Sound speed depends on the temperature and salinity (saltiness) of the water, as well as on depth. To compute the sound speed, we first measure the salinity and temperature using an instrument called a CTD, which stands for Conductivity-Temperature-Depth. The CTD has a thermometer to measure temperature, a pressure sensor to measure depth, and a conductivity sensor to measure how well the sea water can carry an electrical current. Saltier water carries more current, so by measuring conductivity we can determine the salinity of the water.
The CTD is attached to a round frame that is lowered over the side of the ship. Since we're in very deep water (about 5600 meters or 3.5 miles!) it takes about three and a half hours for the CTD to make the trip to the bottom and back. Just to be safe, we stay about 100 meters off the bottom so that we don't risk crashing into the sea floor by mistake!
Once it's back onboard we have all the data we need to compute the sound speed. The plot below shows the measurements we made this morning (temperature, salinity, and sound speed). Note that the temperature is nice at the surface, and gets very cold down deep. The sound speed increases towards the surface due to higher temperature and increases towards the bottom due to higher pressure.
CTD's don't always have to be done so early in the morning, but we needed to get this one out of the way to leave time for some other work today. While it was painful to hear the alarm go off at 4:30 AM, we did get to see a great sunrise!
