Obtaining accurate “stealth rock” detection/recovery capability utilizing shallow water sonars on coastal/waterway survey vessels

A problem that has been facing the coastal and waterway survey community is the consistent acoustic detection and accurate measurement of navigation hazards known as pinnacles or stealth rocks. This paper discusses one region of the US where the problem has been particularly difficult to resolve. Approximately 85 million tons of cargo transit the St. Mary´s River annually. A majority of this cargo is carried in 1000 ft vessels, operating much of the time with less than a ft of clearance above a rock bottom. The Army Corps of Engineers´ survey vessel Bray is responsible for accurately surveying and detecting rock strikes in the St. Mary´s River, while Corps of Engineers´ derrick boat Nicolet is tasked with their recovery and removal. Because of the rock bottom channels and the fact that the water levels can change dramatically in a few hours, a measurement accuracy of 0.2 ft is required for detecting rock strikes. The on-going collaborative effort between the Army Corps of Engineers, Penn State ARL, and NCCOSC to effectively deal with the problem began in Feb.´95. The overall approach to accurate strike detection and efficient removal is designed to utilize the capabilities of single beam multi-channel accuracy for surveying, and multi-beam imagery/visualization for recovery. This approach is based on results of tests performed during the period 11 to 14 July ´95 in the Soo Locks area of Michigan. Sea trials on the survey vessel Bray were conducted utilizing the existing Odom 32 Channel 214 kHz Echo Scan Sonar, Reson 455 kHz Multi-Beam Sonar, Lowrance single beam fish-finder sonar with 50 kHz and 192 kHz transducers, and Humminbird multi-beam fish-finder sonar with 200 kHz and 450 kHz transducers. The trials were conducted to investigate the accuracy of water depth measurements with these sonars in the presence of pinnacle or stealth rocks, and the degree of visualization required for efficient recovery. The Army Corps of Engineers laid a test bed of 25 rocks of various shapes in about 25 ft of water for this purpose. The rocks, which had been photographed and measured prior to installation, were separated by about 60 ft and were laid in a line parallel to and 80 ft off a northwest pier at the west approach to the Sabine Lock