We Need to Map the Ocean Floor

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 ...

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.

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-...

Enhanced with a manual pass: https://www.gannett-cdn.com/presto/2019/10/21/USAT/0a89e2ce-...

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.

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.

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

This will most benefit the deep sea mining industry.

http://nautil.us/blog/we-are-about-to-start-mining-hydrother...

http://www.nautilusminerals.com/IRM/content/default.aspx

https://spectrum.ieee.org/robotics/industrial-robots/seabedm...

I'm surprised no one has mentioned the XPrize foundation: 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.

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.

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?

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.

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

Not until we finish raking the forest floors.