Method for seismic monitoring of an underground zone by simultaneous use of sererval vibroseismic sources

• US 20020191490A1
• Filed: 10/15/2001
• Published: 12/19/2002
• Est. Priority Date: 02/14/2000
• Status: Active Grant

• Current Assignee: Campagnie General De Geophysique, Cggveritas Services SA (CGG), Gaz de France, Institut Francais Du Petrole
• Original Assignee: Campagnie General De Geophysique, Gaz de France, Institut Francais Du Petrole
• Inventors: Meunier, Julien
• Granted Patent: US 6,714,867 B2
• Time in Patent Office: Days
• US Class Current: 367/140
• CPC Class Codes: G01V 1/005 with exploration systems emitting special signals, e.g. frequency swept signals, pulse sequences or slip sweep arrangements
• CPC Class Codes: G01V 1/006 generating single signals by using more than one generator, e.g. beam steering or focusing arrays G01V1/13, G01V1/3861 take precedence

Abstract

Method and system intended for seismic monitoring of an underground zone (1), comprising simultaneously using several seismic vibrators.

The system comprises for example several local units (LU) comprising each a vibrator (5), a seismic pickup antenna (2), a local acquisition and processing unit (6), and a central control and synchronization unit (8) for simultaneously controlling the various vibrators by means of orthogonal signals, local units (6) being suited, by means of particular processing, to isolate and to reconstitute the seismograms corresponding to the contributions of the various vibrators.

Applications: monitoring of a hydrocarbon reservoir during production or of a reservoir used for gas storage for example.

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 [f_b−

f_f].

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 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 (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.