An unexpected encounter and the issue with the motion sensor

On the way further southwards we had a special encounter at sea. Abeam of Portugal we encountered the German sail training ship Gorch Fock coming from south. Both vessels changed their course slightly and we passed each other closely. Salutation with typhon and flag signals and on both ships people waved happily at each other. An encounter at sea is always something very special (fig.1).

For the time being we are at Ampère Seamount – a mighty seamount which rises from a water depth of almost 5000 m till about 100 m beneath the water surface. It is situated about 430 nm west of Gibraltar and belongs to a group of 9 inactive volcanos within this area. It is also very interesting to the biologists as such seamounts often are “oases” in the marine environment. Due to this fact the students are eagerly taking samples and carrying out measurements. During the past few days they have only had little sleep and the laboratories were also used during nighttime (fig. 2). Water samples were filtered, plankton was determined and counted and the samples were preserved for later examination. It is good to see how enthusiastic the students are working and they really use the opportunity to learn something, which cannot be taught during the theory lectures at their home universities.

The seamount is not only interesting for the students, but also for Ralf, Catalina and Sebastian who test and calibrate the hydro acoustic measuring units together with the electronics engineers on board in order to have them ready for the upcoming Antarctic season. This would not be such a difficult job if we did not have the issue with the motion sensor. A basic matter how to orientate oneself at sea is, of course, to know where you are. In the beginning of the seafaring this was not simple. Today it is much easier.  Due to GPS (Global Positioning System) and compasswe know our position and direction i.e. we may specify exactly where we have conducted our measurements. If we, however, want to define the exact water depth or investigate the topography of the sea floor or which kinds of sediment may be found there, we use hydro acoustic measuring techniques. We send out an acoustic signal from the bottom of the ship to the sea floor where it is reflected and received at the bottom of the ship again. Looking at the two-way travel time of the signal, we may calculate the water depth – theoretically. Truth of the matter is, of course, the ship in moving in the sea; it is rolling from portside to starboard, the waves are moving the bow up and down.  Due to this the signal is not always sent straight vertically to the sea floor and miscalculation occur because of the longer travel time. This is where the motion sensor becomes important as it states the position of the ship. With this information we may calculate the error. For this purpose the motion sensor and the measuring units must be able to communicate. I.e. if the signal from the motion sensor is sent too late the measurement data will not be of the quality we aim for. The team led by Ralf has been working hard since we left Bremerhaven and thus ensured that data from the multibeam (topography of the sea floor – fig. 3) and parasound (properties of sediment at the sea floor) is of such high quality that the scientists can work with the systems. Work is done and the three of them may contentedly leave Polarstern in Las Palmas and fly back home.