USING SOUND TO SEE INTO THE OCEAN

Land creatures – including us - are used to using our vision to detect the world around us. Ecologists walking in forests, meadows, grasslands, or deserts can immediately pick out the patterns of the life forms inhabiting the space and easily design sampling protocols to see the relationships to each other and their environment. Not so in the ocean environment. As we stand on the deck of a ship peering into the darkness of the sea surface, we can rarely visualize the animals and plants living just below the surface – much less those living in the depths of the ocean. Divers swimming in the shallow reaches of the ocean have limited visibility (only a few meters in coastal regions, up to 20-30 meters in very clear water), because seawater is a very poor medium for transmitting visible light.  Light is absorbed, scattered, and reflected more in seawater than in air by orders of magnitude.  This limitation affects even the remotely operated and autonomous vehicles with cameras and video systems that can roam the ocean depths, although this technology has given us images of the organisms living deep in the ocean and are leading to new insights about their spatial patterns and behavior on small spatial scales. So how is it possible to view the fascinating 3-dimensional ocean habitat and visualize the spatial arrangement and behaviors of marine organisms on larger spatial and temporal scales?

The transmission of sound at low and moderately high frequencies (1 Hz to 100 kHz) is much more efficient in the ocean than in air. Above 100 kHz, sound is more rapidly attenuated, largely because of absorption due to the salt (principally magnesium sulfate) in seawater.  Despite this limitation, high-frequency sound in the 38 kHz to 500 kHz range is proving exceedingly useful for studies of zooplankton (our target organisms), because it can be used to detect the presence of animals 10's to 100's of meters away from the transducer producing the sound.

Figure 1. Echograms of 38 and 150 kHz acoustic data (Nov. 18, 2011). The vertical axis is depth (m); the horizontal axis is time. Intensity is shown by color (see color bar). The intense scattering shown on both echograms is probably krill patches. Image J.D. Warren

On this cruise, there are several acoustic systems being deployed to provide information about the distribution of zooplankton and larger organisms (such as fish) in the water column. The ship has a hull-mounted Acoustic Doppler Current Profiler (ADCP) with 38 and 150 kHz transducers, which is principally used for measuring current speed and direction with depth under the ship. This system depends on organisms in the water column to reflect sound and produce backscattering (i.e., the portion of the transmitted sound that is reflected off organisms back to the transducer receiver). This can be interpreted as current flow from Doppler shifts (i.e., shifts in the frequency of sound emitted by the transduers) in the returning echoes. Also recorded is the intensity of the sound returned as echoes off the organisms. As the ship steams along, the ADCP provides echograms of the backscattering intensity at two frequencies (Figure 1), providing a continuous indication of high and low concentrations of organisms below the surface.

Figure 2. A) The BioSonics towfish equipped with 38 and 120 kHz transducers being launched from the LM Gould (15 November 2011).  B)The echogram display of the BioSonics frequencies. Photo and Image P.H. Wiebe

A second system is a dual frequency BioSonics echosounder with 38 and 120 kHz transducers mounted in a towed body (Figure 2). This system is being towed off the starboard quarter of the ship for two hours at the end of a station while heading for the next station when sea conditions are good.

Figure 3. A) Joe Warren and MPC Jullie Jackson discuss the zodiac setup; note the orange transducer module on the end of the stainless steel square tubing next to the engine – it will be moved to a down position underwater during the survey. B) The Zodiac is launched. C)  Joe Warren climbs down into the zodiac. D) Kelley Watson, Krista Tyburski, and Joe Warren during a small-boat survey. Photos P.H. Wiebe

The third system is a Simrad echosounder that is battery powered and has transducers operating at 38 and 200 kHz.  It is being used from a Zodiac small boat (Figure 3) to conduct surveys of krill distribution over small spatial scales in areas of interest, where our large vessel is unable to go due to water depth. At our Stn #16, all three echosounders were operated for the first time during this cruise. Conditions were ideal, with low winds and seas – except for a long-period swell running through the survey area.

To help interpret the acoustics data, the small boat survey was conducted along the towing path of the MOCNESS, which provided depth-specific collections of animals and environmental measurements (especially temperature and salinity) in the water column at the station.  The combination of the MOCNESS and IKMT zooplankton samples and the acoustic data will provide a comprehensive picture of the vertical and horizontal distribution of zooplankton living in this Antarctic ecosystem and will allow evaluation of their status in the face of the rapid environmental changes now taking place here.

-- Peter H. Wiebe (Woods Hole Oceanographic Institution) and Joseph D. Warren (Stony Brook University)

P.S. This is Ann Bucklin with a postscript for today’s blog. Bioacoustics is an aesthetically-pleasing (echograms in deep shades of blue and red) and computationally-challenging (huuuuuge data files) field.  The simplicity of the underlying concept – bouncing sound off bugs – is captured in slogans used on Peter and Joe’s T-shirts: “We only measure voltage and time”.  It is also apparent in Peter’s haiku poetry on the subject, including this one:

Echoes

A loud ping goes out
A whisper echo returns
From deep-sea creatures

-- Peter Wiebe