18.1.3 Measurements: Pulse Rate and Sample Locations

A typical DAS measurement cycle consists of the fiber being interrogated by a sequence of pulses at a repetition rate arranged such that the scatter from the distal end of the fiber has time to return along the length of the fiber and be detected before the next pulse being launched. Such an interrogation is often referred to as a ping in the same sense as that used in sonar.

In Figure 18.1.3-1 , the animation shows that the coherent scatter pattern remains constant on successive pings until the fiber is disturbed. Vibration from an acoustic source modulates the fiber altering the physical length of the waveguide and in turn the relative positions of the scatter site. In turn, the coherent sum of the scatter sites in the disturbed region is also modified, thereby affecting the scatter pattern. The data gathered recording the scatter pattern over several pulses makes it possible, with correct processing, to determine a history of the nature of acoustic source as a function of position.

Figure 18.1.3-1 Vibration from an acoustic source modulates the fiber and hence the scatter pattern. The data gathered makes it possible to determine a history of the nature of acoustic source as a function of position. The animation shows that when a sound source is adjacent to the fiber at any location, the back scattered intensity from that location is altered (red line), so that it repeats the undisturbed signal for most of the fiber length, but diverges from it where the noise source is (red). By processing these variations all along the fiber length, the acoustic signal at each point can be obtained. (Images courtesy of OptaSense)