|A view from the acoustics lab of the R/V Neil Armstrong|
Principally, echosounders are able to produce a high resolution biological profile of the water column continuously, given the acoustic returns interpreted correctly. However, as in the case of all other remote sensing approaches, there are significant uncertainties and therefore ground-truthing is required. As a common approach, this will be performed by the stratified net sampling and optics (Video Plankton Recorder, or VPR).
Some structures other than the living organisms are also able produce echoes such as gas bubbles and physical features (e.g. turbulence or temperature /salinity contrast). The new broadband system enables interpretation of these returns in an accurate way thanks to the very high range resolution and the frequency response information along a wide spectrum. There will be an inter-disciplinary effort among biologist, physical oceanographers and acousticians for interpretation of the acoustic data during the cruise.
|Gareth Lawson, Gordon Zhang and Andone Lavery|
Calibration of the echosounders is essential for the accuracy of a quantitative estimation. This is done by introducing a target to the echosounder with known target strength and stable scattering properties. Standard metal spheres are used for this purpose (generally made of tungsten carbide or copper). During calibration, the target is placed into the ensonified volume and moved around.
|Tungsten carbide sphere was used for the initial calibrations.|
|an echogram view during the calibration|
However, logistically, this is not an easy operation. The important task is to position this small sphere exactly below the transducer. On a huge vessel like Armstrong, this is rather difficult. But, thanks to an automated calibration system developed at the Woods Hole National Marine Fisheries Service lab, the whole process runs very smoothly. Development of this system was led by Mike Jech and the engineers working with him.
Once the hardware setup is complete, the whole process can be controlled by the software. Three electrical downriggers are connected to a main hub through wireless communication and this hub is connected to the main computer in the acoustic lab. Once the coordinates of the transducers are entered into the software relative to the sphere’s position, and unless a manual operation is desired, the software takes over the control and moves the sphere based on the desired pattern ( e.g. spiral, starshape or grid).
|operation took time a little longer than expected|
|The tension was constantly checked during the setting up to make sure that it moves freely|