# Detection of Defects Using Spatial Variances of Guided-Wave Modes in Testing of Pipes
## 4 Experimental Setup  
The Teletest device [33], manufactured by Teletest branch of Eddify technology family in Cambridge (UK), used in order to inspect an eight-inch schedule 40 steel pipe with a length of six meters. The test tool included three rings of 24 linearly spaced transducers with 30-mm spacing, which are located 1.5 m away from the back end of the pipe. The main aim of these trials is to assess the ability of the algorithm in highly noisy scenarios and investigate the dependency of each method to the excitation frequency.  
In the first set of tests, two defects of 3% and 2% CSA have been created; these are located 4.5 m from the back end of the pipe, which generates signals with less/almost the same energy as the noise regions. In order to confirm the ability of this thresholding method in detecting another defect in  
Figure 3: System flowchart.  
the same pipe, defects with CSA sizes of 4% and 5% have been created after the initial trials, which are located 5.5 m away from the back end of the pipe. Figure 4 shows the signal reception routes of this setup.  
The excitation sequence is a 10 cycle Hann-windowed sine waves with frequencies of 30 kHz, 34 kHz, and 37 kHz. The selected frequencies are considered the optimum excitation sequences with regard to their resultant forward propagating energy achieved using the backward cancellation algorithm [30]. A summary of the settings used for the first set of tests (first defect), which are numbered from one to six, are shown in Table 1; for the second set of tests, they are numbered from seven to 12 (second defect), and are shown in Table 2.  
Figure 5 shows the baseline signals achieved from the same pipe with no defects, with excitation frequencies of (a) 30 kHz; (b) 34 kHz; and (c) 37 kHz. As can be seen, even though there are no defects in the pipe, the baseline signal is highly polluted by the coherent noise. The first defect signal is expected to be received approximately three meters away from the tool test, between the first and second red dotted lines. The second defect is expected to be located approximately four meters away, between the third and fourth red dotted lines. Three major conclusions can be drawn with regard to the baselines:  
1. Defect 1: The signal with lowest amount of existing coherent noise in the baseline are 34 kHz, 30 kHz, and 37 kHz, respectively.
2. Defect 2: The lowest amount of existing coherent noise in the baseline belongs to 37-kHz, 34-kHz, and 30-kHz signals, respectively  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Test (\#)** & **Defect CSA (\%)** & **Frequency (kHz)** \\ \hline
**1** & & 30 \\
**2** & 3 & 34 \\
**3** & & 37 \\ \hline
**4** & & 30 \\
**5** & 2 & 34 \\
**6** & & 37 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of the first set of tests conditions (first defect). CSA: cross-sectional area.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Test (\#)** & **Defect CSA (\%)** & **Frequency (kHz)** \\ \hline
**7** & & 30 \\
**8** & 5 & 34 \\
**9** & & 37 \\ \hline
**10** & & 30 \\
**11** & 4 & 34 \\
**12** & & 37 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Summary of the second set of tests conditions (second defect).  
Figure 4: Signal reception routes.  
All of the respective frequencies have a higher amount of noise in the Defect-2 region.  
It is expected that the thresholds are required for detecting regions with lower existing coherent noises, which means that overall, Defect 2 will be more difficult to detect.  
Figure 5: Baseline signals of pipe with no defect with frequencies of (**a**) 30 kHz; (**b**) 34 kHz; and (**c**) 37 kHz. The first marked region (3–3.5 m) is the location of the first defect, and the second marked region (4.1–4.5) is the location of the second defect.