## 2 Unburied Pipe Tests
### Experimental Setup and Tests  
The test bed, sketched in Figure 1, simulated a section of the water supply network. A polyvinyl chloride (PVC) pipe with an outer diameter of 90 mm (DN 90) was used as the water main. A high-density polyethylene (HDPE) pipe of length 28 m and smaller diameter (DN 32) was connected to the larger one as customers' service pipe. It was placed at a depth of about 0.5 m on a layer of backfill soil, but left unburied. Two-way shut-off valves were installed at both its extremities. A pressure tank, equipped with a pressure gauge and a pressure regulation system, fed the facility at a constant pressure of about 3.5 bar, which replicates the typical network operation. Artificial 20 mm longitudinal cuts (parallel to the pipe axis) were generated at three different locations along the pipe (Figure 1). The distance of each leak from the terminal valve is reported in Figure 1. The induced damages simulated one of the most common kinds of cracks characterizing burst leaks occurring in the HDPE service pipes managed by the local multi-utility, based on its maintenance records. Such cracks are expected to generate a leaking flow of about 200 L/h in the typical network functioning conditions, which is the target leak rate to detect. Pipe repair clamps were installed to rapidly switch between leaking and non-leaking conditions, and vice versa (Figure 2). An AE sensor (Mistras WS\(\alpha\), sensitivity 55 dB, ref. 1 V/(m/s)) operating in the frequency range of 0.1-1 MHz was installed on the pipe terminal valve (Figure 3). This arrangement simulated the sensor setup possibly achievable in actual service pipes (with the sensor mounted near the customer water meter, i.e., distant from the water main). Indeed, as for the water supply network of interest, customers' water meters are typically installed inside manholes or concrete boxes to be easily accessible for reading operations. The sensor was mounted by using a silicone rubber compound as the coupling agent. Adhesive tape held the sensor in place and provided proper preload during the curing process  
Figure 1: Schematics and characteristics of the test bed with unburied pipe. PVC, polyvinyl chloride; HDPE, high-density polyethylene; DN, outer diameter. Figure 2: Artificially induced leak: (**a**) active leak; (**b**) leak deactivated using the repair clamp. of the bond. Acquisition and conditioning of the signals were performed by means of a Mistras system, namely the USB AE NodeTM (MISTRAS Group Inc., Princeton Junction, NJ, USA) device and its related software AEwinTM [22]. A wideband analysis was performed, since these experiments were meant as a preliminary investigation and, to the authors' best knowledge, no studies on small-diameter plastic pipes could be found in the literature, thus only few data were available. The acquisition setup featured a threshold of 28 db (which defines the minimum signal amplitude considered for identifying an AE event) and a sampling frequency of 5 MHz. Different sets of values were tested for the timing parameters that control the extraction of the signal features and waveform recording, namely [22]: _Peak Definition Time_ (_PDT_), which determines the detection of both the peak amplitude and the rise time of an AE event; _Hit Definition Time_ (_HDT_), which is related to the AE event duration; and _Hit Lockout Time_ (_HLT_), which sets the minimum time interval between two consecutive AE events. The best results, here presented, were obtained by setting _PDT_ = 200 us, _HDT_ = 800 us, and _HLT_ = 1000 us. Time-driven measurements were adopted, namely acquisitions with a preset duration of 90 s. Indeed, water leaks are expected to cause _continuous_ AE signals (although some exceptions have been documented [14]). This expression qualitatively describes AE waveforms characterized by time-overlapping emissions, as opposed to _burst_ AE (i.e., a discrete sequence of AE events), which are typically associated with processes such as crack growth. Measurements were performed in three conditions:  
1.