by Emma Hadré, Tarik Bozkuyu and Ruth Supka
Our workplace is a screen-filled lab in the middle of the ship. This job may appear repetitive at first look, but our workspace is just as interesting and important as the rest of the work carried out on the ship.
You can always find one of us here, as we work in shifts during the day and at night to make sure our instruments are recording data.
The name says it all: hydro = water and acoustics = noise. Our instruments use sound waves, which travel through the water and are reflected back to the ship, to create a map of the seafloor and the uppermost sediment layers. For these two applications we use the Multibeam and the Parasound.
Multibeam
As soon as we wake up, we are greeted by the multibeam. A constant metallic chirping, like that of a bird, can always be heard on the lowest decks of the ship – these are the acoustic ‘pings’ that are sent down to the seafloor. One can easily make a guessing game out of the changing time intervals. This is because the timing of the signals is directly related to the depth of the water. The sound wave is transmitted through the water column to the seafloor, where it is reflected and returned to the ship. If you know exactly how fast the speed of sound is in the water, you can calculate the water depth with a precision of a few meters. Fortunately, the multibeam does that automatically for us. Since the sound waves also propagate to the sides, we can map a strip with a width of about 10km at a water depth of 4000m and produce a precise map of the surface of the seafloor.
Parasound
However, geologists usually want to know from us what the layer below the seafloor looks like. We use Parasound for this purpose. It works similarly to the multibeam, but with sound waves of lower frequencies (4kHz). These can penetrate up to 200 meters deep into the seafloor and provide information about its structure – the presence of sediments, their thickness and stratification. These two methods also give us a chance to detect special surface structures such as small mountains (seamounts), ridges or pockmarks (see below).
On board, this information is mainly needed for sampling. It is used to decide where sediment cores will be taken and how long they might be. While new data are coming in in real time, we are already processing the previous ones. Based on these data, decisions are made for stations throughout the day.
Much of the world’s seafloor is only roughly mapped. The Seabed 2030 project aims to compile accurate maps of the seafloor by 2030. The data we are collecting on this expedition will also contribute to this goal. It is not uncommon to discover underwater seamounts or other small to medium sized structures during these mapping surveys that were not previously shown on any map. For instance, even if it was not a new discovery, crossing a field full of pockmarks was quite interesting and exciting to us. Pockmarks are small crates at the seafloor that are formed by gas escaping from the sediments. We were able to observe the small craters in both multibeam and Parasound. These depressions can vary greatly in size from a few meters to several hundred meters in width and up to 150 meters deep.
We are looking forward to our ongoing journey through the Tasman Sea and send greetings home to the other side of the earth.