A General Guide for Deriving Abundance Estimates from Hydroacoustic Data




















Deadzone Height Calculation


The distance from the bottom at which there is a bias (HBotBias) associated with both processes therefore depends on pulse duration, angle to the fish, and depth (see Ona and Mitson 1996) as follows:

eqn17  [17]


BD = Bottom depth

θ = angle of fish location relative to the acoustic axis

c = sound speed in m•sec-1

τ =  pulse duration in sec

Here this is calculated for the half- power beam angle as an indication of distance from the bottom where we can expect a bias.  For a typical 7o transducer operating with 0.4 msec pulse length, the HBotBias is 0.5 m at 100 m depth (see example below). This increase to 0.76 m with a half-power beam angle of 11o.  Ona and Mitson (1996) present the theory of near-bottom echo integration as well as the equations needed to correct of this bias.

If sampling to a depth of 100 m using a 120 kHz with a 7º transducer, then:



Sv values from within 0.5 m of the bottom are biased