Acoustic sensing system for downhole seismic applications utilizing an array of fiber optic sensors

• US 6,269,198 B1
• Filed: 10/29/1999
• Issued: 07/31/2001
• Est. Priority Date: 10/29/1999
• Status: Expired due to Term

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Litton Systems Incorporated (Northrop Grumman Corporation)
• Inventors: Frederick, Donald A., Hodgson, Craig W.
• Primary Examiner(s): Ullah, Akm E.
• Assistant Examiner(s): CONNELLY, MICHELLE R
• Application Number:
  US09/430,058
• Time in Patent Office: 641 Days
• Field of Search: 385/4, 385/7, 385/12, 385/13, 385/24, 359/115, 359/123, 359/331, 356/345
• US Class Current: 385/13
• CPC Class Codes: G01H 9/004 using fibre optic sensors light guides per se G02B6/00, acousto-optical devices specially adapted for gating or modulating in optical wave guides G02F1/125
• CPC Class Codes: G01V 1/52 Structural details

Abstract

A system for sensing subterranean acoustic waves emitted from an acoustic source includes a plurality of laser sources, each emitting light at different frequencies; a plurality of subterranean optical sensors, receiving and altering the light in response to acoustic waves; at least one optical detector, receiving the altered light and outputting an electrical signal; and electronics, receiving the electrical signal and converting it into seismic data format. Light from the optical sources is modulated at a plurality of modulation frequencies. The electronics can demodulate the signal by mixing the signal with periodic waveforms having frequencies corresponding to one and two times the modulation frequencies. The modulation frequencies are selected such that at least one of the second harmonic frequencies associated with the modulation frequencies is interleaved in a non-interfering manner within the corresponding set of first harmonic frequencies, and preferably such that at least one of the first harmonic frequencies is interleaved in a non-interfering manner within the corresponding set of modulation frequencies. The hydrophone for sensing the acoustic signals is able to operate at pressures and temperatures of at least 5,000 psi and 130° C., respectively. The hydrophone may be housed in a cable having a diameter less than about 1.5 inches. The hydrophone'"'"'s sensor preferably includes a reference mandrel, two sensing mandrels, and a telemetry cam, which are aligned in a coaxial, end-to-end configuration to reduce the profile of the hydrophone. The mandrels have hemispherically-shaped endcaps and are joined by flexible interlinks having grooves for receiving optical fiber.

Claims

1. A frequency division multiplexed fiber optic acoustic sensor system for simultaneously detecting acoustical information at N spaced points, comprising:

N light sources, wherein each light source is modulated at a different modulation frequency;

N distribution fiber lines, wherein each distribution fiber line is connected to a different one of said N light sources;

a return fiber line;

a sensor group comprising N fiber optic acoustical sensors, wherein each acoustical sensor is connected between a different one of said N distribution fiber lines and said return fiber line; and

a detector connected to said return fiber line;

wherein said N modulation frequencies are selected such that;

each modulation frequency has a corresponding signal component that is not overlapped in frequency space by any other modulation frequency component or by any harmonic frequency components of any order associated with any of said N modulation frequencies;

a first harmonic frequency component associated with each modulation frequency is not overlapped in frequency space by any modulation frequency component or by any other harmonic frequency components of any order associated with any of said N modulation frequencies; and

at least one of said N second harmonic frequencies is interleaved within said first harmonic frequencies.

2. The acoustic sensor system of claim 1, wherein:

said N modulation frequencies are equally spaced with a separation Δ

f; and

the lowest frequency of said N modulation frequencies is (2N−

3)Δ

f.

3. The acoustic sensor system of claim 2, wherein N=6.

4. The acoustic sensor system of claim 1, wherein:

said N modulation frequencies are equally spaced with a separation Δ

f, except for a 2Δ

f gap around the frequency 3(N−

2)Δ

f;

and the lowest frequency of said N modulation frequencies is (2N−

4)Δ

f.

5. The acoustic sensor system of claim 4, wherein N=6.

6. The acoustic sensor system of claim 1, wherein:

said N modulation frequencies are equally spaced with a separation Δ

f, except for a 2Δ

f separation centered at the frequency 3(N−

2)Δ

f; and

the lowest frequency of said N modulation frequencies is (2N−

5)Δ

f.

7. The acoustic sensor system of claim 6, wherein N=6.

8. The acoustic sensor system of claim 1, wherein at least one of said N first harmonic frequencies is interleaved within said modulation frequencies.

9. The acoustic sensor system of claim 8, wherein:

N=6; and

said modulation frequencies equal 5⅔

, 7, 8, 9, 10, and 12½

times a minimum frequency separation, Δ

f.

10. The acoustic sensor system of claim 8, wherein at least one of said N second harmonic frequencies is interleaved within said modulation frequencies.

11. The acoustic sensor system of claim 10, wherein at least one of said N third harmonic frequencies is interleaved within said modulation frequencies.

12. The acoustic sensor system of claim 11, wherein:

the two lowest frequencies of said N modulation frequencies equal 3 times and 4 times a minimum frequency separation, Δ

f; and

the remaining N−

2 modulation frequencies equal Δ

f times the first N−

2 consecutive prime numbers beginning at 5.

13. The acoustic sensor system of claim 12, wherein N=6.

14. A frequency division multiplexed fiber optic acoustic sensor system having N light sources modulated at N different modulation frequencies, f₁, f₂, . . . f_N, the N modulation frequencies selected such that none of said N modulation frequencies and their corresponding N first harmonic frequencies is closer in frequency space to any other modulation frequency or to any other harmonic frequency of any order associated with any modulation frequency than some minimum frequency, Δ

f, and such that at least one of said second harmonic frequencies is interleaved within the first harmonic frequencies.

15. The system of claim 14 wherein:

f₁(2N−

3)Δ

f; and

for 1<

n≦

N, f_n=f<sub>n−

1</sub>+Δ

f.

16. The system of claim 15 wherein N=6.

17. The system of claim 14 wherein:

f₁=(2N−

4)Δ

f;

for 1<

n<

N−

1, f_n=f<sub>n−

1</sub>+Δ

f;

f<sub>N−

1</sub>=f<sub>N−

2</sub>+2Δ

f, and

f_N=(3N−

4)Δ

f.

18. The system of claim 17 wherein N=6.

19. The system of claim 14 wherein:

f₁=(2N−

5)Δ

f;

for 1<

n<

N, f_n=f<sub>n−

1</sub>+Δ

f; and

f_N=f<sub>N−

1</sub>+2Δ

f.

20. The system of claim 19 wherein N=6.

21. The system of claim 14 wherein at least one of said first harmonic frequencies is interleaved within said modulation frequencies.

22. The system of claim 21 wherein:

f₁=5⅔

Δ

f;

f₂=7Δ

f;

f₃=8Δ

f;

f₄=9Δ

f;

f₅=10Δ

f; and

f₆=12½

Δ

f.

23. The system of claim 21 wherein at least one of said second harmonic frequencies is interleaved within said modulation frequencies.

24. The system of claim 23 wherein at least one of said third harmonic frequencies is interleaved within said modulation frequencies.

25. The system of claim 24 wherein:

f₁=3Δ

f;

f₂=4Δ

f; and

for 2<

n≦

N, f_n=X_nΔ

f, wherein X_n is the (n−

2)^th consecutive prime number starting at 5.

26. The system of claim 25 wherein N=6.