Geology today, thank you very much!!

Today we are a bit late, lots of work. I have been stuck to the computer all day. But I have also spent a little time looking at the map in the blog and besides the loyal readers like Silvia I have seen that we have several mysterious readers in exotic places, like Malta (although we have already identified you ;-)) and Lipetsk in Russia. I’d swear I've seen a reader in Armenia earlier today, but now I cannot find him. We are also being read in Mjomna, Norway, in several US and British cities. There is Adriana in Greenland and Stefán in Iceland. Hello, Adriana, hello, Stefán! We hope you're not too cold.

We have had flat seas and sun, and a pilgrimage to lie on deck, which is fine because once we start to fish it will be non-stop. Or so I hope. The biologists held another meeting to prepare the sampling and I have been in the acoustic lab almost all day. Last year I told you very briefly that the Spanish General Secretariat of Fisheries (which is the owner of our Vizconde de Eza) had made a very important investment in the acoustic equipment. e-Teo explained to me today in great detail how the acoustics laboratory works and it is truly impressive. This is outside my area of ​​knowledge, but with permission from the geologists I will try to summarize what e-Teo has told me.

The multidisciplinary group of TRAGSA, led by Dr. Araceli Muñoz has carried out several annual surveys on the Vizconde to map the seabed in Spanish waters. They use the multibeam echo sounder and a high resolution seismic system. Simply put, these devices have a transmitter that converts an electrical signal into a sound pulse with a certain frequency that collides with the seabottom and returns an echo that is transformed by a receiver into an electrical signal. A computer measures the time between the pulse “ping” and echo reception. Time increases with depth. This technology allows to obtain the relief of the seafloor. The multibeam echosounder "sweeps" a very wide surface, because these pulses are emitted with a fan shape, perpendicular to the boat direction.  The boat follows as many parallel transects as needed to cover all the study area (weather allowing). The difference between the data obtained by this probe and the seismic system is that the pulse of the latter penetrates several meters into the sea floor, and its echo allows geologists to deduce what kind of substrata we have.

An example of the seismic system output

 But there is more. Because the velocity of sound transmission in the water varies according to the water mass, salinity, temperature…, before starting to work with the echosounder and the seismic system it is necessary to calibrate them with the speed of sound in the work area. A sound profiler is used to this purpose. It is a fairly large sensor to which certain information is uploaded (for example the depth to which the ship is and the depth to which the sensor will be dropped) and then it is cast off to measure the speed of sound transmission it that location. The Vizconde is also equipped with another fixed profiler, located next to the bow propeller, to measure the speed of sound on the surface.

A sound profiler

 And we're not done yet. Now we need to georeference the data that will be collected by the multibeam and the seismic echosounders. Enter the SEAPATH, which is a differential GPS (a system of direction, altitude and positioning sensors). The SEAPATH receives signals from several satellites, represented by circles. The green ones are the satellites from which we receive signals. When cartography surveys are carried out, the signal of two geo-referenced satellites is also received. These satellites orbit farther away from Earth than telecommunication satellites and move with Earth in her rotation and revolution movements. Basically they are always in the same place with respect to Earth. The SEAPATH also receives information from the MRU (Motion Reference Unit), which is a device installed in the ship's center of gravity (in the computer lab) that calculates how much the boat pitches and rolls. These data are used by the SEAPATH so that for calculation purposes based on the data collected by the geologists it is as if the boat were still.

The SEAPATH monitor. The photo would be way better if I was 20 cm taller. Anyway, the circles are the satellites above us. You can see the ship outline in the middle. We receive signal from the green ones. Nr 120 is a georreferenced satellite.

The SEAPATH is also connected to the Vizconde's dynamic positioning system, which allows the vessel to be positioned at a fixed point regardless of weather or sea conditions; and to the TOPAS echosounder, which is the one that shows us the seabottom and depth in the bridge.

If this were not enough, the equipment must also be synchronized so that the pulses of the multi-beam and TOPAS do not interfere with each other, and calculations also include a parameter that cancels the effect of the ship's draft, which varies with its load. And what I don’t know is the time it takes geologists to translate the data from the echosounders to those spectacular maps they produce. Hopefully one of them will drop us a line?

A pretty map showing part of our study area, produced within the frame of the international project NEREIDA, led by IEO. The distance represented in longitude is about 53 nm.