18.1.3 Measurements: Pulse Rate and Sample Locations
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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.