Abstract

This chapter examines the application of distributed acoustic sensing (DAS) in infrastructure, using a single fiber in an optical fiber sensing cable. Usually, the sensor operates using regular monomode optical fiber cables of the same type as those used for telecommunications, but, in some cases, the cable may occasionally be custom designed to withstand severe environments or to improve sensing performance. A DAS system converts a standard optical fiber into an array of acoustic sensors each with the ability to essentially simultaneously determine the time-varying strain at any given position along the long length of the optical fiber. The operational range of systems is usually limited to approximately 50 km without the inclusion of additional amplification or similar measures. However, unlike the technology that preceded it, DAS is able to measure this strain dynamically, with a frequency response of typically several kilohertz. It is usually the case with DAS that it is not possible for the system to recover the absolute or DC strain as the processing method responds only to the AC content of the strain. However, for many applications, DAS systems are superior to more traditional strain sensing optical technologies, as they are able to respond to relatively fast and low-amplitude variations in strain (e.g., AC sensor output signals). In addition, traditional methods are usually based on one or more point sensors, whereas the DAS system can operate in a fully continuous manner over many kilometers of fiber, recovering signals, essentially simultaneously from each and every resolution cell (of length ∼1–10 m, depending on the design) along the length.