The machine that goes ping!
For 20 years the Australian Antarctic Division has used hydroacoustics as a remote sensing tool for studying Antarctic krill, zooplankton, fish and even non-living phenomena such as ice crystals. It was a pioneer in using acoustics in the Southern Ocean - the Simrad EK400 and QD echo-integrator system installed on MV Nella Dan had serial number 1 - in some of the first Australian deep sea biological hydroacoustics research. In the 1990s the next generation Simrad EK500 digital split beam sounder aboard RV Aurora Australis had serial number 3.
With these systems Australia has contributed to multinational, multi-vessel krill biomass surveys (such as FIBEX - the First International BIOMASS EXperiment) and conducted the largest single vessel krill biomass survey, 'BROKE' (baseline research on oceanography, krill and the environment), a biological and oceanographic survey off the East Antarctic coast in 1996 (see Australian Antarctic Magazine # 1). The hydroacoustics data from such surveys is used for regional krill biomass estimates, which in turn are used to set precautionary catch limits on the krill fishery -the region's largest fishery.
How does all this technology work? The systems operate a little like a spotlight or radar illuminating objects ('scatterers') in the water column beneath the vessel as parallel transects lines, each of constant bearing, are systematically sailed over a study region (Figure 1). Pulses of different sound frequencies (like different colours) are used simultaneously to give us more information about a particular type of scatterer or aggregation of scatterers. Higher frequencies of sound tend to give better resolution and stronger signals from smaller scatterers while lower frequencies enable us to see greater distances, as they lose energy at a much lower rate to molecular processes in seawater. With krill, our primary subject for observation, we use frequencies of 38, 120 and 200kHz. Combining data from the different frequencies gives us insight in to the type of scatterer, krill, fish, other zooplankton or even ice crystals (Figure 2).
The primary form for viewing the acoustic backscattered data is the echogram (Figures 2 and 3), a two dimensional image of backscattered energy from beneath the vessel as a function of depth and time (or distance travelled). The form taken by aggregated scatterers in the echogram can also give us information about the identity of the scatterers. A dilute soup of mixed zooplankton species might have the form of a weak, continuous, fuzzy, scattering layer (Figure 3a). Antarctic krill (Euphausia superba) form many varied structures ranging from weak scattering layers through to large dense complex structures (Figures 1 and 3c), whereas coastal cold water krill tends to form tight, dense, strong, scattering blobs (Figure 2).
In an acoustic survey to estimate krill distribution and abundance, echosounders are calibrated using standard reference target spheres suspended within the acoustic beam beneath the vessel. Echogram data for each transect are classified into regions dominated by krill and regions dominated by other species. There are also regions of bad data resulting from noise (for example due to the vessel breaking ice), or reverberation from bubbles in rough seas. We use a combined knowledge of hydrography, multi-frequency analysis, form of the echogram and acoustically targeted scientific net trawls to identify the species. The classified echogram is then integrated over the water column depth being investigated and averaged along the length of the transect to return a mean of the backscattered energy for both intervals along the transect and the whole transect. This information is then converted to mean krill densities. The lower diagram in Figure 1 shows this vertically integrated data as a snapshot of krill distribution for the large-scale BROKE survey.
Acoustic systems can also provide insight into oceanographic features. We have used them to identify plumes of ice crystals streaming out from beneath ice shelves such as the Amery Ice Shelf (Figure 4) and internal waves travelling along an interface of difference water masses of contrasting densities, such as the bottom of the surface mixed layer (Figure 3b).
The AAD has utilised a variety of other acoustic systems, including a Doppler current profiler, simple remote release devices on moored instruments, moored current meters, moored upward looking sonars for measuring sea ice cover, passive towed arrays for whale observations, multi-beam systems and seismic systems.
For future biological studies, scientific acoustic systems are likely to include multi-beam systems that will image close to a 1800 arc of the waters directly beneath the vessel with a feathered beam taking in the waters to the sides of the vessel. This will produce a 3 D picture of the scattering objects in the water column. Current new technologies also include scientific scanning sonars able to look ahead of the vessel. Other nations have also successfully used moored sonar systems and autonomous underwater vehicles carrying sonar systems, such as the United Kingdom's Autosub-2.
Tim Pauly1 JD Penrose2
1. Australian Antarctic Division