FR2765344A1 - METHOD FOR DEVELOPING A COMPOSITE BLOCK FROM SEISMIC RECORDING BLOCKS - Google Patents

Abstract

The invention concerns a method for producing a composite block from seismic recording blocks, characterised in that it consists in: producing independent primary seismic blocks, each seismic block being constructed from seismic data registered along a predetermined acquisition direction, so as to obtain n primary seismic blocks {Ai(x, y, z)} with i varying from 1 to n, n being greater or equal to 2; computing for each point of each primary block Ai a quality criterion value representing the seismic attribute local quality, so as to obtain n quality criterion blocks {Qi(x, y, z)} with i varying from 1 to n, each quality criterion block Qi being associated with the primary block Ai; constructing a composite seismic block C(x, y, z) whereof each sample is computed as a combination of sample values of blocks Ai(x, y, z) weighted by their relative qualities Qi(x, y, z).

Description

Method for making a composite block from blocks
of seismic recordings
The present invention relates to a method for developing a composite block from seismic blocks, obtained by means of seismic data recorded in the form of traces in different acquisition directions.

 When we are in the presence of complex tectonics, as opposed to calm tectonics in which the horizons or reflectors of the subsoil to be explored are tabular layers, slightly deformed, the traditional methods are not always sufficient and precise, especially when the detected events are faults, ruptures or salt domes for example. Indeed, the seismic imagery of the subsoil is strongly disturbed by optical effects, well known to specialists, which are induced by the presence of said faults and / or saliferous, clayey or other intrusions. These effects of optics sometimes obscure certain parts of the basement which are then little and sometimes not even visible at all on the seismic image.

The consequences of these optical effects on said image can vary greatly depending on the geographic layout of the seismic transmission and recording device used. In 3D marine seismic acquisition, the layout is essentially characterized by the acquisition azimuth.

 When the morphology of the subsoil is partially known before acquisition, it is sometimes possible to choose a preferred direction of acquisition which gives a sufficiently complete seismic image.

Unfortunately, when said morphology is little known or when the occulting optical effects are numerous and varied, there is no single direction of acquisition which makes it possible to obtain a sufficiently complete image.

 The seismic image can be in the form of a seismic block migrated time or depth in which the interpreter locates and interprets seismic events appearing in said seismic block.

In reflection seismic, a seismic event is essentially characterized by an extremum of amplitude of the seismic signal having good spatial continuity between neighboring traces,
To distinguish, automatically, the zones which present events from those which do not, one means among others consists in measuring the spatial coherence of the seismic using for example, a technique of intertrace correlation. The various methods and formulas for calculating consistency, well known to specialists, will neither be given nor explained.

 The aim of the present invention is to propose a method for obtaining a seismic image or representation which is more complete and more predictive for the interpreter, this method relating more particularly to the development of a composite block of an attribute. predetermined seismic.

The method according to the invention is characterized in that it consists in
- produce independent primary seismic blocks, each seismic block being constructed from seismic data recorded in a predetermined acquisition direction, so as to obtain n primary seismic blocks {As (x, y, z)} with i varying from 1 to n, n being greater than or equal to 2,
- calculate for each point of each primary block Ai the value of a quality criterion representative of the local quality of the seismic attribute, so as to obtain n blocks of quality criterion (Qi (x, yz)) with i variant from 1 to n, each block of quality criterion Qi being associated with the primary block Ai,
- construct a composite seismic block C (x, y, z) of which each sample is calculated as a combination of the values of the corresponding samples of the blocks A; (x, y, z) weighted by their relative qualities Qj (x, y, z ).

 An advantage of the present invention lies in the fact that it makes it possible to obtain in a secure manner a composite block of seismic data in which the occulting effects induced by the acquisition azimuth are greatly attenuated. The seismic information contained in this composite block is more complete than that obtained in any of the initial blocks. This composite block therefore allows a more precise and complete interpretation of the geology of the subsoil.

 It should be noted that the nature of the seismic data in the seismic blocks (Aj) can be arbitrary. The attribute can be constituted by the classic seismic amplitude, migrated time or depth, but also by any derived seismic attribute or by any transform of the seismic signal.

In addition, the method according to the invention makes it possible to construct, for example, two types of information useful for the interpretation of the composite block
- a block of index I (x, y, z) which gives, for each point (x, y, z), the index Ij of the seismic block (A1) which has, at this point, the best quality (Qi ),
- a block of attribute D (x, y, z), called here "directivity block", which gives for each point (x, y, z), the relative variations of the quality attributes Qj (x, y, z) different blocks A (x, y, z) for a value of the index
I between 1 and the number n of primary blocks The higher the quality values at a different point (x, y, z), the stronger the directivity D (x, y, z), the higher the quality values in one point are homogeneous and the weaker the directivity D.

To help with interpretation, these two blocks of index I and directivity D can be viewed on a screen or on any medium by associating a color tint with each block index and by constructing a color image in which the pixels representing a point (x, y, z) are colored with the hue associated with the index I (x, yz) and have a saturation in this hue all the stronger as the directivity value
D (x, y, z) is high.

 Another advantage of the present invention lies in the fact that in the composite blocks, homogeneous parts appear. In addition, in the same facies, the composite block according to the invention makes it possible to highlight sub-facies and tectonic accidents are detectable.

Other advantages and characteristics will appear more clearly on reading the description of a preferred embodiment of the invention, as well as the appended drawings in which
FIGS. 1 and 2 represent seismic images of the vertical sections of the same plane of the subsoil and extracted respectively from seismic blocks migrated time constructed with seismic data acquired in two perpendicular directions of acquisition.

 Figures 3 to 6 are cross-sections at constant time of four primary migrated time seismic blocks constructed with seismic data acquired according to four directions of acquisition arranged at 45 "from each other.

 FIGS. 7 to 10 are images representing, for each constant time section of FIGS. 3 to 6, the correlation attribute between neighboring traces and which is used as a criterion of local quality.

 FIG. 11 is a section at the same time of the composite block obtained according to the present invention.

 FIG. 12 is a representation of the preferential directivities according to each of the directions of acquisition.

 In reflection and seismic exploration of a medium, a transmission and reception device is generally used consisting of one or more emission sources which emit waves in the medium and by a certain number of receivers which receive and record, in particular function of time, the waves reflected by the various reflectors or horizons of the medium.

The positioning of the device, on the surface of the medium, depends on the type of cover that one wishes to achieve. In marine seismic, for example, a boat is used which generally includes an emission source which is either on the boat or on a support which is towed by the boat at the same time as seismic streamers on which receiver-recorders are mounted. The boat generally moves in a predetermined azimuthal direction by covering the surface to be explored by parallel lines along which said receiver-recorders are aligned.

According to the present invention and in the example of marine seismic, although it can be implemented in terrestrial seismic, it is preferable to implement the recording technique called 4X2D or 4X3D as described in the application FR -A-2 729 766. In the aforementioned request which is integrated here both with regard to the acquisition technique and for the processing of the data in particular for the calculation of the unit speeds and / or for the development of the summation umbrella , a method is described in which the directions of data acquisition are 45 "from each other, that is to say along directions XX,
XY, YY, YX.

 FIG. 1 is a vertical section or seismic image of a plan of the subsoil, said seismic image being obtained from a seismic block migrated in time constructed from seismic data acquired according to a first azimuth or acquisition direction XX of 00 .

 Figure 2 is also a vertical section or seismic image of the same plane of the basement of Figure l, this seismic image being from a seismic block migrated time constructed from seismic data acquired according to a second azimuth or direction of acquisition YY of 90, therefore perpendicular to the first azimuth.

When comparing Figures 1 and 2, there are significant differences between the two seismic images represented in the rectangles
R1 and R and mainly due to the occulting effects of a saliferous intrusion.

These blackout effects have much greater consequences in the YY block (rectangle R2) than in the XX block (rectangle R1). Note that the horizons in rectangle R1 are more marked than in rectangle R2.

 Figures 3 to 6 represent four sections at constant time (t = 2748 ms) and coming from four primary and migrated seismic blocks time A1 (x, y, z) to A4 (x, y, z) obtained according to four directions of acquisition arranged at 45 "from each other and referenced XX, XY, YY and YX. The four sections represent the same horizontal plane of the basement.

 In another step, the local correlation between neighboring traces is calculated for each point (x, y, z) of each block A1 to A4. This correlation locally estimates the spatial coherence of the seismic information which constitutes the example of quality criterion currently chosen. Four blocks of quality criterion Q1 (x, y, z), Q2 (x, y, z), Q3 (x, y, z) and Q4 (x, y, z) are therefore produced. Figures 7 to 10 are images representing for each constant time section of Figures 3 to 6. the correlation coefficient between neighboring traces and which has been chosen as a quality criterion.

 When comparing the images of Figures 7 to 10 with each other, we can note very significant differences. In the image of FIG. 3, the right part is highly disturbed, which results in a relatively low correlation coefficient for this same zone in FIG. 7. A similar result is detected in FIGS. 5 and 10 but in the part left and bottom. Figures 4 and 6 show that some parts are disturbed due for example to a more unfavorable signal / noise ratio.

 In another step, a composite block C (x, y, z) is produced, each sample e (x, y, z) of which is calculated as a combination of the values es (x, y, z) of the blocks A1 (x, y, z) balanced by their relative qualities Qi (x, Y, z), while bearing in mind that the combination rule may vary.

 In the example chosen, the composite block C (x, y, z) is constructed by selecting any point M with coordinates (x, y, z), then we search in the set of blocks Qi for the block which presents, for the point with the same coordinates, the best quality criterion which, in the example of the present invention, corresponds to the highest value. The point M is then assigned the amplitude of the corresponding point in the primary block Ai which is associated with the block Qi selected as having the best quality criterion.

The previous steps are repeated for all the points of the composite block for which there is seismic information.

 FIG. 11 represents the constant time section extracted from the composite block C (x, y, z) for the time t = 2748 ms.

 A block of quality index I (x, y, z) can also be produced by assigning to each point of a blank block the index of the primary block As corresponding to the best value of the quality criterion Qi for the point considered .

 The present invention also relates to the construction of a directivity block D (x, y, z) in which D (x, y, z) is calculated at each point by the formula (IQ / Qmax) where Qmax is the maximum value of Qi (x, y, z), i varying from 1 to n, and Q the average of the (nl) other values Qi (x, y, z).

The closer the quality values Qi (x, y, z) for the four blocks Q1 (x, y, z), the lower the directivity D (x, y, z).

 According to another variant of the method, it is possible to view the information of the blocks XX, XY, YX and YY with different colors, the color associated with a point indicating which block has, at this point, the best correlation. The saturation in color is all the stronger as the directivity D (x, y, z) at this point is strong. A very pastel or even white dot indicates a very weak directivity.

In FIG. 12, the preferential directional zones are represented schematically according to each of the four directions of acquisition
XX, XY, YY and YX instead of representing them with different colors.

The zone which, in color, would be pastel or even white is represented in FIG. 12 with very dense dotted lines, the latter indicating that the corresponding zone is of weak directivity.

Claims (9)

1. Method for developing a composite block of a predetermined seismic attribute, characterized in that it consists of  - produce independent primary seismic blocks, each seismic block being constructed from seismic data recorded in a predetermined acquisition direction, so as to obtain n primary seismic blocks (Ai (x, y, z)} with i varying from 1 to n, n being greater than or equal to 2,  - calculate for each point of each primary block As the value of a quality criterion representative of the local quality of the seismic attribute, so as to obtain n blocks of quality criterion (Qi (x, y, z)) with i varying from 1 to n, each block of quality criterion Qj being associated with the primary block Aj,  1s  - construct a composite seismic block C (x, y, z) each sample of which is calculated as a combination of the values of the corresponding samples of the blocks A1 (x, y, z) weighted by their relative qualities Qj (x, y, z) . 2. Method according to claim 1, characterized in that the seismic data are recorded according to four directions of acquisition arranged at 45 "from each other. 3. Method according to claim 1, characterized in that the seismic attribute is the amplitude. 4. Method according to claim 1, characterized in that the quality criterion is consistency. 5. Method according to claim 1, characterized in that it also consists in constructing a block of index I (x, y, z) by assigning to each point of a blank block the index of the corresponding primary block Aj to the value of the quality criterion Qj for the point considered. 6. Method according to claim 1 characterized in that it also consists in constructing a block of directivity D (x, y, z) in which the directivity D (x, y, z) is calculated at each point by the formula (1-Q / Qmax) where Qmax is the maximum value of Qj (x, y, z), i varying from 1 to n, and Q the average of (nl) other values Q (x, y, z). 7. Method according to claim 6, characterized in that the directivities are displayed on a support with a color code, the color saturation at a point being all the stronger as the directivity is high at said point. 8. Method according to any one of the preceding claims, characterized in that the primary blocks Aj are time migrated blocks. 9. Method according to one of claims 1 to 7, characterized in that the primary blocks Aj are blocks migrated depth.