At dawn on April 1, 2015, scientists and crew of the NOAA Ship Nancy Foster gathered together to take care of an especially tricky task: calibrating the split beam echosounder system. How this job gets done may surprise you. The task involves suspending a tiny tungsten ball directly underneath the split beam sonar mounted underneath the ship. How do you get the ball there? What do you do once you manage to get the ball down there? Why bother? Read on.
Where are fish located in the ocean? How many are there? How big are they? Resource managers and other decision makers need this information so they can make better decisions about how to conserve and manage fish populations and natural resources. One way that scientists gather this data is by using split beam echosounders—a sophisticated tool that uses reflected sound to estimate data such as fish size, direction of travel, and three-dimensional position in the water column. But how do scientists know if the split beam sonar system mounted underneath the ship is collecting accurate information? The answer: calibration.
This is where the metal tungsten ball comes into play. Scientists know precisely the size and density of this ball. If they place the ball under the sonar system and "ping" it, they can compare the results with the known values of the tungsten ball. That information is used to make small corrections to the sonar to ensure it is reading values such as size, position, and density as accurately as possible. Then, when the sonar is used to search for fish, the collected data can be relied upon as accurate.
While the day's calibration was a success, it was cut short. In science, as in life, things don't always go as planned. As this effort was nearing completion, the lines connected to the tungsten ball became entangled with an errant float line from a fishing trap passing beneath the ship. The lines snapped. Luckily, the crew managed to retrieve the (very expensive!) tungsten ball.