Day: 27 November 2020

Why profilers stop massive data processing headaches

When the going gets tough, I like to think about the year 536, and it helps me feel better. The year 536 was a seriously difficult one: a strange fog enveloped most of Europe and large parts of Asia. The mysterious fog was so thick that daytime sunlight was reduced to mere twilight.

You’d think perhaps a stiff wind might clear it up after a few days. Unfortunately, the fog and daytime twilight lasted for 18 months! It turns out the fog was caused by a massive volcanic eruption in Iceland that spewed ash across half the planet. A team of researchers learned more about this by looking at ice core samples. These core samples were ancient, with accumulated layer upon layer of ice over thousands of years. These researchers used super-precise analysis of these ice cores, painstakingly scraping and scanning layer by layer, to examine the ice composition. The ice showed what atmosphere composition looked like through time. These samples revealed massive quantities of volcanic ash throughout the year 536. It took a lot of processing to unravel just what happened back then.

The ash from a massive volcanic eruption blotted out the sun for 18 months back in the year 536

Now most people working with data measured from the ocean are no strangers to lots of processing. While collecting the raw data in the first place can be a challenge in its own right, your headaches may just be beginning when it’s time to look at what you actually recorded. In this article, we will talk about how profilers can help cut through data processing headaches.

Moored profilers are the result of rethinking typical moored oceanographic instrumentation

Most typical oceanographic systems use sensors clamped at a fixed location on a mooring line. It’s a straightforward approach and a simple way to start. But profilers take an entirely different approach: a profiler is mobile on their mooring lines, crawling up and down to new positions in the water column. Each profiler consists of a frame that can be bristling with sensors. These sensors make measurements through the water column as the profiler inches back and forth along the mooring line.

Profilers actively move up and down a mooring line while on-board instruments take measurements

At first glance, there’s an obvious advantage

If you’re using a profiler, you certainly don’t need a whole batch of sensors individually clamped on the mooring line. Longer moorings may have dozens of sensors, and these sensors aren’t cheap! It also can take a lot of time assembling the mooring during deployment. So there is a cost savings factor. But there is a less obvious advantage. While less obvious, it can be significantly more important. Critical, even. This factor is in the data quality profilers can produce.

The problem is in the nature of the ocean itself

The ocean is so big, it may feel intuitive that properties like temperature, salinity, and velocity could only smoothly and slowly vary through the vast depths. But in certain circumstances, nothing can be farther than the truth. In fact, these properties can change extremely abruptly in the water column. There are thin and thick layers of water can form that don’t mix for hours, days, or even years. But these layers can still wobble and move around in the water column through time.

This brings us to the big problem

This big problem is spatial resolution. With fixed sensors, you can completely miss details in the gaps between measurements. This can lead to a severe disconnect between the real ocean and what you read in your data.

A profiler cuts straight through all that confusion: it slides along the mooring, making measurements along the mooring span as it moves. So you get the whole picture along a slice in the ocean.

Traditional moorings have several sensors clamped in place. They can miss a lot of detail as layers of salinity, temperature, and other factors often form in the water column

Profilers are a good idea when there is interest in high resolution measurements

Or perhaps when there’s uncertainty about just what’s going on in the ocean. If there’s some reason to expect complexity and variation in ocean properties, it makes sense to use something like a profiler.

But what about time resolution of the dataset?

It’s true, profilers are a great option, but they can’t do everything. The spatial resolution of the datasets captured by a profiler can be a big advantage. But these measurement profiles can’t be made instantly, or necessarily one right after the other, rapidly. Ultimately, the time resolution of a profile will be limited by how fast the profiler can move. There are other limitations on how often profilers can sample, but it is specific to each kind of profiler.

However, sometimes more than one profiler can be used on a mooring line to help. Sometimes hybrid approaches can also be used with fixed sensors on a mooring line in tandem with a mooring, or sometimes even a profiler mooring with a more standard instrumented mooring nearby as well.

It’s time for a summary

The ocean is a complicated place. There may be many complex layers of temperature, salinity, or other properties that traditional moorings iwth fixed sensors might miss. Profilers are a specialized tool that can help you better understand these complexities by making high resolution measurements as they move along the mooring line. Profilers can slice through the water column, giving you information layer by layer about what’s going on. Not entirely unlike scanning through ice cores to learn more about medieval volcanic disruptions!

Next step

There are a few different kinds of profiler design. But many of them have similar design elements. In all cases, the mooring line slides through the unit, and some kind of clamping mechanism is used. Read more on Del Mar Oceanographic’s WireWalker profiler here and McLane’s Moored profiler here.

Thanks

Thanks to Chris Kontoes and Drew Lucas from Del Mar Oceanographic for the discussion and information on profilers, as well as the inspiring mooring graphics!