## 1 Introduction  
In recent years, the integrity of pipelines and piping systems in industrial plants has moved into the focus of public concern as the global piping infrastructure grows and ages. Moreover, to meet increasingly higher standards in technical safety, environmental compliance and failsafe performance the demand for a holistic pipe monitoring system are expected to rise even further in the future. Such a system should ideally be able to detect, classify, and localize a wide range of different threatening pipe conditions prior to the occurrence of leakages over the whole length of the pipeline and in real time. In the long run it should replace time-consuming and costly routine inspections, which are currently taking place at predetermined time-points [1]. Since piping systems in industrial plants typically are highly branched and stretch over long distances the use of conventional point sensors is usually not practicably feasible to realize a monitoring system without gap. Therefore, the principle of distributed fiber optic sensing (DFOS) [1, 2] has come into focus for this kind of application as it relies on one single optical fiber applied to the structure under question, which simultaneously acts as a spatially continuous sensor as well as the signal transducer. Hence, the sensor fiber--typically embedded in a measuring cable--may be understood as a chain of thousands of sensors stretching over many kilometers. The effective number of these measurement points is solely limited by the total length to be monitored and the spatial resolution of the measuring technique. As a result, extensive structures can be provided with this type of sensor with comparatively low efforts. Present DFOS techniques established in pipeline monitoring are widely based on distributed temperature sensing (DTS) and focus on leak detection [3; 4; 5]. DTS systems allow to identify leakages due to temperature changes induced by escaping gases and fluids (e.g., Joule-Thomson effect). Additionally, distributed strain sensing (DSS) is applied in some cases to detect ground movements and resulting pipe deformations [3; 4; 5]. However, both techniques belong to the class of so-called static sensing mechanisms meaning that a measurement time of at least several minutes, and up to hours, is required before a change in the condition is detected. Besides the established methods to measure temperature and strain, distributed acoustic or vibration sensing (DAS/DVS) has lately received considerable attention as a means to detect and localize third party threats to pipelines, i.e., approach of vehicles, digging, and mechanical manipulation on the pipe [3; 6; 7; 8]. The techniques subsumed under the label of DAS/DVS allow the detection and localization of highly dynamic strain changes along the sensor in the kHz regime. Currently, this technology is a matter of continuous development and refinement for special applications both by commercial suppliers as well as in fundamental research. Additional to the above-mentioned application in the oil and gas industry, the great potential of DAS/DVS was also recognized and is currently rapidly evolving in other application fields like perimeter safety [9; 10; 11], seismic measurements [12; 13; 14; 15; 16] as well as train and track monitoring [17; 18; 19; 20]. The principle of distributed acoustic/vibration sensing relies on the detection of Rayleigh backscatter caused by naturally occurring inhomogeneities along an optical fiber by employing the measuring scheme of coherent optical time domain reflectometry (C-OTDR) [21; 22]. Basically, as in all implementations of OTDR, a short laser pulse is sent into the fiber under test and the back-reflected light is directed onto a photo-detector, while time-correlation between the initial pulse and the detection allows to deduce the spatial coordinate along the fiber via the speed of light.