Thu Jan 14, 2016 6:14 pm

Stunning new technique to accurately map the ocean’s floor via satellites:

Quote:

So how does this map the seafloor? Get this: If there’s a mountain under the ocean, then it has more gravity than the water around it (rock is denser than water, so it has more mass per cubic centimeter, which means it has more gravity than the same volume of water). This slight increase in gravity draws in water on the surface around it, piling it up on the surface—water is incompressible, so it doesn’t just flatten out. So when the satellite flies over a seamount, it sees a little bump in the sea surface.

If there’s a chasm or trench in the seafloor, then it has slightly lower gravity than the rock around it, so there’s a corresponding dip in the sea surface above it.

Mind you, these bumps and dips in the water surface are tiny, on the order of a millimeter. This is usually completely swamped by currents, waves, chop, and the like. The only way to get good measurements is to take a lot of them, and then the bumps and dips that change over time will cancel out. It’s like flipping a coin; do it a few times and they might all come out heads, but do it a million times and you can be pretty sure you’ll get extremely close to half heads and half tails.

Mind you, these bumps are dips in the water are subtle: They may have a height of several meters, which sounds like a lot, but they’re spread out over a long distance, sometimes hundreds of kilometers. The slope of the water is incredibly small and difficult to measure, and this is complicated by currents, waves, chop, and the like. The satellites have to find that gently sloping trend up (in the case of seamounts) or down (in the case of trenches) despite all that noise. The only way to get good measurements is to take a lot of them, and then the noise that changes over time will cancel out. It’s like flipping a coin; do it a few times and they might all come out heads, but do it a million times and you can be pretty sure you’ll get extremely close to half heads and half tails.**

This was the first thing to amaze me: Scientists can measure the sea surface height to incredible accuracy. The map is based on new techniques that improve the measurements by a factor of two or so.

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** Correction, Jan. 14, 2016: Arg! OK, I made a mistake. When I originally wrote this, I said that the measurements had to be accurate on the order of a millimeter to measure the water piling up or flowing away from gravitational anomalies. However, the reality is subtly different: The satellites measure the slopes of the mounds or troughs in the water over great distances to map the topography beneath them. I have to admit: This is even more amazing to me. You're not just looking for a bump in your data that pokes up or down; you're looking for a looooong, slooooow trend, which is in many ways more difficult. So my admiration for this work stand; it just points in a slightly different direction. For more info, read this, and this, and my thanks to Peter Guth for his help.
[links omitted, italics and bold included]

Last edited by DEyncourt on Thu Jan 14, 2016 8:07 pm.

Thu Jan 14, 2016 10:51 pm

Kirk posted:

We had that posted here many pages back I think.

Fri Jan 15, 2016 2:16 pm

TOS posted:

man, they were just kids

Fri Jan 15, 2016 7:28 pm

Geesie posted:

Fri Jan 15, 2016 7:38 pm

obvs posted:

Geesie posted:

What does this mean?

Fri Jan 15, 2016 9:01 pm

obvs posted:

Where can I get one of those?

Fri Jan 15, 2016 10:44 pm

DukeofNuke posted:

obvs posted:

Where can I get one of those?

You're not talking about the cake cutter, are you ?