Before we could set course back to the home port of Scheveningen, there was one last task to be done.
In the morning of Monday 11.04., we headed again to a sheltered location. Though it was not because the weather had turned against us. This time, the reason was that we needed a quiet place to perform a second calibration. We did this to check whether things were similar to when we calibrated at the beginning of the survey. Any significant changes to the equipment during the survey may have major effects on the echo integration and subsequent biomass estimation!We headed to a well known calibration location in the north of Scotland, Loch Eriboll:
Although not totally flat calm, the conditions were just good enough for calibration.
To calibrate the echosounder, we had to attach a small sphere made of copper to three strings and suspend it below the transducers. If they were situated below the ship, the whole set up would look something like this:
However, since our transducers are on a towed body, that we tow next to the ship during surveying, we attached three metal arms to it. The sphere can then be suspended below the towed body, attached to three strings via the arms:
This is the small copper sphere 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.Here Eric is demonstrating the use of the control board:
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:
Eventually we can fit the points to a parabola to define and adjust the acoustic beam pattern and gain levels to match the theoretical values:
Luckily the calibrated values closely matched the ones from the first calibration done before the survey!
After a few hours spent calibrating, we finally finished and took the towed body system back on board for a last time. On Tuesday afternoon we said good bye to Loch Eriboll and headed out to the open sea again.
Before we knew it, we were sailing through Pentland Firth between the Orkney Islands and the Scottish mainland.
Time to go home!