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PoorlyAttired t1_iy2osn5 wrote

Reply to comment by BobbyDropTableUsers in TIL More than 80 percent of the ocean has never been mapped, explored, or even seen by humans and as a result, we know more about the surface of Mars than Earth by [deleted]

What's the definition of 'mapped'? I've seen global sea depth maps and presumably they have been mostly covered by depth surveys/sonar at some point or another. On land 'mapped' normally means recording the topography (roads,, buildings, forest, rivers) but for the seabed, what do you record? Depth? Depth and material? Salinity? There's not much to map.

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tutier09 t1_iy2q58y wrote

There are still seamounts we haven't discovered yet. For instance, in 2005 the USS San Francisco collided with one which caused serious damage. They nearly didn't make it. Google it, there are impressive pictures of the submarine.

Edit: I just googled the most recent collision with a seamount. That was the USS Connecticut last year.

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bak3donh1gh t1_iy2tp3u wrote

How could sonar not detect those while the ship was moving toward it? I could see it if they were for some reason trying to be stealthy, but aside from Russian waters and North Korean, subs should be ok.

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tutier09 t1_iy2v5fs wrote

I don't know for submarines but I've nearly crashed a scientific device because the sonar gave the false depth on our research vessel.

You have to set the sonar to a certain depth which you get from nautical charts. I don't remember the exact value but let's say that the digital nautical chart said 2000 m so the sonar was set to let's say 1500-2500 m. We deployed our oceanographic device when one of the officers double checked with an old nautical chart on paper and it said 1000 m. We've already been down for a few hundred meters when he called us to stop. We then slowly advanced and, believe it or not, the old chart was right. It was unlucky that the real depth was exactly half of what we initially thought it would be. Because of reflection the sonar gave us the way to the bottom and back to the surface as depth. If the officer hadn't listened to his gut feeling we would have wrecked equipment worth more than a big family house. That was off the beaten track though, far away from shipping routes - I think they should be mapped accurately by now. If the setting is correct, sonar gives you a good idea of what is down below. More detailed than satellites ever could.

But as I said no idea how that works with submarines. Maybe they didn't have the sonar on for some tactical or training reasons.

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EggKey5513 t1_iy3wfu2 wrote

I believe the differences in ocean current and temperatures in different layers of ocean causes sonar readings to be inaccurate. I’m relying on 15 years of past readings to answer this.

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tutier09 t1_iy47ble wrote

If you keep measuring that stuff you can factor that in. But it's not that much of a difference honestly. I mean not like double or half the actual depth.

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Smart_Ass_Dave t1_iy45r5o wrote

Submarines are (almost) always in total stealth mode. This started during the Cold War. Soviet and American subs would track each other and try to find each other while avoiding detection themselves because total global thermonuclear war could start like...any second and knowing where the enemy's ballistic missile submarines were was not something you wanted to wait on. Keeping submarine locations secret is so important that even commanders didn't know beyond a vague area they were ordered to patrol.

Active sonar (the pings) is rarely used in general, as it's sort of like sneaking around in a forest and turning on a spotlight. If you've been seen but you don't know where the enemy is, it is worth shining it around, but if you have no idea where the enemy is, active sonar gives away your position at a much further distance than it reveals enemy positions. Instead passive sonar is used, listening for the displacement of water by the hull and propeller.

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Paladin327 t1_iy3o1b8 wrote

When underway, a submarine will generally not use their active sonar as using it kinda defeats the whole purpose of a submarine. It’d be like turning on a flashlight in a dark room when you’re trying to not be seen. Subs rely on passive sonar mostly, so unless an uncharted undersea mountain is transmitting noise, it’s invisible

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TheAdmiralMoses t1_iy2q36v wrote

Not exactly, I believe those are mostly extrapolation based off of the known depths, scanning ships go along major routes and scan the sea floor to measure it directly, but they don't know more remote areas, which is 70% of the ocean.

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