## 3 Results
### Quasi-Distributed Detection and Localization of Short Impacts on Pipes  
Localization of short impacts on a pipe equipped with a quasi-distributed fiber-optic sensor configuration as described in Figure 2b was tested in a similar manner by hitting the pipe mechanically in positions A and B. The data evaluation procedure is exemplarily presented for position A and a pulse length of 50 ns in Figure 6. The two sensor regions are visible in the raw data (Figure 6a) between bin 17 and bin 67 as well as bin 114 and bin 163, corresponding to ranges of 29 m and 28 m. Further processing is carried out in a range between bin 25 and bin 59 and between bin 122 and 155, respectively, to evade influences from the loosely hanging fiber regions. In each sensor region, the signal is detected simultaneously over all bins and is recurring periodically as the acoustic wave is reflected on both ends of the pipe. Therefore, the two sensor regions indeed act as point sensors with respect to the propagating acoustic waves, and signal processing using summation over the range of each localized sensor (e.g., RQD) is a valid approach. The potential of such an approach is illustrated in Figure 6b-d. While (b), (c) illustrate the temporal evolution of the signal intensity for one bin taken from the center of either region without and with filtering, (d) shows  
Figure 6: (a) Temporal excerpt of filtered data (differentiation with temporal shift of 1) acquired when hitting the pipe on the front face using a quasi-distributed sensor configuration with two localized sensor ranges, (b–d) time traces recorded by the sensor closer to (blue) and further away (orange) from the point of impact, when observing raw data of one bin, filtered data from the same bin or filtered data summed over the full range of bins relevant to the point sensor, respectively. the signal obtained after summation of filtered data over the total range of bins in each region. The latter exhibits a significantly improved signal-to-noise ratio and hence much more expressed signal onset. The time points of signal onset may be readily obtained from these traces by setting an appropriate threshold and the velocity calculated from the time difference and the known distance between the localized sensors is (\(5320\pm 180\)) m/s. The measurement uncertainty is given by error propagation of the position uncertainty introduced by the finite length of the two localized sensors and the uncertainty in the determination of the time difference between the two sensor regions. Additionally, the recurring nature of the signal may be used for data evaluation by correlation of the two time traces presented in Figure 6d. Figure 7a shows the resulting correlation function. At the absolute maximum, the correlation coefficient is 0.78, and a velocity of (\(5040\pm 170\)) m/s is deduced for the corresponding time shift. Considering the first eleven maxima of the correlation function a mean value of (\(5080\pm 170\)) m/s is derived. Time traces shifted according to this time offset are presented in Figure 7b. If excitation takes place on the side of the pipe (Figure 2, position B) the periodicity seen above is not as expressed, which may again be attributed to blurring caused by signal crossings and to a less effective excitation of the acoustic mode resulting in an overall lower signal-to-noise ratio. Therefore, signal onset is used for the determination of velocities. Setting appropriate thresholds, we find (\(5710\pm 210\)) m/s, (\(5510\pm 200\)) m/s, and (\(5510\pm 200\)) m/s using optical pulse lengths of 100 ns, 50 ns, and 30 ns, respectively. Time traces shifted with respect to signal onset are shown together with the ones obtained after excitation in position A (pulse length: 50 ns) in Figure 8.  
All velocities determined using the quasi-distributed sensor configuration 2 are compatible with the ones derived using the fully distributed configuration 1.