Systems and methods for spread spectrum distributed acoustic sensor monitoring
A method and device for monitoring oil field operations with a fiber optic distributed acoustic sensor (DAS) that uses a continuous wave laser light source and modulates the continuous wave output of the laser light source with pseudo-random binary sequence codes.
1. A system for monitoring regions of interest for occurrences that generate acoustic perturbations, comprising:
a. an optical fiber span positioned into a region of interest;
b. a light source for generating a continuous coherent signal of a pre-determined wavelength into the optical fiber;
c. a pseudo-random binary code sequence generator driven by a master clock;
d. an optical modulator having first and second ports for receiving the primary coherent light signal from the light source and a generated pseudo-random binary codes from the pseudo-random binary code sequence generator to produce a modulated light signal;
e. wherein the pseudo-random binary sequence codes are binary sequences of ones and negative ones;
f. and wherein the pseudo-random binary sequence codes are periodic with a period of T b and the minimum period that the code stays at a certain value is T c ;
g. and wherein the fiber optic cable length L, the desired spatial sampling Δ z , and the acoustic signal bandwidth σ A are specified in advance for the application; and
i. N is chosen so that NT c =T b ;
ii. T b is chosen so that
T
b
=
2
L
c
L
+
ρ
,
where ρ is small compared to
2
L
c
L
;
and
iii. T c is chosen so that
Δ
z
=
c
L
ε
=
c
L
T
c
;
h. an optical circulator/coupler to receive the modulated light signal from the optical modulator and pass it into the optical fiber span positioned into the region of interest;
i. a detector system driven by the master clock for de-modulating, correlating, and de-multiplexing returned backscattered Rayleigh signals from the optical fiber span positioned into the region of interest, wherein the detector system has a processor to detect coherent Rayleigh noise generated by the optical fiber span positioned in the region of interest to identify acoustic events in the regions of interest; and
j. wherein the returned backscattered Rayleigh signals from the optical fiber span positioned into the region of interest are directed to the detector system by the optical circulator/coupler.
2. The system for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 1 , wherein the light source for generating a continuous coherent signal of a pre-determined wavelength is a laser.
3. The system for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 1 , wherein the detector system driven by the master clock comprises:
a. a heterodyne or homodyne demodulator;
b. a decoder; and
c. an FM demodulator.
4. The system for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 3 , wherein the detector system further comprises photo detectors and a processor for controlling all of the functions and computations of the detector system and providing the output of acoustic pressure signals.
5. The system for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 3 , wherein the decoder comprises:
a. circuitry for separating the electronic signal from the heterodyne or homodyne demodulator into separate branches representing positions along the sensing fiber optic;
b. circuitry for separating and time delaying the binary coding sequence with a delay proportional to the time it takes for the code to arrive at a defined position of the optical fiber;
c. circuitry for multiplying in time filtering the separated electronic signals from the heterodyne or homodyne demodulator and the corresponding binary coding sequences to obtain signals that contain only the information representing certain positions in the optical fiber; and
d. wherein the circuitries can be implemented either analogically or digitally.
6. The system for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 5 wherein the demodulator is a heterodyne demodulator and the circuitry for multiplying in time and filtering the separated electronic signals utilizes band-pass filtering.
7. The system for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 5 wherein the demodulator is a homodyne demodulator and the circuitry for multiplying in time and filtering the separated electronic signals utilizes low-pass filtering.
8. The system for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 5 wherein the region of interest can include a subsurface wellbore, an oil reservoir, or a pipeline.
9. The system for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 5 wherein the region of interest can include structures such as subsea umbilical's or risers.
10. The system for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 5 wherein the region of interest can include perimeters encircling high security areas.
11. A method for monitoring regions of interest for occurrences that generate acoustic perturbations, comprising:
a. deploying a fiber optic cable into a region of interest;
b. transmitting a continuous wave laser light source through the fiber optic cable;
c. modulating the continuous wave output of the laser light source with pseudo-random binary sequence codes; wherein the pseudo-random binary sequence codes are binary sequences of ones and negative ones;
d. and wherein the pseudo-random binary sequence codes are periodic with a period of T b and the minimum period that the code stays at a certain value is T c ;
e. and wherein the fiber optic cable length L, the desired spatial sampling Δ z , and the acoustic signal bandwidth σ A are specified in advance for the application; and
i. N is chosen so that NT c =T b ;
ii. T b is chosen so that
T
b
=
2
L
c
L
+
ρ
,
where ρ is small compared to
2
L
c
L
;
and
iii. T c is chosen so that
Δ
z
=
c
L
ε
=
c
L
T
c
.
f. detecting backscattered Rayleigh signals from the deployed fiber optic cable; and
g. using the detected backscattered Rayleigh signals to identify and measure the acoustic perturbations from locations in the region of interest.
12. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 11 wherein the occurrences are generated by impacts of sand grains.
13. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 11 wherein the occurrences are generated by proppant noise in hydraulic fracturing operations.
14. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 11 wherein the occurrences are generated by high frequency wellbore leaks.
15. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 11 wherein the occurrences are generated by wireline sonic logging.
16. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 11 wherein the occurrences are generated by inter-zone leaks in wellbores.
17. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 11 wherein the occurrences are generated by flow cavitation.
18. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 11 wherein the occurrences are generated by flow vortex shedding.
19. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 11 wherein the occurrences are generated by a particular flow regime.
20. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 11 wherein the occurrences are generated by a particular flow rate.
21. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 11 wherein the occurrences are generated by a particular fluid fraction.
22. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 11 wherein the occurrences are part of an active ultrasonic flow monitoring system.