Through-the-Earth Mine Communications | Hacker News Reader

Through-the-Earth Mine Communications

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39 pointsslow_typist9 comments

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Recently some people experimented with LoRa in mines.

Seems like the extreme sensitivity (down to -146dBm) is able to deal with the reflections there pretty well down there.

But: that's not Through-the-Earth. It's Within-The-Earth.

(I have no details regarding the used band, antenna nor exact setup)

There seems to be so much excitement around LoRa, but I don’t really get it…

What is the use/appeal?

it's low power and long range, but low bandwidth. For a lot of applications, that's pretty useful.

It's extremely sensitive to weak signals (-146dBm.. for comparision Wifi becomes unreliable below -80dBm and unuseable at -90dBm) and thanks to its chirp character very robust in presence of noise (jammers, alien stations, etc.).

You either can parametrize it for speed or for robustness - there are a lot of parameters to fine tune it. Also it's easy to switch even faster modes (that are not LoRa) by just sending a few SPI commands.

The documentation of the chips is stellar. There are good software libs around, also Zephyr supports a few of them.

Also Semtech is extremely hobbyist friendly.

If you want to play around with them: use LoRaWAN and The Things Networks. There are a lot of basestations around - at least in Europe.

Getting >30km links with a still a metric ton of SNR left pretty much the standard. Even with lightly obstructed LoS. I've seen experiments with balloons that hat >400km links with plenty to SNR left.

Practically: we are using a single LoRaWAN basestation to gather telemetry from a giant industry complex. No problem even if the sensors are placed deep in the basement.

Cool!

Not claiming to understand lora but I guess that is probably because it kinda sweeps through the band so reflections which belong together come on the same frequency and therefore can add to the reconstruction of the signal (waves hands)

> Radio communication within caves is problematic because rock is a conductor and therefore absorbs radio waves.

It's funny, I would have bet my house that rock is an insulator.

Depends on the rock.

Enormous asteroid of pure copper? That thing gonna conduct.

You'd have kept your house if you had a reasonable judge, but only if the statement was "rock is more like an insulator than a conductor" and not if the statement was "radio will work in a mine deep underground".

Everything is an imperfect conductor. There's just an awful lot of rock in a mine, and in practice it's often mixed with highly conductive mineral-bearing water.

Looking up some numbers, "rock" has a conductivity on the order of 10^-5 sieverts per meter. That's billions of times less conductive than something like copper, and comparable to something like wood, which we think of as an insulator. You could touch a piece of wire energized with high-voltage AC to a granite or quartz countertop, touch the countertop a short distance away with your hand, and connect your other hand to the ground reference and feel no shock at all.

I suspect that someone was taught that radio doesn't pass into a faraday cage or escape a microwave because the conductivity of the metal mesh absorbs the radio waves, and separately that radio does not work underground, and synthesized those facts to "rock is a conductor" without actually getting out a 4-wire Kelvin probe and measuring the conductivity of the rock.

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