ACOUSTICS UNPACKED

A General Guide for Deriving Abundance Estimates from Hydroacoustic Data

 

 

 

 

Near surface and near bottom dead zones

Although acoustic methods are efficient for water column measurements, they are less effective at measuring backscattering by organisms near boundaries such as the sea surface or sea floor.  Surface and bottom dead zones (Ona and Mitson 1996) inherent in the design and application of typical echosounders are important limitations in many survey areas. 

Fish that are near the sea surface are not observed with echosounders as vessel-mounted or surface towed downward-looking transducers do not sample the water column above the depth of the transducer.  Additionally, data within the transducer near-field are not valid for survey estimates.  For post-processing, a surface exclusion zone is selected, accounting for both transducer depth and the near-field.  Surveys interested in near surface species should consider horizontally oriented echosounders or sidescan and multibeam sonars. 

 

The near-field distance (Rnf) may be calculated as:

Equation 16  [16]

 

where:
a is the radius of the active elements of the transducer (m), and

λ is the wavelength (m)

To be safely in the far-field region, we need to multiply this value by 2 (Simmonds and MacLennan 2005) or 3 (Medwin and Clay 1998). The example on near-field distance calculation provides a sample calculation for determining the near-field.

The bottom dead zone is important in the Great Lakes because bloater, kiyi, rainbow smelt, and alewife in some of the lakes are often closely associated with the bottom during the day (Janssen and Brandt 1980; Tewinkel and Fleischer 1998; Yule et al. 2007).  The detection of a fish close to the bottom is not possible after the wave front of the sound pulse first strikes the bottom, as the bottom generates a much stronger echo than any fish.  When the beam is circular, a fish located at the angle θ relative the acoustics axis cannot be detected if it is closer to the bottom than the bottom depth (BD) multiplied with (1-cos(θ)) (Ona and Mitson 1996).  In addition, fish will be only partially integrated when closer to the bottom than the resolution (cτ/2).  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]

Note that if the bottom slope is steep (as is the case in e.g., Lake Champlain), the bottom dead zone is larger.

(See also the Deadzone example for more details.)