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Comment by mcdonje | Hacker News Reader

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.

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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.

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.

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.

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.

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.

Wouldn’t the water-ice and ice-air boundaries reflect separately in a detectable way?

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.

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.

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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