We just entered the harbour of Galway, where we will stay over the weekend.
Yesterday evening and this morning, we did calibrate our echosounders (38 kHz and 120 kHz) in the Galway Bay (see figure 2). The calibration was a little bit more difficult then expected as we had to face quite strong currents and winds. Hence, we couldn’t manage to get the sphere to cover the entire beam as the current was just forcing it out of range all the time. This left us with no other option than to wait for better weather conditions, which we expected to occur at around midnight and at 6 in the morning. Luckily the predictions were quite accurate and we did manage to do two good calibrations on our 38 kHz echosounder (main frequency) as well as 1 good and 1 almost good calibration of the 120 kHz echosounder.
What, why and how – Calibration of echosounders
Calibration of acoustic equipment is very important for us, as it gives us an estimate of how accurate the equipment is working (this is important as the sounders may perform differently under varying conditions i.e. salinity, temperature). Without a calibration no quantitative results can be taken out of the acoustic data, e.g. no biomass estimates and species determination using multiple frequencies, further calibrating the equipment allows us to detect errors in the system and to solve those before we start with the ‘real’ survey. Calibration should be done in a sheltered area, close to the area of interest, with a water depth of at least 20 m. To calibrate, we use a copper or tungsten carbide sphere, with a known target strength (echo), attach it to 3 lines, which can be steered from deck, and then the sphere is placed underneath the echsounders, in our case the towed body (metal frame with the acoustic equipment, floating next to the ship, see figure 1). The next step is the real calibration, where we have to move the sphere around in the acoustic beam, trying to get the entire beam covered. Finally if we have covered the beam we can look at the result and look at the strengths of the detections in the entire beam which should be close to the reference values of the sphere. In a next step we can update our beam information with these values, e.g. apply a correction filter to the echo. To finalise the calibration, we do a second run to make sure that the applied correction is performing well and we did not worsen the set up.