## 3 Experimental Results and Analyses
### Experiment in Waters with Flat Seabed Topography
#### 3.1.2 Experiment in Constant-Depth and Fixed-Point Modes
Figure 15 displays the image data captured by the AUV at speeds of 3, 4, and 5 knots while positioned at the same location, The red box shows that the shape of the anchor hook changes with the speed:  
Based on the analysis of Figure 15, it can be observed that the change in texture angle in the upper right corner indicates a correlation between speed and image compression/stretching in the direction of travel. This observation is consistent with traditional side-scan sonar operations, where an increase in speed results in image compression, while a decrease in speed leads to image stretching. Therefore, it is essential to maintain speed control during operation to ensure accurate image interpretation. Figure 17 presents the position deviation of anchor ruts after accounting for the offset distance between tracks. The black lines illustrate the position of anchor ruts in different tracks. It can be observed that all six tracks detected anchor ruts, which redundantly supplies information and thereby compromises efficiency. The overlap degree and occurrence ratio of corresponding targets within each group are listed in Table 5.  
Figure 14: Image data under the first experimental conditions. (**a**) AUV’s distance from the seabed and coordinate information. (**b**) Displayed side-scan sonar band image. (**c**) Waterfall chart containing model information. Figure 15: Waterfall chart under different speeds: (**a**) 3 knots; (**b**) 4 knots; (**c**) 5 knots. The red box shows that the shape of the anchor hook changes with the speed. To optimize the efficiency of side-scan sonar operations, the number of lines required to cover the area should be minimized while ensuring that spacing and range settings meet the requirements of not missing any targets and ensuring adjacent strip connections. However, it is also crucial to avoid having too many occurrences of the same target. By satisfying the criterion of having the same target appear twice, co-visibility target-to-strip connections can be applied to enhance operational efficiency. According to the data in the table, increasing overlap does not increase the number and proportion of target occurrences. Hence, having numerous target occurrences is meaningless for underwater terrain exploration. Shrinking both the spacing and range of the lines will increase the number of lines, resulting in a decrease in efficiency. Therefore, the spacing and range of the lines should be set to ensure that the number of lines is as small as possible to  
Figure 16: Image data under the second experimental conditions. (**a**) AUV’s distance from the seabed and coordinate information. (**b**) Displayed side-scan sonar band image. (**c**) Waterfall chart containing model and texture information. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Experiment 1**} & \multicolumn{2}{c}{**Experiment 2**} \\ \cline{2-5}  & \multicolumn{1}{c}{**Medium**} & \multicolumn{1}{c}{**Small Group**} & \multicolumn{1}{c}{**Group 1**} & \multicolumn{1}{c}{**Group 2**} \\ \hline Overlap (one-sided) & 150\% & 100\% & 160\% & 140\% \\ Number of lines required to cover the area & 4 & 6 & 14 & 16 \\ Number of occurrences of the same target & 3 & 2 & 6 & 4 \\ Proportion of occurrences of the same target & 75\% & 33.3\% & 42.9\% & 25\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Overlap parameter table. improve operational efficiency while still meeting the minimum overlap requirement. The four overlap degrees in the table are relatively high, with a one-sided overlap of 100% for the degree with the target occurring twice. Due to AUV positioning accuracy issues underwater, it is necessary to increase the overlap percentage based on shipboard side-scan sonar operational standards to ensure co-visible target acquisition. The tests in fixed-depth mode demonstrated that this mode was not superior to constant-altitude mode in flat waters.