I’ve often wondered why the oceanographic community shows so little interest in working with the merchant marine (MM), considering that the MM could give us incredibly broad and sustained coverage the high seas water column, what we could never expect to achieve on our own. Is it because their vessels are kind of intimidating (surely they wouldn’t let me work with them?), is it because they operate on rigid schedules (UNOLS is far easier to work with), or because it is easier to explore the vast archives of satellite and Argo data (plenty of excellent science there)? A few of us do in fact work with the MM, but we tend to view them as extensions to what we do on UNOLS vessels; we have not explored the possibilities the MM might open up for ocean observation. Earlier, I wrote about the need for scanning currents to greater depths on M-vessels. Here’s another example.
First, a bit of background. I sense that the lack of MM-interest has historical roots besides the technical challenges. Namely that hydrography has always been and continues to be the backbone of physical oceanography. Much of our research focuses on the distribution of ocean water properties, and the mechanisms that shape their spread from source regions. MM-vessels can’t help, they are limited to dropping expendable bathythermographs (XBT). As a result, we have made little (no) effort to develop hydrographic instruments and sensors optimized for MM use. There are two exceptions. One is what European oceanographers call ‘ferry boxes’, instruments that record surface temperature and salinity, and in few cases nutrients. The other development measures dissolved CO2 in the surface waters; it has been deployed to a number of MM-vessels.
Amazingly, the XBT technology is over 50 years old. It was designed for Navy use, not for oceanography, but thankfully the XBT has been a major contributor to what we know about the warming of the oceans. The interest in the XBT has waned a bit with the advent of the Argo float program. Which is unfortunate because Argo is a broadcast low spatial resolution system whereas XBTs allow for high resolution, targeted coverage; they complement each other. XBTs excel at resolving structures and fronts, free-drifting Argo floats were not designed for this. This begs the question whether it might be possible to evolve the XBT into a cost-effective expendable CTD (XCTD). While XCTDs can be bought today, and they are good, but very costly. A fresh approach is needed.
The conductivity sensor on the original Neil Brown CTD consisted of a tiny ceramic tube, perhaps a ¼” in diameter (there’s a sketch of it in the Yvette instrument blog). While it was a challenge to fabricate, the advantage of using ceramic is that it is extremely stiff and chemically inert. This matters because the calibration of the sensor depends on a absolutely stable electrode geometry regardless of pressure. Can we take this thinking to modern integrated chip (IC) technology? ICs use silicon as a substrate, which is a close relative to ceramic. As you can imagine given the millions of transistors operating on a single μ-processor chip, the dimensional control of modern IC technology is phenomenal. I like to think that it might be possible to put a conductivity sensor on a chip, and when you calibrate one, you’ve calibrated them all. This is admittedly a wild assertion, but let it fly for the moment. Let’s also imagine that the analog-to-digital converter, very-well established technology, can be put on the same chip. While we’re at it, why not put the temperature sensor on the chip as well? All that remains is the pressure sensor. Chip-based ceramic pressure sensors exist, most likely they would be a separate component in an XCTD. What all this suggests is that with modern chip and fabrication technologies we can imagine the possibility of mass-producing XCTDs. We haven’t asked this kind of question because with have first-class CTDs on research vessels and Argo floats, so the notion of using XCTDs on MM-vessels hasn’t entered our minds, kind of a chicken and egg problem. But imagine we had XCTDs optimized for use on MM-vessels and even for RVs if it meant we didn’t have to stop as often? I’m quite certain an XCTD could include an O2 sensor. I tried working with a ceramic Clark sensor 40 years ago. I haven’t followed the technology since but a quick check on the web shows they are widely used. Clark sensors are flow sensitive. The XBT approach becomes particularly attractive because of their very high and steady high descent rate, presently ~6 m/s.
Developing these sensors will cost, but in volume production the unit cost might be quite attractive. The XBT housing and wire technology are so well-established that elapsed time gives depth to ~2% accuracy, but uncertainties in fall rate can be reduced if the probes include pressure sensors. A major production challenge will be to streamline calibration and handling of the calibration data so the measurements have been scaled and calibrated into proper units for immediate dissemination as is the case with XBTs today. As for calibrating the conductivity sensor, my gut feeling is focus on resolution rather than absolute accuracy. The reason is simple, much of the salinity variability is in the upper ocean. Use archived salinity data from the deep ocean to ‘render accurate’ the chip. There should be an X-Prize for the XCTD!
Imagine being able to couple hydrography with our ability to scan ocean currents from MM-vessels. This would enable us to measure fluxes of various water properties, giving us a heads-up on future states of the ocean!