Systems and methods for spread spectrum distributed acoustic sensor monitoring

Abstract

A method and device for monitoring oil field operations with a <span class="c21 g0">fiberspan> optic distributed acoustic sensor (DAS) that uses a <span class="c0 g0">continuousspan> wave laser light source and modulates the <span class="c0 g0">continuousspan> wave output of the laser light source with pseudo-random binary sequence codes.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

where c(t) is a spread-spectrum code signal, such that ∫c(t)c(t+τ)dt=δ(τ), and ω_s is the carrier frequency. This results in the reception of the signal:
E_b(t)=∫₀^zr(z)μ_Ac(t−2c_L⁻¹{circumflex over (z)}(t,z))E_sscos(ω_s(t−2c_L⁻¹{circumflex over (Z)}

where c_L is the speed of light, E_ss and μ_A are constants, r(z) is the distributed reflection along the fiber, and

$[[\hat{z}\left(t,z\right)=z+{\mu }_L{\int }_0^{Z}p\left(t,x\right)\mathrm{dx}]]$ $\hat{z}\left(t,z\right)=z+{\mu }_L{\int }_0^{Z}p\left(t,z\right)\mathrm{dz}$

with p(t,z) being the pressure at position z and time t.

Then upon heterodyne (or homodyne) demodulation to a baseband signal (but with the signal still spread):

$b\left(t\right)=\left[E_b\left(t\right)\cos \left(\left({\omega }_s+{\Delta }_{\omega }\right)t\right)\right]*h_{\mathrm{LP}}\left(t\right)\approx {\int }_0^{Z}r\left(z\right){\mu }_{A}c\left(t-2c_L^{-1}\hat{z}\left(t,z\right)\right)E_{\mathrm{ss}}\cos \left({\Delta }_{\omega }t-2{\omega }_s{c}_L^{-1}\hat{z}\left(t,z\right)\right)dz,$

where h_LP(t) is a low-pass filter that removes the undesired spectral components around 2ω_s. In the case of homodyne demodulation Δ_ω=0.

Then the demodulated baseband signal can be decoded by:

$\stackrel{\_}{b}\left(t,z_i\right)=\left[c\left(t-2c_L^{-1}{z}_i\right)b\left(t\right)\right]*h_{\mathrm{PB}}\left(t\right)=\left[{\int }_0^{Z}r\left(z\right){\mu }_{A}c\left(t-2c_L^{-1}{z}_i\right)c\left(t-2c_L^{-1}\hat{z}\left(t,z\right)\right)E_{\mathrm{ss}}\cos \left({\Delta }_{\omega }t-2{\omega }_s{c}_L^{-1}\hat{z}\left(t,z\right)\right)dz\right]*h_{\mathrm{PB}}\left(t\right)\approx r\left(z_i\right){\mu }_A{E}_{\mathrm{ss}}\cos \left({\Delta }_{\omega }t-2{\omega }_s{c}_L^{-1}\hat{z}\left(t,z_i\right)\right).$

Where h_PB(t) is a band pass filter for heterodyne demodulation or a low-pass filter for homodyne demodulation.

More information regarding decoding is provided in the next section.

Decoding Analysis

Incorporating the information from the pass-band filter h_PB(t) into the de-spreader:

$\stackrel{\_}{b}\left(t,z_i\right)=\left[c\left(t-2c_L^{-1}{z}_i\right)b\left(t\right)\right]*h_{\mathrm{PB}}\left(t\right)=\left[{\int }_0^{Z}r\left(z\right){\mu }_{A}c\left(t-2c_L^{-1}{z}_i\right)c\left(t-2c_L^{-1}\hat{z}\left(t,z\right)\right)E_{\mathrm{ss}}\cos \left({\Delta }_{\omega }t-2{\omega }_s{c}_L^{-1}\hat{z}\left(t,z\right)\right)dz\right]*h_{\mathrm{PB}}\left(t\right)={\int }_0^{\infty }{\int }_0^Z{h}_{\mathrm{PB}}\left(t-\tau \right)r\left(z\right){\mu }_{A}c\left(\tau -2c_L^{-1}{z}_i\right)c\left(\tau -2c_L^{-1}\hat{z}\left(\tau ,z\right)\right)E_{\mathrm{ss}}\cos \left({\Delta }_{\omega }\tau -2{\omega }_s{c}_L^{-1}\hat{z}\left(\tau ,z\right)\right)dzd\tau ={\int }_0^Z{E}_{\mathrm{ss}}r\left(z\right)\left[{\int }_0^{\infty }{h}_{\mathrm{PB}}\left(t-\tau \right){\mu }_{A}c\left(\tau -2c_L^{-1}{z}_i\right)c\left(\tau -2c_L^{-1}\hat{z}\left(\tau ,z\right)\right)\cos \left({\Delta }_{\omega }\tau -2{\omega }_s{c}_L^{-1}\hat{z}\left(\tau ,z\right)\right)d\tau \right]dz$

If we now assume that:

$c\left(\tau -2c_L^{-1}\hat{z}\left(\tau ,z\right)\right)=c\left(\tau -2c_L^{-1}\left(z+{\mu }_L{\int }_0^{z}p\left(t,x\right)\mathrm{dx}\right)\right)\approx c\left(\tau -2c_L^{-1}z\right),$

that is, that the time delay variation caused by the acoustic pressure is negligible when compared to the time delay caused by the time of flight of the optic wave; it is possible to write:

$\overline{b}\left(t,z_i\right)\approx {\int }_0^{\propto }{E}_{\mathrm{ss}}r\left(\text{⁠}z\right)\left[\text{⁠}{\int }_0^{\infty }{h}_{\mathrm{PB}}\left(t-\tau \right){\mu }_{A}c\left(\tau -2c_L^{-1}{z}_i\right)c\left(\tau -2c_L^{-1}z\right)\cos \left({\Delta }_{\omega }\tau -2{\omega }_s{c}_L^{-1}\hat{z}\left(\tau ,z\right)\right)d\tau \right]\mathrm{dz}.$

It will be considered that the code c(t) has bandwidth σ_c, and also has the following property:

$c\left(t\right)c\left(t+\delta \right)\approx \{\begin{array}{cc}1,& \mathrm{if}❘\delta ❘\le ϵ\\ d\left(t\right),& \mathrm{if}❘\delta ❘>ϵ\end{array}$

where function d(t) is the result of spreading the code twice, and has a bandwidth of 2σ_c. Hence, the integration region in the z variable can be decomposed into two disjoint sets:
₁={z|z≤z_i+|c_Lϵ⁻¹|}
₂={z|z>z_i+|c_Lϵ⁻¹|}.

Thus the received signal can be written as:

$\overline{b}\left(t,z_i\right)\approx {\int }_{z_1\in {ℛ}_1}{E}_{\mathrm{ss}}r\left(z_1\right)\left[{\int }_0^{\infty }{h}_{\mathrm{PB}}\left(t-\tau \right){\mu }_A\cos \left({\Delta }_{\omega }\tau -2{\omega }_s{c}_L^{-1}\hat{z}\left(\tau ,z_1\right)\right)d\tau \right]{\mathrm{dz}}_1$

If the FM signal bandwidth is σ_FM, then most of the information of region in ₂ is spread by the function d(t), and has bandwidth 2(σ_c+σ_FM) and is centered around frequency Δ_ω, and most of the information of region in ₁ is concentrated in frequency, has a bandwidth of σ_FM, and is centered around frequency Δ_ω.

With that information, it is possible to specify the filter h_PB(t) with center frequency Δ_ω and passband of σ_FM that removes most of the information from the region ₂ while leaving the information from ₁ unaltered.

The decoded signal, then, can be written as:
b(t,z_i)≈ω_{zi−cLϵ−1}^zi+cLϵ−1E_ssr(μ_A cos(Δ_ωτ2ωsc_L⁻¹{circumflex over (z)}(τ,z))dz+v(t,z),

where v(t,z) is a nuisance signal. It is also possible to note:

• • The larger the bandwidth of σ_c relative to σ_FM, the easier it is to remove the interference from other spatial regions of the fiber.
  • The smaller the value of ϵ, the better the tuning of the spatial information, thus allowing more spatial points to be sampled.
  • On the other hand, the value of E cannot be so small as to make the approximation that the time delay variation caused by the acoustic pressure is negligible compared to the time delay caused by the time of flight of the optic wave invalid.
  • The higher the beat frequency Δ_ω, the more selective the filter h_PB must be.
  • The center frequency should be high enough so that it is possible to retrieve the acoustic pressure signal.

Acoustic Signal and FM Signal Bandwidth

Ideally, the decoded FM signal captured at position z_i is:

${\overline{b}}^{†}\left(t,z_i\right)=r\left(z_i\right)E_{s}s{\mu }_A\cos \left({\Delta }_{\omega }t-2{\omega }_s{c}_L^{-1}{z}_i-2{\omega }_s{c}_L^{-1}{\mu }_L{\int }_0^{z_i}p\left(t,x\right)\mathrm{dx}\right).$

Carson's rule states that for a signal of the form:
s_FM(t)=A_c cos(ω_ct+f_ΔΨ(t)),

and its bandwidth is:
σ_FM=2(f_Δ+σ_A),

where σ_A is the bandwidth of the modulating signal.

Adapting the Carson's rule for the decoded signal, one obtains:

${\sigma }_{FM}\approx 2\left(\frac{2{\omega }_s{\mu }_L}{c_L}+{\sigma }_A\right)=2\left({\sigma }_{\Delta }+{\sigma }_A\right),$

where this approximate σ_FM usually covers 98% of the energy of the FM signal. It should also be noted that σ_A is actually the bandwidth of the derivative of p(t, z_i). In practice, since there are an infinite number of p(t,z) influencing the FM signal, the worst-case (largest possible value of σ_A) should be selected. Alternatively, a bandwidth for the acoustic pressure can be arbitrarily chosen and then the assumed FM signal bandwidth can be determined.

With this background and term definition we are now in a position to propose a code design.

Code Design

We have found that for the applications of this disclosure Maximal Length Sequences (M-Sequences) and the use of auto-correlation provide excellent code candidates. In particular, two parameters are of interest for the spread spectrum sensing using fiber optics: the ϵ of the sequence and its bandwidth. ϵ (epsilon) is the smallest delay to the signal for which the sequence can be recovered. Any delay larger than epsilon, produces a noise-like sequence.

M-Sequences are bipolar sequences that can be generated through the use of a feedback-shift register (FSR). For the sake of the following discussion, it will be considered that c(t)ϵ{−1,1} and that it is periodic with period equal T_b, also the minimum period that the code stays at a certain value is T_c.

The following properties are true for an m-sequence.

• • Its auto-correlation is

$R_c\left(\tau \right)=\{\begin{array}{cc}1-\frac{N+1}{{\mathrm{NT}}_c}❘\tau ❘,& ❘\tau ❘\le T_c\\ -\frac{1}{N},& T_c<❘\tau ❘\le T_b\end{array},$

• • where T_b=NT_c.
  • The product of two time-aligned codes is c²(t)=1.
  • Its power spectral density is

$S_c\left(f\right)=\frac{1}{N^2}\delta \left(f\right)+\frac{1+N}{N^2}\sum _{n=-\infty }^{n=\infty }{\mathrm{sinc}}^2\left(\frac{n}{N}\right)\delta \left(f-\frac{n}{N{T}_c}\right),$

• • and the spectrum is discrete-valued and has an envelope that follows that of a sinc²(⋅) function. Using this information it is possible to approximate the signal bandwidth. Hence, the bandwidth of σ_c can be approximated as 2/Tc.

Code Requirements

Using the properties just defined in the previous section, the following specifications can be defined for a coding sequence.

The symbol period T_c is related to the autocorrelation properties of the sequence. Also, it can be seen that the shorter the period the more different two time shifted codes become. Hence, the parameter ϵ is directly proportional to T_c:
ϵ∝T_c,

The smaller the T_c, the better is the ability of the code to pick out the signal from a desired position.

The possible spatial sampling Δ_z of the z axis is also governed by the choice of T_c. A conservative separation between positions equal to

${\Delta }_z=\frac{c_L}{ϵ}=\frac{c_L}{T_c}.$

Thus, the smaller the period of the code the greater the number of positions that can be sampled.

The symbol period is also related to the code bandwidth. In order to yield a good separation of signals from neighboring regions, the code bandwidth should be greater than the bandwidth of the FM signal:
σ_c=2/Tc>>2(σ_Δ+σ_A),

where σ_Δ is the spread in frequency introduced by frequency modulation and σ_A is the acoustic signal bandwidth. so that,

$\frac{6\pi }{\left({\sigma }_{\Delta }+{\sigma }_A\right)}\gg \left({\mathrm{NT}}_c\right).$

Since the code is periodic, its period T_b should be greater than that of the time it takes for the light to transverse the whole fiber optic cable and arrive back at the receiver. Mathematically

$T_b={\mathrm{NT}}_c>\frac{2L}{c_L},$

where L is the length of the fiber optic.

Combining the equations above, one has

$\frac{6\pi }{\left({\sigma }_{\Delta }+{\sigma }_A\right)}\gg {\mathrm{NT}}_c>\frac{2L}{c_L},$

which gives a loose upper bound and a more tight lower bound for the requirement for the code length. Considering these bounds, a good strategy would be to use a code with length close to (but not equal to)

$\frac{2L}{c_L}.$

The following steps would then be employed to specify the system:

• • 1. Specify the fiber optic cable length L, the desired spatial sampling Δ_z, and acoustic signal bandwidth σ_A.
  • 2. Choose T_b so that

$T_b=\frac{2L}{c_L}+\rho ,$
where ρ is small when compared to

$\frac{2L}{c_L};$

• • 3. Choose N so that NT_c=T_b and T_c so that

${\Delta }_z=\frac{c_L}{ϵ}=\frac{c_L}{T_c}.$

Turning now to FIG. 2, a system for monitoring region of interest for occurrences that generate acoustic perturbations is described. A fiber optic waveguide 2 is positioned into a region of interest, which may be an oil or gas wellbore, oil or gas reservoir, or an extended pipeline. Some possible deployments will be illustrated in a later figure. A light source 1 is used to generate a continuous primary coherent signal of a pre-determined wavelength that is fed to the fiber optic waveguide. A binary code sequence generator 4, coupled to a master clock 6 supplies an electronic code c(t) to an optical modulator 3 that receives the primary coherent light signal and modulates it based on the input from the binary code sequence generator. The now modulated light signal from modulator 3 then enters an optical circulator/coupler 5 that receives the modulated light signal and passes it into the optical fiber span positioned in the region of interest. Positions Z₁, Z₂, . . . Z_n along the deployed optical fiber span represent locations at lengths L₁, L₂, . . . L_n at which the modulated light signal interacts with the optical fiber and returns backscattered Rayleigh signals. The numeral 8 represent the terminal end of the deployed optical waveguide. The backscattered Rayleigh signals are directed by the optical circulator/coupler 5 into a detector 7 that performs functions of heterodyne demodulation, decoding, and FM demodulation. Detector 7 also has photo detectors for detecting and measuring the light signals and a processor for directing all of the functions of demodulation and decoding necessary to produce measured the desired acoustic pressure signals p(t,z) along the length of the deployed optical fiber span.

A more detailed depiction of the detector system 7, to explain the separate functions of heterodyne or homodyne demodulation, decoding, and FM demodulation is shown in FIG. 3. In section 201 the optical signal is heterodyne demodulated by combining the optical signal E_b(t) with another optical signal cos((ω_s+Δ_ω) t that is shifted in frequency by ω_s+Δ_ω relative to the received signal. For homodyne demodulation Δ_ω=0. The output of demodulator 201, now an electronic signal, is submitted to decoder 202, which extracts the information of the positions Z₁, Z₂, . . . , Z_n of the fiber which are being sensed. The phase of each of the signals are then extracted by the FM Demodulator and the acoustic pressure signal p(t,z₁) . . . p(t,z_n) associated with each position along the fiber is obtained. Not shown in the elements of the detector system would be 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.

FIG. 4 exhibits more details regarding decoder 202 of FIG. 3. The decoder provides circuitry for separating the electronic signal b(t) from the heterodyne demodulator into separate branches representing the positions Z₁, Z₂, . . . , Z_n along the sensing fiber optic. The binary coding sequence c(t) is also split into several signals, each signal being time delayed with a delay proportional to the time it takes for the code to arrive at a defined position of the fiber. The circuitry for providing this functionality could be provided either analogically or digitally. The electronic signal and the delayed coding sequences are then multiplied in time and band-pass filtered (low-pass filtered in the case of homodyne demodulation) to obtain a signal that only contains the information of a certain position of the optical fiber.

Some possible configurations for deployment of distributed sensing systems in and around a wellbore are shown in FIG. 5, as 27, 28, and 29. These configurations are examples and not meant to be exhaustive. Configuration 27 is a fairly typical retrievable wireline in which a fiber optic cable 33 is deployed within metal tubing 34 and down to a bottom hole gauge or termination 36. The metal tubing 34 is surrounded by production casing 32, which is surrounded by a surface casing 31 near the surface. Configuration 28 represents a permanent tubing installation in which a fiber optic cable 33 is attached to metal tubing 34. And configuration 29 represents a casing attachment in which the fiber optic cable 33 is attached outside the production casing 32. As discussed earlier there are other possible configurations (not shown) when using distributed sensing systems in applications such as perimeter security systems, monitoring of subsea umbilical's, risers, or pipelines.

FIG. 7 spells out the preferred code requirements for the Maximal Length Sequences (M-Sequences) proposed in this disclosure along with the use of auto-correlation. In step 410 the practitioner specifies the fiber optic length, the desired spatial sampling, and the acoustic bandwidth. Then in step 420 Tb is chosen so that it is very close to

$T_b=\frac{2L}{c_L}.$
Then in step 430 N is chosen so that NT_c=T_b and T_c so that

${\Delta }_z=\frac{c_L}{ϵ}=\frac{c_L}{T_c}.$

Although certain embodiments and their advantages have been described herein in detail, it should be understood that various changes, substitutions and alterations could be made without departing from the coverage as defined by the appended claims. Moreover, the potential applications of the disclosed techniques is not intended to be limited to the particular embodiments of the processes, machines, manufactures, means, methods and steps described herein. As a person of ordinary skill in the art will readily appreciate from this disclosure, other processes, machines, manufactures, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes. machines, manufactures. means, methods or steps.

Claims

27. A method, comprising:

modulating a <span class="c0 g0">continuousspan>-wave light beam with pseudo-random binary sequence codes;

transmitting the modulated <span class="c0 g0">continuousspan>-wave light beam through a <span class="c21 g0">fiberspan>-optic cable;

wherein the pseudo-random binary sequence codes are periodic with a <span class="c3 g0">periodspan> of T_b and the minimum <span class="c3 g0">periodspan> of each code is T_c;

and wherein a length of the <span class="c21 g0">fiberspan>-optic cable is L, a <span class="c11 g0">spatialspan> <span class="c12 g0">samplingspan> <span class="c3 g0">periodspan> along the <span class="c21 g0">fiberspan> is Δ_z, and a <span class="c15 g0">bandwidthspan> of a sensed acoustic perturbation is <span class="c16 g0">σspan>_A; and

N is chosen so that NT_c=T_b;

T_b is chosen so that

e####

$T_b=\frac{2L}{c_L}+\rho ,$

and

T_c is chosen so that

$\Delta z=\frac{c_L}{\in }=\frac{C_L}{T_c};$

detecting backscattered rayleigh signals from the <span class="c21 g0">fiberspan> optic cable; and

using the detected backscattered rayleigh signals to identify and to measure one or more acoustic perturbations from one or more locations along the <span class="c21 g0">fiberspan>-optic cable.

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

a. deploying a <span class="c21 g0">fiberspan> optic cable into a region of interest;

b. transmitting a <span class="c0 g0">continuousspan> wave laser light source through the <span class="c21 g0">fiberspan> optic cable;

c. modulating the <span class="c0 g0">continuousspan> 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 <span class="c3 g0">periodspan> of T_b and the minimum <span class="c3 g0">periodspan> that the code stays at a certain value is T_c;

e. and wherein the <span class="c21 g0">fiberspan> optic cable length L, the <span class="c10 g0">desiredspan> <span class="c11 g0">spatialspan> <span class="c12 g0">samplingspan> Δ_z, and the acoustic <span class="c2 g0">signalspan> <span class="c15 g0">bandwidthspan> <span class="c16 g0">σspan>_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

$T_b=\frac{2L}{c_L}+\rho ,$

 where ρ is small compared to

$\frac{2L}{c_L};$

 and

iii. T_c is chosen so that

${\Delta }_z=\frac{c_L}{ϵ}=\frac{C_L}{T_c}.$

f. detecting backscattered rayleigh signals from the deployed <span class="c21 g0">fiberspan> optic cable; and

g. using the detected backscattered rayleigh signals to identify and measure the acoustic perturbations from locations in the region of interest.

26. A <span class="c6 g0">systemspan>, comprising:

a light source <span class="c7 g0">configuredspan> to generate a <span class="c0 g0">continuousspan> <span class="c1 g0">coherentspan> <span class="c2 g0">signalspan> of a predetermined wavelength into an <span class="c20 g0">opticalspan> <span class="c21 g0">fiberspan>;

a pseudo-random binary code sequence generator <span class="c7 g0">configuredspan> to be driven by a master clock;

an <span class="c20 g0">opticalspan> modulator having first and second ports <span class="c7 g0">configuredspan> to receive the primary <span class="c1 g0">coherentspan> light <span class="c2 g0">signalspan> from the light source and a generated pseudo-random binary codes from the pseudo-random binary code sequence generator to produce a modulated light <span class="c2 g0">signalspan>;

wherein the pseudo-random binary sequence codes are binary sequences of ones and negative ones;

and wherein the pseudo-random binary sequence codes are periodic with a <span class="c3 g0">periodspan> of T_b and the minimum <span class="c3 g0">periodspan> that the code stays at a certain value is T_c;

and wherein an <span class="c20 g0">opticalspan>-<span class="c21 g0">fiberspan> length L, a <span class="c10 g0">desiredspan> <span class="c11 g0">spatialspan> <span class="c12 g0">samplingspan> Δ_z, and an acoustic <span class="c2 g0">signalspan> <span class="c15 g0">bandwidthspan> <span class="c16 g0">σspan>_A are specified in advance for the application; and

N is chosen so that NT_c=T_b;

T_b is chosen so that

e####

$\mathrm{Tb}=\frac{2L}{c_L}+\rho ,$

where ρ is small compared to

$\frac{2L}{c_L};$

 and

T_c is chosen so that

$\Delta z=\frac{c_L}{\in }=\frac{C_L}{T_c};$

an <span class="c20 g0">opticalspan> circulator/coupler <span class="c7 g0">configuredspan> to receive the modulated light <span class="c2 g0">signalspan> from the <span class="c20 g0">opticalspan> modulator and to pass it into the <span class="c20 g0">opticalspan> <span class="c21 g0">fiberspan>;

a <span class="c5 g0">detectorspan> <span class="c6 g0">systemspan> <span class="c7 g0">configuredspan> to be driven by the master clock for de-modulating, correlating, and de-multiplexing returned backscattered rayleigh signals from the <span class="c20 g0">opticalspan> <span class="c21 g0">fiberspan>, wherein the <span class="c5 g0">detectorspan> <span class="c6 g0">systemspan> has a <span class="c4 g0">processorspan> <span class="c7 g0">configuredspan> to detect <span class="c1 g0">coherentspan> rayleigh noise generated by the <span class="c20 g0">opticalspan> <span class="c21 g0">fiberspan> to identify acoustic events; and

wherein the <span class="c20 g0">opticalspan> circulator/coupler is <span class="c7 g0">configuredspan> to direct the returned backscattered rayleigh signals from the <span class="c20 g0">opticalspan> <span class="c21 g0">fiberspan> to the <span class="c5 g0">detectorspan> <span class="c6 g0">systemspan>.

1. A <span class="c6 g0">systemspan> for monitoring regions of interest for occurrences that generate acoustic perturbations, comprising:

a. an <span class="c20 g0">opticalspan> <span class="c21 g0">fiberspan> span positioned into a region of interest;

b. a light source for generating a <span class="c0 g0">continuousspan> <span class="c1 g0">coherentspan> <span class="c2 g0">signalspan> of a pre-determined wavelength into the <span class="c20 g0">opticalspan> <span class="c21 g0">fiberspan>;

c. a pseudo-random binary code sequence generator driven by a master clock;

d. an <span class="c20 g0">opticalspan> modulator having first and second ports for receiving the primary <span class="c1 g0">coherentspan> light <span class="c2 g0">signalspan> from the light source and a generated pseudo-random binary codes from the pseudo-random binary code sequence generator to produce a modulated light <span class="c2 g0">signalspan>;

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 <span class="c3 g0">periodspan> of T_b and the minimum <span class="c3 g0">periodspan> that the code stays at a certain value is T_c;

g. and wherein the <span class="c21 g0">fiberspan> optic cable length L, the <span class="c10 g0">desiredspan> <span class="c11 g0">spatialspan> <span class="c12 g0">samplingspan> Δ_z, and the acoustic <span class="c2 g0">signalspan> <span class="c15 g0">bandwidthspan> <span class="c16 g0">σspan>_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

$T_b=\frac{2L}{c_L}+\rho ,$

 where ρ is small compared to

$\frac{2L}{c_L};$

 and

iii. T_c is chosen so that

${\Delta }_z=\frac{c_L}{ϵ}=\frac{C_L}{T_c};$

h. an <span class="c20 g0">opticalspan> circulator/coupler to receive the modulated light <span class="c2 g0">signalspan> from the <span class="c20 g0">opticalspan> modulator and pass it into the <span class="c20 g0">opticalspan> <span class="c21 g0">fiberspan> span positioned into the region of interest;

i. a <span class="c5 g0">detectorspan> <span class="c6 g0">systemspan> driven by the master clock for de-modulating, correlating, and de-multiplexing returned backscattered rayleigh signals from the <span class="c20 g0">opticalspan> <span class="c21 g0">fiberspan> span positioned into the region of interest, wherein the <span class="c5 g0">detectorspan> <span class="c6 g0">systemspan> has a <span class="c4 g0">processorspan> to detect <span class="c1 g0">coherentspan> rayleigh noise generated by the <span class="c20 g0">opticalspan> <span class="c21 g0">fiberspan> 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 <span class="c20 g0">opticalspan> <span class="c21 g0">fiberspan> span positioned into the region of interest are directed to the <span class="c5 g0">detectorspan> <span class="c6 g0">systemspan> by the <span class="c20 g0">opticalspan> circulator/coupler.

2. The <span class="c6 g0">systemspan> for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 1, wherein the light source for generating a <span class="c0 g0">continuousspan> <span class="c1 g0">coherentspan> <span class="c2 g0">signalspan> of a pre-determined wavelength is a laser.

3. The <span class="c6 g0">systemspan> for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 1, wherein the <span class="c5 g0">detectorspan> <span class="c6 g0">systemspan> driven by the master clock comprises:

a. a heterodyne or homodyne demodulator;

b. a decoder; and

c. an FM demodulator.

4. The <span class="c6 g0">systemspan> for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 3, wherein the <span class="c5 g0">detectorspan> <span class="c6 g0">systemspan> further comprises photo detectors and a <span class="c4 g0">processorspan> for controlling all of the functions and computations of the <span class="c5 g0">detectorspan> <span class="c6 g0">systemspan> and providing the output of acoustic pressure signals.

5. The <span class="c6 g0">systemspan> for monitoring regions of interest for occurrences that generate acoustic perturbations of claim 3, wherein the decoder comprises:

a. circuitry for separating the electronic <span class="c2 g0">signalspan> from the heterodyne or homodyne demodulator into separate branches representing positions along the sensing <span class="c21 g0">fiberspan> 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 <span class="c20 g0">opticalspan> <span class="c21 g0">fiberspan>;

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 <span class="c20 g0">opticalspan> <span class="c21 g0">fiberspan>; and

d. wherein the circuitries can be implemented either analogically or digitally.

6. The <span class="c6 g0">systemspan> 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 <span class="c6 g0">systemspan> 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 <span class="c6 g0">systemspan> 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 <span class="c6 g0">systemspan> 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 umbilicals or risers.

10. The <span class="c6 g0">systemspan> 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 high-security areas.

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 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 <span class="c6 g0">systemspan>.

23. The method of claim 11 wherein the region of interest includes at least a portion of a perimeter of a high-security area.

24. The method of claim 11 wherein the region of interest includes at least one straight portion.

25. The method of claim 11 wherein deploying the <span class="c20 g0">opticalspan> <span class="c21 g0">fiberspan> includes deploying the <span class="c20 g0">opticalspan> <span class="c21 g0">fiberspan> to have at least one straight portion.