VENUS Tests New Broadband Inverted Echo-Sounder

VENUS is an innovative facility that can support a wide variety of instruments. Since the Saanich Inlet array was installed in February 2006, there have been many different instruments deployed and connected to the sub-sea infrastructure. Many of these instruments will be permanent and generate long time series for a range of research including process oriented studies, the detection of short and seasonal variations, and over time, climatic shifts or trends. The facility can also support user provided instruments for short and long-term studies, and instrument testing. Our latest addition is a broadband echo-sounder, similar to our Zooplankton Acoustic Profiler (ZAP).

The SciFish 2100-B Broadband Echo-Sounder was provided by Dr. Tetjana Ross of Dalhousie University in Halifax, NS. Mounted on the VENUS Camera Platform (Figure 1), the SciFish was deployed in Saanich Inlet on February 29, 2008, at a water depth of 100m. Dr. Ross is a Physical Oceanographer with an interest in using high frequency acoustics to measure and distinguish between physical and biological phenomenon. In general terms, sound in water scatters from both physical structures associated with stratified turbulence and suspended particulates, including zooplankton and fish. However, the intensity of the scattering depends on the frequency of the sound. Turbulent structures will more efficiently scatter lower frequency sound, while small particulates will scatter only high frequency acoustics.

Traditional narrowband sonar is limited to “viewing” the ocean in a single acoustic frequency, analogous in the visual world to looking at a black and white image. By using a broadband sonar signal, we can capture far more details, in essence changing our black and white image into a colour one. Clearly, we have a much easier time identifying objects when we see them in full colour. Once per second, the SciFish 2100 transmits a broadband “chirp” signal that sweeps through the acoustic frequencies between 85 and 155 kHz.

With a typical underwater sound speed of 1480 m/s, the acoustic wavelengths contained within the chirp vary between 1.75 and 0.95 cm. The sonar logs the raw transducer voltage time series digitized at 500 kHz. Two information rich products can be calculated from these data. The first involves calculating spectra as a function of range for each ping. This yields estimates of volume scattering strength (Sv) as a function of depth, time, and now also acoustic frequency. Spectrograms of Sv, with the vertical axis as water depth and the horizontal axis as the acoustic frequency, can be stacked into an animation. Here is a link (http://venus.uvic.ca/gallery2/main.php?g2_itemId=3057) to a streaming video showing ten minutes of spectragrams. The Sv data can also be displayed as an “acoustic colour” plot, seen in Figure 2, more akin to what is output by traditional narrowband sonar. The acoustic colour plot reduces the avi-movie into a single plot by selecting three acoustic frequencies and assigning each a colour. Lighter means stronger scattering, darker less scattering; redder means more scattering at lower frequencies, bluer means more at high frequencies. Note the bluish colour of the migrating zooplankton layer, while the discrete targets below, which are likely fish, often have a reddish tint.

Performing pulse compression on the raw returns creates a second data product, seen in Figure 3. Pulse compression takes advantage of the chirp form of the transmitted signal, not to gain spectral information, but to increase both the spatial resolution and signal-to-noise ratio (SNR) of the returns. An ideal pulse compressed signal can temporally resolve scatterers separated by 1/BW, where BW is the bandwidth of the signal. With the 70 kHz bandwidth of the SciFish 2100, this leads to a resolution of 1 cm (as compared to a resolution of around 1 m for both the spectral calculation from this sonar and for a typical narrowband sonar). Again for ideal signals, the SNR increases by 2*BW*PL through pulse compression, where PL is the length of the chirp pulse. The PL is typically 1 ms for the SciFish 2100, leading to a SNR increase of around 140. The increased resolution of the scattering data seen in Figure 3 is quite remarkable. The increase in SNR is less remarkable, which is a consequence of the careful selection of noise-free acoustic frequencies in the creation of the acoustic colour plot.

Shown in Figure 4 is a complete 24 hour section of acoustic colour revealing the diel migration of the zooplankton. Shades of red represent various intensities at lower (95 kHz) frequencies, green intermediate (120 kHz) frequencies and red higher (145 kHz) frequencies. Zooplankton are showing up mostly only in the very high frequency back-scatter. Fish and other larger volume scatterers show up as red and green, or even white (broadband).

The SciFish 2100 is a sophisticated oceanographic echosounder. It is a 48 VDC Ethernet instrument, and can easily generate 1 GB of data per hour. It is therefore ideally suited for a cabled observatory. Dr. Ross has VPN (virtual private network) access to communicate directly with the SciFish from Halifax, where she will adjust the sample configuration and collect data for her research. Data plots and products will be posted on the VENUS web site as this research progresses.

by Tetjana Ross, Dalhousie University and Richard Dewey, VENUS, a division of ONC, University of Victoria.

Figure 2. An 'acoustic colour' plot from the SciFish 2100 in Saanich.
Figure 3. Two five minute echograms from the SciFish 2100 showing the compressed pulse (top) and acoustic colour (bottom).	Figure 4. Twenty four hour section of Scifish 'acoustic colour' spanning July 19, 2008. Time is in local Pacific Daylight Savings Time (PDT).