Under the Hood by Brandyn Lucca, Blog 23 June 2018
I had previously alluded to the “acoustic fingerprint” (or how objects reflect sound in the water) of marine critters we detect via our echosounders in my first blogpost, and I’d like to expand a little bit on that today. An animal’s acoustic fingerprint, or target strength (TS, dB re: 1 m2), is affected by a lot of things, such as an animal’s size (e.g., length, fatness) and shape (e.g., round, skinny), and how the animal is oriented relative to the surface (e.g., horizontal, head facing the surface).
Although it is difficult to know how animals are oriented in the water column, we can measure some of their other acoustical properties. We take a lot of photographs, which are important for getting basic shape information. These data are then fed into an acoustic model to predict how certain animals reflect sound at the frequencies we are interested in.
Another factor that affects an animal’s acoustic fingerprint is their internal anatomy. Animals with lungs and swim bladders tend to produce stronger target strengths than smaller, fluid-filled critters. The same goes for animals that produce bubbles, which scatter a lot of acoustic energy. Dolphins are an example of an animal that both have lungs and produce bubble streams. During one of our echosounder deployments, a large pod of common dolphins were spotted near our boat. So we very scientifically put a camera in the water (i.e., a GoPro on a large boat pole) to record some video of the dolphins frolicking. It should go without saying, it was pretty neat, although maybe not as much for the folks sleeping since the dolphin whistles could be heard through the hull of the boat!