## 2 Materials and Methods
### Technical Design for AUV-Mounted Side-Scan Sonar Operations
#### 2.2.1 Survey Line Deployment  
According to the principles governing side-scan sonar operations, the layout of survey lines should be parallel and optionally aligned with the direction of water flow, the long edge of the survey area, the isobath, or shipping channels. Therefore, before the  
Figure 6: The workflow of AUV-mounted side-scan sonar operations. strategic deployment of survey lines, a preliminary scan of the survey sea area from the surface is essential to map the seafloor features. The planning of survey lines follows this assessment. Typically, the preliminary extensive scanning is performed in parallel with the long edge of the survey area, maintaining the same line spacing as in official operations. However, it is essential to note that data collected during the preliminary scan should only serve as a reference dataset due to potentially varying surface water velocities and greater seafloor depths. Integrating other empirical data, such as nautical charts, is required for a comprehensive seafloor assessment in areas where water depths exceed the instrument's measuring range. The deployment of survey lines can be categorized into two scenarios based on seabed topographic characteristics, as Figure 7.  
1. In cases where the seafloor is relatively flat or displays gradual depth variations with a slope gradient of less than 14\({}^{\circ}\), maintaining image clarity and resolution while optimizing equipment performance necessitates the adherence to the following principles during survey line deployment: 1. The constant-altitude mode shall be employed as the preferred vertical motion mode, setting the altitude within the range from 15 to 30 m. In sea areas with depths below 15 m, consideration shall be given to increasing the submersion depth while prioritizing safety to maintain a submersion depth of at least 5 m. 2. Given the minor fluctuations in water flow velocity and depth after submersion, and taking into account the AUV's customary operation in non-harbor shipping channels, it is recommended that survey lines be oriented parallel to the longer side of the survey area. 2. In scenarios featuring complex seabed topography and significant depth variations, significantly when the slope gradient exceeds 14\({}^{\circ}\), the following principles guide the layout of survey lines: 2. Figure 7: Method of line layout. In such water environments, the constant-altitude mode would trigger multiple protection mechanisms, resulting in operational interruptions. Given that, the mode of vertical motion shall be depth-keeping. At the same time, submersion depth changes shall correspond to the varying water depths, ensuring that the distance from the seafloor consistently falls between 15 and 30 m.  
The survey lines shall be parallel with the local directions of isobaths. The survey line deployment strategies mentioned earlier are tailored to the unique characteristics of the survey area. When determining the survey line spacing, it is crucial to arrange the lines in a way that minimizes the number of lines needed to adequately cover the survey area. The range should be set based on the selected line spacing and the desired level of overlap, aiming for a 100% or more overlap on one side of the range. However, considering the AUV's design for submersion, steering, and positioning, each survey line must extend beyond the confines of the survey area. Extending the initial survey line facilitates the submersible to descend to greater depths, enabling it to cover a distance that allows the vehicle to reach the designated depth at its maximum speed. Moreover, extending the remaining survey lines eliminates any invalid side-scan sonar data generated during pivot steering. For fixed-point motion control, a guiding line should be added to the initial survey line to ensure the submersion depth remains within the measuring range of the DVL.