BIOLOGY - SCATTERING LAYERS

Acoustic investigation of the structure of the biological scattering layers concerns the determination of the variability of backscattering strength SV. This variable characterises the scattering properties of the medium and depends on the number of scattering centres and their scattering cross-sections. It is calculated from the sonar equation for the volume scattering that describes the dependence of a measured intensity of the signal scattered by medium inhomogeneities on the physical parameters of the medium and technical parameters of the measuring system

SV = 20logUi - (SL+VR) - G - DI - 10log(ct/2) 

where
		Ui	- voltages of successive samples
		SL	- source level
		VR	- voltage response
		G	- receiver gain
		c	- sound speed in the sea
		t	- pulse duration
		DI	- directivity index

SL, VR and DI are calibration values that are constants for any given echosounder. They depend on the operating frequency and transceiver construction. The sum SL+VR can be inferred from the calibration test using the copper spheres of the known target strength. In order to compensate for the geometrical spreading of the acoustic beam and absorption loss in the seawater, the standard time-varied gain is used

TVG = 20logR + 2aR

where R is the one-way distance, and a is the absorption coefficient expressed in dB/m.

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Joanna Szczucka<szczucka@iopan.gda.pl>, 2.03.1996