
We Need to Map the Ocean Floor - dnetesn
http://nautil.us/blog/why-we-need-to-map-the-ocean-floor
======
breatheoften
I used to operate multibeam sonar system on an oceanographic research ship.
Getting good data from those kinds of systems is an interesting challenge.

The speed of sound in water is proportional to temperature and conductivity —
which are approximately dynamic functions of water column depth and
temperature (and time depending on the properties of the region you are in —
the tenperature can change a lot over short time scales). You need to measure
to measure these properties in order to obtain a sufficiently accurate sound
speed profile to be able to trace the path taken by acoustic energy projected
from the transceiver, off the sea floor and back to the transceiver.
Inaccurate sound velocity quickly becomes one of the largest error terms in
measurement accuracy. We computed sound speed profiles as often as we
reasonably could — sometimes the science objective involved measuring these
properties and we’d use data from those measurements but sometimes we were
just transiting along and would draw from a supply of expendable bathy
thermographs that we could use to make sound speed profiles periodically (but
certainly not often enough to maintain accurate calibration all the time.)

The ship is moving and the orientation of the array needs to be accurately
calibrated — shipboard MRUs are expensive and imperfect.

The water column is rising and falling and vertical position measurement
accuracy is very low from systems like gps compared to horizontal measurement
accuracy.

The ship is pitching and with many ship designs this can create bubbles which
can be swept beneath the ship — a small amount of bubbles close to the
transceiver head can absolutely overwhelm the noise budget of these systems.

I’d be curious to know how much data from ship mounted sonar systems are
actually useable for large scale high resolution mapping. I believe the best
teams that build high resolution global sea floor maps rely on satellite
gravimetric measurements for baselines — and use very sophisticated algorithms
to fuse shipboard datasets with the global gravimetry — which im guessing
probably have to throw away huge amounts of information from the higher
resolution shipboard systems ...

~~~
mr_overalls
Does multibeam sonar function well in the presence of temperature/salinity
stratification layers in the water column?

I don't have the first clue about sonar, but as someone who crunches data for
environmental assessments, stratification occasionally plays hell with a lot
of chemical & biological models.

~~~
breatheoften
Those layers are why you need to measure the sound speed profile -- speed of
sound as a function of depth. The variation in sound speed in the water column
affects not only the travel time of acoustic energy -- but also the path --
(snell's law).

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habnds
A quick summary:

Only 6% of the ocean floor has been mapped. People at the Seabed 2030 project
plan to map the ocean floor in high res by 2030.

Reasons to do this include navigation safety, ability to lay fiber optic wire
and pipelines, because we don't know what's there, weather forecasts since
water/temperature flows along ocean floor contours affect atmospheric flows,
tsunami predictions, we could find sea wrecks.

~~~
FillardMillmore
I hadn't thought about the possibility of sunken ships and sea wrecks until
you mentioned it. I wonder how many mysteries this sea-floor mapping endeavor
will put to rest? If they can put even one big mystery (something like finding
Malaysia Airlines Flight 370) to bed, they'd get a lot of great publicity.

~~~
habnds
Yeah on the one hand it's cool possibility for historians and stuff, on the
other it kind of ruins the adventurous scuba diver studying old maps and stuff
searching for lost treasure.

~~~
mynameisvlad
Firstly, who still does this? Nobody I know in the scuba diving community
"[studies] old maps and stuff searching for lost treasure". That'd be a good
way to not have a dive plan and end up in trouble.

Secondly, no scuba diver is about to dive on the ocean floor. Maybe you're
thinking of deep sea divers, who dive in a submarine or other deep sea vessel?

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pp19dd
Here are "high frequency" sonar images of the Japanese WWII aircraft carrier
Akagi, sunk in the Battle of Midway. They were supposedly taken by an AUV.

Low-resolution sonar: [https://www.gannett-
cdn.com/presto/2019/10/21/USAT/ecaf19d4-...](https://www.gannett-
cdn.com/presto/2019/10/21/USAT/ecaf19d4-e0ae-4380-84c6-f412e1026e02-AP_Battle_of_Midway_Sunken_Warships_4.JPG)

Enhanced with a manual pass: [https://www.gannett-
cdn.com/presto/2019/10/21/USAT/0a89e2ce-...](https://www.gannett-
cdn.com/presto/2019/10/21/USAT/0a89e2ce-a059-4bae-88ce-65e180a296b0-AP_Battle_of_Midway_Sunken_Warships.JPG)

Best guess they used the Remus 6000 to find the ship. It looks like a torpedo.

    
    
      - 3.8 meters long
      - 9 km/h speed
      - 22 hour 'charge'
      - 6000 meter max. diving depth
      - EdgeTech 2205 sidescan array (best guess)
    

The WWII ship is about 250 meters long, so the broad image captures about ...
1.5-2 km? Anyhow, take all that and apply it to roughly 360 million square
kilometers (161 for the Pacific). Ignoring an average current here and marine
traffic and conditions, just going by ideal specs, that's an imaging speed of
about 13 - 18 square km per hour. It would take something like two - three
thousand years to cover the oceans.

Logistical support alone for this AUV is incredible: crew of 20, four two
thousand horsepower diesel engines eating 54 gallons of diesel per hour each
at low throttle. Like 216 - 283 gallons of diesel per hour on average, and not
counting electric generators on that vessel. The engines have to be overhauled
every 4,000 hours. In other words, this effort would be a massive undertaking
under current conditions, and there's not enough diesel on the planet to make
it happen.

~~~
jpm_sd
We're working on a cheaper way to do it:

[https://www.saildrone.com/news/autonomous-vehicles-cost-
effe...](https://www.saildrone.com/news/autonomous-vehicles-cost-effective-
global-ocean-mapping-bathymetry)

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eternalny1
There is no way that the nuclear submarine fleets of the world's countries
haven't mapped a significant portion of the seabed since the 1980s when the
advanced techniques were invented.

I don't see why that data should be held classified, at least make this
project a little bit easier.

~~~
jonknee
> I don't see why that data should be held classified, at least make this
> project a little bit easier.

Really, you don't see why the military would want to keep the strategic
advantage of keeping what they know about the sea floor secret?

~~~
mcbits
I think it should be all be declassified for the public good, but I can
understand why they would want to keep it classified: by revealing where
they've mapped, they reveal where they've been. Quality variations in
different parts of the map could also reveal where they spend more time, or at
what depths. But I'm skeptical that it really matters.

~~~
Razengan
Or they awoke eldritch leviathans who now command them.

~~~
homonculus1
Nothing is Beyond Their Reach

~~~
Razengan
Reference to
[https://en.wikipedia.org/wiki/USA-247](https://en.wikipedia.org/wiki/USA-247)

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Tepix
Does this scanning with sonar harm whales? I'm surprised they are not
mentioned in the article.

~~~
michaelwilson
I believe that the sonar they are talking about here is much "quieter" and
directional than the type which they is harming sea life. The "harmful" type
is either the intense omni directional sonar used to locate other ships, and
the extremely loud sounds used by the petroleum industry to map subsurface
deposits of oil and gas.

Reference: [https://www.nytimes.com/2019/01/22/science/oceans-whales-
noi...](https://www.nytimes.com/2019/01/22/science/oceans-whales-noise-
offshore-drilling.html)

~~~
i_am_proteus
Indeed. It's also possible to further minimize harm by not transmitting when
marine mammals can be seen or heard nearby.

~~~
lstodd
With the speed of sound and its characteristic attenuation in the sea water
and marine mammals' general stealthiness this is just not possible.

Not possible, period.

~~~
i_am_proteus
Interesting conjecture! I have seen whales with my eyes and heard them on
sonar, and disagree with you.

Note that minimizing harm and preventing it are different things.

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maxwell
This will most benefit the deep sea mining industry.

[http://nautil.us/blog/we-are-about-to-start-mining-
hydrother...](http://nautil.us/blog/we-are-about-to-start-mining-hydrothermal-
vents-on-the-ocean-floor)

[http://www.nautilusminerals.com/IRM/content/default.aspx](http://www.nautilusminerals.com/IRM/content/default.aspx)

[https://spectrum.ieee.org/robotics/industrial-
robots/seabedm...](https://spectrum.ieee.org/robotics/industrial-
robots/seabedmining-robots-will-dig-for-gold-in-hydrothermal-vents)

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pmohun
I'm surprised no one has mentioned the XPrize foundation:
[https://oceandiscovery.xprize.org/prizes/ocean-
discovery](https://oceandiscovery.xprize.org/prizes/ocean-discovery)

> As part of its post-prize impact work, XPRIZE announced a partnership with
> Seabed 2030, a collaborative project between The Nippon Foundation and The
> General Bathymetric Chart of the Oceans (GEBCO) to inspire the complete
> mapping of the world’s ocean by 2030 and to compile all bathymetric data
> into the freely-available GEBCO Ocean Map.

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Razengan
Just gotta say, nothing quite drives home the point of how much _water_ there
is on Earth, like even a short oceanic flight and looking out the window.

Imagining you could dive into any spot at any time kinda gives the feeling of
having an entire alien planet to explore.

Our always-connected society gives us the illusion that we've discovered
everything there is to discover, but apparently most of the world is
uninhabited (by humans) and as this says, many parts aren't even mapped.

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savingGrace
Doesn't the ocean floor change often due to currents? Sediment consistently is
moved from point A to point B similar to sand dunes? Even if this is related
to just 'bedrock' or whatever more solid mass is down there, doesn't that get
impacted by the faults a lot? Or is that around just the fault lines?

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redog
It's probably a lot like trying to document the internet. Once you've begun
you have to start over because of how much will have changed.

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jhallenworld
Raising money for such a project should be easy. What investors would not want
to partake in a treasure hunt?

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rootlocus
Not until we finish raking the forest floors.

