[HN Gopher] How thick is sea ice and how do we know?
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How thick is sea ice and how do we know?
Author : mooreds
Score : 41 points
Date : 2023-01-26 14:56 UTC (8 hours ago)
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| mcdonje wrote:
| Well that's depressing.
|
| It's also a bit confusing to me. Sonar can't measure thickness.
| It only measures the distance between the sub and the bottom of
| the ice. Altimeters likewise only measure the height.
|
| I suppose if the maps are comprehensive enough and you overlay
| them, you can get a thickness. However, the computer models
| appear to be looking at ice age and surface temperature, not
| sonar or altimeter mapping.
|
| Surface temperature seems like the sort of thing that would be
| well tracked if satellites are capable of taking those
| measurements. Ice age seems like something they'd probably need
| human run surveys for.
| soperj wrote:
| Generally they track the ice and where it's moving, so they
| know when the older ice is moving in a direction where it'll
| melt out. The older ice normally piles in around ellesmere
| island, especially during the summer.
| FoomFries wrote:
| I would imagine sonar coupled with water pressure could measure
| thickness (or however a sub measures its current depth).
|
| Surface temp is done via IR satellite imagery, although clouds
| will muck the readings.
| abruzzi wrote:
| How accurate are depth gauges? (I see them in submarine movies
| all the time, but I don't really know anything about them.).
| But if your depth gauge says 100 ft, and the sonar says the ice
| is 80 ft above you, then you should be able to estimate 20 ft.
| Also does the presence of ice affect the depth gauge? I'd think
| not, but I don't know.
| mcdonje wrote:
| You could be onto something there, but it'd be complicated by
| displacement. If it pushes the water boundary down 20', how
| thick is the ice above sea level? If they have a pretty
| consistent way to estimate that at the relevant level of
| granularity, then that would work.
| Retric wrote:
| Ice has a reasonably uniform density as does sea water.
| This approach isn't going to provide micron level
| precision, but should get to within inches.
| LarryMullins wrote:
| If the density of both the ice and the water are known and
| if the ice is floating (not resting on the bottom), then
| you can calculate how much ice is below the water by
| observing how much ice is above the water, and vice versa.
| wefarrell wrote:
| If you know the altitude above sea level and the density is
| uniform then it should be possible to calculate how how much
| ice is below the surface and the thickness.
|
| Sonar should be able to measure the distance from the sub to
| the ocean floor and the sub to the bottom of the ice above it.
| Add them together and you get the distance from the ice to the
| ocean floor.
|
| With those two datapoints it should be possible to get an
| accurate estimate of the ice thickness.
| misnome wrote:
| Wouldn't the water-ice and ice-air boundaries reflect
| separately in a detectable way?
| echelon wrote:
| Could you fire projectiles or lasers at the ice from drones?
| This could be automated and cover a large area.
|
| A small penetrator with built in radio could conceivably make
| it through a lot of ice sheet depth and relay all of the data
| back.
|
| Optical methods could penetrate a certain thickness of ice and
| give us absorbance / reflectance spectra.
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