Title of Invention

METHOD INTENDED FOR SEISMIC MONITORING OF AN UNDERGROUND ZONE SIMULTANEOUSLY USING SEVERAL VIBROSEISMIC SOURCES

Abstract

The present invention relates to a method of seismic monitoring of an underground formation comprising the steps of emitting seismic waves in the formation, receiving signals emanating from the formation in response to the emitted seismi.c waves, recording the received signals and forming a seismogram by processing the recorded signals, characterized in that the method comprises the steps of: continuously emitting seismic waves in the formation by coupling with the formation at least two vibrators which emit seismic waves simultaneously and are controlled by orthogonal pilot signals consisting of sinusoids of different frequencies, in the fundamental components thereof as well as in the respective harmonies thereof, so as to form a composite vibrational signal; and processing the recorded signals comprising discriminating the respective contributions of the vibrators to the composite vibrational signal and reconstructing seismograms equivalent to seismograms that would be obtained by actuating the vibrators separately.

Full Text

FIELD OF THE INVENTION The present invention relates to a method and to a device intended for seismic monitoring of an underground zone such as a reservoir, comprising simultaneously using several seismic vibrators. BACKGROUND OF THE INVENTION It is well-known to monitor the long-term state variations of a reservoir during production, either a hydrocarbon reservoir or a reservoir intended for gas storage, by means of a seismic system comprising an impulsive seismic source or a seismic vibrator emitting seismic waves in the ground and a reception device comprising seismic pickups arranged at the surface or in wells and coupled with the formations to be monitored. At predetermined time intervals, seismic investigations are carried out with wave emission, reception of the waves reflected by the subsoil discontinuities and recording of seismograms, so as to determine by comparison the changes that take place in the reservoir as a result of the development thereof Various long-term seismic monitoring systems are described for example in patents EP-591,037 (US-5,461,594X FR-2,593,292 (US-4,775,009), FR-2,728,973 (US- 5,724311) or FR-2,775,349. Patents FR-2,728,973 and FR-2,775,349 notably describe systems intended for seismic monitoring of an underground zone during development, either a hydrocarbon reservoir or a gas storage reservoir for example. As diagrammatically shown in Figs. 1 to 3, they comprise for example a network of seismic antennas 2 consisting each of a series of seismic pickups 4 arranged at regular intervals along a well 3 drilled in the ground. This network can be regular as shownin Fig. 2, or irregular. The pickups can be one-directional geophones oriented vertically or multi-axis geophones (triphones) and/or hydrophones. A seismic source 5 is arranged in the vicinity of each antenna 2. Piezoelectric type vibrators such as those described in patent application FR-99/04,001 filed in the joint names of the applicants are advantageously used as sources and permanently installed in the immediate neighbourhood of each antenna 2. The seismic waves generated by the or by each seismic source 5 are propagated downwards (downgoing waves 9). These incident waves are first recorded by receivers 4 in each well 3, The waves reflected by the discontinuities of the zone (seismic interfaces) are propagated upwards. These upgoing waves 10 are also recorded by the various receivers 4. The upgoing waves and the downgoing waves are thus superimposed on the seismograms. They are usually processed by means of a method similar to the VSP (Vertical Seismic Profiles) processing method well-known to the man skilled in the art. The various sources of the seismic system can be actuated successively by providing, between each triggering, a sufficient time interval for reception of the waves reflected by the investigated zone. Several seismic sources emitting the same signals can also be used and triggered simultaneously in order to increase the power emitted. Patent FR-2,589,587 (US-4,780,856) also describes a marine seismic prospecting method comprising emission of seismic waves by a vibrator or simultaneously by several vibrators controlled by coded vibrational signals according to a pseudo-random code. SUMMARY OF THE INVENTION The method according to the invention allows to carry out operations intended for seismic monitoring of an underground formation. It comprises : - emission of seismic waves in the formation by coupling with the formation at least two vibrators emitting simultaneously and controlled by orthogonal signals so as to form a composite vibrational signal 1, - reception of the signals reflected by the formation in response to the emission of seismic waves, - recording the signals received by at least one seismic pickup, and - formation of seismograms by processing the signals recorded, comprising discrimination of the respective contributions of the vibrators to the composite vibrational signal and reconstruction of seismograms equivalent to those that would be obtained by actuating the vibrators separately. Sinusoidal signals of different frequencies, in their fundamental components as well as in their respective harmonics, or signals based on wavelets, on Legendre polynomials or on random series, etc, are for example used as orthogonal signals. In the case notably where the orthogonal signals emitted are sinusoids, discrimination of the respective contributions of the vibrators is for example carried out by determining the amplitude and the phase of the composite vibrational signal at the fundamental frequencies of the pilot signals applied to the vibrators. Discrimination of the respective contributions of the vibrators comprises for example weighting the recorded signal by a bell weighting (or tapering) factor and determining the amplitude and the phase of the composite signal. In order to carry out discrimination of the respective contributions of the vibrators, a selection by Fourier transform of the lines of the complex spectrum respectively associated with the various weighted signals is for example performed. Reconstruction of the seismograms specifically corresponding to the various vibrators is performed for example by applying, after separation thereof, an inverse Fourier transform to the lines respectively associated with the various weighted signals. According to an implementation mode, the frequencies of the orthogonal pilot signals respectively applied to the various vibrators are shifted by frequency intervals, at predetermined time intervals, so as to sweep a certain emission frequency band. The system intended for seismic monitoring of an underground formation according to the invention comprises means allowing emission of seismic vibrations in the formation comprising at least two vibrators and means for generating orthogonal signals and for applying them respectively to the vibrators so as to generate in the formation a composite vibrational signal, means allowing reception of the signals reflected by the formation in response to the emission of seismic waves, means for recording the signals received by the reception means and means for processing the signals recorded in order to form seismograms, comprising at least one computer suited to carry out discrimination of the respective contributions of the vibrators to the composite vibrational signal and reconstruction of seismograms equivalent to those that would be obtained by separately actuating the vibrators. According to a first implementation mode, the system comprises at least two local units arranged at a distance from one another and coupled with the formation, each unit comprising at least one seismic pickup, a seismic vibrator, a local device intended for acquisition and processing of the signals received, and a central control and synchronization unit connected to the various imits, comprising a generator suited to apply to the vibrators the orthogonal vibrational pilot signals. According to another implementation mode, the system comprises at least two local units arranged at a distance from one another and coupled with the formation, each unit comprising at least one seismic pickup, a seismic vibrator, and a central control and synchronization unit connected to the various local units by a material (cables for example) or immaterial link (radio) and comprising a signal generator suited to form the various orthogonal vibrational pilot signals, and means intended for acquisition of the signals received by the various antennas and for reconstruction of seismograms corresponding to the contributions of the various vibrators. The reception means comprise for example at least one antenna consisting of several seismic pickups arranged along a well drilled in the formation, this antenna being connected to the recording means. BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the method and of the system according to the invention will be clear from reading the description hereafter of non limitative examples, with reference to the accompanying drawings wherein : - Figure 1 diagrammatically shows a system intended for monitoring of an underground formation, comprising several signal emission and acquisition units, - Figure 2 shows an example of distribution, at the surface, of monitoring devices, - Figure 3 diagrammatically shows a signal emission and acquisition unit comprising seismic pickups arranged so as to form antermas, - Figure 4 shows a variant of the monitoring system of Fig. 1 where the seismic signal acquisition means are centralized in a central station, - Figure 5 illustrates the various stages of the algorithm for implementiag the method, and - Figure 6 diagrammatically shows the path of the waves between two emission points XI, X2 and a common reception point. DETAILED DESCRIPTION The method thus allows to carry out seismic monitoring operations in an underground zone by using a series of seismic pickups and a plurality of vibrators simultaneously actuated by signals at different frequencies selected so as to allow discrimination of the contributions of each source on the seismograms formed from the signals received and recorded* This is generally done through control of the various sources by « orthogonal» signals representing functions referred to as orthogonal functions, well-known to the man skilled in the art, and by using well-known numerical calculation techniques such as the inverse Fourier transform for separating the contributions to the seismograms obtained of the various vibrators, as explained hereafter by means of the following notations : • Convolution * • Correlation * We consider two unit sinusoidal signals P1 and P2 of respective frequencies f1 and f2 emitted by two sources S1 and S2 located at points X, and X2 (Fig,6) for a duration te whose value is great compared to 1/fi and l/fj. If Si and S2 emit simultaneously, the linearity of the transmission of the seismic waves implies that the seismogram of the waves received at R is the sum of T1 and T2, Equation (A) expresses the orthogonality of signals P1 and P2; equations (B) and (C) express the possibility of separating composite signal T into its two components. This property theoretically applies to any number of sources emitting sinusoids of different frequencies or more precisely orthogonal signals but, in practice, the number of sources has to be limited because of the following phenomena : a) the distortion which cannot be disregarded with mechanical sources. As it emits b) the necessarily truncated nature of emission length (ts), which is expressed in the frequency domain by means of a convolution of the line (impulse) by the Fourier transform of the truncation. If the latter is sudden (multiplication by a boxcar of length ts), it is a diffraction function of great width. If it is progressive (multiplication by a bell- shaped curve, a Gaussian curve or a Hanning function for example), it is another bell function whose width is inversely proportional to the length of the truncation, and c) the imperfection of the sources, which affects their stability and the precision of the frequencies emitted. In practice, it can be considered that this imperfection simply contributes to the increase in the line width. different. Separation of the signals received by the pickups in the field, in response to the simultaneous emission of the various signals, is thus obtained by selection of the line at the suitable frequency. Figure 5 diagrammatically illustrates the various stages of the method. Sinusoidal pilot signals 11 of respective frequencies af0, bf0), cf0, df0), etc., are simultaneously applied to the various seismic sources 5 installed in the field, coefficients a, b, c, d, etc., being so selected that these frequencies are different from one another and different from their respective harmonics. These frequencies are whole multiples of a fundamental frequency ^. The seismogram 12 that is obtained by recording the waves received by the pickups of the various antennas 4 is a linear combination of the seismograms that would have been obtained by exciting sources 5 sequentially. The recorded signals are then weighted by multiplying them by a bell weighting factor referred to as tapering factor 13 in order to form tapered or weighted signals 14. The real part 15 and the imaginary part 16 of the Fourier transform of the tapered signals are then calculated. Each part consists of impulses separate from one another. For each source 5, only the real number 17 and the imaginary number 18 forming the complex value of the Fourier transform at the frequency emitted by the source are then kept. The sets of various numbers 17 and 18 when the source emits all the programmed frequencies form the real part 19 and the imaginary part 20 of the seismogram 21 associated with the source. This seismogram is obtained by inverse Fourier transform. According to a first example of implementation of the method, the system comprises a plurality of local units LU comprising each an antenna 2 connected by cables (not shown) and a local acquisition and processing device 6 (Figs1, 2), and the various vibrators are connected by cables C for example to a central control and synchronization unit 8 comprising a signal generator (not shown) suited to generate, for the various vibrators 5, the orthogonal pilot signals defined above. According to another implementation mode (Fig.4), the various reception antennas 2 are connected by cables C for example to central control and synchronization unit 8 which fulfils the tasks of generation of the composite signals for the various sources 5 and acquisition and recording of the signals received by pickups 4, as well as processing of the acquired signals. Of course, cables C can in general be replaced by any material or immaterial link (radio link, optical fiber, etc.). Local acquisiton and processing devices 6 and/or central control and synchronization unit 8 comprise computers such as PCs programmed to carry out processings intended to isolate and to reconstitute the seismograms corresponding to the specific contributions of the various vibrators 5 as defined in the description. CLAIMS 1) A method intended for seismic monitoring of an underground formation (1) comprising emission of seismic waves in the formation, reception of the signals reflected by the formation in response to the emission of seismic waves, recording of the signals received by at least one seismic pickup (4) and formation of seismograms by processing the recorded signals, characterized in that: - emission is carried out by coupling with the formation at least two vibrators (5) emitting simultaneously and controlled by orthogonal signals consisting of sinusoids of different frequencies, in the fundamental components thereof as well as in the respective harmonics thereof, so as to form a composite vibrational signal ; and processing comprises discrimination of the respective contributions of the vibrators to the composite vibrational signal and reconstruction of seismograms equivalent to those that would be obtained by actuating the vibrators separately. 2) A method as claimed in claim 1, characterized in that orthogonal signals based on wavelets, Legendre polynomials or random series are emitted. 3) A method as claimed in claim 1 or 2, characterized in that discrimination of the respective contributions of the vibrators is carried out by determining the amplitude and the phase of the composite vibrational signal at the fundamental frequencies of the pilot signals applied to the vibrators, 4) A method as claimed in any one of claims 1 to 3, characterized in that discrimination of the respective contributions of vibrators (5) comprises weighting the recorded signals by a bell weighting factor (13) and determining the amplitude and the phase of the composite signal. 5) A method as claimed in the claim 4, characterized in that discrimination of the respective contributions of the vibrators comprises selection, by Fourier transform, of lines (15-18) of the complex spectrum respectively associated with the various weighted signals. 6) A method as claimed in any one of claims 1 to 5, characterized in that reconstruction of the seismograms specifically corresponding to the various vibrators is carried out by applying, after separation thereof, an inverse Fourier transform to the lines (19, 20) respectively associated with the various weighted signals. 7) A method as claimed in any one of the preceding claims, characterized in that the frequencies of the orthogonal pilot signals respectively applied to the various vibrators are shifted by frequency intervals, at predetermined time intervals, so as to sweep a certain emission frequency band [4 - ff]. 8) A system intended for seismic monitoring of an underground formation, comprising emission means allowing emission of seismic vibrations in the formation, means allowing reception of the signals reflected by the formation in response to the emission of seismic waves, recording means for recording the signals received by the signal reception means and processing means for processing recorded signals so as to form seismograms, characterized in that: the emission means comprise at least two vibrators (5) and means (8) for generating orthogonal signals consisting of sinusoids of different frequencies, in the fundamental components thereof as well as in the respective harmonics thereof, and for applying them respectively to vibrators (5) so as to generate in the formation a composite vibrational signal, and the processing means comprise at least one computer (6) suited to carry out discrimination of the respective contributions of the vibrators to the composite vibrational signal and reconstruction of seismograms equivalent to those that would be obtained by actuating the vibrators separately. 9) A system as claimed in claim 8, characterized in that it comprises a plurahty of local units (LU) arranged at a distance from one another and coupled with the formation, each unit comprising at least one seismic pickup (4), a seismic vibrator (5), a local device (6) intended for acquisition and processing of the signals received, and a central control and synchronization unit (8) connected to the various local units, comprising a signal generator suited to apply to vibrators (5) the orthogonal vibrational pilot signals. 10) A system as claimed in claim 9, characterized in that the central control and synchronization unit (8) is connected to the various local units by material or electromagnetic links. 11) A system as claimed in claim 9 or 10, characterized in that it comprises a plurality of local units (LU) arranged at a distance from one another and coupled with the formation, each unit comprising at least one seismic pickup, a seismic vibrator (5), and a central control and synchronization unit (8) connected to the various local units (LU) comprising a signal generator suited to form the various orthogonal vibrational pilot signals, and means intended for acquisition of the signals received by the various antennas (2) and for reconstruction of the seismograms corresponding to the contributions of the various vibrators (5). 12) A system as claimed in claim 10 or 11, characterized in that the reception means comprise at least one antenna (2) consisting of several seismic pickups (4) arranged along a well (3) drilled in the formation, this antenna being connected to recording means. 13. A method intended for seismic monitoring of ah underground formation substantially as herein described with reference to the accompanying drawings. 14. A system substantially as herein described with reference to the accompanying drawings. Dated this 9 day of October 2001

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