We saw from the last post on CTDs that the ocean in the area where we are working is about 3-1/2 miles deep. The ocean is not necessarily flat at that depth, but it can have hills and even mountains, called seamounts, and deep trenches. When we deploy an acoustic mooring we don't want the anchor to land on the side of a seamount and go sliding down, so before we do a mooring deployment we always take a bathymetric survey to get a map of what the bottom of the ocean looks like. We then pick out a nice flat spot to put in the mooring.
We take bathymetry (bottom depth) measurements using the ship's multibeam sonar. It sends out high frequency signals (called "pings") from transducers at the bottom of the ship. When the pings hit the bottom of the ocean, they bounce back to the ship. If we measure the time it takes for the sound to travel to the bottom of the ocean and back again and we know the sound speed of the water from the CTD, we can figure out how far away the bottom is. The multibeam sonar keeps pinging as it moves, so we put together the results from thousands of pings to get a picture of the bottom like the one below. Here, blue is deep and red is more shallow. You can see there is a seamount with a red peak next to the valley where we put the mooring (indicated by the red arrow).
As you can imagine, if we drop an anchor from a ship to the ocean floor 3-1/2 miles below, it does not always land directly below the point where we dropped it. In our next post we'll describe how we find out exactly where the anchor landed. Here's a hint: acoustics again!