## 3 Laboratory Testing  
An experiment was carried out on a 7 m long 8-inch schedule 40 steel pipe. The transducers driven by Teletest MK3 system device were located at around 2.6 m from the front end of the pipe. A detailed sketch of this experiment has been described, as shown in Figure 6. The experiment had been performed by The Welding Institute Ltd. as shown in Figure 7. The steel pipe was placed on roller supports. The received signal only contains two echoes reflected from the each pipe end. The experiment used a 30 kHz sine signal, variable bits of 30 kHz Barker code, variable bits of 30 kHz Golay code pair and the BCG code pair at frequencies (from 30 kHz to 50 kHz) as excited signals. Guided torsional wave mode T(0,1) was excited by a transmitter. Figure 5: The theoretical analysis results. (**a**) The 3-bit Barker code combined 4-bit Golay code compared to simple pulse, 7-bit Barker code, 11-bit Barker code, 4-bit Golay code and 8-bit Golay code. (**b**) The theoretical intensity comparison of nine different sequences of BCG codes. The experimental results were collected from a 7 m long steel pipe without any defect and picked up the data from eight segments in a circumferential transducer array using the Teletest MK3 interface. Due to the sine signal is relevant to the input waveform used generally by the Teletest system, all the selected input signals are modulated by the sine waveform as shown in Table 1.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Excitation Sequence**} & \multicolumn{3}{c}{**Characteristics**} \\ \cline{2-4}  & **Carrier Frequency (kHz)** & **Duration (\(\upmu\)s)** & **Binary Code** \\ \hline
1-cycle sine & 30 & 33 & - \\
3-bit Barker & 30 & 100 & 11-1 \\
5-bit Barker & 30 & 160 & 111-11 \\
7-bit Barker & 30 & 230 & 111-1 -11-1 \\
11-bit Barker & 30 & 360 & 111-1-1-11-11-11-11 \\
13-bit Barker & 30 & 430 & 11111-11-11-11-11 \\
2-bit Golay & 30 & 66 & 11 \& 1-1 \\
4-bit Golay & 30 & 132 & 111-1 \& 11-11-11 \\
8-bit Golay & 30 & 264 & 111-111-11 \& 111-1-1-1-1 \\
16-bit Golay & 30 & 528 & 111-111-11111-1-1-11-11 \\
11-11-11-Figure 8 shows an example of the raw echo signal that is a 3-bit Barker code at 30 kHz. This figure is a three-dimensional surface plot reconstructed from the echo signals received by eight segments. The echo signals from the front end (FE) and the echo signals from the back end (BE) have been marked in Figure 8. The cross-correlation results of echo signals from the front end are shown in Figure 9. Figure 9 contains the results of variable bits of Barker code at 30 kHz, variable bits of Golay code at 30 kHz and the BCG code at frequencies. It is obvious from the figure that the main lobe of the BCG code has the highest intensity as expect. The BCG code ability to increase the intensity of echo signal is much better than other codes. In this paper, the work forced on the echo signal SNR and PSL. The method could also be used to investigate the information, such as axial resolution, dead zone, inspection range, etc. Table 2 indicates the increased SNR and PSL of Barker codes of different lengths at 30 kHz. The experimental data show that the length of Barker code increased can improve  
Figure 8: The raw experimental echo signals of a 3-bit Barker code at 30 kHz from eight segments of the pipeline inspection system. Notes: the experimental setup detail of the pipeline is shown in Figure 6.  
Figure 9: Comparison of results for the intensity of the Barker codes, Golay codes and BCG codes in the time domain. Please noted: The average power of the input signal for all excitation methods is the same. both the echo SNR and PSL. Therefore, the length of the code needs to be considered when the excitation code is selected. The increments of echo SNR using a 3-bit Barker code signal from the front and back pipe ends are 16.7 dB and 19.1 dB compared to the sine wave at 30 kHz, and the peak sidelobe levels of this code signal are \(-11.8\) dB and \(-13.3\) dB, respectively.