Method and apparatus of distributed acoustic sensing
There is provided a method and an apparatus of fiber optic distributed acoustic sensing (DAS) which use a simple, reliable and robust signal processing for amplitude-based DAS measurements. 1) The lack of linearity of the amplitude-based DAS measurements (due to its unpredictable variation of the transfer function along the fiber) can be improved by normalizing the amplitude vs distance using an amplitude-normalization trace obtained from either a) coherent laser OTDR measurements with laser frequency dithering or b) computing a normalization trace from the coherent OTDR/DAS traces and the un-coherent OTDR/DAS traces. 2) Instrument offset (e.g., due to low vertical sampling resolution) may further be corrected using un-coherent OTDR/DAS traces to extract an instrument system offset.
1 . A fiber optic distributed acoustic sensing (DAS) method for performing acoustic and vibration measurements, the method comprising:
performing repetitive DAS acquisitions toward an end of an optical fiber link using a DAS acquisition device comprising a coherent Optical Time Domain Reflectometer (OTDR) to provide DAS traces P (n,k) (z), wherein each DAS acquisition is performed by propagating in the optical fiber link, a pulsed test signal and detecting corresponding return light from the optical fiber link so as to obtain a DAS trace representing backscattered and reflected light as a function of distance in the optical fiber link;
processing said DAS traces to produce a DAS signal;
obtaining an amplitude-normalization trace Γ(z) with respect to distance along the optical fiber link; and
normalizing said DAS signal using the amplitude-normalization trace, thereby improving a linearity of the DAS signal.
2 . The DAS method as claimed in claim 1 , further comprising extracting an instrument offset and correcting said DAS signal to remove the extracted instrument offset.
3 . The DAS method as claimed in claim 2 , wherein said instrument offset is extracted from un-coherent DAS acquisitions.
4 . The DAS method as claimed in claim 2 , wherein said instrument offset is extracted from said DAS acquisitions by filtering said DAS acquisitions to extract a low-frequency signal.
5 . The DAS method as claimed in claim 1 ,
wherein the DAS traces are acquired and processed in groups; and
wherein DAS acquisitions are repeated to obtain a plurality of independent groups of K DAS traces P (n,k) (z) and a corresponding plurality of DAS signals, such that independent DAS signals are obtained using said independent groups of DAS traces.
6 . The DAS method as claimed in claim 5 , wherein processing said DAS traces to produce a DAS signal comprises:
for each independent group of DAS traces P (n,k) (z), calculating a Root Mean Square (RMS) difference as a function of the distance z along the optical fiber link, to obtain a corresponding DAS signal √{square root over ( ΔP (n,k) (z) 2 K )}.
7 . The DAS method as claimed in claim 6 ,
wherein the amplitude-normalization trace Γ(z) is computed by RMS averaging un-dithered acquired DAS traces as:
Γ
(
z
)
=
{
Δ
P
(
j
,
z
)
(
z
0
)
2
〉
Δ
z
〈
P
0
(
z
0
)
〉
×
〈
P
0
(
z
)
〉
.
8 . The DAS method as claimed in claim 7 , further comprising:
repeating the step of performing repetitive un-dithered DAS acquisitions to provide multiple groups of un-dithered DAS traces P (j,z) (z 0 ); and
averaging over the multiple groups of un-dithered DAS traces P (j,z) (z 0 ) to obtain an averaged amplitude-normalization trace Γ(z) calculated as:
Γ
(
Z
)
=
∑
i
=
1
I
(
{
Δ
P
(
j
,
z
)
(
z
0
)
2
〉
Δ
z
〈
P
0
(
z
0
)
〉
×
〈
P
0
(
z
)
〉
)
/
I
wherein I is the number of groups of un-dithered DAS traces P (j,z) (z 0 ).
9 . The DAS method as claimed in claim 6 ,
wherein obtaining the amplitude-normalization trace Γ(z) comprises:
performing repetitive DAS acquisitions while applying frequency dithering to provide a group of frequency-dithered DAS traces P dit (i,k) (z); and
processing said frequency-dithered DAS traces to produce the amplitude-normalization trace Γ(z).
10 . The DAS method as claimed in claim 9 , further comprising:
repeating the step of performing repetitive DAS acquisitions while applying frequency dithering to provide multiple groups of frequency-dithered DAS traces P dithe (i,k) (z); and
averaging over the multiple groups of frequency-dithered DAS traces P dither(i,k) (z) to obtain an averaged amplitude-normalization trace Γ(z) calculated as:
Γ
(
z
)
=
∑
i
=
1
I
(
(
Δ
P
dithe
(
i
,
k
)
(
z
)
2
〉
K
)
/
I
wherein I is the number of groups of frequency-dithered DAS traces P dither(i,k) (z).
11 . The DAS method as claimed in claim 9 , further comprising:
performing repetitive acquisitions toward an end of an optical fiber link using an un-coherent Optical Time Domain Reflectometer (OTDR) to provide un-coherent traces P 0(k) (z).
12 . The DAS method as claimed in claim 11 , wherein said amplitude-normalization trace Γ(z) is calculated as:
Γ
(
z
)
=
〈
(
P
dither
(
i
,
k
)
(
z
)
-
∑
k
=
1
K
(
P
dither
(
i
,
k
)
(
z
)
)
/
K
)
2
〉
K
-
〈
Δ
P
0
(
k
)
(
z
)
2
〉
K
.
13 . A fiber optic distributed acoustic sensing (DAS) system for performing acoustic and vibration measurements, the DAS system comprising:
a DAS acquisition device connectable toward an optical fiber link and comprising a coherent Optical Time Domain Reflectometer (OTDR) for performing repetitive DAS acquisitions, wherein each DAS acquisition is performed by propagating in the optical fiber link, a pulsed test signal and detecting corresponding return light from the optical fiber link so as to obtain a DAS trace representing backscattered and reflected light as a function of distance in the optical fiber link;
a memory to store said DAS traces, at least one group of DAS traces at a time; and
a processing unit receiving said DAS traces and configured for:
processing said DAS traces to produce a DAS signal;
obtaining an amplitude-normalization trace Γ(z) with respect to distance along the optical fiber link; and
normalizing said DAS signal using the amplitude-normalization trace, thereby improving a linearity of the DAS signal.
14 . The DAS system as claimed in claim 13 ,
wherein obtaining the amplitude-normalization trace Γ(z) comprises:
performing repetitive DAS acquisitions while applying frequency dithering to provide a group of frequency-dithered DAS traces P dith (i,k) (z); and
processing said frequency-dithered DAS traces to produce the amplitude-normalization trace Γ(z).
15 . The DAS system as claimed in claim 13 ,
wherein the amplitude-normalization trace Γ(z) is computed by RMS averaging un-dithered acquired DAS traces as:
Γ
(
z
)
=
{
Δ
P
(
j
,
z
)
(
z
0
)
2
〉
Δ
z
〈
P
0
(
z
0
)
〉
×
〈
P
0
(
z
)
〉
.
16 . The DAS system as claimed in claim 13 ,
wherein the DAS traces are acquired and processed in groups; and
wherein DAS acquisitions are repeated to obtain a plurality of independent groups of K DAS traces P (n,k) (z) and a corresponding plurality of DAS signals, such that independent DAS signals are obtained using said independent groups of DAS traces.
17 . The DAS system as claimed in claim 16 , wherein processing said DAS traces to produce a DAS signal comprises:
for each independent group of DAS traces P (n,k) (z), calculating a Root Mean Square (RMS) difference as a function of the distance z along the optical fiber link, to obtain a corresponding DAS signal √{square root over ( ΔP (n,k) (z) 2 K )}.
18 . A non-transitory computer-readable storage medium comprising instructions that, when executed, cause a processor to perform the steps of:
receiving DAS acquisitions performed toward an end of an optical fiber link using a DAS acquisition device comprising a coherent Optical Time Domain Reflectometer (OTDR), wherein each DAS acquisition is performed by propagating in the optical fiber link, a pulsed test signal and detecting corresponding return light from the optical fiber link so as to obtain a DAS trace P (n,k) (z) representing backscattered and reflected light as a function of distance in the optical fiber link;
processing said DAS traces to produce a DAS signal;
obtaining an amplitude-normalization trace Γ(z) with respect to distance along the optical fiber link;
normalizing said DAS signal using the amplitude-normalization trace, thereby improving a linearity of the DAS signal.
19 . The non-transitory computer-readable storage medium as claimed in claim 18 ,
wherein obtaining the amplitude-normalization trace Γ(z) comprises:
receiving DAS acquisitions performed repetitively while applying frequency dithering to provide a group of frequency-dithered DAS traces P dithe (i,k) (z); and
processing said frequency-dithered DAS traces to produce the amplitude-normalization trace Γ(z).
20 . The non-transitory computer-readable storage medium as claimed in claim 18 ,
wherein the amplitude-normalization trace Γ(z) is computed by RMS averaging un-dithered acquired DAS traces as:
Γ
(
z
)
=
{
Δ
P
(
j
,
z
)
(
z
0
)
2
〉
Δ
z
〈
P
0
(
z
0
)
〉
×
〈
P
0
(
z
)
〉
.