Sonar “Look” at Hydrothermal Flows

Today we are discussing hydrothermal flows (flows of hot water coming from the center of the earth). In particular, theses flows are diffuse (spread out) and relatively low temperatures (one was measured at 7 to 13 C, or 45 to 55 F) compared to those that can reach 250 C (482 F) or more. These low-temperature flows can occur wherever there are the higher-temperature flows; sulfide mounds, lava tubes, isolated cracks, or other seafloor areas where water can pass though.
Despite being cooler, the output of heat from these flows is potentially larger than those of “smokers”, as the high temperature vents may be called. However, it is difficult to determine the heat output of low-temperature diffuse flows, as temperatures vary both in space in time; there are episodic changes locally, as well an effect from the tides. Addditionally, they behave somewhat differently depending on the source; cracks as opposed to lava tubes, for example.
Scientists have a fair amount of information on non-diffused flows and how they behave; their interactions with current, volume over time, and heat output over time, using sonar. Today’s article worked towards these same things using sonar on diffuse flows. “Sonar observation of diffuse hydrothermal ows”, written by D. R. Jackson, A. N. Ivakin ,G.Xu, and K. G. Bemis was published in April 2017 in the American Geophysical Union’s journal Earth and Space Science
A focused (non-diffuse) hydrothermal vent, or “black smoker”. We know a lot about these; today’s article looked more at their diffuse cousins. Image credit: National Ocean Service/National Oceanic and Atmospheric Administration
The authors used data from COVIS, the Cabled Observatory Vent Imaging Sonar run by Ocean Networks Canada. At a depth of 2200 meters (1.37 miles), COVIS gave four years worth of data.
The basics of sonar (or radar) is to send out a signal; if it hits something, it will bounce back. If you know how fast the signal is going, you can use this and the time it takes for the signal to return to determine how distant the object is.
COVIS uses this along with the fact that the speed of sound is not constant. It varies based on things like temperature and, in a case such as this where the sound travels through water, salinity. In the case of COVIS, the return ‘ping’ would be expected at a certain time; if it is not, it indicates a change in the speed of sound, which therefore indicates a change in the temperature. Changes in salinity would have to be much greater than present in the area COVIS was deployed in order to change the speed of sound, and any effects from ocean currents would be negated by the same effect in reverse during the return trip. Therefore, changes in the return time from one ping to another ping in the same place suggest a change in the temperature.
The actual mathematics used are beyond the scope of this blog, but were able to use the basic idea above to work out temperature fluctuations and the likely locations of diffuse flows. However, the seafloor in the area was complex, and a simpler geometry would be very helpful in creating a geophysical model to combine with the mathematics here could lead to finding the output of heat as well. As always, there’s something more to study–the underwater world is less known than its above surface counterpart.

 

Bibliography
Jackson, D. R., A. N. Ivakin, G. Xu, and K. G. Bemis (2017), Sonar observation of diffuse hydrothermal ows, Earth and Space Science4doi:10.1002/2016EA000245 
Today’s main article.
National Ocean Service/National Oceanic and Atmospheric Administration. What is a Hydrothermal Vent? NOS/NOAA, 22 June 2016. Web.
A short page about hydrothermal vents from NOAA, and the source of the image in today’s post.
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