Ping! The survey of the seafloor

IN THE HYDROACUSTIC ROOM

Color surfaces build up on the monitors in all hues, revealing even the smallest structures on the seabed. Unknown areas that nobody has seen before, and that cannot be detected by the naked eye or satellite.

For example, if we zoom in on "Google Earth" to an area of 30 meters within a city, we can clearly see streets, houses, cars and even people in the satellite image. If we zoom in on the ocean with the same setting, we only see a blue box.

With satellites, we cannot look directly through the water to the ocean floor. Only the sonar can penetrate the ocean. "That's why we use research vessels to survey areas of the sea using the echosounder systems, areas where we have previously had no, or only imprecise, records," says Paul Wintersteller.

On this expedition, we search for cold seeps and hot vents on the seafloor. Using sonar, we cannot only capture the structure and depth of the seabed, but we can see signals in both the water column and the sediment indicating that gas, such as methane, is leaking out from the seafloor.

This bathymetric map is an example of one of the maps created on our expedition, showing us the structures of Kemp Caldera, a submarine island-arc volcano at 1,600 meters of water depth.

If the engines on our ship were not so loud, we might even hear it, the sound we send to the bottom of the sea, and the echoes we pick up to calculate the depth. "Like a songbird twittering," says Paul, pursing his lips, and yes, it could be a small bird our hydro-acoustic expert is trying to make audible.

By the way, from our hydro-acoustic "birdsong", five times faster in the water than in the air, each ping produces 320 beams, i.e. angles, forming a fan below the ship. While the ship is in motion, the fans are put together and form the basis of the map. Depending on the water depth, “Polarstern’s” multibeam can map a swath up to 16 km wide.

One of the first to realize that sound spreads well in water and could be used for locating objects, was Leonardo da Vinci. In 1490, he noted, "If you dip a pipe into the water and hold the end to your ear, you can hear ships at very great distances."

Fortunately, we do not have to keep a pipe in the water today. Hydro-acoustic systems have developed immensely, especially in the second half of the past century and at the beginning of this century. Paul took 40 terabytes of data storage on this expedition.

DEEPEST POINT IN SOUTH ATLANTIC

At night, we drove over the deepest point in the South Atlantic. The systems showed 8,264 meters. We reached this point, the so-called "Meteor deep", a depth in the South Sandwich Trench that scientists discovered during the "German Atlantic Expedition" in the years 1925-1927.

It is almost unimaginable that they could capture such depth at that time. From aboard their ship, the first "Meteor", they used one of the first reliable depth probes, an invention of the German physicist Alexander Behm (1880–1952).

It was a single beam sounding line - just a "ping" whose echo we still carry in our thoughts today while we try to go to the bottom of the sea, here in the stormy waters around South Sandwich Trench.

Paul is already sitting at one of the computers, preparing the data, so that the next morning a new map of the seabed is on the table. It shows us the way, and shows positions that we want to explore, live and in color, using our remotely operated vehicle.