Method and apparatus for optical sensing

A method of distributed acoustic sensing, comprising:

• determining acoustic signals incident along a pipe using a distributed acoustic sensor (DAS) system having a fibre optic cable deployed in use along the pipe, the distributed acoustic sensor system receiving backscattered and/or reflected light from along the optical fibre;
• and monitoring fluid flow in the pipe using the distributed acoustic sensor system by determining the speed of sound in one or both directions of fluid flow.

An apparatus, comprising:

• a distributed acoustic sensor (DAS) system having a fibre optic cable deployed in use along a pipe to be monitored, the distributed acoustic sensor system receiving backscattered and/or reflected light from along the optical fibre and determining acoustic signals incident along the fibre optic cable therefrom;
• wherein the distributed acoustic sensor system is arranged to monitor fluid flow in the pipe by tracking eddies generated within the flow over a section of the pipe to determine the flow rate.

A method of distributed acoustic sensing, comprising:

• determining acoustic signals incident along a pipe using a distributed acoustic sensor (DAS) system having a fibre optic cable deployed in use along the pipe, the distributed acoustic sensor system receiving backscattered and/or reflected light from along the optical fibre;
• and monitoring fluid flow in the pipe using the distributed acoustic sensor system by tracking eddies generated within the flow over a section of the pipe to determine flow rate.