MXPA98002292A - Seismic reflection data acquisition and processing method and device for prospecting in tectonically complex environments - Google Patents

Abstract

A seismic reflection data acquisition and processing method and device for prospecting in tectonically complex environments are disclosed. In particular, the seismic reflection data acquisition and processing method is useful for providing a summation rate tensor field and a 3D image unit, e.g. in 3D earth seismics or complex tectonics.

Description

DEVICE AND METHOD OF OBTAINING AND TREATMENT OF SEISMIC DATA OF REBOUND FOR THE EXPLORATION OF A MEDIUM TECTONIC COMPLEX The present invention relates to a device and method for obtaining and processing seismic data for the exploration of a complex tectonic medium. The multiple cover is a technique for obtaining bounce seismic data, in which the emission sources and the receivers are arranged on the surface of the medium to be explored. The records, in the form of dashes, are grouped in such a way that the same middle point regroups several registers. The series of records associated with the same midpoint form what can conveniently be called a collection of common midpoint strokes. To obtain such trace collections, it is convenient to distribute the emission sources or emitters and receivers on the surface of the medium following a predetermined geometric organization. In general, you can define two categories of devices: 2D or 3D. In a 2D device, the transmitters and receivers are displayed theoretically in line on the surface of the medium, in order to provide a distribution of common midpoints (PMC) linear, or treated as such, with a single horizontal coordinate for each PMC . In the 3D device, the transmitters and receivers are arranged in order to provide a distribution at the surface level of the midpoints, or treated as such, with two horizontal coordinates for each PMC. In FR 2 729 766 of January 23, 1995, which is integrated in the present application with regard to the treatment of the characteristics or parameters of the medium to be explored, a seismic data treatment method specially adapted to the study is described. of complex tectonic means and that allows obtaining parameters that characterize at least the field of velocities of addition, associated with elements of rebound. The method described in FR 2 729 766 proposes, in particular, to record four 2D seismic of the serial type, analogous to the conventional nautical 3D, and oriented in four directions, two successive directions any one forming an angle approximately equal to 45 °. An important advantage of this method lies in the fact that a three-dimensional procurement can be performed with 2D acquisitions for media that can only be explored in three dimensions. The main object of the present invention is a method for obtaining and processing rebound seismic data that allows to obtain a stress field of summation velocities and a 3D image block, for example in terrestrial 3D seismic and complex tectonic. The method according to the invention is of the type in which at least one emitter of elastic waves and receivers of rebound waves is used in at least one reflector of the medium, the rebound waves being recorded in the form of seismic traces, and characterized in that: a) a number of sufficiently large seismic traces is generated following a first predetermined direction to have in said first direction, a first dense distribution of the midpoints (PM) of emitter-receiver pairs that have produced said traces seismic, and is conformed for each of the PM distributed in the first direction, a collection of traces in PMC that regroup all the traces produced by the emitter-receiver pairs that this one sensibly aligned with the first direction, the midpoint of each pair being confused with the PMC of the trace collection, b) a second dense distribution of PM is generated in the same way following a second direction perpe ndicular with respect to the first address and conforms for each of the PMs distributed in the second direction, a collection of traces in PMC that regroup all the traces produced by the sender-receiver pairs that this substantially aligned with the second direction, the average of each pair being confused with the PMC of the trace collection, in such a way that series of PMC lines are obtained parallel to the first and second directions that constitute an analysis grid whose intersections are analysis nodes, the number of seismic traces in each trace collection in PMC being sufficient to perform an analysis of curvature of indicators. According to another characteristic of the invention, a third dense distribution of PM is generated in a third direction and a collection of traces in PMC is grouped for each of the PMs distributed in the third direction, grouping together all the traces produced by the emitter-receiver pairs that are substantially aligned with the third direction, the middle of each pair being confused with the PMC of the trace collection, the number of seismic traces in each stroke collection in PMC being sufficient to perform an analysis of curvature of indicators. According to another characteristic of the invention, a fourth dense distribution of PM is generated in a fourth direction and a collection of traces in PMC is grouped for each of the PMs distributed in the fourth direction, grouping together all the traces produced by the emitter-receiver pairs that are substantially aligned with the fourth direction, the mean of each pair being confused with the PMC of the trace collection, the number of seismic traces in each trace collection in PMC being sufficient to perform a curvature analysis of indicators. According to another characteristic of the invention, each of the third and fourth directions form an angle of approximately 45 ° with each of the first and second directions.

According to another characteristic of the invention, the analysis grid constitutes a support for a device for obtaining bounce seismic data. According to another feature of the invention, a 3D cover is produced at dense surface level simultaneously with the constitution of the analysis grid and in which the PMs distributed following each of said directions are selected from those obtained for the 3D cover. According to another feature of the invention, the analysis grid is used with a 3D cover previously made. According to another characteristic of the invention, for each PMC corresponding to an analysis node, a value of the curvature of the indicators in each direction is determined, the values thus obtained being used to determine the components of the velocity field at time t0 associated with said PMC and for a given bounce element of the medium, t0 being the vertical propagation time with bounce for zero inhibition between the emitter and the receiver. According to another characteristic of the invention, from the compounds of the velocity field associated with a reflector, the parameters are determined such as: ®, t, pmin and tpmax characterizing the geometry of said rebound element,? representing the angle forming the first direction with a reference direction, pmi-n and tpmax designating respectively the values of the weakest dip and the strongest of the rebound element and t designating the transit time. According to another characteristic of the invention, the meshes of the analysis grid are regular. According to another characteristic of the invention, the meshes of the analysis grid are square. According to another characteristic of the invention, the analysis meshes have the form of parallelograms. According to another characteristic of the invention, the device for applying the method according to the invention is characterized in that it comprises at least two lines of receivers arranged in the analysis grid 51 and at least one emitter, the receivers being activated depending on the type of cover to be made, said emitter being, at each shot, in or near one of the receiver lines. An advantage of the present invention resides in the fact that a single seismic data acquisition bell is sufficient to provide the velocity data following a 2x2D, 3x2D or 4x2D analysis grid and simultaneously a 3D cover. Instead of performing the 3D cover simultaneously with the realization of the analysis grid, the 3D seismic data of a previous extraction campaign made for the medium for which the analysis grid is made can be used. Other features and advantages will become apparent upon reading the description of the method according to the invention, as well as the accompanying drawings on which: Figure 1 is a schematic representation of a 2x2D analysis grid, Figure 2 is a schematic representation of a 3x2D analysis grid, Figure 3 is a schematic representation of a 4x2D analysis grid, Figure 4 is an example of a 2x2D and lx3D cover map. Fig. 5 is a schematic representation of the analysis grid and of the transmitting and receiving device according to the invention, Fig. 6 is a schematic and partial representation of the device for a cover of the type A, Fig. 7 is a schematic representation and part of the device for a cover of the type C, figure 8 is a schematic and partial representation for making a combination of covers A and C. A device called seismic data acquisition is arranged on the surface of the medium to be explored, said surface for example, the terrestrial soil or the seabed (figures 5 to 7). The obtaining device comprises at least one emitter that emits elastic waves in the medium to be explored and the receivers that receive the rebound waves in one or more reflectors. Rebound waves are recorded in the form of solid lines. The method according to the invention consists in establishing an analysis grid that allows, above all, the determination of the voltage velocity field in said medium.

For this purpose, a number of sufficiently large seismic traces is generated by following a first predetermined azimuth direction 1 or forming an angle * with a reference direction. The number of seismic traces generated in the first direction 1 must be sufficient to obtain a dense distribution of midpoints (PM) in said first direction 1. A midpoint or PM is the middle site of the distance separating an emitter from a receiver of a given transmitter-receiver pair. In another step, for each of the PMs distributed in said first address 1, a collection of traces in common midpoints (PMC) is constituted and which regroups all the traces produced by the emitter-receiver pairs that are substantially aligned with the first address 1 and that are centered in the PMC, that is to say, whose midpoint (PM) is centered in the common midpoint (PMC) of said collection of strokes under consideration. In another step and preferably simultaneously with the previous steps, a number of seismic traces sufficient to have a dense distribution of PM in a second direction 2 that is perpendicular to the first direction 1 is generated equally and in the same way. also constitutes for each of the PMs distributed following the second direction 2, a collection of traces in PMC that groups together all the traces produced by the pairs receiver-receiver that are substantially aligned with the second direction D2 and centered on the PMC of the Collection of strokes in consideration. Series of lines of PM 11 are made parallel to address 1 and series of lines of PM 21 parallel to address 2. The two series of lines 11 and 21, called analysis lines, constitute an analysis grid 5 whose meshes 3 have regular shape, for example in the form of a diamond, as shown in figure 1. The nodes 4 of the meshes constitute the analysis nodes for the velocity field. The analysis grid 5 is called the 2x2D analysis grid. According to a variant of the method, as shown in FIG. 2, an analysis grid 6 is created, called the 3x2D analysis grid, deriving from the analysis grid 5 when a third direction 7 is added, which preferably forms an angle of approximately 45 ° with each of the directions 1 and 2. In the same way, the PM lines 71 are made which are parallel to the third direction 7. The lines 71 intersect the lines 11 and 21 in the points that are already points intersection of lines 11 and 21. Such points of triple intersection 8 are analysis and sum knots for grid 6 while double points of intersection 9 (only between a line 11 and a line 21) are only addition nodes . Likewise, in each of the lines 71, in the same way as for the lines 11 and 21, a number of traces sufficient to obtain a dense distribution of PM following the lines 71 is generated and is also constituted for each of the PM distributed along lines 71, a collection of traces in PMC that brings together all the traces produced by the emitter-receiver pairs that are substantially aligned with the third direction 7 and centered in the PMC of the collection of traces under consideration. In another variant shown in FIG. 3, an analysis grid 10 is formed for the 4x2D, deriving from the grid 6 when adding a fourth direction 12, which preferably forms an angle of approximately 45 ° with each of the directions 1 , 2 and 7. In the same way, the PM lines 121 are made which are parallel to the fourth direction 12. The lines 121 are substantially perpendicular to the lines 71 and pass through the triple intersection points of the lines 11, 21 and 71, which are then converted into quadruple intersection points 13. Intersection points 13 are analysis and summation nodes for grid 10, the other points of intersection 14 not quadruples being addition nodes. The density of the analysis knots for each of the grids 5, 6 and 10 is selected to provide a regular spatial sampler on the surface for the obtaining device used and adapted to the analysis grid that serves as support. The regular spatial sampler of the surface is adapted to the spatial fluctuations of the velocity field near this surface. The density of the PMC along the lines of analysis is selected to collect samples correctly from the most inclined reflectors of the medium or from the diffraction present in the medium, for the analysis of curvature. Each PMC of the analysis lines regroups a sufficient number of traces whose azimuth, that of the associated emitter-receiver pair, is close to that of the direction under consideration, to carry out a curve analysis of the indicators in that azimuth. The term "substantially aligned" previously used means that this corresponds to the angular tolerance of the analysis for which a deviation of an order of magnitude of approximately 15 ° in relation to the collinearity of the transmitting-receiving pairs and the analysis lines can be be used In the method according to the invention, the 2x2D, 3x2D or 4x2D cover is associated with a 3D cover for the medium under consideration. The 3D cover can be made simultaneously with the constitution of one of the analysis grids and has a dense surface level distribution. The PM distributed following the directions of the grid used are selected from those obtained for the 3D cover. In the same way it is possible to associate the 2x2D, 3x2D or 4x2D cover to a 3D seismic cover previously made for the medium under consideration. Figure 4 represents an example of a cover map for the analysis grid 5 shown in Figure 1. Along the lines of analysis 11 and 21, there is a large number of MW of the order of 160, compared to the number ( 32) of PM that are located in the non-analytical lines. The data are composed of two parts di s t int a s: a) those that are registered with common midpoints along the lines of analysis; and b) those that are registered with the common midpoints outside the lines of analysis. In a first time, the lines whose common midpoints are located in the lines of analysis are extracted. They are classified in common midpoints as if those traces had been recorded in 2D seismic. The PMCs are extracted from these traces, each of which is a double, triple or quadruple analysis node according to the information recorded in the direction of the determined azimuth and in the orthogonal direction 2x2D or in the three directions (3x2D) or four directions at 45 °. In each analysis node and for each direction, a measure of parameters t0 and tp is obtained for each real measurement in the curvature analysis, this is how it is explained in FR 2 729 766.

In the case of the two directions, the hypothesis is posed that there is an exact orientation DIP and STRIKE. In that case, the two measured values of tp for the same t0 are the two extreme values tpmin and tpmax. In the case of four directions, the parameters a, t, tp? N, tpmax describing the shape of a sum umbrella are derived from the four measures of tp performed for the same t0 in each direction and by tensional inversion. local. The angle a represents the orientation of the large axis of the summing umbrella in relation to a reference axis. If an angle ® is defined in relation to the large axis of the summing umbrella, the value of tp (®) is given by the formula: min rr x _ _T, m? n; ~ v, ma.- • 2 r ~ \ l / Q-) - 1"* r p cos O + tp sm? j, sin + tp eos t) - r The azimuth F of the trace in relation to the reference axis is given by the formula: = a- T (2) For each plot recorded on the ground, defined by its azimuth F and its deviation, the parameter tp (®) is associated with the help of equations (1) and (2).

The correction of the obliquity & t that this stroke must suffer to be able to proceed by continuing the sum in phase of the different strokes of a binary that has different azimuths and deviations is given by the following formula: Thus, thanks to 4x2D associated with a 3D cover, a very efficient sum can be made on all the binaries while the sum parameters depend on the azimuth, which is the case in the complete tectonic j o. Between two diagonally opposite and consecutive analysis nodes, the measured tp are interpolated linearly perpendicular with respect to the direction of the registration lines. Four interpolations will be necessary to grid the terrain. Any midpoint located outside the diagonal lines will be supplied with four tp interpolated bilineally from the interpolated values along the diagonals of each respective square. It is calculated by continuing the parameters, t, tpmin, tpmax describing the shape of the local stack umbrella.

In the case of 2x2D, an estimate of the parameter t can not be made directly. Two solutions are therefore possible: the first solution is to resort to a third direction of oblique obtaining, of approximately 45 ° of one of the previous two (step to 3x2D) to obtain an independent measurement and calculate the parameter t. The second solution is, if the medium is not too complex, to do either an approximation, for example t = o, or to search for the optimal value of that parameter for the sum, for example by scanning values of t in each binary or group of 3D binaries. In the case of a single structural axis responding to the 3x2D hypothesis, the orientation of the two perpendicular directions of the analysis grid following the DIP and STRIKE axes is desirable, since the tension equations to be solved are simplified, but not it is necessary insofar as it is available, in each 3x2D analysis node, of three independent measurements of cover tp] _, tp2 and tp3 to solve the three unknown parameters (tpmi-n, tpmax and t). To obtain the 4x2D + 3D surface spatial sample adapted to the treatment as described below, it is possible to apply any devices for the same number of 2D and 3D covers obtained, so that the desired cover is made for the treatment. Advantageously, a basic device 2x2D + lx3D is used to obtain data, which, combined with other obtaining devices, makes it possible to obtain complete covers 1 x3D + 4x2D and 2x3D + 4x2D as well as the sub-covers of type 2x3D + 2x2D; 2x3D + 3x2D; lx3D + 2x2D; lx3D + 3x2D. For example, cover lx3D + 4x2D can be obtained by combining the cover 1X3D + 1X2D with the 3x2D cover or even with the cover lx3D with the cover 4x2 D. To make these covers, the two variants of the device of figures 6 and 7 can be used. A particularly interesting device for applying the method according to the invention is of the type described below. In the surface to be covered (figure 5) or more specifically in which it is desired to carry out the covers indicated below and which is represented by a so-called analysis grid 51 as in figure 1, there is an emission set and a reception set that constitute the device itself. The receiving assembly is constituted by a stride 52 that is composed of receivers located following the lines of the analysis grid and consists of a number of regular square meshes or in diamonds 53. Stride 52 is displaced after each emission or series of emissions, as clarified further to cover said grid On each side of a 53 mesh of the stride 52 is arranged an elementary thread 54 which is in fact a chain of receivers 55, for example to the number of four, distributed in a manner regular and separated between them at a distance e that is called intertrazo. The length 1 of each thread is less than that of the L side of the grid of the analysis grid 51 and is equal to Le with e / 2 at each end, such that the four consecutive threads arranged in cross and belonging to four Adjacent tranche meshes are separated from each other in the two perpendicular directions, at a distance equal to e. In other words, the threads are arranged in the perpendicular directions in such a way that a gap 56 is created at the junction of said directions and of square shape on the side e U2 / 2 and that no receiver is at the cusps of the meshes of the analysis grid 51. Thus, the ends of each elementary yarn 54 are separated from the ends of the mesh side of the analysis grid on which said yarn is arranged. The number of threads or the succession of threads of each receiver line depends essentially on the size of the step that you wish to move in the analysis grid. The emission set can take various forms depending on the nature of the source and the number of sources used in each shot. When a single vibrator is used as a source of emission, it is necessary to move it to each shot from one point to the other, but it is possible to use several vibrators that can be operated simultaneously or sequentially one after the other. When explosives are used as sources, they are available in the stride depending on the type of cover that you want to obtain. When you want to make a simple cover 2D, it is necessary to emit and receive in the same DI offset direction; that is, that the emitter (s) 57 are "collinear" with the receivers 55. The collinearity is obtained when the emitter (s) 57 is in or next to the chain of receivers 55, but it is nevertheless on the same line. The simple 3D cover is obtained with the receivers 55 'which are arranged following two directions forming an angle between them which is, in the selected example, a multiple of 45 °. In the embodiment shown in FIG. 6, the shot is made between two receivers 55 and substantially in the middle of the distance separating two consecutive receivers 55. In this case, there will be an emission to the center 56 'of the vacuum 56 created between the threads. When the receivers 55 are only activated, a simple 2D cover is obtained. When the receivers 55 and 55 'of the two perpendicular wires are activated, a 3D cover and a 2x2D cover are obtained. When the receivers 55 and 55 'of three zigzag wires are activated (arrows D2), then a cover lx3D + 3x2D is obtained. The emission being made to the center 56 'of the voids 56, what is called here cover A. is obtained. When the emission is made in vertical direction with respect to the receivers 55 or 55' (figure 7), that is to say when there is no emission to the center 56 'of the voids 56, is performed in the same way as above as to the 2D covers (colinear emitters and receivers), lx3D + 2x2D (emitters and receivers arranged in the perpendicular directions) and 1X3D + 3X2D (emitters and receivers). zigzag receivers). But this type without shooting at the center 56 'of the voids 56 is called cover C. Of course, it is also possible to combine the two types of covers A and C when shooting successively in a vertical direction with respect to the receivers of each elementary yarn 54 and the middle of the distance separating the two consecutive receivers 55 or 55 'of each elementary yarn, as well as the crosses 56' of the directions of said elementary yarns. An analysis node 80 of this combination is shown in FIG. 8, in which the black points 81 represent the midpoints, the crosses 82 represent the emitters and the squares 83 represent the receivers, the node 80 corresponds to the quadruple node of the Figure 3. The combination can be carried out respecting the geometric conditions indicated in figure 8. It can for example be binar a device type A, 3D cover equal to 8, with another type C, the same cover, but dilated a factor D2 and rotated at 45 ° in relation to a device of type A, so that under these conditions, in each binary, an average of 16 points-means can be obtained whose azimuths and offsets cover an area between minus of an intertrazo and the semidiagonal of the largest size of the stride. It should be noted that the interest of the method is to allow, for example, through the treatment, to apply an indicator curvature correction (known from the study of the 4x2D velocity field in the general case), allowing the addition in phase of all the azimuths and all the deviations, even in the case where that correction varies depending on the azimuth. It benefits in this case fully, even in the complex tectonic, with the redundancy of the bounced signal, while the incoherent signals and noise, coming from records whose sources and receivers are distributed on the surface and statistically aligned with each other , they are going to strongly decrease in average value by addition. As it is schematized in figures 6 and 7, each wire is provided with a transmitter 90 which is connected, by means of a wireless connection, for example radio, to the registration laboratory, in charge of receiving for each shot, the data of a certain number of threads, to organize them in the form of strides and to register them in a magnetic support, for example, for treatment according to the method of the invention. In order to avoid problems of crossing of reception wires between them and in the case where the vibrators are used as shown in figures 6 and 7, it is preferable to have two emission lines parallel and placed in both parts of the wire as along which the two emission lines are displaced, the space between each emission line and the wire being equal for example ae / 4. The application of the obtaining can be, for example, carried out in the following way: the reception and emission points of the analysis grid 51 are defined by means of a topographic survey, a stride 52 is arranged on a sufficient area of the analysis grid, so that all the desired reception is made, the length of the wires is emitted either in a straight line, or in a zigzag, covering the entire surface of the stride, recording at each emission point only the data of the receivers of the threads useful to obtain the 3D and / or 2D cover, - for a cover of type A, the emission will be made each time in longitudinally centered direction between two successive receivers and to the center of the voids between the wires (figure 6), - for a cover of type C, the emission will be made each time in the longitudinal vertical direction of each receiver or by both parts of each receiver (figure 7), - if the set of This is made up of more than one active thread, other emission sections can be extracted (in a zigzag or straight line) in order to complete the cover in the stride (figure 5). - once the appropriate 2D and 3D covers have been obtained in a stride, said stride will be moved on an adjacent part of the analysis grid, so that the continuity and homogeneity of the 3D cover are obtained by pursuing the emission. The device described below can also be used in the background cable type technique and called OBS (Ocean Bottom Survey). Fn • 'n,', e has inceptors of t ino <Compounds and / or hydrophones are not used, eg when unwinding the < , Bl. - ¡o and L) -. from the i. uques claveros. Rsi < 1 '> and '.'m connected to a r f-j > 'OR. T. This is done, for example, from the s' i 'l' i e < -. n a source ship equipped with a pipe i! CI i í P, To obtain the 2 x 2 D + 1 x 3 D corresponding to the type OC ^ eptooede as follows: we gave $ p, G to the alida of a sufficient number of cables and 'oi The registration numbers to cover a "?" X? D tri-lete (full 3D cover obtained by the i? R? L? Oe said tranche), will be unwound from the cable network in In the direction of the river, to the north, to define the first day of the road in the tunnel, the same cable line is developed in an erpendicular direction, for example I did it in the liaison, in order to obtain intersections < at irections and in the middle of two receivers - > - > or < used for each of them. One then obtains, in the case of reception, which is used for a device of Unit A or C, the emission can immediately be made, for example, first in a series of lines navigated vertically of each of the lines north, then in another series of vertical emissions of the west lines, in one or the other direction, when setting the positions of the emissions between the two reception points (type A) or horizontally of those points (type C) , - it is possible, for example, to record the data of each one of the stride receivers for each shot, then reclassify the records in order to reconstitute the desired 2D and 3D covers desired in the stride. It is sufficient to move the stride immediately over a suitable distance, for example first towards the north then towards the west, to gradually cover the set of the desired surface in both the 2x2D and 3D, in a homogeneous way, for the meeting of data, for example with the complementary campaign 2x2D + lx3D and their joint treatment.

Claims (22)

1. Method of treatment of rebound seismic data for the exploration of a complex tectonic medium, of the type in which at least one emitter of elastic waves and rebound wave receptors is used in at least one reflector of the medium, The waves of bounce being recorded in the form of seismic traces; characterized in that: a) a sufficiently large number of seismic traces is generated following a first predetermined direction (1) to have in said first direction, a first dense distribution of midpoints (PM) of emitter-receiver pairs having produced said seismic traces, and it is constituted for each of the PM distributed in the first direction, a collection of traces in PMC that regroup all the traces produced by the sender-receiver pairs that are substantially aligned with the first direction (1), the midpoint of each pair being confused with the PMC of the trace collection, b) a second dense distribution of PM is also generated following a second direction (2) perpendicular to said first direction and is constituted for each of the PM distributed in the second direction, a collection of traces in PMC that regroup all the traces produced by the sender-receiver pairs that are sensibly alienated with the second direction, the mean of each pair being confused with the PMC of the trace collection, so that series of PMC lines are obtained parallel to the first and second directions that constitute an analysis grid (5) whose intersections are analysis knots, the number of seismic traces in each collection of PMC traces being sufficient to perform a curve analysis of indicators.

2. Method according to claim 1, characterized in that a third dense distribution of PM is also generated following a third direction (7) and is constituted for each of the PM distributed in the third direction, a collection of traces in PMC that regroup all the traces produced by the emitter-receiver pairs that are substantially aligned with the third direction, the mean of each pair being confused with the PMC of the trace collection, the number of seismic traces in each trace collection in PMC is sufficient to perform an analysis of curvature of rests.

3. Method according to claim 1, characterized in that a fourth dense distribution of PM is also generated following a fourth direction (12) and is constituted for each of the PM distributed in the fourth direction, a collection of traces in PMC that regroup all the traces produced by the emitter-receiver pairs that are substantially aligned with the fourth direction, the mean of each pair being confused with the PMC of the trace collection, the number of seismic traces in each trace collection in PMC being sufficient to perform an analysis of curvature of s.

4. Method according to claim 2 or 3, characterized in that each of the third and fourth directions (7, 12) forms an angle of approximately 45 ° with each of the first and second directions (1,2).

5. Method according to one of claims 1 to 4, characterized in that the analysis grid (5, 6, 10) constitutes a support for a device for obtaining bounce seismic data.

6. Method according to one of claims 1 to 4, characterized in that a 3D covering is made at a dense surface level simultaneously with the constitution of the analysis grid (5, 6, 10) and because the PMs distributed following each of said directions they are selected from those obtained for the 3D cover.

7. Method according to one of the rei indications 1 to 5, characterized in that the analysis grid (5, 6, 10) is used with a 3D cover previously and fec tuada.

8. Method according to one of claims 1 to 7, characterized in that for each PMC corresponding to an analysis node, a value of the curvature of the indicators in each direction is determined, the values thus obtained being used for the determination of compounds of the field of velocities at the time t0 associated with said PMC and for a given bounce element of the medium, t0 being the vertical propagation time with bounce for a zero deviation between the emitter and the receiver.

9. The method according to claim 8, characterized in that, from the compounds of the velocity field associated with a reflector, parameters such as: & , t, tpmin, tpmax characterizing the geometry of said rebound element,? representing the angle forming the first direction with a reference direction, t min and ^ max respectively designating the values of the weakest dip and the strongest of said rebound element and t designating the transit time.

10. Method according to one of claims 1 to 7, characterized in that the meshes of the analysis grid are regular.

11. Method according to claim 10, characterized in that the meshes of the analysis grid are square.

12. Method according to claim 10, characterized in that the analysis meshes are in the form of a parallelogram.

13. Device for applying the method according to the indications 1 to 12, characterized in that it comprises at least two lines of receivers (55, 55 ') arranged in the analysis grid (51) and at least one emitter (57), receivers being activated depending on the type of cover to be made, said emitter (57) being, at each shot, on or near one of the lines of the receivers (55 or 55 ').

14. Device according to claim 13, characterized in that the emitter (57) is at each shot, placed vertically with respect to a receiver (55, 55 ').

15. Device according to claim 13, characterized in that the emitter (57) is, at each shot, placed in the middle of the distance separating the two consecutive receivers (55, 55 ').

16. Device according to one of claims 13 to 15, characterized in that the two lines of receivers are perpendicular.

17. Device according to one of claims 13 to 16, characterized in that each line of receivers is constituted by a succession of elementary threads (54), each elementary thread being arranged on one side of a mesh of the analysis grid and comprising a determined number of receivers (55, 55 '), the length (1) of each elementary thread being less than the side (L) of said malia.

18. Device according to claim 17, characterized in that the ends of each elementary yarn are separated from the ends of the side of the mesh in which the yarn is arranged.

19. Device according to claims 15 and 18, characterized in that the shots are made between the consecutive receivers of the threads and the intersections of the directions of the eteen threads.

20. Device according to claims 14 and 18, characterized in that the shots are made in the vertical direction of the receivers of each of the threads.

21. Device according to claims 19 and 20, characterized in that the shots are made successively in a vertical direction with respect to the receivers of each elementary thread and in the middle of the 1st distance separating the two consecutive receivers of each elementary thread, as well as the crosses of the directions of the hi. the elementals

22. Device according to one of the I5 rei indications 13 to 19, characterized in that a set of elementary threads constitutes a stride (52) that is displaced in the analysis grid (51).