What Lies Beneath

Ping... ping... ping... chirp... chirp... chirp...

At first, some of us thought it was a dripping faucet, others a captive bird. But it turns out to be a sound of science aboard the Palmer—the constant background chirping of the vessel's multibeam sonar system.

Sonar is simple in theory. Sonar systems work by emitting a sound pulse, or "ping," and measuring how long it takes for the pulse to reflect off the seafloor and back to the ship. Multiplying the time required by the speed of sound in seawater gives the distance traveled. Halving the travel distance gives the depth. You use the same principle to estimate the distance to a thunderstorm by counting the seconds between lightning flash and thunderclap.

But sonar systems can be quite complex in practice, especially on a ship. Shipboard sonars must account for the ship's roll, pitch, and yaw; and correlate the sonar readings with the ship's constantly changing position. They must also account for changes in water temperature, as sound travels faster in warm water, slower in cold. Choppy seas and breaking waves can also interfere with sonar readings, as they can cause bubbles to build up on the keel-mounted hydrophones. The bubbles can interfere with both sound emission and reception.

Early sonars used a single sound beam to map a narrow track directly beneath the ship. Multibeam sonar is a relatively new technology that uses a directed series of pings to map a wide swath of seafloor bathymetry beneath and to either side of the ship's track.

The Palmer's state-of-the-art, multibeam sonar uses 191 separate beams to map a continuous swath of seafloor up to 12 miles wide. Moreover, the system's high-frequency pulses (12 kilohertz) allow detection of seafloor features as small as 10 centimeters across. That's a quantum leap in resolution for seafloor maps of the Ross Sea and Southern Ocean, lightly traveled areas where large parts of the seafloor have never been mapped at all, even by physical soundings.

To put this in perspective, consider that the most-detailed map for large parts of the Southern Ocean and Ross Sea is a recent satellite image with a resolution of about 7 kilometers per pixel. The Palmer's sonar can provide 10,000 times more detail. Of course, that detail is only available in areas where the ship has traveled, and the Southern Ocean is a big place. It would take thousands of transits to generate a detailed map of the entire area.

Accurate seafloor maps are important for both navigation and science. Tomorrow we'll see how seafloor maps can be combined with magnetic readings to help reconstruct the geologic history of the Southern Ocean.