Day: 8 January 2021

When profilers are optimized for control

It’s impossible to see a Frogfish. It’s not because of their camouflage, even though it is incredibly intricate. It’s not because they swim quickly, either: they are not very streamlined, and many Frogfish will lumber around slowly and mainly try to stay in one spot. What I mean is that you literally won’t be able to see them. When it comes time for them to strike their prey, they can do so in as little as 6 milliseconds. How fast is this? It is well below normal human reaction time – and most other animals’ – of 200 milliseconds. But while the speed is impressive, it means nothing if they miss their target.

So how do Frogfish control their attack to make sure they don’t miss? Their jaws are extremely flexible: when they expand outward, they increase their mouth volume by a factor of 12. This helps them contain their prey more easily. In addition to this, they suck in a massive amount of water around their target to draw them in. The excess water is simply filtered out their gills while their meal gets sucked right into their stomachs. Enveloping their prey and sucking them in makes sure their attacks are well controlled.

Control is key to getting consistent and specific results. In the case of oceanography, there’s more than one way to measure profiles in the water column. Wave powered profilers are impressive, but they have limitations. Making consistent and specific profiles may require a powered profiler, and in this article, we’re going to talk about how these technologies work.

Frogfish envelop their prey and suck in water to control their attacks.
Picture credit: Betty Wills (Atsme), Wikimedia Commons, License CC-BY-SA 4.0

A powered profiler has an onboard energy supply

These powered profilers carry their own battery and electric motors and use them to move on the mooring. This is in contrast to wave powered profilers that ratchet along a mooring that moves from ocean waves.

The powered profiler drive mechanism is straightforward: it uses a traction wheel to crawl in either direction along the mooring line. The onboard batteries and electric motor ultimately drive this traction wheel.

Powered profilers may seem a bit overkill

After all, there’s extra complexity with carrying their own batteries and motors – particularly in contrast to a simpler alternative like wave powered profilers. However, wave powered profilers just won’t work if there are no waves or no surface buoy on the mooring. But there is another advantage to powered profiles, and that is in their control capabilities.

Recovery of a McLane Moored Profiler. Picture credit: Scripps MOD

Powered profilers have a lot of ways to make controlled profiles

There are lots of options to control when profiling. Powered profilers can make profiles on a regular schedule. This schedule can be adjusted over time to sample seasonal variations in the environment, too. Either way, regularly scheduled profiles will generate a consistent dataset with evenly spaced samples in time, reducing data post-processing later on.

When powered profilers are used in a real-time mooring with satellite communication, they can even profile on-demand. But when to profile is only half the story. The other half is where to profile.

There’s also a lot of control over the profile span

A basic approach is to use mechanical stops on a mooring line. But powered profilers have many programming options, too. They can run between specified depths. Different depth ranges can even be used in separate profiles, as well.

All these time and spatial measurement capabilities give a lot of control over profiling. While this is a significant advantage, there are also a few instances when they are the only choice for making profiles.

Mooring motion is vital for wave powered profilers

Wave powered profilers need the mooring motion from surface waves to work correctly. Subsurface moorings, by their nature, have no surface buoy. So if you want to make profilers on subsurface moorings, your only option will be a powered profiler.

But the longer moorings are, the less the lower portion of the line moves when there is wave action. If profiling is required in the lower part of the mooring that is isolated from surface waves, or even in particularly deep water, powered profilers may be the only option.

Like everything in life, there are advantages and limitations

There is only so much energy in the batteries. The traction motors can only exert so much force, too. If there are high current regimes or excessive wave action, it can slow down profiling or use more energy than expected. The drag forces can also exceed the traction motor’s ability to move on the mooring. So there are some limits for use in extreme environments. While wave powered profilers thrive on ocean waves and excel in profiling near the surface, powered profilers typically operate in deeper than 20m water depth to avoid disruption from wave forces.

Another limitation is that the mooring deployment length may be governed by how much energy the batteries can hold for profiling. So how easily you can get out to the mooring to service it is a factor for how they can be used effectively.

Let’s look at an example

Scripps Institute of Oceanography used powered profilers on subsurface moorings to measure internal ocean waves in the Tasman Sea. You can see a schematic that highlights the use of two McLane Moored Profilers below. Note the mooring schematic is not to scale: each powered profiler traversed over a kilometre of line independently. The profilers needed to cover this distance because the internal waves Scripps Oceanographers were looking for were hundreds of meters in size, though also slow-moving, as they propagated through the ocean.

Two McLane Moored Profilers were used on this subsurface mooring to track internal ocean waves propagating in the Tasman Sea. Picture credit: Gunnar Voet from Scripps MOD

It’s summary time

Powered profilers carry their batteries and an electric traction motor on board to work. This means they can also control the depth and rate of profiling. These parameters can vary specifically over time – such as to account for seasonal changes in the ocean. But their onboard energy supply may be a limiting factor for the mission duration. They also have a limit to how much force they can use to crawl along the mooring line – so be wary of extreme environments with large surface waves or currents.

Even though a Frogfish can strike in the blink of an eye, it still needs a way to control its attack so they don’t miss. Similarly, control may be a significant factor when you are planning profiles for your oceanographic project – and a powered profiler may be just what your project needs.

Next step

McLane Research Laboratories makes both powered and wave powered profilers. Read more on their powered profiler MMP here and the wave powered profiler Prawler here.

Thanks

Thanks to Tom Fougere at McLane Research Laboratories for the discussion and information on powered profilers. The helpful mooring example was also provided by Gunnar Voet at Scripps MOD.

PS

You can see a video of how fast a Frogfish can strike below. Don’t blink!