Acoustically enhanced optical cables
Methods and apparatus to control the acoustic properties of optical cables used as in-well oil and gas probes for acoustic monitoring, such as distributed acoustic sensing (DAS). One example aspect provides a solid path for the acoustic wave to propagate from an outside armor layer of the cable to the sensing optical waveguide embedded therein. Another example aspect offers ways to spatially dispose the optical sensing elements to create response delays indicative of the propagation speed and/or direction of an acoustic wave. Yet another example aspect provides ways to utilize additional spectral interrogation to increase ultimate spatial resolution. Yet another example aspect provides ways to locally vary the acoustic properties along the length of the cable.
1. An optical cable comprising:
a tube;
an optical waveguide disposed in the tube; and
one or more solid structures disposed between an inner surface of the tube and an outer surface of the optical waveguide, the one or more solid structures being configured to form at least a portion of a solid path for acoustic waves to travel from an environment outside the tube to the optical waveguide.
2. The optical cable of claim 1 , wherein the one or more solid structures contact the optical waveguide.
3. The optical cable of claim 1 , wherein the one or more solid structures are attached to the inner surface of the tube at one or more locations along a length of the tube.
4. The optical cable of claim 3 , wherein the one or more solid structures are attached to the inner surface of the tube via at least one of solder or an adhesive.
5. The optical cable of claim 1 , wherein the one or more solid structures comprise a strip of material having a plurality of bends.
6. The optical cable of claim 5 , wherein the plurality of bends are periodic for at least a portion of the strip.
7. The optical cable of claim 6 , wherein the plurality of bends have a first periodicity for a first portion of the strip and a second periodicity for a second portion of the strip.
8. The optical cable of claim 5 , wherein a periodicity of the plurality of bends corresponds to a resolution of a distributed acoustic sensing system based on backscattering of light introduced into the optical waveguide.
9. The optical cable of claim 5 , wherein the strip of material having the plurality of bends forms a spring capable of being compressed radially between the tube and the optical waveguide.
10. The optical cable of claim 1 , wherein the tube has a non-circular cross-section.
11. The optical cable of claim 10 , wherein the non-circular cross-section comprises an oval cross-section having a short axis and a long axis and wherein the one or more solid structures are disposed along the short axis of the oval cross-section.
12. The optical cable of claim 1 , wherein the tube comprises a wall having varying thicknesses in at least one of a length of the optical cable or a cross-section of the optical cable.
13. The optical cable of claim 1 , wherein a cross-section of the tube comprises a first thickness of a wall between an outer diameter and an inner diameter of the tube that is different from a second thickness of the wall in the same cross-section of the tube.
14. The optical cable of claim 1 , wherein the one or more solid structures include two solid structures disposed on opposite sides of the optical waveguide.
15. The optical cable of claim 14 , wherein the optical waveguide is positioned by the two solid structures to follow a central axis of the tube.
16. The optical cable of claim 1 , further comprising one or more transducers mechanically coupled to an outer surface of the tube.
17. An optical cable comprising:
a tube;
a first acoustic sensor comprising a first optical waveguide disposed in the tube; and
a second acoustic sensor comprising a second optical waveguide disposed in the tube, wherein the second acoustic sensor has different acoustic properties from the first acoustic sensor.
18. The optical cable of claim 17 , wherein:
the first acoustic sensor further includes a first capillary;
the first optical waveguide is disposed in the first capillary;
the second acoustic sensor further includes a second capillary; and
the second optical waveguide is disposed in the second capillary.
19. The optical cable of claim 18 , wherein the different acoustic properties are due to the second capillary having different acoustic characteristics from the first capillary.
20. The optical cable of claim 17 , wherein:
the first acoustic sensor is disposed in a first channel;
the second acoustic sensor is disposed in a second channel; and
at least one of the first channel or the second channel is disposed adjacent an inner surface of the tube.
21. The optical cable of claim 20 , wherein at least one of the first channel or the second channel contains ultrasonic soldering fluid.
22. The optical cable of claim 17 , wherein the different acoustic properties include different frequency ranges, such that the first acoustic sensor has a different frequency range from the second acoustic sensor.
23. A method for determining at least one of a direction or a speed of an acoustic wave, the method comprising:
providing an optical cable comprising a tube and one or more optical waveguides disposed in the tube, each optical waveguide having one or more cores surrounded by a cladding, wherein at least one of an angular or radial position of the one or more cores in the one or more optical waveguides is known at one or more locations along a length of the optical cable;
measuring times at which the acoustic wave stimulates the one or more cores in the one or more optical waveguides at one of the locations; and
based on the measured times and on the at least one of the known angular or radial position of the one or more cores in the one or more optical waveguides, determining the at least one of the direction or the speed of the acoustic wave at the one of the locations.
24. The method of claim 23 , wherein:
the one or more optical waveguides comprise:
a first optical waveguide having a first core surrounded by a first cladding; and
a second optical waveguide having a second core surrounded by a second cladding; and
measuring the times at which the acoustic wave stimulates the one or more cores comprises:
measuring a first time at which the acoustic wave stimulates the first core at the one of the locations; and
measuring a second time at which the acoustic wave stimulates the second core at the one of the locations, the second time being different from the first time.
25. The method of claim 24 , wherein at least one of an angular or radial position of the first core is known at the one of the locations and wherein at least one of an angular or radial position of the second core is known at the one of the locations.
26. The method of claim 23 , wherein the one or more optical waveguides includes an optical waveguide disposed helicoidally in the tube.
27. The method of claim 26 , wherein the optical waveguide has a core and wherein the measuring comprises:
measuring a first time at which the acoustic wave stimulates a first part of the core at the one of the locations; and
measuring a second time at which the acoustic wave stimulates a second part of the core at the one of the locations.
28. The method of claim 27 , wherein the first part of the core and the second part of the core are disposed in a single coil of the helicoidally disposed optical waveguide at the one of the locations.
29. The method of claim 23 , wherein:
the one or more optical waveguides comprise a single optical waveguide having two or more cores; and
measuring the times at which the acoustic wave stimulates the one or more cores comprises monitoring a chronological sequence at which the acoustic wave is stimulating each of the two or more cores.
30. The method of claim 23 , wherein the optical cable is a noncircular optical cable, and wherein a position and an orientation of the optical cable are known.