Fibre optic cable with tuned transverse sensitivity

1. A fibre optic cable having a longitudinal cable axis comprising;

at least one optical fibre;

a compliant core material surrounding and mechanically coupled to the at least one optical fibre such that a longitudinal force acting on the compliant core material induces a longitudinal strain in the at least one optical fibre; and

at least one deformable strain transformer coupled to the compliant core material and configured such that a force acting on the strain transformer in a direction transverse to the cable axis results in a deformation of the strain transformer thereby applying a longitudinal force to the compliant core material; wherein the at least one strain transformer comprises at least one helically coiled member wherein the helically coiled member is coiled around said compliant core member, wherein the at least one optical fibre is tightly coupled to a buffer material and the buffer material is coupled to the compliant core material.

2. A fibre optic cable as claimed in claim 1 wherein at least a portion of the strain transformer has a shape with a resting longitudinal length in the absence of any external force applied to the cable and is configured such that deformation of the strain transformer in response to a force transverse to the cable axis over a first portion of the cable causes a change in the longitudinal length of strain transformer.

3. A fibre optic cable as claimed in claim 1 wherein the at least one strain transformer is at least partly embedded with the compliant core material.

4. A fibre optic cable as claimed in claim 1 wherein the helix angle of said helically coiled member is less than 45 degrees.

5. A fibre optic cable as claimed in claim 1 wherein the helix angle of said helically coiled member is greater than 5 degrees.

6. A fibre optic cable as claimed in claim 1 wherein the diameter of said helically coiled member is in the range of 3 to 10 mm.

7. A fibre optic cable as claimed in claim 1 wherein at least one portion of the fibre optic cable comprises a plurality of strain transformers, each strain transformer comprising a helically wound coiled member.

8. A fibre optic cable as claimed in claim 7 wherein at least one helically wound coiled member is wound in the opposite direction to another helically wound coiled member.

9. A fibre optic cable as claimed in claim 7 wherein at least one helically wound coiled member is interleaved with another helically wound coiled member.

10. A fibre optic cable as claimed in claim 1 wherein the strain transformer and compliant core material have a strain response of between 0.05% to 0.01% strain per Newton tensile load.

11. A fibre optic cable as claimed in claim 1 wherein the strain transformer comprises a shaped steel member.

12. A fibre optic cable as claimed in claim 1 wherein the compliant core material comprises extruded nylon.

13. A fibre optic cable as claimed in claim 1 comprising a first jacket layer wherein the at least one optical fibre, the compliant core material and the at least one strain transformer are disposed within the first jacket layer.

14. A fibre optic cable as claimed in claim 13 wherein at least part of the strain transformer is coupled to the first jacket layer.

15. A fibre optic cable as claimed in claim 13 wherein the strain transformer and compliant core material are disposed so as to be moveable with respect the first jacket layer.

16. A fibre optic cable as claimed in claim 15 wherein the at least one optical fibre, the compliant core material and the at least one strain transformer may be disposed within a liquid contained within the first jacket layer.

17. A fibre optic cable as claimed, in claim 1 wherein the strain transformer is configured with respect to the compliant core material and the optical fibre to provide a relatively low transverse sensitivity.

18. A fibre optic cable as claimed in claim 17 wherein the strain transformer is configured with respect to the compliant material and optical fibre such that a transverse strain on a given portion of the cable results in substantially no change in the effective optical path length of that portion of the optical fibre.

19. A method of distributed fibre optic sensing comprising interrogating an optical fibre of a fibre optic cable as claimed in claim 17 which is relatively insensitive to transverse acoustic signals.

20. A method as claimed in claim 19 comprising sensing for temperature variations.

21. A fibre optic cable having a longitudinal cable axis comprising:

at least one optical fibre:

a compliant core material surrounding and mechanically coupled to the at least one optical fibre such that a longitudinal force acting on the compliant core material induces a longitudinal strain in the at least one optical fibre; and

at least one deformable strain transformer coupled to the compliant core material and configured such that a force acting on the strain transformer in a direction transverse to the cable axis results in a deformation of the strain transformer thereby applying a longitudinal force to the compliant core material: wherein the at least one strain transformer comprises at least one helically coiled member wherein the helically coiled member is coiled around said compliant core member; and

a first jacket layer wherein the at least one optical fibre, the compliant core material, and the at least one strain transformer are disposed within the first jacket layer so as to be moveable with respect to the first jacket layer, wherein the at least one optical fibre, the compliant core material, and the at least one strain transformer are contained within a second jacket layer which is disposed within the first jacket layer, with the second jacket layer being moveable with respect to the first jacket layer.