Acoustic sampling and signal processing near the seabed: the deadzone revisited

There are particular difficulties in making acoustic estimates of the abundance of demersal and semi-demersal fish. One possibility which exists in any survey situation is that the fish may move from the direct path of the vessel because of the noise it is radiating. However, the problems addressed here are primarily due to the physical characteristics of the transmitted acoustic pulse from the echo-sounder and its interaction with fish close to the seabed. This paper looks at the factors controlling the detection of these fish in terms of the acoustic sampling volumes near the bottom, the discrimination theoretically possible between fish and seabed echoes and the “depth anomaly”. The acoustic deadzone is defined and its volume is determined. Practical aspects of signal processing in this near seabed situation are then described, including seabed recognition and safeguarding fish signals from contamination by the bottom echo and from noise. Next, an echo-integrator deadzone comprising the acoustic deadzone, the backstep zone, and the partial integration zone (related to pulse length) is described and defined. Equations for calculating the effective volume or effective height of this deadzone are developed. Estimation errors due to the echo-integrator deadzone are investigated and equations derived for the necessary corrections. An example is shown of partial failure of the bottom recognition system and how the echo integrator result can be corrected to compensate.