Ongoing maintenance is critical to the performance and longevity of underwater structures such as cooling water intake tunnels. Commercial divers or robotic vehicles can do the needed inspections, but such manual methods require a costly plant shutdown and provide only qualitative results. AS[ Group Ltd. has designed and built an advanced, dual-axis sonar system that works in fast-moving water and can deliver quantitative data about the extent and location of debris buildup on submerged assets.