## 3 Buried Pipe Tests
### Results and Discussion  
The acquisitions related to the test conditions _Lk-3b, Lk-4b_, and _Lk-5b_ did not show any significant difference with respect to those concerning the non-leaking state, thus leak detection was not achievable for the most distant leaks. Consequently, data concerning such leaks are not reported and not considered hereafter. Figure 9 shows the values of two AE parameters, namely Hits and Cumulative Amplitude, measured for the non-leaking state and for the leaking conditions _Lk-1b_ and _Lk-2b_, with both threshold settings. Most of the AE events detected in all the tests were characterized by signal levels notably lower than those measured in the previous unburied pipe tests. Many recorded waveforms exhibited notable distortion due to quantization. This behavior is reasonably ascribable to a higher signal attenuation caused by the surrounding medium, since the presence of soil is the only variation in the tested boundary conditions. The measurements show an acceptable repeatability, with the standard deviation being lower than 10% of the mean value for the conditions _Lk-1b_ and _Lk-2b_, and about 20% for the non-leaking state. The acquisitions with a threshold at 26 db include numerous AE events that are ascribable to external perturbations unrelated to the leaks, since they are detected even in the non-leaking state. Conversely, the higher threshold can filter out most of these spurious AE. Leak detection is still possible with both thresholds, as shown by the percentage variation of the AE parameters (Figure 10). The trend of all parameters, indeed, shows a clear increment related to the presence of active leaks. Differing from the results of unburied pipe tests, none of the parameters revealed a superior sensitivity to the leaks. The increment in the parameter trends is more evident for the threshold of 27 db, which therefore appears more suitable for leak detection purposes. Unlike previous tests on the unburied pipe, the trends of the AE parameters do not exhibit a close correlation with the leak distance, as the percentage variation of all parameters is greater for condition _Lk-2b_. This unexpected behavior may be associated with the actual flow rate characterizing this condition, which is about 50% higher than for condition _Lk-1b_. The experiments confirmed that leak detection can be achieved within a range of about 8 m by means of AE monitoring. However, the detection performance does not appear completely satisfactory, in particular, for the nearest leak. Improving the method reliability (i.e., incrementing the percentage variation of AE parameters related to leaks) would be required for practical applications, in order to limit as much as possible the rate of false positive detections. Figure 9: Buried pipe tests (both threshold levels): (**a**) total Hits; (**b**) Cumulative Amplitude. _NLb_, no leak; _Lk-1b_, first leak (3 m); _Lk-2b_, second leak (8 m). The distribution of the AE events in the AF vs RA diagram for the leaking conditions _Lk-1b_ and _Lk-2b_, with both threshold settings, is reported in Figures 11 and 12, respectively. Data related to the non-leaking state are also plotted. The effects of signal quantization are evident for all conditions, since several points have one or both coordinates that are identically repeated. The comparisons do not reveal any specific correlation between the pattern of the AE events and the presence of active leaks. Indeed the patterns of AE events in both leaking and non-leaking conditions basically overlap for all conditions. Hence the effectiveness of this kind of analysis, that was apparently proven for the unburied pipe tests, was not confirmed for the buried condition. Since the test bed replicates the main features of buried service pipes of the network, the results provided by the buried pipe experiments are considered applicable to actual pipelines.