Seismic prospecting method and device using simultaneous emission of seismic signals obtained by coding a signal by pseudo-random sequences

• US 6,704,245 B2
• Filed: 12/21/2001
• Issued: 03/09/2004
• Est. Priority Date: 12/21/2000
• Status: Expired due to Fees

• Current Assignee: Institut Francais Du Petrole (IFP Energies Nouvelles)
• Original Assignee: Institut Francais Du Petrole (IFP Energies Nouvelles)
• Inventors: Becquey, Marc
• Primary Examiner(s): Moskowitz, Nelson
• Application Number:
  US10/023,697
• Publication Number:
  US 20020067659A1
• Time in Patent Office: 809 Days
• Field of Search: 367/39-41, 175/50, 166/250.6, 702/17
• US Class Current: 367/39
• CPC Class Codes: G01V 1/005 with exploration systems emitting special signals, e.g. frequency swept signals, pulse sequences or slip sweep arrangements

Abstract

Seismic prospecting method and device using simultaneous emission of seismic signals obtained by coding a signal by pseudo-random sequences, and notably a periodic signal phase modulated by such sequences.

Signals such as, for example, periodic signals phase modulated according to a pseudo-random code (binary for example), produced by a control unit, are for example emitted in the ground by means of vibrators, the seismic signals reflected by the subsoil discontinuities in response to the periodic signals emitted are picked up by receivers coupled with the formation and recorded in an acquisition and recording system. The periodic signals are emitted simultaneously by the vibrators. The control sequences of the coded periodic signals intended for all the vibrators are obtained either from the same random binary sequence with respective time lags evenly distributed over the length of the control sequence according to the number of vibrators used simultaneously (first mode), or from different sequences selected from among a group of sequences of minimum crosscorrelation (second mode), or by combining the two aforementioned modes. The respective contributions of the various vibrators are separated by correlating the signals received and recorded with signals constructed from the various control sequences obtained with time lags. The vibrators can be divided into several groups, the vibrators of each group being controlled by a binary sequence belonging to a minimum crosscorrelating group.

Application: seismic prospecting or seismic monitoring of reservoirs for example.

Claims

1. A method of prospecting or monitoring of an underground formation by elastic waves, comprising:

emitting in the ground elementary sequences formed by coding of a signal by pseudo-random sequences, receiving and recording the signals reflected by subsoil discontinuities in response to the emitted signals, and processing the recorded signals by correlation with signals formed from the emitted signals; and

wherein

the signals are emitted simultaneously by seismic sources each controlled by a signal of an elementary sequence of a length at least equal to a product of a number of seismic sources vibrating simultaneously by a listening time, a respective contribution of each one of the seismic sources being separated by correlating the received and recorded signals by signals constructed from the elementary sequence, extended with parts of a length at least equal to the listening time of the elementary sequence, obtained by circular permutation.

2. A method as claimed in claim 1, wherein:

the elementary sequences are formed by phase modulation of a periodic signal applied to vibrators.

3. A method as claimed in claim 2, wherein:

a same control sequence is applied to all the vibrators, with a time lag and a circular permutation, the time lag between two vibrators being at least equal to the listening time, and a length of the elementary control sequence being at least equal to a sum of the time lag and of the listening time.

4. A method as claimed in claim 2, wherein:

the vibrators are controlled by a connected set minimum crosscorrelation sequences, each vibrator emitting a vibration with a sequence independently of the other vibrators.

5. A method as claimed in claim 3, wherein:

the vibrators are divided into groups, and a pseudo-random sequence belonging to the same group of minimum crosscorrelation sequences is assigned to each group, the vibrators of a group being controlled by a same elementary sequence with the time lag.

6. A device intended for prospecting or monitoring of an underground formation by elastic waves, comprising:

at least a group of m vibrators, a control unit including a signal generator which controls the vibrators by phase modulated periodic signals, a connection which applies to the vibrators of each group the signals produced by the control unit, seismic receivers coupled with the formation, a system which acquires and records the seismic signals reflected by subsoil discontinuities in response to the emitted periodic signals and a system which processes the recorded seismic signals, by correlation with part of the emitted periodic signals, wherein the control unit includes a modulator which forms m elementary sequences phase modulated by a pseudo-random sequence, the signals generated by the control unit being simultaneously applied to the different vibrators by the connection; and

wherein the vibrators are permanently installed for seismic monitoring of an underground zone.

7. A device as claimed in claim 6, wherein:

the modulator comprises a generator which generates at least one pseudo-random control sequence, a time lag which forms at least one set of m elementary sequences with a time lag between each one respectively, a phase modulator which phase modulates the periodic signals generated by the generator respectively by the m elementary sequences to generate m phase modulated periodic signals.

8. A device as claimed in claim 6, wherein:

the modulator comprises a generator which generates at least one pseudo-random control sequence, a modulator which modulates, by the sequence, the periodic signals generated by the generator, and a time lag which produces m modulated periodic signals with the time lag between each one.

9. A device as claimed in claim 6, wherein:

the signals emitted by the control unit depend on a length of each control sequence and of a received vibrational signal listening time.

10. A device as claimed in claim 6, comprising:

p groups of vibrators, the control unit generating p minimum crosscorrelation elementary sequences and applying the sequences to the vibrators of each group with a time lag.

11. A device as claimed in claim 6, wherein:

the vibrators are permanently installed for seimic monitoring of an underground zone.

12. A device as claimed in claim 7, wherein:

the signals emitted by the control unit depend on a length of each control sequence and of a received vibrational signal listening time.

13. A device as claimed in claim 8, wherein:

the signals emitted by the control unit depend on a length of each control sequence and of a received vibrational signal listening time.

14. A device as claimed in claim 7, comprising:

p groups of vibrators, the control unit generating p minimum crosscorrelation elementary sequences and applying the sequences to the vibrators of each group with a time lag.

15. A device as claimed in claim 8 comprising:

p groups of vibrators, the control unit generating p minimum crosscorrelation elementary sequences and applying the sequences to the vibrators of each group with a time lag.

16. A device as claimed in claim 9, comprising:

p groups of vibrators, the control unit generating p minimum crosscorrelation elementary sequences and applying the sequences to the vibrators of each group with a time lag.

17. A device as claimed in claim 6, comprising:

p groups of vibrators, the control unit generating p minimum crosscorrelation elementary sequences and applying the sequences to the vibrators of each group with a time lag.

18. A device as claimed in claim 7, comprising:

p groups of vibrators, the control unit generating p minimum crosscorrelation elementary sequences and applying the sequences to the vibrators of each group with a time lag.

19. A device as claimed in claim 7, wherein the vibrators are permanently installed for seimic monitoring of an underground zone.

20. A device as claimed in claim 8, wherein the vibrators are permanently installed for seimic monitoring of an underground zone.

21. A device as claimed in claim 9, wherein the vibrators are permanently installed for seimic monitoring of an underground zone.

22. A device as claimed in claim 10, wherein the vibrators are permanently installed for seimic monitoring of an underground zone.

23. A device as claimed in claim 12, wherein the vibrators are permanently installed for seimic monitoring of an underground zone.

24. A device as claimed in claim 13, wherein the vibrators are permanently installed for seimic monitoring an underground zone.

25. A device as claimed in claim 14, wherein the vibrators are permanently installed for seimic monitoring of an underground zone.

26. A device as claimed in claim 15, wherein the vibrators are permanently installed for seimic monitoring of an underground zone.

27. A device as claimed in claim 16, wherein the vibrators are permanently installed for seimic monitoring of an underground zone.

28. A device as claimed in claim 17, wherein the vibrators are permanently installed for seimic monitoring of an underground zone.

29. A device as claimed in claim 18, wherein the vibrators are permanently installed for seimic monitoring of an underground zone.

30. A device intended for prospecting or monitoring of an underground formation by elastic waves, comprising:

at least a group of m vibrators, a control unit including a signal generator which controls the vibrators by phase modulated periodic signals, a connection which applies to the vibrators of each group the signals produced by the control unit, seismic receivers coupled with the formation, a system which acquires and records the seismic signals received by the seismic receivers which are reflected by subsoil discontinuities in response to the emitted periodic signals and a system which processes the recorded seismic signals, by correlation with part of the emitted periodic signals, the control unit includes a modulator which forms m elementary sequences phase modulated by a pseudo-random sequence, the signals generated by the control unit being simultaneously applied to the different vibrators by the connection; and

wherein

the modulator comprises a generator which generates at least one pseudo-random control sequence, a time lag which forms at least one set of m elementary sequences with a time lag between each one respectively, a phase modulator which phase modulates the periodic signals generated by the generator respectively by the m elementary sequences to generate m phase modulated periodic signals; and

the signals emitted by the control unit depend on a length of each control sequence and of a received vibrational signal listening time.

31. A device intended for prospecting or monitoring of an underground formation by elastic waves, comprising:

at least a group of m vibrators, a control unit including a signal generator which controls the vibrators by phase modulated periodic signals, a connection which applies to the vibrators of each group the signals produced by the control unit, seismic receivers coupled with the formation, a system which acquires and records the seismic signals received by the seismic receivers which are reflected by subsoil discontinuities in response to the emitted periodic signals the, the control unit includes a modulator which forms m elementary sequences phase modulated by a pseudo-random sequence, the signals generated by the control unit being simultaneously applied to the different vibrators by the connection; and

wherein

the modulator comprises a generator which generates at least one pseudo-random control sequence, a modulator which modulates, by the sequence, the periodic signals generated by the generator, and a time lag which produces m modulated periodic signals with the time lag between each one and the signals emitted by the control unit depend on a length of each control sequence and of a received vibrational signal listening time.

32. A method of prospecting or monitoring of an underground formation by elastic waves, comprising emitting in the ground elementary sequences formed by coding of a signal by pseudo-random sequences, receiving and recording the signals reflected by the subsoil discontinuities in response to the emitted signals, and processing of the recorded signals by correlation with signals formed from the emitted signals, and wherein

the signals are emitted simultaneously by seismic sources, each controlled by a signal of an elementary sequence of a length at least equal to the product of a number of simultaneously vibrating seismic sources, which are extended before and after a listening time with parts of a length at least equal to the listening time and a respective contribution of each one of the seismic sources is separated by correlating the received and recorded signals by signals constructed from the elementary sequence.

33. A method as claimed in claim 32, wherein:

the elementary sequences are formed by phase modulation of a periodic signal applied to vibrators.

34. A method as claimed in claim 33, wherein:

a same control sequence is applied to all the vibrators, with a time lag and a circular permutation, the time lag between two vibrators being at least equal to the listening time, and a length of the elementary control sequence being at least equal to a sum of the time lag and of the listening time.

35. A method as claimed in claim 33, wherein:

the vibrators are controlled by a connected set of minimum crosscorrelation sequences with each vibrator emitting a vibration with a sequence independent of the other vibrators.

36. A method as claimed in claim 34, wherein:

the vibrators are divided into groups, and a pseudo-random sequence belonging to a same group of minimum crosscorrelation sequences is assigned to each group with the vibrators of a group being controlled by a same elementary sequence with the time lag.