Monday, June 13, 2016

Armstrong Cruise - Acoutic calibration (1)

A view from the acoustics lab of the R/V Neil Armstrong
R/V Neil Armstrong is equipped with the cutting edge Simrad EK 80 scientific echosounder running at central frequencies of 18 kHz, 38 kHz, 70 kHz, 120 kHz and 200kHz. This echosounder, using pulse compression technique, produce frequency modulated (i.e., broadband) sound signals and can resolve targets with high precision. Thanks to high signal to noise ratio, it is capable of detecting weakly scattering zooplankton accurately with sizes as small as copepods.

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.
The principle idea is to ensure that the echosounder is measuring what it is expected to measure. Traditionally the calibration is done to compare the amplitude of the received signal with respect to the expectation based on the transmitted signal (transducer power gain).
an echogram view during the calibration
Calibration also involves a test for the geometry of the acoustic beam. Most modern echosounders are equipped with a split beam system which means the transducer is divided into quadrants. After the sound transmission, each of these quadrants is able to listen independently. This enables locating the exact position of the targets based on the time delay in received signal between different quadrants. This allows calculation of the angle of the target relative to the transducer. As a result, the shape of the acoustic beam (beam pattern) can be tested accurately.


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.

Mike Jech

Communication hub
Wireless communication

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).

Jennifer Johnson
operation took time a little longer than expected
Jennifer, the assistant of Mike, is adjusting the length of the lines through the software. She is trying to make sure that the sphere is correctly placed below the transducers. Since this was a dockside calibration there were several obstacles that the monofilament line can get entangled in. So this operation took time a little longer than expected.

The tension was constantly checked during the setting up to make sure that it moves freely


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