Tuesday, June 14, 2016

Armstrong Cruise - Acoutic calibration (2)

Since this is the first onboard  broadband calibration for the Armstrong’s EK80, each frequency has been tested for different settings. While Andone Lavery performed the calibration, Gareth noted the calibration details. In the meantime senior acousticians, Kenneth Foote and Tim Stanton made valuable comments and contributions.
From left to right: Tim Stanton, Kenneth Foote, Gareth Lawson, Andone Lavery and Jennifer Johnson.

With the new wideband echosounders the frequency response of the target sphere is also tested. There are specific frequencies where, theoretically, the sphere should produce very weak echoes (nulls) due to the phase cancellation.  Since the wideband system uses the entire spectrum of the sound produced by the transducers, it is important to calibrate the full band width of each transducer and nulls observed in the expected location. This is done through comparing the observed frequency response curve relative to the theoretical curve.
The main factor determining the position of the nulls is the water density (temperature and salinity) and the material properties of the target. Andone tells a story how she struggled to get correct curve when she was doing a calibration experiment with sphere which she was initially thought that it was tungsten carbide. Her Fourier transform was not able to fit the nulls into the correct theoretical positions. But after two days of failure, she was happy to find out that the sphere was actually stainless steel. This explains the importance of the material properties in determining the backscattering properties. 

Andone Lavery of the WHOI Applied Ocean Physics and Engineering Lab
Another logistical challenge for the calibration is to find an appropriate location. The dockside in Woods Hole is a good compromise as it is very well sheltered and deep enough for the higher frequencies. However, the downside is the presence of other targets such as fish and the water depth is shallower than the suggestion by Simrad ( 20m for 18 kHz). However, Ken and Tim indicated that the only limitation would be the range where the far field is formed.  They practically calculated the nearfeld depth with the following equation: N=(2D)^2 /λ. where “N” is transition from near field to far field,” D” is the diameter of the transducer and “λ” is the wave length. This corresponds to depth convenient for 18kHz which is available at the dockside. According to Ken, the requirement to keep calibration range at such great depths was due to potential instability of the earlier, relatively primitive transducers and analog time varying gain. However with the highly precise new system there is less reason to worry about the calibration depth. 
Kenneth Foote ( WHOI Applied Ocean Physics and Engineering) , Tim Stanton WHOI Applied Ocean Physics and Engineering), Gareth Lawson (WHOI Biology Department)


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