Isn’t it amazing that we can use sound to profile currents so easily from vessels underway, but not something as simple as temperature. This, even though sound refracts so readily in temperature gradients – sound always bends away from warmer water. For example, this is why submarines would hover close to the surface on warm afternoons because any sound they emit would bend or refract away from the surface – what was called the afternoon effect. Naval ships wouldn’t have a clue a submarine was hiding nearby. For the longest while I thought one could use this refractive effect to reconstruct the upper ocean temperature profile - and gave it quite a bit of thought.
What I had in mind was simple enough: transmit acoustic pulses and measure the round-trip travel time (TTM) to a reflector (read fish) as the ship steams by. As the ship approaches, the travel time will decrease until it reaches a minimum when overhead after which it starts to increase. The resulting TTM vs. time plot will have a hyperbolic shape. The minimum gives the reflector’s depth and knowing the speed of the vessel relative to the particle one can from the shape of the hyperbola derive determine the average speed of sound and hence temperature. Underway, the ship will get travel times to reflectors over a range of depths. From a set of mean speeds of sound to various depths one can then back out the temperature profile. It won’t have the detail of an XBT or Argo profile, but assuming enough reflectors along the vessel’s route, one should be able to reconstruct the upper ocean thermal structure and heat content.
These thoughts were developed some 20+ years ago so perhaps they have little currency today given the global coverage of the 4000+ Argo floats. But I wonder, there are several aspects that come to mind. First, continuity in the horizontal. This means capturing and detailing mesoscale features such as fronts and eddies the sparse Argo array can’t. Second, the repeat sampling along specific routes allows one to monitor temporal developments with greater confidence, that of fronts for example. Third, the acoustic technology is very straightforward and could be applied to any ship without the need to drydock so it could readily be applied to a vast array of vessels to scan the ocean.
I know, I know this has the flavor of wishful thinking. But think about it for a moment. If a major objective of the global XBT and Argo programs is to inventory upper ocean heat content, then perhaps this is worth further consideration. Not to replace but to augment.
Hardware-wise, all that is required is a linear array of acoustic transducers (that transmit and receive) mounted athwartships attached to the hull of the ship. The ensures that the radiated signal spreads radially in a vertical plane aligned in the ship’s direction of travel. The transducers would be mounted in a frame that can be secured magnetically to the hull without the need to drydock. The one assumption I have to make is that it is possible to install a wet-mateable connector in the hull to power the acoustic transceiver without the need to drydock. Since the array of transducers will work as a single unit, only one electrical connection is required (transmit/receive) if one can use the ship as ground. I know nothing about this, but assume it is possible.
There are serious issues that will need to be addressed. First, the timing requirements are demanding, which means that the assumption that a backscatterer (fish) doesn’t move during the vessel’s transit overhead becomes a challenge. Can its motion be averaged out or can we use the acoustics (Doppler shift) to measure its motion? The linear transducer is designed to insonify the water in a narrow vertical plane. How sensitive will the results be to backscatterers that are near but at some distance from the vertical plane. It is assumed that discrete backscatterers are available in sufficient quantities. Software will need to be developed to pick out the hyperbolae and determine the TTMs patterns accurately. Knowing vessel speed will be of paramount importance.
If this strikes you as interesting or worth further study, let me know as I still have the slides from a talk I gave on these ideas 20 years ago.