile features of shot-receiver distance gathers from different azimuths after superposition; Based on the wavy characteristics of the co-directional axis and the differences in the profile characteristics, it is determined whether the target stratum has an azimuth anisotropy problem.

4. The apparatus according to claim 3, characterized in that, The anisotropy parameters of the wide-azimuth seismic data of the target stratum were obtained in the following manner: Select seismic data from multiple different azimuths from the wide-azimuth seismic data of the target stratum; Anisotropic parameters are updated for seismic data in each azimuth. The anisotropic parameters are obtained by performing elliptic linear fitting on the updated seismic data with anisotropic parameters.

5. An electronic device comprising a memory and a processor, the memory storing a computer program, the processor executing the method as claimed in claim 1 or 2 when running the program.

6. A storage medium for storing a computer-readable program, which, when executed, performs the method as claimed in claim 1 or 2.

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