Comment by willis936 | Hacker News Reader

Comment by willis936 | Hacker News Reader

>Dr Alastair Lyndon, from Heriot-Watt University, said: “Underwater cables emit an electromagnetic field. When it’s at a strength of 500 microTeslas and above, which is about five percent of the strength of a fridge door magnet, the crabs seem to be attracted to it and just sit still.

For reference, this is the magnitude of Earth's magnetic field.

What's the strength of magnetic fields around undersea cables?

https://www.mdpi.com/2077-1312/9/7/776/htm#B15-jmse-09-00776

From the paper: >EMF strengths predicted around subsea power cables, as reported in the literature, vary from 140–8000 µT [15,17,18]. A commonly utilised cable operating at 1600 A is expected to produce an EMF of 3200 µT in a perfect wire, at the cable surface [17]. As with all EMF, the values will decrease with distance from the source, resulting in a field strength of 320 µT and 110 µT at 1 m and 4 m respectively [17].

15: https://dspace.lib.cranfield.ac.uk/bitstream/handle/1826/778...

426 pages sheesh. Page 25 (37 in the pdf) discusses the magnetic field strength model. No math but references. I'm getting more nervous because the references are all related to modeling power lines in air.

Table B-1 shows a list of submarine power lines and Table B-2 contains their simulated magnetic field strengths.

Medium does affect magnetic field geometry. It'd be nice if someone took some actual measurements if they're not going to use a decent model.

My understanding is that, in an ideal case, the medium only affects epsilon and mu, the permittivity and permeability, which are easily found and substituted from lookup tables.

"In theory, practice and theory are the same. In practice, they are not."

Do you have a reason why lookup tables don't work here or are you regurgitating this quote without evidence that it makes sense in this context?

The lookup tables were made based off of power lines in air.

So much effort has been spent studying the effects but presumably no one has actually checked the magnitude of the magnetic fields around submarine power lines.

I was not referring to the lookup tables in the appendix, rather known lookup tables for relative permittivity and permeability in salt water at 60Hz. If you know those relative parameters you can simply adjust the air-based calculation by multiplying by some function of the ratio of the water parameter to the air parameter (effectively cancelling out).

I'm no expert though, only have some high level undergraduate fields and waves course work.

Which would be fine to do, if it was done. The relatively permeability is implicitly assumed to be 1 in the references that even mention it (17).

Maybe that's a reasonable assumption, but is it? It at least warrants discussion in one of these papers.

Have you ever tried to pick up seawater with an electromagnet?

Is it really so trivial? Water is diamagnetic. Salt water is conductive.

All I expect is a "yes we thought about it" in the form of measurements of ocean floor salt water magnetic permeability.

If the models prove too much they could always drop a magnetometer off a boat.

I just don't understand why you think this matters. Even if the relative permeability were as high as, say, 2, it would only move the 500 uT effect radius by some tens of centimetres. But just from common sense, it must be several orders of magnitude less than that, because otherwise your compass wouldn't work near the ocean. In practice, you need a very sensitive instrument to even measure the relative permeability of water, which already means that it's completely negligible for the purposes of this discussion. Wikipedia quotes water's permeability as 0.999992.

By that measure, an eighty-times stronger argument against the researchers' due diligence would be "how do they know the cable isn't actually carrying 1599 amps? Did they measure the current?"

Considering running ADSL over wet string didn't work until the water was salted there's obvious EMF considerations at play.

https://www.revk.uk/2017/12/its-official-adsl-works-over-wet...

I'm pretty sure that's due to saltwater's conductivity, not any EMF considerations. Conductivity will result in some partial cancellation of the AC magnetic field. But not enough to matter at 60 Hz, which is within the Extremely Low Frequency band used for communications with submarines.

https://en.wikipedia.org/wiki/Communication_with_submarines

Relative permeability is very close to 1 in non-metallic materials like water.

Reply on news.ycombinator.com