FR2648562A1 - Method and system for determining the azimuthal anisotropy in a domain of a physical medium - Google Patents

Abstract

The invention relates to a method and system for determining the azimuthal anisotropy in a physical medium. With the physical medium having at least one substantially plane separation (splitting) surface S, the method consists in arranging on the surface S a plurality of transmitter/receiver pairs Ei, Ri of a pressure wave directed towards the physical medium to be studied. Each pair Ei, Ri is arranged along a direction alpha i which is determined with respect to a reference N around a common centre CK. A pressure wave Pi is generated by each transmitter Ei and detected by the associated receivers Ri in a propagation-time window defining an elementary domain of the domain D. The preceding step is repeated for each transmitter/receiver pair Ei, Ri. The fluctuations in amplitude and in propagation time, as a function of the orientation alpha i make it possible to establish a propagation variation law for the reflected pressure waves or echoes as a function of the corresponding azimuth.

Description

 The invention relates to a method and a system for determining azimuth anisotropy in a domain of a material medium.

 The study of isotropy of material environments is currently of increasing interest.

 In the technical field of geophysical prospecting or mining, it is particularly interesting to be able to determine the parameters of anisotropy of environments such as the underlying geological layers on the surface of the Earth's crust.

 Recent work has been carried out, in France in particular, to establish a methodology for determining the anisotropy characteristics of material media from transverse shear waves, S waves or "shearwaves" in Anglo-Saxon language.

The proposed methodologies allow for a given material medium to determine the transfer matrices of the medium to the aforementioned shear waves.

 For a more detailed description of the proposed methodology, reference may be made to French patent application No. 2,599,508 published on behalf of the applicant on December 4, 1987. The above-mentioned methodology is entirely satisfactory. of sources generating shear waves and receivers of these waves as a function of their direction of polarization, for example tri-axis omnidirectional geophones Finally, the aforementioned methodology is above all advantageous in well seismic for the determination of the transfer function of a given area of land.

 The present invention aims to implement a method and a system for determining the azimuth anisotropy of a material medium to. from pressure waves, so longitudinal.

 Another object of the present invention is the implementation of a method and a system for determining the azimuth anisotropy of a material medium that can be implemented in surface earth seismic from survey equipment. acquisition profiles in three-dimensional seismic for example.

 Another object of the present invention is finally the implementation of a method and a system for determining the azimuth anisotropy of a material medium capable of allowing the establishment of a three-dimensional map of anisotropy by a tomography process.

 The method for determining the azimuth anisotropy in a domain of a material medium having at least one substantially planar separation surface, object of the present invention, is remarkable in that it consists, for an acquisition for an elementary domain, disposing on the separation surface a plurality of transmitter-receiver pairs of a pressure wave directed towards the material medium to be studied. Each transceiver pair is arranged in a determined direction with respect to a reference direction around a common center and a transmitter is shifted by a corresponding receiver by an offset distance. A pressure wave is generated from a transmitter and the reflected echoes or pressure waves are recorded at the receiver associated therewith in a propagation time window defining the elementary domain. The previous step of transmitting and recording is repeated for each of the transceiver pairs for the considered elementary domain. The law of variation of propagation of the reflected pressure waves or echoes as a function of the corresponding azimuth is established from the fluctuations of amplitude and propagation time for each received echo.

 The system for determining the azimuth anisotropy in a domain consisting of elementary domains of a material medium having at least one substantially planar separation surface, in accordance with the subject of the present invention, is remarkable in that it comprises a network of transceiver pairs arranged in a plurality of azimuths and means for controlling the transceiver pairs interconnected to the transceiver pairs to provide transmit control of transmitters and receive receivers defining the domain elementary. Calculation means are provided for performing a calculation of amplitude and delay fluctuations of the received echoes as a function of the azimuth.

 The method and the system, objects of the invention, are applicable to the determination of the azimuth anisotropy of material media of any kind, in particular the stratified material media.

A particularly advantageous application of the method and system of the invention relates to three-dimensional earth seismic in tectonic .calme.

A more detailed description of the method and system, objects of the invention, will be given in more detail on reading the description and on the observation of the drawings in which
FIG. 1 represents an illustrative diagram of the implementation of the method which is the subject of the invention,
FIG. 2 represents the law of variation of the propagation time between a transmitter Ei and a receiver Ri as a function of the orientation of these latter with respect to a reference direction,
FIG. 3 represents the law of variation of the amplitude of the echoes received by a receiver Ri as a function of the angle of orientation relative to a reference direction,
FIG. 4 represents a three-dimensional diagram of the echo signals received by a receiver Ri determined according to the vertical axis graduated in propagation time, the horizontal axis graduated in offset distance between transmitter Ei and receiver Ri, an additional graduation in an orientation angle of 3600 with respect to a reference direction further being carried on the horizontal axis for different discrete values of offset distances.

FIG. 5 represents an illustrative diagram of the implementation of the method according to the invention for different depths
of analysis, that is to say for different successive elementary domains of
likely to constitute a total area D,
FIG. 6 represents an advantageous embodiment not
limiting a system allowing the implementation of the method which is the subject of the invention,
. Figure 7 shows the flowchart of a program
integrated into a control and processing computer allowing the
of the method which is the subject of the invention for a set of domains
elementary constituting a domain D.

The method, object of the invention, will first be described in
connection with the illustrative diagram of figure I.

According to the above figure, and as shown therein, the
process, object of the invention, consists, in order to determine the anisotropy
azimuth of a material medium having at least one surface of
substantially plane separation, noted S in this figure, to have on this
separation area a plurality of transceiver pairs El, E2,
Ei, respectively R1, R2, Ri of a pressure wave directed towards the middle
material to study. To perform an acquisition on a domain
elementary of FIG. 1, each pair of transceiver Ei, Ri
is arranged in a direction ai determined with respect to a direction
reference N. The transceiver pairs Ei, Ri are arranged
around a common CK center.

By way of non-limiting example, the reference direction N
may be arbitrary, but it may, in the case where the process is subject to
the invention is used for the study of the underlying geological layers at
the terrestrial surface, be constituted by the direction of the north. In all the
case, and regardless of the reference direction used, the anisotropy
in evidence for the studied material medium will be defined by anisotropy
azimuth angle, the angle ai being then considered as the azimuth of each
transceiver pair Ei, Ri relative to the reference direction
N, whether it is constituted by the direction of the geographical north or not.

o '
As shown in FIG. 1, a transmitter Ei is separated from a corresponding receiver Ri by an offset distance Oi. It will be noted that the offset distances of the different transmitter-receiver pairs can in principle be arbitrary. However, for the convenience of implementing the method, object of the invention, a predetermined offset value will be used, as will be described later in the description.

 According to the above-mentioned FIG. 1, the method which is the subject of the invention then consists in generating, from a transmitter Ei, a pressure wave Pi and recording at the receiver Ri associated therewith the echoes or pressure waves. reflected in a propagation time window defining an elementary domain in the domain D to be studied.

We will understand ,. of course, that the recording by the receiver
Ri is assigned to the corresponding emitter Ei echoes or pressure waves reflected on a layer underlying the separation surface S, this reflection being, of course, partial, and the aforementioned recording being performed during a width window of time 6 t offset from the instant of emission of the pressure wave Pi by the transmitter Ei by a time t, makes it possible to obtain in fact the amplitude and phase or propagation delay information. of the wave reflected on a slice of thickness o Z corresponding to the width of the recording time window 6 t.

The 6 Z thickness wafer on an elementary reflection surface determined makes it possible to constitute the elementary volume or elementary domain, a set of these elementary domains constituting the above-mentioned field of study D. It will be understood, of course, that depending on the width of the time window 6 t, it is possible to make the thickness of the elementary domain slice of fl Z as small as possible so as to obtain an elementary domain of appropriate size.

With regard to the elementary surface of reflection to it is determined by the conditions of incidence of the pressure wave
Pi, as will be described later in the description.

 According to another characteristic of the method, which is the subject of the invention, the preceding step consists in generating from a transmitter Ei a pressure wave Pi and in recording at the associated receiver Ri the reflected echoes or pressure waves. is repeated for each of the transceiver pairs Ei, Ri for the elementary domain of considered.

 Since the preceding steps have been repeated for each pair of transceivers mentioned above, the method which is the subject of the invention then consists in determining for each received echo the amplitude and propagation time fluctuations in order to establish a variation law of propagation of reflected pressure waves or echoes as a function of the azimuth or orientation angle corresponding to the reference direction.

 In the case where the elementary surface a is located above terrains or material layers presenting the anisotropy around the vertical axis passing through the common center CK, the aforementioned anisotropy results in a time deformation of the corresponding reflections. to paths crossing the anisotropic lands following a sinusoid of period 1800 and by a variation of the amplitude of these reflections according to a sinusoid of also period 1800.

 Given the aforementioned observation, the method, object of the invention, in its step of determining the law of variation of.

propagation of the reflected pressure waves or echoes as a function of the azimuth or angle of orientation ai with respect to the reference direction N then consists in memorizing the values of propagation time Ti for each azimuth value oi, then at comparing for an azimuth value ai the value of the propagation time Ti corresponding to the average value of the propagation times T at the considered point.

 According to another characteristic of the method, which is the subject of the invention, the step of determining the propagation variation law of the reflected pressure waves or echoes as a function of the corresponding azimuth may also consist in memorizing the amplitude values. Ai reflected pressure waves or echoes for each azimuth value ai or orientation angle relative to the direction N, then to compare for a given azimuth value the value of the amplitude Ai corresponding to the average value of amplitudes A at the point considered.

 According to an advantageous nonlimiting characteristic of the method, object of the invention, and in order to facilitate the mathematical analysis of the previously described phenomena, the emission and reception of the reflected pressure waves or echoes are carried out for a range of azimuths or angles of orientation ai between 0 and 1800 constant incidence.

In this case, as shown in particular in FIG. 1, the transceiver pairs E 1, R 1 have the same offset distance O 1 and are arranged in a pattern forming the same circumference with respect to the common center CK. By way of nonlimiting example, the emission parameters, such as the offset Oi between each emitter Ei and each corresponding receiver Ri, and, finally, the angle of incidence of the pressure waves emitted Pi on the elementary reflecting surfaces. can be chosen equal so that
Oi = 2 x Z where
Z denotes the depth of the elementary reflecting surface with respect to the separation surface S.

 FIG. 2 shows the law of variation of the propagation time of the reflected pressure waves Pi for different angles of azimuth or orientation ai for a range of values of this angle of orientation between 00, substantially corresponding to in the reference direction N, with respect to the common center CK, and 1800 corresponding to the opposite direction with respect to this same common center CK. By way of nonlimiting example, the increments of orientation angles α1 have been taken to be 150. In FIG. 2, the vertical axis is graduated to a value of propagation time with respect to an average propagation time. , and it is found that the propagation time values, as a function of the orientation angle or azimuth ai, substantially follow a sinusoldale law.

Thus, the determination of the weakest propagation time makes it possible to determine, on the one hand, the direction of the axis of anisotropy called "fast axis", this direction being denoted ar in FIG. 2, and, d On the other hand, for the maximum propagation time, the direction of the axis of anisotropy called the "slow axis" corresponding to the direction I on FIG. 2. The aforementioned fast and slow directions constitute the principal directions of anisotropy of the medium. material considered at the aforementioned CK Joint Center.

 In the same way, as shown in FIG. 3, the law of variation of the amplitude Ai of the pressure waves Pi as a function of the angle of orientation or azimuth ai of the transceiver pairs Ei, Ri has been represented in FIG.

 In FIG. 3, the vertical axis represents amplitude values with respect to the average amplitude and the horizontal axis of the orientation angle values with respect to the reference direction N.

 The law of amplitude variation of the echoes as a function of the angle of orientation a i or of the abovementioned azimuths also follows a sinusoidal variation. The amplitude variation law, as represented in FIG. 3, was obtained for increments of orientation α equal to 15. As in the case of the determination of the law of variation of propagation time. amplitude variation law, as represented in FIG. 3, makes it possible to determine the differential attenuation of amplitude according to the aforementioned slow and fast axes for the values, ar, al previously determined, that is to say for the fast and slow directions constituting the principal directions of anisotropy or natural coordinates of the material medium considered.

 Of course, with regard to FIGS. 2 and 3, the axes of time and amplitude values can be graduated in relative value with respect to the value of propagation time. average, respectively the average amplitude value.

 With regard to the conduct of the method, object of the invention, in particular, in order to obtain stable amplitude values of the echoes or pressure waves reflected during the tests, it is desirable to work at constant incidence, The amplitude of the reflected echoes depends, of course, incidence of the pressure wave Pi generated by each transmitter Ei. As has been seen in the course of tests, low incidences, ie small values of offset distance between emitters Ei and associated receivers Ri, have the effect of causing very small differences in propagation time. Consequently, the previously indicated incidence value, corresponding to an angle of incidence of the pressure waves Pi with respect to the vertical substantially equal to 450, has been retained.

 FIG. 4 shows a three-dimensional diagram of the echo signals received by a receiver Ri determined as a function of the vertical axis graduated in propagation time T, this axis being graduated in millisecond. The horizontal axis is graduated in offset distance between emitters Ei and receivers Ri for offset distances expressed in feet (one foot = 0.3048 meters), an additional graduation in angle of orientation or azimuth of 0 to 3600 relative to a direction, reference being furthermore focused on this same horizontal axis for different discrete values of offset distance.

 In the above diagram of FIG. 4, it is possible to observe, on the one hand, the periodicity according to a sinusoidal law of the fluctuations of the propagation time T as a function of the azimuth for each corresponding offset value, as well as a periodic variation. period 1800 amplitude values of reflected echoes or pressure waves, these periodic amplitude values appearing due to the black color intensity for a given average propagation time.

 FIG. 4 also shows the isoincidence curves of the pressure waves Pi, these curves having been calculated for the incidence values 200, 300 and 45, for the set of offset values between each transmitter Ei and each associated receiver Ri for depth values and finally corresponding T propagation time.

Finally, it is found that the most favorable angle of incidence for the conduct of the tests is between 300 and 45 ° C. The diagram of FIG. 4 justifies for the study of the phenomenon of anisotropy, in accordance with the process object of the invention, the study of the fluctuations of propagation times and amplitudes of echoes as a function of the orientation angle or azimuth relative to the reference direction on an orientation or azimuth value included between 0 and 1800 to determine the main directions and the differential attenuations along these directions, such a finding is justified by the fact that the propagation anisotropy is identical by rotation of 1800 of a pair of emitter Ei receiver Ri considered around a common center CK, the corresponding offset distance Oi being retained.

An alternative embodiment or implementation of the method, object of the invention, will be described in connection with Figure 5. The variant embodiment or implementation of the method, object of the invention, described in connection with the FIG. 5 makes it possible to carry out a measurement or an acquisition in a succession of elementary domains of incremented successive depth. For this purpose, as shown diagrammatically in FIG. 5, the method according to the invention consists in making a plurality of acquisitions, the pattern forming the same circumference with respect to the common center CK being transformed by a sequence of homotheties of center CK and ratio Kr. Thus, the initial pattern can be transformed so that the offset distance Oir takes successive values.
Oir Kr x Oir
The values of the coefficient or ratio of homothety Kr can be chosen so that Oir and Oi rI represent, for example, the offset distances of the emitter-receiver pairs, r Ei, Ri for a constant depth increment Pf, as well as represented in FIG. 5 and corresponding to a pitch shift in depth of the elementary domain of considered.

 FIG. 5 shows by way of nonlimiting example with respect to a common center CK a pattern consisting of a plurality of transmitter-receiver pairs E 1, R 1, Rital resulting from an initial pattern not represented and successively followed. a pattern consisting of transceiver pairs Eir, Rir and Ei +1> Ri r + 1 The successive elementary domains are then denoted by 1. der, der + 1 They correspond, of course, to different elementary surface elementary domains has shifted by an increment of depth Pf, as shown in FIG.

Of course, depending on the incidence of pressure waves
Pi on each elementary surface a and the width of the temporal iron space 6 t, it is thus possible to ensure successive overlapping of the successive elementary domains of, der, of 1 to constitute all or part of a domain of D analysis, as will be described below in the description.

 As furthermore schematized in FIG. 5, in order to perform an acquisition in a succession of elementary domains of shifted in position according to a first x or second y direction parallel to the separation surface s; the pattern, forming a same circumference relative to the common center CK, and the latter, are then shifted in translation on the surface S of one or more increments px, py respectively. Thus, and purely for illustrative purposes, the above-mentioned x and y directions are shown in FIG. 5, these being a priori purely arbitrary on the surface S. The common center CK can thus be brought to a later position CK + 1 in Figure 5, the common center has been moved a step py in the direction y and a step px in the direction x for example. It is thus possible, according to the method, object of the invention, to perform in a new position of the common center CK + I a new acquisition, as previously described for the previous position CK.

 A recovery of all the elementary domains from I to 1 and the related elementary domains obtained from the common center CK + 1 I to obtain an acquisition in a domain D constituted by all the aforementioned elementary domains.

 The method which is the subject of the invention is thus analogous and particularly remarkable in that it is analyzed in a process comparable to a tomography method allowing a fine analysis of the material in elementary domains.

 Of course, the overlapping of the successive elementary domains, both in the direction of depth Z and in the directions of lateral translation, along the x or y directions, can be carried out and determined by image intercorrelation products of two successive domains, in order to determine the corresponding average Z, X or Y limits according to the usual picture correlation techniques.

 Additional indications will be given with regard to the implementation of the method, object of the invention, with regard to the implementation of the emission of each pressure wave Pi considered.

According to a first particularly advantageous aspect of the method, object of the invention, the repetition for each transceiver pair Ei, Ri can be carried out by a sequential emission-recording of the reflected pressure waves for the directions of azimuth or orientation have considered. So, if we go back to the figure again
I, the method, object of the invention, can thus consist in performing first emission by the transmitter El of the pressure wave P1 and.

together the reception of the pressure wave reflected by the elementary surface a of the domain of and the measurement of the propagation time TI and the amplitude values Al of the reflected pressure or echo wave, then successively for the transmitter -receiver E2, R2, then the transmitter-receiver, ur
Ei, Ri and so on until exhaustion of the transceiver pairs.

The processing of the recorded values can then be carried out so as to determine the main directions or azimuths of slow and fast axes, as mentioned above.

On the other hand, in the case where the method, object of the invention, is to be implemented for example in the context of a terrestrial three-dimensional seismic acquisition, the number of transmitter-receiver pairs, as will be described later. in the description, can be very important although the method, object of the invention, can be implemented from a number of transceiver pairs greater than or equal to 3. In the case cited above, however, the repetition for each pair of transceiver Ei, Ri may more advantageously be effected by simultaneous emission-recording of the reflected pressure waves. In this case, the emission of the pressure wave
Pi by each transmitter Ei consists of a coded periodic pressure wave, each transmitter Ei emitting a pressure wave Pi coded according to a code of its own. By way of non-limiting example, the coding of the emission of the pressure wave Pi by each transmitter Ei may consist of a phase coding. The phase coding can then be advantageously representative of the azimuth ai or corresponding orientation angle of the pair Ei, Ri corresponding transceiver. By way of nonlimiting example, the phase coding may be performed according to a binary code of the aforementioned order number i. The principle of phase coding at the emission of sources, vibrations will not be described in more detail because it corresponds to coding principles perfectly known to those skilled in the art.

 A more detailed description of a system for determining the azimuthal anisotropy in a domain D consisting of elementary domains of a material medium, according to the subject of the invention, this material medium having at least one separation surface substantially planar will now be given in connection with Figures 6 and 7.

According to FIG. 6 in particular, the system which is the subject of the invention comprises a network of transceiver pairs El, RI, Ei,
Ri to E6, R6 in this figure.

 Of course, as already described in connection with FIGS. 1 and 3, illustrative of the implementation of the method, object of the invention, the above-mentioned transceiver pairs are arranged in a plurality of orientation directions. relative to the reference direction N around the common center CK. In this same FIG. 6, the network of transceiver pairs is denoted R. This network R is represented by the dashed circle in FIG. 6.

 By way of nonlimiting example, the emitters Ei can be constituted by sources of seismic vibrations and the receivers Ri by omnidirectional geophones.

 In addition, as shown in this figure, control means are provided for controlling the transceiver pairs Ei, ri interconnected with the transceiver pairs.

The aforementioned means make it possible to control the transmitters Ei and receive the receivers. Ri in a window of time
Propagation Gt defining the elementary domain of, as previously described. In the three-dimensional figure. These sources may also possibly be constituted by explosive sources. In the latter case, the emission of the pressure waves Pi can be carried out sequentially, as described above in connection with the method, object of the invention. The command and transmission lines. aforementioned are then connected from each pair Ei, Ri to the control means 1 by an interfacing system known in itself, for example the calculating means 100 in a conventional manner.

 The system, object of the invention, can furthermore, as shown in FIG. 6, be implemented in a particularly advantageous manner in the case of the existence of a grid G for acquiring seismic data in three-dimensional earth seismic as will be described below.

 Such a case appears particularly advantageous insofar as the gate G, as shown in FIG. 6, has already been implemented, the system which is the subject of the invention can then be implemented by configuring a network R on the gate G, as will be described below.

In the aforementioned case, the network R of transceiver pairs Ei, Ri can then advantageously be formed on a grid G of lines of emitters El-lk and receivers Rlk, spaced apart by a pitch Px for example. Two successive lines are then formed by an alignment of emitters respectively of receivers, these two lines being spaced apart by the same pitch Py, equal for example to pitch Px so as to constitute in a direction perpendicular to the lines, that is to say say the x direction, a similar column arrangement of alternating El Ik-receiver Rlk transmitters. Each transceiver pair Ei, Ri is then selected by addressing transceiver pairs Ei, lk, Rlk to form a network R in a substantially circumferential pattern. Of course, in the aforementioned case, the network R is also represented in FIG. 6 by the dot-and-dash circle, the transceiver pairs El,
R1 to E6, R6 being constituted by transceivers El.lk, Rlk of the gate G.

 In the latter case and as also shown in FIG. 6, the control means I of the transceiver pairs Ei, Ri interconnected to the aforementioned transceiver pairs and the calculation means can be constituted respectively by an interface circuit 200 interconnecting the lines of transmitters and receivers. The interface circuit makes it possible to trigger the emission of the pressure waves Pi by addressing the transmitters Ei considered, that is to say by addressing in the gate C of the transmitters El I k and the receivers Rlk addresses chosen accordingly. The interface circuit 200 also ensures reception and registration by addressing the receivers Ri considered reflected pressure waves or echoes in the time window 8 t defining the elementary domain of considered.

Of course, the allocation of the receivers Ri is carried out from the addresses Rlk of the lines of. considered receivers.

Finally, the interface circuit 200 makes it possible to perform the analog-to-digital conversion of the echo signals of the pressure waves.
Pi and the storage of the digitized echo signals obtained.

In addition, an acquisition and processing line computer 100 is interconnected with the aforementioned interface circuit 200 in order to enable the latter to control the transmitters Ei and the receivers.
Ri. The acquisition and processing line computer 100 also makes it possible to process the recorded and digitized echoes to determine, for a center CK considered, the direction of the slow and fast axes with respect to the reference direction N as well as the differential attenuation of amplitude along the aforementioned slow and fast axes.

 The acquisition and processing conduct computer can be constituted in a conventional manner when the system, which is the subject of the invention, corresponds to a system for acquiring three-dimensional seismic data, for example by a VAX SYSTEM computer, for example, equipped with its resources. peripheral devices.

 In order to effect the processing of the recorded echoes, the acquisition and processing line computer 100 may comprise a sub-program for sorting the propagation time values Ti making it possible to determine the azimuths or angles of orientation a 1 and ar corresponding to on the one hand, the maximum propagation time and, on the other hand, the minimum propagation time for the common center CK considered. These directions correspond and respectively define the directions of the slow and fast axes, as described above.

 In addition, the acquisition and processing driving computer also comprises a sub-program for sorting the amplitude values and calculating the peak-to-peak amplitude of the amplitude variation of the echoes received as a function of the azimuth or angle of orientation a i. The aforementioned subroutine also makes it possible to calculate the differential attenuation along the slow and fast axes of the amplitudes of the reflected echoes.

 The two aforementioned sorting programs can be constituted by sorting modules of conventional type, such as bubble sorting modules allowing by. for example, determining the largest value of a set of stored delay values, as well as the smallest stored delay value and the corresponding orientation angle or azimuth values a i. The same is true for the stored amplitude values, the amplitude values for the orientation values of the slow and fast axes o 1 being determined by simple reading of the corresponding amplitudes. Finally, the calculation of the differential attenuation, along the slow and fast axes, can be carried out simply by calculating the ratio of the values of the amplitudes to the aforementioned values a 1 and a r relative to the average amplitude value.

These programs correspond to programs whose algorithms are perfectly known to those skilled in the art.

 A more detailed description of the algorithm of a program allowing automatic control of the method, object of the invention, by means of the system described in FIG. 6 will be given in conjunction with FIG.

According to the aforementioned figure, in order to perform an acquisition on a domain D constituted by a set of elementary domains of, the acquisition and processing line computer 100 may comprise an acquisition control program. This acquisition driving program can comprise, as represented in FIG. 7, an initialization module 1000 allowing the definition on the transceiver gate G El lk Rlk of the common center.
CK of the network R of transceiver pairs Ei, Ri. The aforementioned initialization module 1000 makes it possible, for example, to introduce the coordinates Ik of the aforementioned common center CK or to read them in an embodiment which will be described later. it also allows the introduction of the offset distance Oi of each transceiver pair Ei, Ri, this offset constituting the diameter of the circular pattern or network R of center CK. The initialization module 1000 also allows the introduction of the initial depth Z and the incidence of the pressure wave Pi generated by each transmitter Ei. The acquisition driving program, represented in FIG. 7, also comprises a calculation module 1001 of the addresses k, I on the gate G of the transmitters El Ik-receivers Rlk of this gate G making it possible to constitute the pairs of transceivers Ei, Ri of the network R.

It will be understood, of course, that the initialization module 1000 and the address calculation module 1001 make it possible to define on the grid G an initial pattern corresponding to the network R. By way of non-limiting example, the module for calculating the addresses ki on the gate G, module 1001, allows, from the reading of the initial values entered or respectively read with regard to the common center CK by the module 1000, to calculate the corresponding addresses k1 on the gate G constituting the gate acquisition in three-dimensional seismic. By way of nonlimiting example, the coordinates of the common center CK having been entered, as well as the offset distance Oi, the address calculation module may comprise a sub-module for calculating the radius of the pattern corresponding to the network R, this radius being equal to Q, followed by a calculation module of
2 the distance to the common center CK of all the transmitters El Ik and the receivers Rlk included in a domain delimited by a square C1 circumscribing the network R at a pitch Px or Py corresponding distance, this square being noted Cl in the figure 6 and between a cross domain C2 inscribed in the network R, as shown in FIG. 6.

It will be understood, of course, that the domain, thus delimited by the curves C1 and CZ, can very easily be defined from the addresses in lines I and in columns K of emitters and receivers El I k
Rlk of the grid G. A calculation of the distance to the common center CK of each corresponding transmitter located between the curves C1 and C2 then makes it possible to choose the transceivers El Ikw Rlk whose difference in the distance to the common center CK to the radius of the R pattern is minimum.

All of these emitter-receivers thus constitute the network R constituted in circular pattern. In order to facilitate the calculations, advantageously but not limitatively, it will be possible to choose the center CK as the center of symmetry of a mesh constituted by two receivers of a first line and two emitters of a successive line. In this case, it is advantageous to choose an offset or offset distance between transceiver pairs Ei, Ri equal to an odd multiple of steps
Px or Py so as to obtain a maximum of transceiver forming pairs in the vicinity of the pattern constituted by the circumference.

 In FIG. 6, these transceivers have been denoted El, R2, E4, ES, A6 and R1, E2, E3, R4, R5 and E6, respectively. It will be understood, in fact, that under these conditions, from the point of view of the single symmetry, the transceivers El, E2, R1, R2, E4, ES, and RS, R4 play essentially the same role with respect to the common center CK.

 The calculation of the corresponding addresses of the transmitters and receivers having been carried out, as previously described, it is then possible to assign to the emitters of a half-circumference of the network R, thus determined, a serial number which corresponds to the number of order i, the corresponding corresponding order receivers being automatically symmetrical with respect to the center CK.

 As will also be noted in FIG. 7, the acquisition control program also comprises a module 1002 for transforming the addresses k1 of the transmitters El Ik-receivers Rlk, previously determined to form the network R constituting the circular pattern. of origin, to choose a succession of new motives obtained by homothety of the initial motive. The new successive patterns allow a variable acquisition in depth of one.

not Pf determines.

 Thus, the above-mentioned module 1002 allows the input of the depth step parameter Pf, which, as a function of the incidence previously chosen, makes it possible to determine the new offset value 01r + 1 relative to the preceding offset value ir. from a simple trigonometric calculation and by calling the address calculation module again to determine the new addresses of the transceivers corresponding to the new patterns, as shown in FIG. 5, the new k1 addresses of the new transceivers constituting the new pattern can be obtained from curves C'I and C'2 obtained by homothety with respect to curves C1 and C2 above. Of course, the address transformation module 1002 is iterative in order to obtain the number of elementary domains of, of, spaced along the axis of depth Z, as previously described in FIG.

 In addition, as shown in FIG. 7, the initialization modules 1000 for calculating the addresses 1001 and for transforming the addresses 1002 are integrated in an iterative loop making it possible to translate along the x and y of the surface S of the material medium.

Successive positions CK-I, CK, CK + I of
successive common centers, shifted in translation successively according to the
direction y, then on the direction x of two steps Py, respectively Px
successively.

It goes without saying that any path of movement of the center
common CK can be considered to constitute an appropriate scan of the
surface S via the transmitters Ellk, Rlk of the grid G for
obtain an acquisition in a domain D of dimension substantially
any.

As will be seen in Figure 7, the iterative loop
includes an initialization module, which is rated 1005, allowing
to enter in particular the coordinates on the grid G of a succession of a number
NK of shifted positions in translation of a pitch Px or Py or a multiple
of these steps, common centers CK of coordinates Ik. The positions of
CK common centers can be entered by their Ik coordinates
expressed in relation to the gate G. This module 1005 makes it possible in particular
the entry of the number NK 'of the positions of common centers necessary to
ensure the scanning of the aforementioned surface S, as well as the initialization of a
count parameter K to value 1.

The aforementioned iterative loop also includes a counting loop from an initialized count variable K initialized
previously at 1, this counting loop comprising a test 1003 of the
count variable K compared to the previously entered NK value.

A negative response to this test has the effect of incrementing the
count variable of a value I and return to the initialization module
input, which allows, by reading the data of the common center
C corresponding K values, then call the 1000 module
initialization, which again allows the conduct of the process, as well as
previously described, thanks to the modules 1000, 1001 and 1002. A complete scan of the underlying layers of the surface S is thus performed.

On positive response to test 1003 above, a module 1006
allows to call up the sorting subprograms and propagation time and amplitude values, as previously described in the description, in order to ensure the processing of these values and the determination of the fluctuations of propagation time and corresponding amplitude for the different common centers CK distributed on the surface S.

 There is thus described a method and a system for determining the azimuthal anisotropy of a D domain of a particularly powerful material medium insofar as such a system and such a method can be used in fields as varied as the study of materials and, very significantly, in the field of three-dimensional seismic.

 It will be understood, in particular, that in the latter case the method and system, objects of the invention, appear particularly advantageous insofar as a three-dimensional seismic acquisition grid can easily be used, it being already implemented thanks to to appropriate software, which was the subject of this description. In the aforementioned case, depending on the frequency of the signals received, the elementary domain of can have a dimension of a few meters to a few tens of meters, the elementary reflection surface may be constituted by a square or a rectangle of a few tens of meters of side corresponding to an elementary cell designated in Anglo-Saxon language, by "bin" in the technical field of three-dimensional seismic.

Claims (13)

1. A method for determining the azimuth anisotropy in a domain (D) of a material medium having at least one substantially planar separation surface, characterized in that said method consists of an acquisition for a basic domain of - to dispose on said separation surface a plurality of transceiver pairs (Ei, Ri) of a pressure wave directed towards the material medium to be studied, each pair of transceiver being arranged in a direction (ai) determined by reference to a reference direction (N) around a common center (CK) and a transmitter (Ei) being separated from a corresponding receiver (Ri) by an offset distance (Oi), - to be generated from an emitter (Ei) a pressure wave (Pi) and recording at the receiver (Ri) associated therewith the echoes or pressure waves reflected in a propagation time window defining said elementary domain area (of), - repeating the preceding step for each of said transceiver pairs (Ei, Ri) for the elementary domain (of) considered, - to determine for each echo received the amplitude and time fluctuations of propagation for establishing a propagation variation law of said reflected pressure waves or echoes as a function of the corresponding azimuth.  2. Method according to claim 1, characterized in that the step of determining the propagation variation law of said reflected pressure waves or echoes as a function of the corresponding azimuth consists in - memorizing the values of propagation time (Ti ) for each value  of azimuth (a i), - to compare for an azimuth value (ai) given the value of the time of  propagation (Ti) corresponding to the mean value of the  propagation (T) at the point considered.  3. Method according to one of claims I or 2, characterized in that the step of determining the law of variation of propagation of said reflected pressure waves or echoes as a function of the corresponding azimuth consists in - memorizing the values of amplitude (Ai) of reflected pressure waves  or ecHos for each azimuth value (a i), - to be compared for a given azimuth value (ai) the value of the amplitude  (Ai) corresponding to the average value of the amplitudes (A) in the  considered.  4. Method according to one of the preceding claims, characterized in that the transmission and reception of the reflected pressure waves or echoes are performed for a range of azimuths (ai) between 0 and 1800 constant incidence, said couples transceiver (Ei, Ri) having the same offset distance (Oi) and being arranged in a pattern forming the same circumference with respect to said common center (CK).  3. Method according to claim 4, characterized in that, in order to perform a measurement in a succession of elementary domains (of) of incremented successive depth, said method consists in making a plurality of acquisitions, said pattern forming a same circumference with respect to said common center (CK), being transformed by a sequence of homotheties of center (CK) and of ratio (Kr) such that  Oir = K r x Oi ,, l where Oir and ir I represent the offset distances of couples  transceiver (Ei, Ri) for a depth increment (Pf)  corresponding to a no depth shift of the domain  elementary (of) considered.  6. Method according to claim 4 or 5, characterized in that, in order to perform an acquisition in a succession of elementary domains (of) shifted in position according to a first (x) or second (y) direction parallel to the surface of separation, said pattern forming a same circumference with respect to said common center (CK), as well as the latter, are offset in translation, on said surface of an increment (Px, Py) respectively.  Method according to one of the preceding claims, characterized in that the repetition for each transceiver pair (Ei, Ri) is effected by a sequential emission-recording of the reflected pressure waves for the azimuth directions ( have) considered.  8. Method according to one of claims 1 to 6, characterized in that the repetition for each pair of transceiver (Ei, Ri) is performed by a simultaneous emission-recording of the reflected pressure waves, the emission of the pressure wave (Pi) by each of the emitters (Ei) consisting of a coded periodic pressure wave, each emitter (Ei) emitting a pressure wave (Pi) coded according to a code of its own.  9. Method according to claim 8, characterized in that the coding consists of a phase coding, said coding being representative of the order number (i) of the azimuth (a i) or corresponding orientation angle.  10. System for determining the azimuth anisotropy in a domain (D) consisting of elementary domains (of) of a material medium having at least one substantially plant-like separation surface, characterized in that it comprises a network of pairs transceivers (Ei, Ri) arranged according to a  plurality of orientation directions (a i) with respect to a direction of  reference (N), around a common center (CK), - means (I) for controlling said transceiver pairs (Ei, Ri)  interconnected to said transceiver pairs to provide the  emission control of transmitters (Ei) and reception of receivers  (Ri) in a propagation time window defining said domain  elementary (de), - means (100) for calculating amplitude and time fluctuations of  propagation of the received echoes according to the direction of orientation  ( have).  He. System according to claim 10, characterized in that said network (R) of transceiver pairs (Ei, Ri) is formed by a grid (G) of lines of transmitters (El Ik) and receivers (Rlk) , spaced apart by one step (Px), two successive lines being formed by an alignment of transmitters respectively of receivers and spaced from uh same pitch (Py), equal to (Px), so as to constitute in a direction perpendicular to said lines a similar column arrangement of alternating transmitters (El Ik) receivers (Rlk), each pair of transceiver (Ei, Ri) being selected by addressing couples (El.lk), (Rlk) of transmitter- receiver to form a network (R) in a substantially circumferential pattern.  12. System according to claim 10 or 11, characterized in that said means for controlling said transceiver pairs (Ei, Ri) interconnected to said transceiver pairs and said calculating means are. constituted by - an interface circuit (200) interconnecting the. lines  transmitters and receivers and to ensure  triggering the emission of pressure waves (Pi) by addressing  issuers (Ei) considered,  . receiving and recording, by addressing receivers (Ri)  considered, reflected pressure waves or echoes in a window  temporal definition defining the elementary domain (of) considered,  . the analog digital conversion of the echo signals and stored it  digitized echo signals obtained, - a calculator (100) for acquisition and processing  enabling control by the interface circuit (i00) of the transmitters  (Ei) and receivers (Ri), and the processing of echoes recorded for  determine, for a center (CK) considered, the direction of the axes slow and  fast relative to the reference direction and mitigation  Amplitude differential along the slow and fast axes.  13. System according to claim 12, characterized in that, in order to effect the processing of the recorded echoes, said acquisiton driving calculator comprises a sub-program for sorting the values of the propagation times.  to determine the corresponding azimuths (a I) and (a r) of a  the maximum propagation time and, on the other hand, the time of  minimum propagation, said directions or azimuths (a 1), (a r)  respectively defining the directions of the slow and fast axes, - a sub program for sorting the values of the amplitudes and calculating the  the peak-to-peak amplitude of the amplitude variation of the echoes received in  function of the azimuth (a i) and the differential attenuation according to said  slow and fast axes.  14. System according to one of the preceding claims 10 to 13, characterized in that, in order to perform an acquisition on a domain (D) constituted by a set of elementary domains (de), said acquisition conduct calculator comprises an acquisition control program, said acquisition control program comprising - an initialization module (1000) allowing the definition, on the grid  (G) transceiver (El Ik) (Rlk) of the common center (CK) of the network  of transceiver pairs (Ei, Ri) of the offset distance (Oi)  diameter of the center circular pattern (CK), the initial depth (Z)  and the impact of the pressure wave (Pi) generated by each  transmitter (Ei), - a calculation module (1001) of the addresses (k, I) on the grid (G) of the  transmitters (E, Ik) receivers (Rlk) making it possible to constitute the couples  transceiver (Ei, Ri), said initialization and calculation modules  addresses for defining on the grid (G) an initial pattern, - a module for transforming said addresses (k, I) of the transmitters  (Ei, 1 k) receivers (Rlk) to choose a succession of new patterns obtained by homothety of the initial pattern, said new successive patterns allowing a depth-variable acquisition of a determined pitch (Pf).  15. System according to claim 14, characterized in that said modules for initialization, address calculation, and address transformation are integrated in an iterative loop to ensure a translation along the directions (x) and (y). of the surface (S) of the material medium by a step or a determined number of steps of the successive position of the common center (CK-I, CK, CK + I) of the network (R) constituting the initial pattern, of to ensure an acquisition for all the elementary domains (of) constituting the domain (D).  16. System according to claim 15, characterized in that said iterative loop comprises - an initialization module (1005) for entering the coordinates on  the grid (G) of a sequence of a number (NK) of positions shifted in  translating a step (Px), (Py) or a multiple of these steps, - a counting loop in which said modules are integrated.