MXPA06006002A - Method for determination of sufficient acquisition coverage for a marine seismic streamer survey - Google Patents

Abstract

Seismic data sets representative of a marine seismic streamer survey are constructed with test coverage holes in the data sets. The data sets are processed and data quality degradation in the processed data due to the test coverage holes is evaluated. Maximum acceptable coverage holes for the survey are determined from the evaluation of data quality degradation.

Description

METHOD TO DETERMINE AN ENTRY COVERAGE SUFFICIENT FOR AN EXPLORATION WITH SEA SEISMIC CABLE CROSS REFERENCES TO RELATED REQUESTS Does not apply RESEARCH OR DEVELOPMENT OF FEDERAL SPONSOR Not applicable LIST OF SEQUENCES, TABLE OR LIST OF COMPUTER Does not apply BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates generally to the field of geophysical exploration. More particularly, the invention refers to the field of marine seismic exploration.

DESCRIPTION OF THE RELATED TECHNIQUE In the oil and gas industry, geophysical exploration techniques are commonly used to assist in the search for hydrocarbon deposits that are located in underground formations. In seismic exploration, seismic energy sources are used to generate a seismic signal that propagates in the earth and that is reflected at least partially by seismic reflectors of the subsoil. Said seismic reflectors are usually interfaces that are between underground formations that have different elastic properties, specifically the wave velocity and the density of the rock, which result in differences in the acoustic impedance at the interfaces. Reflections are detected by seismic receivers at or near the surface of the earth, in an underlying body of water, or at known depths in boreholes. The resulting seismic data can be processed to produce information related to the geological structure and the properties of the underground formations, and their potential hydrocarbon content. The objective of data processing, seismic is to extract from them all the possible information that is related to the underground formations. In order for the seismic data processed to accurately represent the geological properties of the subsoil, the reflection amplitudes must be accurately represented. Non-geological effects can cause measured seismic amplitudes to deviate from the amplitude caused by reflection from the geological target. The amplitude distortions resulting from the irregular distribution of source and receiver positions during data acquisition turn out to be a particularly problematic non-geological effect. If not corrected, these non-geological effects can dominate the seismic image and obscure the geological representation. A seismic wave source generates a wave that is reflected from, or that illuminates a portion of reflectors at different depths. The reflected seismic wave is detected by sensors and the detected signals are recorded. In a three-dimensional (3D) scan, seismic signals are generated at a large number of source locations, and scanning generally illuminates large regions of the reflectors. Conventional pre-stacked 3D migration algorithms can produce good images of subsurface horizons only if the distribution on the surface of the sources and receivers is relatively uniform. In practice, there are always irregularities in the distribution of sources and receivers. It is usually too expensive to obtain a perfectly regular acquisition geometry. As a result, frequently pre-stacked 3D migrated images are contaminated with non-geological aberrations. These aberrations can interfere with the interpretation of the seismic image and attribute maps. An objective of seismic acquisition is to balance the regularity of the distribution of the source and the receiver with a reasonable acquisition cost.

In the explorations with marine seismic cable, the cables do not form straight lines. Normally, currents cause cables to bend, a phenomenon known as flocculation, and curvature is usually measured in degrees. Changes in currents often cause changes in flocculation. In such circumstances, if the separation of the planned navigation line of the seismic vessel is maintained, then flocculation will cause coverage holes in certain displacements or displacement ranges, at certain depths. The term "cover hole" as used herein, refers to a surface area in which, for a given displacement or displacement range, inadequate data is recorded. It is defined that the data is located in the midpoint positions of the surface, between the source and receiver pairs. The coverage holes may have several kilometers in the direction of the navigation line (in line), but they are in the order of ten to a few hundred meters in the orthogonal direction (transverse line) to the navigation line. In marine seismic cable surveys, often portions of the surface are not adequately covered by the receiver's records due to cable flocculation. In this way, to cover these areas that were omitted in the first pass, additional passes of the seismic vessel are required through the prospecting exploration area. Additional numbers of navigation lines may also arise from the orientation of the vessel, to achieve acceptable coverage. This means that the distance between the passes averages less than the original acquisition plan. These additional passes significantly increase the time and cost associated with a full scan. These additional passes of the exploration vessel are known as "fill shot". A large portion of the marine seismic data collection may be directed to the fill draft portion of a scan. The filling portion can take several weeks or even months to complete. Thus, it is not uncommon to have to invest 15 to 20% of the total acquisition costs in the acquisition of landfill. Any reduction in these large filler costs would provide an economic advantage. Typically, the maximum data hole sizes that would provide acceptable subsoil coverage before acquisition are determined, and are usually independent of local factors such as geology and exploration objectives. Criteria for seismic exploration, such as acceptable subsoil coverage, are commonly referred to as "fill specifications". In the past, the assessment of whether an exploration acquisition plan would provide acceptable subsoil coverage was made during, or after, the acquisition was made. However, waiting until after the acquisition means either incurring the cost of retaining the equipment and personnel in the exploration area until the evaluation is made, or running the risk of having to bring the equipment back and forth. personnel to the exploration area for an additional acquisition of landfill at a considerable cost. Making fill acquisition decisions during acquisition means being able to initiate an additional fill acquisition without having to wait. For example, the document Brink, M., Jones, N., Doherty, J., Vinje, V., and Laurain, R., "Infill decisions using simulated migration amplitudes", SEG Int'l. Exp. And 74th Ann. Mtg., Denver, Colorado, Oct. 10-15, 2004, p. 57-60 describes a method for making decisions about filling during acquisition. The seismic data is modeled in a velocity depth model using navigation data and migration amplitudes across key horizons. The data and navigation speeds can be acquired during the acquisition, and then they can be generated by the migration amplitudes simulated during the acquisition. Then the need for an additional filler shot can be determined. However, it would be more efficient to determine the maximum acceptable coverage hole sizes before starting the acquisition. After, any difference discovered could be corrected during the acquisition, considerably reducing the need for a subsequent additional fill acquisition. However, Brink et al. 2004 does not describe how to make filler specifications regarding data hole coverage before starting the acquisition. A series of three parts (Muerdter, D., and Ratckiff, D., "Understanding subsalt illumination through ray-trace modeling, Part 1: Simple 2D models", The Leading Edge, Vol. 20, 6, June, 2001 p. 578-594, (Muerdter et al., 2001a); Muerdter, D., Kelly, M., and Ratckiff, D., "Understanding subsalt Illumination Through Ray-trace Modeling, Part 2: Dipping Salt Bodies, Salt Peaks, and Nonreciprocity of Subsalt Amplitude Response," The Leading Edge, m Vol. 20, July 7, 2001, p. 688-697, (Muerdter et al., 2001b); and Muerdter, D., and Ratckiff, D., "Understanding subsalt illumination through ray-trace modeling, part 3: Salt ridges and furrows, and the impact of acquisition orientation", The Leading Edge, Vol. 20, August 8, 2001, pgs. 803-816, (Muerdter et al 2001c) describes the application of trajectory modeling to clarify the image problems under various salt structures, such as salt and body salty detached with irregular shape. The modeling involves building salt models in 3D and speed, applying ray tracing to pre-stacked depth migration scans, and then classifying the data into common 2D and 3D reflection point groups to compare them with the migrated seismic data. Muerdter et al. 2001c claim that modeling can be used to predict the expected illumination and to determine the best acquisition parameters before acquisition, but the only parameter studied is the effect of the orientation of the acquisition (direction of shooting) relative to the structural orientation (edges and grooves) of the salt structure (although also the length of the image is mentioned but not discussed in Muerdter et al.2001a). However, Muerdter et al. 2001a does not describe how to make padding specifications related to data hole coverage as surface coverage before starting acquisition.

Thus, there is a need for a method to make an a priori determination of the efficiency of the acquisition coverage for a given exploration with marine seismic cable, that is, to determine the size of the coverage holes that is acceptable before carrying out the acquisition.

BRIEF DESCRIPTION OF THE INVENTION The seismic datasets representative of a marine seismic cable exploration are constructed with test coverage holes in the data sets. The data sets are processed and the degradation in the quality of the processed data due to the coverage holes is evaluated. The maximum acceptable coverage hole for the scan is determined from the evaluation of the degradation of the data quality.

BRIEF DESCRIPTION OF THE DRAWINGS The invention and its advantages can be more easily understood by reference to the following detailed description and the accompanying drawings, in which: Figures 1A-1 D are examples of coverage holes in marine seismic cable data in four ranges of displacement; Figure 2 is a flow chart illustrating the steps of the method of one embodiment of the method of the invention, to determine the suitability of the acquisition coverage for a marine seismic exploration in a scanning area; Figure 3 is a synthetic cross-section without cover holes, after pre-stacked migration; and Figure 4 is the cross section of Figure 3 with a 75 meter coverage hole, after the pre-stacked migration. Although the invention will be described in connection with its preferred embodiments, it should be understood that the invention is not limited thereto. On the contrary, the invention is intended to cover all alternatives, modifications and equivalents that may be included within the scope of the invention, as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION The invention is a method for the a priori determination of a sufficient acquisition coverage for a marine seismic cable exploration. The determination is based on the evaluation of the impact of the coverage holes in the acquired data at different displacements and depths in the seismic data processed. Figures 1A-1 D show examples of coverage holes in marine seismic cable data in four ranges of displacement at a selected depth. Figures 1A-1D cover the displacement ranges 165-1665 meters 1665-3165 meters, 3165-4665 meters, and 4665-6165 meters, respectively. The cover holes are the dark gray areas 21, and are typical for marine seismic cable data. The cover holes 21 are usually elongated in the direction of the navigation line and are thinner in the direction of the transverse line. The width of the cover holes 21 varies with the image, as can be seen in Figures 1A-1 D. The high coverage areas are the light gray areas 22, and are similarly elongated in the navigation line direction and have a variable width. The cover holes 21 and their elongated shape are due to occurrences during explorations with marine cable, such as flocculation or obstacle avoidance. When performing an embodiment of the invention, a size scale is selected for the test coverage holes, test displacements and test depths. Complete coverage seismic datasets are constructed in such a way that they are representative of the geology of the exploration area and the proposed configuration of the exploration vessel. From these full coverage data sets, partial coverage seismic data sets are constructed with test coverage holes. Partial coverage seismic data sets are constructed by removing data to model the test coverage holes by expanding the size scale of anticipated coverage holes in the proposed exploration. These complete and partial seismic data sets can be constructed from real data (if available), synthetic data, or a combination of real and synthetic data. Seismic data sets are processed through standard data processing, such as pre-stacked migration. The degradation of the quality of the life data to the selected test coverage holes in the processed data sets for the test offsets and the test depths is evaluated. The degradation of the data quality in the processed seismic life data to the test coverage holes is evaluated by comparing the quality of the data in the partial coverage sets with the data quality in the complete coverage set. The degradation in the quality of the data is usually evaluated by determining the extent of the aberrant particles in the data of the processed data. The estimated degradation in data quality for the different sizes of the test coverage holes is used to determine the maximum acceptable coverage holes as an image and depth function that will meet the data quality requirements for the scan . This determination of the maximum acceptable coverage holes can then be used to maximize the efficiency of acquisition in the scan. Figure 2 shows a flow diagram illustrating the steps of the method of one embodiment of the method of the invention to determine the adequacy of acquisition coverage for a marine seismic cable scan. First, in step 101, a size scale is selected for the test coverage holes. The size scale is selected to cover the sizes of the coverage holes that would be anticipated during the proposed exploration with marine seismic cable. The test coverage holes will be generated within the test data sets to test the subsoil coverage for the proposed exploration. The size scale of the test coverage holes can be the size scale that is anticipated to occur as a result of the flocculation of the marine seismic cables, the deviation of the navigation line to avoid anticipated obstacles in the area of exploration, the malfunctioning of the equipment in the cables or in the parts of the cables, or any other known source of the cover holes, as is well known in the geophysical investigation technique. In step 102, the test offsets are selected.

The test displacements are selected to cover the range of displacements that could be anticipated to occur during the proposed exploration with marine seismic cable. In an alternative mode, the scrolling ranges are selected to test the test data sets. In the quality specifications for the acquisition of marine seismic data, contributions are often grouped into different displacements, joining the displacements in ranges of displacement, as is well known in the technique of seismic acquisition. The method of the invention will be illustrated by referring to a set of test offsets, rather than displacement ranges for convenience only, but is not intended to be a restriction of the invention. In step 103, the depths are selected. The depths of the test are selected to cover the scale of depths that would be of interest during the proposed exploration with marine seismic cable. In an alternative mode, travel times are selected to test the coverage of the test subsoil, instead of the depths. Travel times are an equivalent method for measuring depth, as is well known in the seismic data processing technique. In step 104, it is determined whether real data sets or synthetic data sets are to be used to test the subsoil coverage in the proposed seismic exploration. In one embodiment, the seismic data sets comprise real seismic data selected from the exploration area or in other similar areas that are representative of the geology of the exploration area. In another embodiment, the seismic data sets comprise synthetic seismic data selected to be representative of the geology of the exploration area. In another modality, the seismic data sets comprise a combination of both real and synthetic seismic data, selected to be representative of the geology of the exploration area.

Full coverage seismic data sets are selected (or constructed) and partial coverage data sets are constructed from the full coverage data sets. Partial coverage seismic data sets will be constructed to simulate the effects of the test coverage holes within the size scale selected in step 101. The seismic data sets are preferably selected from available seismic data representing better the geology, as well as the configuration of the geometry of the exploration, anticipated in the exploration. If synthetic data is to be used, then the procedure proceeds to step 105 to construct the synthetic data sets. If real data is to be used, then the procedure proceeds to step 109 to construct real data sets. If a combination of both synthetic and real data is to be used, then the procedure proceeds to two steps 105 and 109. In step 105, a velocity model is constructed for the area that will be explored in the proposed exploration with seismic cable Marine. The velocity model will be used to generate the synthetic data sets. In one embodiment, a single velocity model is selected to represent the known velocity in the scanning area. In an alternative mode, a set of speed models is selected to represent the known speed of the scanning area. The model or speed models are selected from any speed information that is available, to better represent the speed of the scanning area. If the velocity models are not already available for the exploration area, then appropriate velocity models are developed for the exploration area. The speed models used can be constructed with depressions representative of the geology of the exploration area, at least as measured in the depressions in line and in section of the known horizons. The method of the invention will be illustrated by referring to a single velocity model for convenience only, but this reference is not intended to be a restriction of the invention. In step 106, the configuration of the exploration vessel for the proposed exploration with marine seismic cable is defined. The configuration of the scan vessel will be used to generate the synthetic data sets. The configuration of the scan vessel determines the geometry of the sources and the locations of the receiver in the scan. Typical important parameters for the configuration of the exploration vessel include, but are not limited to, the number of cables, the length of the cables, the separation of the cables, the number of source arrangements, and the separation that There are between the source arrangements and the cables. In step 107 seismic data traces are generated for a full coverage data set. The traces of synthetic seismic data are generated using the speed model of step 105 and the configuration of the scanning vessel of step 106. Typical important variations in the modeling of synthetic seismic data sets include, but are not limited to, target depth, frequency content, image scale, cross-sectional and inline horizons. The fact that the image data are only useful for a rare depth should also be taken into account. After construction of the full coverage synthetic data set (without cover holes), further processing is performed in step 111. In step 108, synthetic seismic data sets with test cover holes are constructed, starting from Complete coverage synthetic data set from step 107. Coverage holes can be modeled by removing data from the full coverage data set. The data removed may be some or all of the seismic traces in the full coverage data set. After construction of the partial coverage synthetic data sets, further processing of the partial coverage data sets is performed in step 111. After processing, the partial coverage data sets with test coverage holes will be compared to the full coverage data set without coverage holes. In step 109, available real seismic data is selected to form a complete coverage data set. In one embodiment, the seismic data comprises real seismic data selected from the exploration area that are representative of the geology of the exploration area. In another modality, the seismic data includes real seismic data selected from another area or area sufficiently similar to the exploration area that are representative of the geology of the exploration area. In another embodiment, the seismic data comprises a combination of real seismic data selected from the exploration area and other similar areas. After construction, the full coverage real data set (without cover holes) is further processed in step 111. In step 110 real seismic data sets are constructed with test cover holes from the data set full coverage of step 109. Test coverage holes can be simulated by removing data from the full coverage data set. The data removed may be some or all of the seismic traces in the full coverage data set. For example, the removed data could simulate missing data due to flocculation in the marine seismic cables, anticipated obstacles in the exploration area, malfunctioning of the equipment in the cables or in the parts of the cables, or any other known source of cover holes. You can simulate any different size of coverage holes, to cover the scale of sizes selected for the test in step 101, by the amount of data removed. After construction, the actual partial coverage data sets are further processed in step 111. The partial coverage data sets with test coverage holes will be compared after processing with the full coverage data set which does not have cover holes. In step 111, the seismic data sets are processed.

These seismic data sets include any set of synthetic or real data and any data set of partial or complete coverage that was selected or constructed in the manner described above. Normally the processing will comprise a pre-stacked migration. However, this particular form of processing is not intended to be a restriction of the invention. For example, processing may include, but is not restricted to, partial migration (DMO), post-stack migration, inversion, interpolation and extrapolation. In step 112 the effect of the test coverage holes on the quality of the data in the processed data sets of step 111 is evaluated. The degradation of the data quality due to the effects of the test coverage holes it is evaluated on the test offsets (or Image scales) selected in step 2 and the test depths selected in step 103. The degradation of the data quality can be evaluated by comparing the quality of the data in the sets of Partial coverage data processed with the quality of the data in the complete coverage data sets processed. The degradation of the data quality in the seismic datasets processed is usually estimated by observing the levels of the aberrations caused by the test coverage holes. Typical aberrations include, but are not limited to, changes in event time, decreases in event amplitude, phase distortion, and increases in migration noise as represented, for example, with migration curves. In another modality the data removed from the full coverage data set to model a test coverage hole in the full coverage data set, can be processed, and the processed removed data is compared to the full coverage data set processed to evaluate the data degradation resulting from the coverage hole. The effect of the test coverage holes on the quality of the data depends on the bandwidth (the frequency content) of the signals in the processed data. The frequency content usually depends on the depth and the image. The expected frequency content of the data that will be acquired will normally be taken into account when evaluating the effect of the data quality degradation on the partial coverage data sets with coverage holes, after the processing that was done in step 111. Figures 3 and 4 show examples of aberrations in processed seismic data. These aberrations are typical indications of the degradation of the quality of life data for the purposes of the test coverage holes. Figure 3 shows a cross section 31 of the synthetic data after the pre-stacked time migration. Figure 4 shows the same cross section 41 showing the effect of the cover hole, again after the pre-spaced time migration. A 75 meter coverage hole is simulated by extending beyond the migration opening in the in-line direction. A time deviation of 2.5 msec is shown at position 42 of Figure 4, compared to the corresponding position 32 of Figure 3. A smaller seismic amplitude can be seen at position 43 of Figure 4, compared to the corresponding position 33 in Figure 3. An example of migration curve, which is not present in Figure 3, can be seen at position 44 of Figure 4. In step 113, the maximum acceptable sizes of the Coverage holes for the proposed exploration with marine seismic cable. The maximum acceptable sizes are determined by analyzing the evaluations that are made in step 112 of the degradation of the data quality due to the effects of the test coverage holes. The maximum acceptable sizes of the cover holes normally depend on the objectives of the exploration and the geology of the exploration area. In this way, the maximum acceptable sizes determined at this point can then be expressed as fill specifications for the proposed scan. For example, these filler specifications can express the maximum acceptable sizes of coverage holes as a function of displacements and depths. The maximum acceptable sizes of the coverage holes will also vary with the required degree of coverage. Full coverage means that coverage is as planned for a particular configuration of a scouting vessel, with no coverage holes. Zero coverage means no data in a specific size or area. Degree of coverage, normally represented as a percentage of coverage, corresponds to the fraction of coverage between zero coverage and full coverage. In step 114 it is determined if an improved acquisition coverage is needed during the proposed exploration with marine seismic cable. If additional coverage is needed, then coverage can be improved, for example, by reducing the separation between the navigation lines of the seismic vessel towing the seismic cables. Alternatively, additional fill lines can be planned. However, this invention is not intended to be limited by these examples, and would include other conventional means of improving coverage, which are well known in the geophysical research art. Thus, the method of the invention makes possible an a priori determination of the filling specifications for a proposed exploration with marine seismic cable. By comparing the degradation of data quality due to the test coverage holes in full coverage and partial coverage seismic data sets, maximum acceptable coverage holes can be determined for the exploration before the acquisition begins. Corrections to fill specifications, if necessary, can be made before the exploration, thus reducing or avoiding the high cost of the fill shot after the acquisition. It should be understood that the above description is simply a detailed description of the specific embodiments of this invention, and that numerous changes, modifications and alternatives can be made to the described embodiments, in accordance with the present disclosure without departing from the scope of the invention. The foregoing description, therefore, is not intended to limit the scope of the invention, rather the scope of the invention will be determined solely by the appended claims and their equivalents.

Claims (21)

NOVELTY OF THE INVENTION CLAIMS

1. - A method for determining a sufficient acquisition coverage for a marine seismic wire exploration, comprising: constructing seismic data sets, representative of the exploration, with test coverage holes; process the seismic data sets with test coverage holes; evaluate the degradation of the data quality due to the test coverage holes; and determining the maximum acceptable coverage hole for the exploration from the evaluation of the degradation of the quality of the data.

2. The method according to claim 1, further characterized in that it also comprises: constructing a set of seismic data of complete coverage representative of the exploration, where the seismic data sets with test coverage holes are constructed from said complete coverage seismic data set representative of the exploration; and process the full coverage seismic data set, where the evaluation of data quality degradation due to the coverage holes, involves comparing the quality of the data of the complete coverage seismic data set processed with the quality of data from seismic data sets with test coverage holes.

3. - The method according to claim 1, further characterized in that said sets of seismic data with test coverage holes are constructed for the test coverage holes of a selected size scale, for selected test displacements and for test depths selected.

4. The method according to claim 3, further characterized in that the size scale of the test cover holes is selected in such a way as to cover the anticipated cover holes in the scan.

5. The method according to claim 3, further characterized in that the test offsets comprise test shift ranges.

6. The method according to claim 3, further characterized in that the test depths comprise test depth scales.

7. The method according to claim 2, further characterized in that the full coverage seismic data set comprises synthetic seismic data.

8. The method according to claim 2, further characterized in that the complete coverage seismic data set comprises real seismic data.

9. - The method according to claim 2, further characterized in that the complete coverage seismic data set comprises a combination of synthetic and real seismic data.

10. The method according to claim 7, further characterized in that the construction of said complete coverage synthetic seismic data set comprises: constructing a velocity model for the exploration area; determine a configuration of the exploration vessel for exploration; and using said velocity model and said configuration of the exploration vessel to construct said complete coverage seismic data set.

11. The method according to claim 10, further characterized in that the speed model comprises a set of speed models.

12. The method according to claim 10, further characterized in that the construction of said sets of seismic data with test coverage holes, comprises: removing data from the complete coverage data set to model extending test coverage holes the size scale selected.

13. The method according to claim 8, further characterized in that the construction of said complete coverage seismic data set comprises: selecting available real data to construct a complete coverage seismic data set.

14. - The method according to claim 13, further characterized in that the construction of seismic data sets with test coverage holes comprises: removing data from the full coverage data set to model test coverage holes extending the scale of selected size.

15. The method according to claim 1, further characterized in that the processing of the seismic data sets comprises a pre-stacked migration.

16. The method according to claim 12, further characterized in that the evaluation of the degradation of the data quality that results from the test coverage holes, also comprises: processing the data removed from the full coverage data set to model a test coverage hole; and compare the quality of data in the processed data with the data quality in the seismic data set of complete coverage processed.

17. The method according to claim 14, further characterized in that the evaluation of the data quality degradation due to the test coverage holes, also comprises: processing the data removed from the full coverage data set for modeling a test coverage hole; and compare the quality of data in the processed data with the data quality in the seismic data set of complete coverage processed.

18. - The method according to claim 2, further characterized in that the evaluation of the data quality degradation that results from the test coverage holes, also comprises: measuring the aberrations in the processed seismic data sets caused by the holes of test coverage.

19. The method according to claim 18, further characterized in that the measured aberrations are taken from a set that includes the decrease in amplitude, time changes, phase distortion, and noise induced migration curves.

20. The method according to claim 18, further characterized in that the measured aberrations are based on the expected frequency content of the processed seismic data sets.

21. The method according to claim 1, further characterized in that it also comprises: determining fill specifications from the determined acceptable maximum of cover holes.