Optical range-finding methods are well established to determine the distance of events along a fibre. Optical time-domain reflectometry is just one example of a range-finding method. We can use range-finding to establish the point at which an unspooled FLI fibre becomes spooled on the bobbin by measuring the light loss along the length of the fibre. This allows us to measure the depth to the top of the FLI tool when it has reached the bottom of the well. However, the design of the FLI tool itself enables us to use methods that greatly improve upon the accuracy of conventional range-finding:
1. We know the precise length of each layer of fibre wound on the bobbin
We wind the fibre on to a bobbin inside the FLI tool in layers with a precisely determined length. During a job we can 'see' the layers of fibre wound on the bobbin. This means that we can count the layers left on the spool and convert that count into an accurate length of fibre that has exited the FLI tool. In effect the layers act as markers along the fibre, without needing to resort to expensive doping techniques to fix chemical markers into the glass itself
2. The FLI tool is a highly sensitive microphone that can be found acoustically
Spooled fibre on the bobbin acts as an extremely sensitive microphone. During an acoustic survey we can detect the obvious difference in sensitivity between the spooled and unspooled fibre, from which we can then determine the location of the FLI probe.