Acoustic imagery and scanning sonar returns!

This guide aims to help users interpret sonar images and offers best practices and techniques for using them on ROVs. While specific examples feature the Ping360 and BlueROV2, the concepts discussed are applicable to any mechanical scanning sonar and ROV combination.
 
Scanning SONARs, or SOund Navigation And Ranging, are active sonars that emit sound pulses into the water and capture the echoes reflected off objects. These sonars have a fan-shaped acoustic beam, combining a wide vertical and a narrow horizontal beam. This configuration allows them to create acoustic cross-sections of the environment, presenting the data as images on a viewer.
 
Determining distances is achieved by calculating the time it takes for the echo to return and combining it with the known speed of sound in water. The speed of sound can vary depending on factors such as water temperature, salinity, and sonar operating depth. Users can adjust this value in the viewer to obtain accurate range measurements.
 
The reflectivity of targets varies based on their material density compared to water. Targets with significantly different densities, such as metal or rock, produce strong echoes. In contrast, materials like mud, silt, sand, and plants generate weaker echoes due to their similar density to water or acoustic energy absorption. Brightness levels on the viewer's color palette indicate the strength of the echo, with darker colors representing weaker echoes and brighter colors indicating stronger echoes.
 
Scanning sonars have fixed horizontal and vertical beam patterns, much like a flashlight's beam illuminating a specific area. The transducer head inside the scanning sonar rotates through a stepper motor, creating slices of the environment that build up into a complete image on the display, similar to how a camera captures snapshots as a flashlight beam sweeps across an area.
 
Targets within the sonar's beam pattern are "illuminated" acoustically, and their echoes are reflected back to the sonar, appearing as images on the viewer. However, targets outside the beam pattern, either above, below, or outside the horizontal range, will not be displayed on the viewer.
 
To gain the best bottom imaging range, operators can adjust the sonar's altitude above the seabed and its angle downwards. This allows for the illumination of a wider swath of the seabed. Following the "10% Rule," which recommends a specific altitude and angle for optimal seabed coverage, helps ensure efficient scanning.
 
Acoustic shadows are present behind illuminated targets, providing valuable information about target height, shape, and orientation. Short acoustic shadows may be hard to interpret, so maintaining an altitude closer to the seabed with a shallower down angle can create longer acoustic shadows, making target assessment easier.
 
Targets that are perpendicular to the sonar's beams produce the strongest echoes, while surfaces at other angles may yield weaker returns. Understanding these echo patterns helps in identifying and interpreting the sonar images more effectively.
Scanning sonars are valuable navigation tools when mounted on ROVs, especially in low visibility conditions. They extend the range at which targets can be located, offering an advantage over relying solely on visual interpretation through cameras.
To locate targets on the seabed or water column effectively, ROV operators can use scanning sonars in combination with specific techniques. Maneuvering the ROV slowly allows time for new images to generate. By placing the ROV in a stable position, generating a 360-degree Polar Scan, and identifying the target's relative bearing, operators can then narrow the sector scan to improve target tracking during movement.
 
In conclusion, scanning sonars are versatile tools for ROV operations, providing valuable data and enhancing navigation capabilities in challenging conditions. By understanding their operating principles and implementing the appropriate techniques, users can optimize the use of scanning sonars to achieve their intended objectives.