Tracking objects in conduits

A method of tracking movement of an object in a conduit comprising:

• taking data corresponding to signal returns from interrogation of an optic fibre positioned along the path of said conduit to provide distributed acoustic sensing;
• and
• processing the returns from each of a plurality of discrete longitudinal sensing portions to detect an acoustic signature associated with movement of said object in said conduit and determine the origin of said acoustic signature,
• wherein the step of processing the returns comprises the steps of, for each of a plurality of test positions along the conduit:
• forming a first subset of data corresponding to a plurality of frames of data from discrete sensing portions corresponding to portions of the conduit on one side of the test position ;
• forming a second subset of data corresponding to a plurality of frames of data from discrete sensing portions corresponding to portions of the conduit on the opposite side of the test position;
• and
• processing the first and second subsets to determine the presence of an acoustic disturbance in the first and second subsets propagating away from the test position.