## 2 Materials and Methods
### Technical Design for AUV-Mounted Side-Scan Sonar Operations
#### 2.2.1 Survey Line Deployment
The top view of the survey line deployment is illustrated in Figure 8.  
Because of the design structure and performance constraints of autonomous underwater vehicles (AUVs), it is crucial to maintain the equipment's optimal horizontal motion state while underwater, with communication equipment positioned above the waterline. Generally, the maximum allowable pitch angle for the equipment should not exceed 20\({}^{\circ}\). Exceeding this maximum pitch angle can result in a weakening or loss of control over the equipment. When an AUV transitions into a vertical state, the equipment gradually loses its ability to move within a plane. Vertical motion requires specific structural strength of the propeller and the state structure of the carrier, making it necessary to limit the pitch angle. However, the pitch angle limitations may vary based on the specific equipment's performance. During hill climbing maneuvers, the equipment's slope limitations primarily depend on the maximum allowable pitch angle. If the slope angle surpasses the pitch angle limit, the AUV will trigger a near-bottom alarm and eventually activate floatation after climbing for a certain duration. Figure 8: Top view of the survey line deployment. In constant-altitude mode, we establish a set of assumed AUV constant-altitude navigation parameters, as illustrated in Table 1. This set of parameters is based on the working principle of constant-altitude mode and the continuous altitude failure alarm principle. As shown in Figure 9, this analysis examines the limiting situation where the AUV does not activate a continuous height-holding failure alarm during fixed-altitude mode. Under the assumed parameter conditions, the maximum slope angle for the AUV to maintain a constant height over the seabed is 14.068\({}^{\circ}\). This indicates that under the assumed parameter performance, the fixed-altitude mode of the AUV has limited applicability to large slope seabed terrains. Additionally, based on the collision avoidance principle of the safety system, the assumed parameters are presented in Table 2. \begin{table}
\begin{tabular}{c c c} \hline \hline Altimeter operating frequency & 2 Hz & Two measurements per second \\ \hline Response time & 1 s & At least two measurements are required to initiate response \\ \hline Maximum diving speed & 0.4 m/s & \\ \hline Height tolerance error & 0.3 m & The closer the plan is to the bottom, the smaller the allowable error \\ \hline AUV speed & 3 kn & Comprehensive efficiency and resolution requirements \\ \hline AUV height from bottom & 20 m & Comprehensive safety and resolution \\ \hline Duration required for alarm & 30 s & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: AUV assumed parameters. Figure 9: Schematic diagram of non-triggering alarm limit. As depicted in Figure 10, when analyzing the critical conditions triggering a collision avoidance alarm while ensuring equipment safety, it becomes evident that equipment collisions occur when the slope angle exceeds 84.797\({}^{\circ}\). Considering the above analysis under assumed parameter performance conditions, it is important to note that the applicability of the constant-altitude mode for certain types of terrains may vary due to differences in equipment performance. For instance, the installation of oil pockets and sliders can enhance the equipment's ability to maintain non-horizontal positions and relax the maximum pitch angle limitations. Consequently, the choice of the vertical motion mode should be made based on the specific performance characteristics of the AUV.