## 3 Results  
### Fully Distributed Detection and Localization of Short Impacts on Pipes  
As a first test of the ability of the system to localize short mechanical impacts on the pipe, the pipe was hit on the front face (Figure 2, position A) while a continuous measurement by C-OTDR was carried out using the fully distributed sensor configuration 1. A pulse length of 100 ns was chosen to achieve high signal intensity at acceptable spatial resolution. Figure 4a shows a temporal excerpt of the acquired raw data. Signal intensity is plotted as a function of time and bin number representing the position along the optical fiber. The signal appears as a zigzag pattern between bins 19 and 101. Considering, the spatial resolution of 10 m along the optical fiber for the chosen pulse length of 100 ns, data outside the region between bins 28 and 92 was discarded for further evaluation. The remaining range of 64 bins or \(64\times 0.57\) m \(=36.5\) m corresponds well to the region of fiber which is in close contact to the pipe surface due to the helical wrapping. No signal is visible outside this fiber region, that could be attributed to the mechanical impact. Therefore, the influence of air-borne sound caused by the impact can be neglected and the pattern observed in Figure 4 is understood as the structure-borne acoustic wave traveling in the pipe material, which is reflected from both ends of the pipe segment. It extends the full length of the pipe without any obvious damping and can be observed for several seconds. This underlines the necessity to capture the propagation of the acoustic wave at the onset of the signal on a fast time-scale in order achieve a localization of the event on the pipe. The slope of the edges appearing in Figure 4 allows to deduce the velocity of signal propagation throughout the pipe material. After filtering of raw data for improved edge detection (differentiation with temporal shift of 2), the signal onset could be automatically detected by setting an appropriate threshold. Points of signal onset are used for linear fitting as shown in Figure 4b. The slope of the fitted line amounts to (\(5370\pm 190\)) m/s. Although propagation with this velocity appears to be dominant over time, a second mode of acoustic propagation with a considerably lower velocity can be discerned. In Figure 4c a temporal excerpt is shown, where manifestation of this mode is hardly overlaid by other signals. However, for this mode, automatic edge detection is not readily obtained. Manually, a velocity of approximately 3000 m/s may be estimated from the slope marked in Figure 4c, which was drawn along the corresponding edge visible in the image. For a pipeline with greater length less reflections of acoustic waves and therefore less crossings of the acoustic signatures are expected. Hence, detection of additional edges like the one observed here may be simplified. Figure 3: **(a)** Schematic illustration and photo of the optical fiber configuration used in the experiments involving gas explosions in pipes. Positions of the two sensor penetrations for piezoelectric pressure transducers are indicated. **(b)** Detail photos of the optical fiber wrapped helically around the pipe under test. In a second step, a more realistic scenario of impact detection and localization was realized by hitting the pipe in an arbitrary position on the side (Figure 2, position B). Such a scenario may also be encountered in industrial plants with long connected pipe systems. Additionally, this setting allows to assess localization accuracy and precision for different settings of the DAS system, i.e., different optical pulse lengths. Raw data and filtered data recorded for such events using different pulse lengths are compared in Figure 5. As expected, acoustic waves were observed traveling from a common source to either side of the pipe, where they were reflected. As a result, many crossing signals lead to blurring of the acoustic signatures after the first reflections on the edges.