FLI is fixed at the wellhead, but drum-spooled cables are not

With a drum-spooled optical cable, a substantial portion (perhaps even the majority) of the cable is on the drum at surface. Furthermore, as the cable unspools from the drum it is moving relative to the wellhead. This means that working out how much of the unspooled cable is in the well at any time is not trivial. People often have to rely on causing “events” at the wellhead to work out its position along the optical cable. Typical examples of such events are placing ice bags at the wellhead to cause a sharp temperature drop or tapping on the wellhead to cause a sharp acoustic response.

To complicate matters further, the length of fibre inside a drum-spooled cable is not the same length as the cable itself. Optical fibre has almost no stretch before it snaps; metal wire can stretch considerably under its own weight as it gets deeper in the well. This issue is overcome by installing “slack” in the fibre. As the fibre is mobile inside the cable, the amount of slack and its location is an additional error that has to be corrected for.