Monday morning, 25.06. we left Scheveningen and headed north towards Scapa Flow on the Orkney Islands (see map).
On the way, the fishing gear had to be tested and herring was soon present on the conveyor belt. Here a few impressions from the last few days:
During Tuesday night, we arrived in Scapa Flow where the vessel was anchored and the calibration of the acoustic equipment started first thing Wednesday morning. All acoustic frequencies used during the survey (38, 200 and 333 kHz) had to be calibrated and the whole procedure lasted the whole day. Conditions were not optimal, with some wind and currents giving problems to steer the calibration sphere through the acoustic beam. Eventually, after 9 hours, everything was ready and we could head to the start of the first transect at 58º17’N 2º45’W.
Here a bit more explanation about the calibration procedure:
To calibrate the echosounder, a small sphere made of copper or tungsten carbide has to be attached to three strings and suspend below the transducers on the towed body:This is one of the small metal spheres we use for calibration:
Because we know the echo returned by this sphere, we can then change the echosounder settings until the echo we measure is the same as the theory – therefore calibrate it!
The idea behind using the three strings is that we can then move the sphere around in the acoustic beam until we have enough readings. Due to some ingenious engineering, we can actually steer the sphere around remotely, using a control board and little motors winding the three strings up and down.
The calibration software then shows us the readings from the copper sphere collected inside the acoustic beam. First there are none…
… but with time, while manoeuvring the sphere around the beam, we slowly fill up the beam area, resulting in an even spread of readings: