Grating position dithering for improved distributed acoustic sensing engineered fiber performance

Patent metadata

Field	Value
Publication number	US-2023141615-A1
Application number	US-202117524615-A
Country	US
Kind code	A1
Filing date	Nov 11, 2021
Priority date	Nov 11, 2021
Publication date	May 11, 2023
Grant date	—

Abstract

An optical system employs a method for measuring an acoustic signal in a wellbore. The optical system includes an optical interrogator and an optical fiber. The optical fiber has a plurality of nominal sites uniformly spaced apart along a longitudinal axis of the optical fiber. A plurality of gratings are formed in the optical fiber. Each of the plurality of gratings is associated with a nominal site and is separated from its associated nominal site by an offset distance. The offset distance is selected to reduce a destructive interference between reflections from the plurality of gratings. The optical interrogator transmits a light pulse into the optical fiber to measure the acoustic signal via a reflection of the light pulse from at least one of the plurality of gratings.

First claim

What is claimed is:

1 . An optical system for measuring an acoustic signal, comprising:
an optical interrogator configured to transmit a light pulse; an optical fiber for receiving the light pulse, the optical fiber having a plurality of nominal sites uniformly spaced apart along a longitudinal axis of the optical fiber; and a plurality of gratings formed in the optical fiber, each of the plurality of gratings associated with a nominal site and separated from its associated nominal site by an offset distance, wherein the offset distance is selected to reduce a destructive interference between reflections from the plurality of gratings.

2 . The optical system of claim 1 , wherein the light pulse transmitted by the optical interrogator defines a light pulse length.

3 . The optical system of claim 2 , wherein the plurality of nominal sites are spaced apart from each other to at a distance of about one or more nominal sites within a light pulse length.

4 . The optical system of claim 1 , wherein the plurality of gratings further comprises a plurality of chirped Fiber Bragg gratings.

5 . The optical system of claim 1 , wherein the offset distance is greater than a quarter wavelength of the light pulse.

6 . The optical system of claim 5 , wherein the offset distance is selected using a dithering function defined within a dithering range, wherein a range limit of the dithering range is between about 1 micron and about 1 centimeter.

7 . The optical system of claim 5 , wherein the plurality of gratings includes a first grating separated from a first nominal site by a first offset distance and a second grating separated from a second nominal site by a second offset distance, wherein the first offset distance is a first value selected using the dithering function and the second offset distance is a second value selected using the dithering function.

8 . The optical system of claim 1 , wherein a signal-to-noise ratio obtained from the plurality of gratings located at the offset distances is greater than the signal-to-noise ratio obtained from the plurality of gratings located at the plurality of nominal sites.

9 . A method of measuring an acoustic signal in a wellbore, comprising:
disposing an optical fiber in the wellbore, the optical fiber having a plurality of gratings formed therein, each of the plurality of gratings being located at a selected offset distance from a respective one of a plurality of nominal sites of the optical fiber, the plurality of nominal sites being uniformly spaced along a longitudinal axis of the optical fiber, wherein each offset distance is selected to reduce a destructive interference between reflections from the plurality of gratings; and an optical interrogator configured to transmit a light pulse into the optical fiber to measure the acoustic signal via a reflection of the light pulse from at least one of the plurality of gratings.

10 . The method of claim 9 , wherein the light pulse defines a light pulse length.

11 . The method of claim 10 , wherein the plurality of nominal sites are spaced apart from each other at about one or more nominal sites within a light pulse length.

12 . The method of claim 9 , wherein the plurality of gratings further comprises a plurality of chirped Fiber Bragg gratings.

13 . The method of claim 9 , wherein the offset distance is greater than a quarter wavelength of the light pulse.

14 . The method of claim 9 , wherein the offset distance is selected using a dithering function defined within a dithering range, wherein a range limit of the dithering range is between about 1 micron and about 1 centimeter.

15 . The method of claim 14 , wherein the plurality of gratings includes a first grating separated from a first nominal site by a first offset distance and a second grating separated from a second nominal site by a second offset distance, wherein the first offset distance is a first value selected using the dithering function and the second offset distance is a second value selected using the dithering function.

Assignees

• Johnston William
• Wysocki Paul
• Provenzano Dan
• Baker Hughes Oilfield Operations Llc

Inventors

• Johnston William
• Wysocki Paul
• Provenzano Dan

Classifications

• G02B6/2931

  Diffractive element operating in reflection · CPC title

• G01H9/004

  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

• G01V2210/1429

  Subsurface, e.g. in borehole or below weathering layer or mud line · CPC title

• G01V1/226 Primary

  Optoseismic systems · CPC title