During our cruise we have been working in shifts around the clock to sample the biological, physical, and chemical properties of the off shore Gulf of Mexico from the surface to thousands of meters depth. The efforts required for the collection of these samples utilizing an array of oceanographic equipment are great. However, the acoustical instruments we are using require very little effort in their deployment, and continuously sample 24 hours a day, even when weather conditions are too rough for deck operations.
In water, sound travels faster than it does though air by a factor of five and dissipates less energy than in air. Given these properties, acoustic instruments provide an excellent means for remotely sensing the underwater environment in large volumes over great distances at a high spatial resolution. For our cruise we are using active acoustics. These instruments operate by sending sound impulses, or pings, and waiting for the pulse to hit an object. When it does, the sound is scattered in multiple directions creating an echo to which the instrument’s receiver listens for. Knowing the time between a ping and an echo to return, we can determine how far away an object is with high precision.
Acoustics typically ping sound at one or several frequencies. Low frequencies travel long distances while high frequencies travel a shorter distance as the sound is attenuated more quickly. For our application we are using a Sentinel Work Horse Acoustic Doppler Current Profiler (ADCP) with a 1200 Hz frequency, and an ASL Environmental Sciences Acoustic Water Column Profiler (AWCP) with dual frequencies, 446 Hz and 720 Hz. Organisms in the sea scatter sound differently across a spectrum of sound frequencies. By using multiple frequencies simultaneously, it is easier to determine specifically what organisms are responsible for a proportion of the total backscatter measured in an acoustical survey.
There are two problems in resolving acoustic data. First is the “inverse problem” which is the issue of using target strength (TS), which is the sound scatter strength of an object, to calculate the number of individual objects (N) that are present from the measured scatter volume (Sv). The second is the “forward problem”, which is the application of ground-truthing data (data from nets and optical samplers) to acoustical data to see what was actually present in the water during an acoustic survey. Data on physical parameters of the water column are equally important to determine the effect they have on sound in water.
It is easy to see that there is no single tool that is perfect for sampling plankton. Optics and nets help verify the cause of sound scatter in the water column, while acoustics give information relative to the large scale abundances and distributions of organisms collected by optical and net sampling instruments. Despite so many difficulties inherent with acoustic instruments, the inversion of acoustic data for accurate biological estimations is regularly accomplished. Acoustics are fast becoming standard tools for surveying fish stocks, and are regularly used to map the ocean floor with incredible precision.
-Fredrick D. Marin
The Acoustic Doppler Current Profiler, 1200 Hz (top) and the Acoustic Water Column Profiler, 446 Hz & 720 Hz (bottom) mounted to a pole which is turned to point down in the water while conducting acoustic surveys.
A plot of acoustic data collected over multiple days. The red undulation is the deep-scattering layer (DSL), which is the massive daily vertical migration of small fishes and zooplankton into the upper water column at night and back down to the deep during the day.