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

Abstract

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.1996 International Council for the Exploration of the Sea

Acoustic data collected during and between bottom trawl stations: consistency and common trends

“…It would also tend to increase tilt angle and hence reduce target strength (MacLennan et al 1987;Kloser and Horne 2003;McQuinn and Winger 2003). Fish may also move into the acoustic dead zone (Ona and Mitson 1996;Lawson and Rose 1999) and be inaccessible to the echosounder. In the present study, the statistically nonsignificant but systematic stability or increase of NASC value in the bottom layers during trawling is associated neither to a corresponding systematic decrease of NASC values in the midwater layers nor to a change in height of the mean energy in any of the bottom or midwater layers.…”

Northern cod comeback

“…Standard methods of acoustic integration of 38 kHz signals were employed throughout (Simmonds and MacLennan 2005) as in previous reports (e.g., Rose 2003;Mello and Rose 2009). A linear seafloor dead-zone correction was made based on the mean backscatter in the bottom 5 m but seldom surpassed 20% of the total integrated backscatter from cod (Ona and Mitson 1996). A length-based target strength model (TS dB = 20 log 10 length -67.5, after Rose 2009) incorporating mean length of cod in the catch was used to convert backscatter to cod densities.…”

Precision in estimates of density and biomass of Norwegian spring-spawning herring based on acoustic surveys

“…However, the effect of depth-varying acoustic target strength is being investigated in a separate study. A number of authors have focused on acoustic deadzones near the bottom (Godø & Wespestad 1993;Ona & Mitson 1996;McQuinn et al 2005;Kotwicki et al 2012). The IESNS is a pelagic survey conducted in a deep ocean, but an acoustic deadzone near the surface might be a source of bias.…”

“…Such biases can be caused by vessel avoidance, acoustic shadowing and depth-dependent acoustic target strength (Skaret et al 2005;Løland et al 2007;Hjelvik et al 2008). For other species authors have focused on varying catchability related to bottom trawl surveys (Thorson et al 2013), acoustic deadzones near the bottom (Ona & Mitson 1996), or the use of combined acoustic and bottom trawl data to improve abundance estimates (Godø & Wespestad 1993;McQuinn et al 2005;Kotwicki et al 2012). We acknowledge that the effect of varying catchability is also relevant for herring in the present survey.…”