Systems and methods for multiple-code continuous-wave distributed acoustic sensing

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 optical signal of a specified frequency;

c. a frequency modulator for providing a number of shifted frequencies to produce multiple optical signals from the continuous optical signal, each having a unique frequency;

d. a binary code sequence generator driven by a master clock for generating multiple pseudorandom binary codes, wherein the multiple pseudorandom binary codes are selected to minimize cross-correlation between the multiple pseudorandom binary codes;

e. multiple optical modulators having first and second ports for receiving the continuous optical signal from the light source and the multiple pseudorandom binary codes from the binary code sequence generator to produce multiple modulated light signals; and

f. an optical circulator/coupler to combine the multiple modulated light signals from the multiple optical modulators and pass the combined multiple modulated light signals into the optical fiber span positioned into the region of interest, and to receive returned backscattered Rayleigh signals from the optical fiber span positioned in to the region of interest, wherein the returned backscattered Rayleigh signals are directed to a detector system by the optical circulator/coupler;

g. wherein the detector system comprises:

i. a series of demodulators, each of which corresponds to one of the multiple optical modulators for transforming the optical signal from each corresponding optical modulator into an electronic signal and extracting phase information;

ii. a phase differencer, for receiving the electronic signals from the series of demodulators so that the phase information sensed by each code from each region can be combined to obtain pressure signals corresponding to pressure impinging on a determined length of the optical fiber span; and

iii. a processor to detect coherent Rayleigh noise generated by the optical fiber span positioned in the region of interest to identify acoustic perturbations in the region of interest.

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 optical signal of a specific frequency is a laser.

3. The system for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 1 , wherein each of the series of demodulators in the detector system 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 a demodulator of the series of demodulators is a heterodyne demodulator and the decoder utilizes band-pass filtering.

5. The system for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 3 wherein a demodulator of the series of demodulators is a homodyne demodulator and the decoder utilizes low-pass filtering.

6. The system for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 3 wherein the region of interest can include a subsurface wellbore, an oil reservoir, or a pipeline.

7. The system for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 3 wherein the region of interest can include structures such as subsea umbilical's or risers.

8. The system for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 3 wherein the region of interest can include perimeters encircling high security areas.

9. A method for monitoring regions of interest for occurrences that generate acoustic perturbations, comprising:

deploying a fiber optic cable into a region of interest;

transmitting a continuous wave light signal to a frequency modulator;

modulating a frequency of the continuous wave light signal to generate a finite number of optical signals, each having a unique frequency;

transmitting each of the finite number of optical signals to a corresponding number of optical modulators along with bipolar pseudorandom binary sequence codes, each of which is delayed relative to a reference coding sequence, wherein the bipolar pseudorandom binary sequence codes are selected to minimize cross-correlation between the bipolar pseudorandom binary sequence codes, wherein the optical modulators generate modulated optical signals;

combining each of the modulated optical signals in an optical circulator/coupler;

transmitting the combined optical signals from the optical circulator/coupler into the fiber optic cable deployed in the region of interest;

collecting and separating backscattered Rayleigh signals from the deployed fiber optic cable, and transmitting the separated backscattered Rayleigh signals to a series of demodulators corresponding to the number of optical modulators, transforming the separated signals into electronic signals, de-coding the electronic signals by their appropriate codes, and extracting phase information; and

transmitting the electronic signals to a phase differencer so that the phase information sensed by each code from each region can be combined to obtain pressure signals representing pressure impinging on a determined length of the fiber optical cable;

identifying acoustic perturbations from locations in the region of interest based on the separated backscattered Rayleigh signals.

10. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 9 wherein the bipolar pseudorandom binary sequence codes are binary sequences of ones and negative ones.

11. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 9 wherein the occurrences are generated by impacts of sand grains.

12. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 9 wherein the occurrences are generated by proppant noise in hydraulic fracturing operations.

13. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 9 wherein the occurrences are generated by high frequency wellbore leaks.

14. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 9 wherein the occurrences are generated by wireline sonic logging.

15. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 9 wherein the occurrences are generated by inter-zone leaks in wellbores.

16. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 9 wherein the occurrences are generated by flow cavitation.

17. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 9 wherein the occurrences are generated by flow vortex shedding.

18. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 9 wherein the occurrences are generated by a particular flow regime.

19. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 9 wherein the occurrences are generated by a particular flow rate.

20. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 9 wherein the occurrences are generated by a particular fluid fraction.

21. The method for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 9 wherein the occurrences are part of an active ultrasonic flow monitoring system.