U.S. Geological Survey were out on the Missouri River at Williston, North Dakota, USA, performing a hydrographic survey to monitor the state of riverbed erosion. They were using a multibeam echo sounder which transmits sound energy and analyzes the return signal (echo) that has bounced off the riverbed or other objects. Multibeam sonars emit sound waves from directly beneath a ship's hull to produce fan-shaped coverage of the riverbed.
