Developing active broadband acoustic methods to investigate the pelagic zone of the Great Australian Bight

Investigating the pelagic community of the Great Australian Bight (GAB) remotely and pervasively remains an elusive challenge to the scientific community. Advanced broadband acoustic sounders offer a new tool to help characterise and subsequently classify the dominant scatterers via the frequency dependence of their target strengths. Broadband acoustic data were collected from the GAB for the first time on a recent scientific expedition on board RV Investigator in December 2015. A broadband sonar system (EK80), mounted on a custom designed vertical profiling acoustic platform was used to record acoustic data in the frequency range of 45-160 kHz. A processing method was developed to allow the system to be calibrated at depth with a reference target so that the frequency dependence of scattering from biological targets could be measured independently of their positions in the acoustic beam. The results of some initial tests of the method are reported here and include the measurement of the frequency dependent target strength of two targets at a depth of 600 m. The characteristics of these frequency response curves indicated that one was an animal with a gas inclusion whereas the other behaved like a fluid scatterer. No independent ground truthing was available to confirm this; however, these results indicate that the broadband acoustic method has considerable promise as a new tool to study the pelagic community of the region. This study is being undertaken as part of the Great Australian Bight Research Program, a collaboration between BP, CSIRO, the South Australian Research and Development Institute (SARDI), the University of Adelaide, and Flinders University. The Program aims to provide a whole-of-system understanding of the environment, economic and social values of the region; providing an information source for all to use.