Drift correction in a fiber optic distributed acoustic sensing system

Patent metadata

Field	Value
Publication number	US-11796382-B2
Application number	US-201916690950-A
Country	US
Kind code	B2
Filing date	Nov 21, 2019
Priority date	Nov 21, 2019
Publication date	Oct 24, 2023
Grant date	Oct 24, 2023

Abstract

An apparatus for sensing acoustic energy in a borehole penetrating the earth includes an optical interrogator and a sensing optical fiber having a length Ls optically coupled to the optical interrogator and configured to sense the acoustic energy to provide sensed acoustic data. The apparatus also includes a reference optical fiber having a length Lr optically coupled to the optical interrogator to provide reference data, wherein the optical interrogator corrects the sensed acoustic data using the reference data to provide corrected sensed acoustic data.

First claim

What is claimed is:

1. An apparatus for sensing acoustic energy in a borehole penetrating the earth, the apparatus comprising:
an optical interrogator comprising a light source that emits light at a frequency,
a sensing optical fiber having a length Ls optically coupled to the optical interrogator and configured to sense the acoustic energy to provide sensed acoustic data, the sensing optical fiber having a gauge length Lg over which a strain measurement is made; and
a reference optical fiber having a length Lr optically coupled to the optical interrogator to provide reference data, wherein the length Lr is greater than or equal to the gauge length Lg; and
a processor that receives the sensed acoustic data and the reference data and is configured to implement an algorithm comprising one or more equations to remove a phase measurement error in the sensed acoustic data due to drift Δf in the frequency f of the light emitted by the light source to provide sensed acoustic energy data with reduced error, the one or more equations comprising an index of refraction of the sensing optical fiber and an index of refraction of the reference optical fiber;
wherein the algorithm implements the following equations:

ϕ
s

≈

ϕ
′

s

-

Δϕ
r

,

ϕ
′

s

=

2
⁢

π
⁡
(

f
+

Δ
⁢
f

)

⁢

nL
g

(

1
+
ϵ

)

c

,
and

Δϕ
r

=

2
⁢
π
⁢

nL
g

⁢
Δ
⁢
f

c

,

wherein:
ϕ s represents phase measurement of sensed acoustic energy with reduced error;
ϕ′ s represents resultant phase measurement of sensed acoustic energy due to frequency drift of the light source;
Δϕ r represents change in phase in the reference optical fiber due to frequency drift of the light source;
n represents index of refraction of corresponding optical fiber;
L g represents gauge length;
ϵ represents dynamic strain on the sensing optical fiber;
Δf represents drift in frequency of light emitted by the light source; and
c represents the speed of light.

2. The apparatus according to claim 1 , wherein the sensing optical fiber and the reference optical fiber comprise reflective imperfections for Rayleigh scattering.

3. The apparatus according to claim 1 , further comprising an optical circulator configured to optically couple the optical interrogator to the sensing optical fiber and to the reference optical fiber.

4. The apparatus according to claim 1 , wherein the reference optical fiber is directly in series with the sensing optical fiber with no intervening optical device.

5. The apparatus according to claim 4 , wherein the reference optical fiber is disposed between the optical interrogator and the sensing optical fiber.

6. The apparatus according to claim 1 , wherein the reference optical fiber is in a coil.

7. The apparatus according to claim 6 , further comprising a stabilizer environment that contains the reference optical fiber, the stabilizer environment comprising at least one of an acoustic energy absorber and insulation to limit at least one of acoustic energy contamination and temperature drift.

8. The apparatus according to claim 1 , wherein the optical interrogator interrogates the reference optical fiber and the sensing optical fiber with the gauge length Lg.

9. The apparatus according to claim 4 , wherein the gauge length Lg is a distance between two successive light pulses emitted by the optical interrogator.

10. The apparatus according to claim 1 , wherein the optical interrogator corrects for drift of frequency of light emitted by the optical interrogator.

11. The apparatus according to claim 10 , wherein the reference data comprises a reference phase value and the sensed acoustic data comprises a sensed phase value and the drift of frequency of the light is corrected by subtracting (i) a phase error resulting from strain error due to the drift of frequency from (ii) the sensed phase value.

12. The apparatus according to claim 1 , wherein the sensing optical fiber and the reference optical fiber comprise fiber Bragg gratings.

13. A method for sensing acoustic energy in a borehole penetrating the earth, the method comprising:
obtaining sensed acoustic data from a sensing optical fiber having a length Ls disposed in the borehole using an optical interrogator optically coupled to sensing optical fiber, the optical interrogator emitting light from a light source at a frequency f the sensing optical fiber having a gauge length Lg over which a strain measurement is made;
obtaining reference data from a reference optical fiber having a length Lr using the optical interrogator that is optically coupled to the reference optical fiber, wherein the length Lr is greater than or equal to the gauge length Lg; and
processing, by a processor, the sensed acoustic data using the reference data by implementing an algorithm comprising an one or more equations to remove a phase measurement error in the sensed acoustic data due to drift Δf in the frequency f of the light emitted by the light source to provide sensed acoustic energy data with reduced error, the one or more equations comprising an index of refraction of the sensing optical fiber and an index of refraction of the reference optical fiber;
wherein the algorithm implements the following equations:

ϕ
s

≈

Assignees

• Johnston William Albert
• Barry Alexander Michael
• Baker Hughes Oilfield Operations Llc

Inventors

• Johnston William Albert
• Barry Alexander Michael

Classifications

• G01V2210/123

  Passive source, e.g. microseismics · CPC title

• G01H9/004 Primary

  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 title

• E21B47/135 Primary

  using light waves, e.g. infrared or ultraviolet waves · CPC title

• G01V1/44

  using generators and receivers in the same well (G01V1/52 takes precedence) · CPC title

• G02B6/02342

  characterised by cladding features, i.e. light confining region · CPC title