The high-resolution acoustic mapping (HRAM) system was developed in response to a stated need by the United States Army Corps of Engineers to evaluate the floor of a navigation lock that could not be dewatered. Navigation Lock #26 on the Mississippi River was constructed on piles and mats over a sandy bottom. Over the years the footings had shifted, probably damaging the floor. The lock could not be dewatered because of leakage around the lock-wall footings. Based on our work in ultrasonic inspection; using B-scan, C-scan and holographic imaging to display hidden faults in metal; we were asked to propose a solution to the navigation-lock imaging problem. The C-scan ultrasonic method employs a single ultrasonic transducer stepped over a regularly spaced grid to collect a set of data that can be displayed on an oscilloscope screen to evaluate the material being inspected. It is well suited for the inspection of large flat areas. We proposed to build in essence a large C-scan system. A boat supporting several ultrasonic transducers would move in a regular X-Y pattern over the floor of the lock. The data from the scan would be computer processed to provide a plot of the surface of the floor. We called this a 3D plot since it would be a nearly three-dimensional view of the inspected surface. I should point out that this work was proposed in 1975 and conducted in 1976, when small high-powered computers were still well in the future. The results of the program were spectacular. The 3D plot of the Lock 26 floor showed that individual concrete slabs had broken, some slabs had been tilted, the entire river side of the lock floor had settled nearly two meters, and piles of silt had built up in front of the lock gates.
Ted O. Price,
"Underwater terrain mapping of dam stilling basins", Proc. SPIE 2457, Nondestructive Evaluation of Aging Structures and Dams, (12 May 1995); doi: 10.1117/12.209392; https://doi.org/10.1117/12.209392