Miners/Rescuers Don't Have To Be "In The Dark" After Disasters : by: Me
Do you get the feeling that American coal/mineral mines are inspected by the same people who check highway bridges ?
The recent Utah mine cave-in and other similar incidents have shown that a reliable way for rescuers and miners to communicate would be highly desireable. Too often there is little clue as to where miners might take shelter in a large complex or as to their condition and immediate needs.
Conventional communications methods are generally worthless in a deep mine. Cell phones and common radios won't work because of the shielding effect of the earth and/or ores. Wired communications lines are generally cut during a serious cave-in - be they conventional wired telephones or phones that use ore-car tracks as 'wires'. Even the convention of tapping on walls or rails is often rendered useless.
However, one relatively simple and inexpensive technology that can facilitate communications does exist - VLF or ELF radio (Very-Low-Frequency / Extremely-Low-Frequency).
ELF radio is used by the US Navy to communicate with submarines. Where high-frequency radio waves tend to skim the earths, beam into space or are simply absorbed by earth and water, low-frequency radio waves can easily penetrate THROUGH earth and water.
There are two downsides to VLF/ELF radio. The first is because low frequencies mean long wavelengths - thus a very long antennat is needed for medium/long-range transmission. Submarines trail a wire antenna, possibly a mile long.
The second issue is data-transmission capability. Data, be it voice or code, requires modulation of the radio signal. Either its strength (amplitude) or frequency or both have to be twiddled. At the low frequencies there isn't much "room" to modulate the signal before you're out of the bandspace entirely. ELF uses frequencies of only 30 Hz to 3 kHz ... the equivalent of a deep bass note to vocal-range tones. VLF runs from 3 kHz to 30 kHz. Very little modulation can be applied to such signals, they're literally radio waves in the audio-spectrum zone as opposed to 550-1600 kHz for AM broadcast radio or megahertz fequencies for CBs, portable phones, police and avaiation radios and such.
It's easy to superimpose an audio-frequency variant on a megahertz-frequency radio signal. Not so with a radio signal that's an even lower freqency than most audio. Military ELF/VLF communications generally just turn the signal on and off to achieve a sort of Morse-code effect. Data comes in at only a couple of characters per second at best - slower than the slowest typist. Voice ? Forget it !
So how to get around these problems ?
First of all, since we're talking short ranges, we can skip the mile-long antennas. Ordinary slug-loaded coil antennas (take apart an old AM radio - it's the long black bar with a coil of wire around it) would serve OK under those conditions. Not efficient, but who cares ?
Secondly, since we're interested in penetrating rock instead of seawater, we can probably use the "higher" frequencies - maybe up to five or eight kHz. This gives us a *little* room for some kind of modulation.
Now here's where modern technology is a big help. It IS possible to put a voice signel even on VLF radio - but you have to "cheat". The trick is to forget about "real time" back and forth banter. Instead you store speech in a small memory buffer and then, slowly, encoded it into as much modulation as your VLF channel CAN handle. It literally may come across almost as "Morse code", switching the signal on and off, or represent only a one percent change in signal frequency.
Neverthless, the voice signal CAN be encoded. It's just very, very *slow*. The miner would speak a sentence and then wait awhile - maybe a minute or two. Aboveground the reciever would translate the code back into an audio signal. When enough of the message had been translated, it would 'speak' it electronically in that famous 'Stephen Hawking' electrovoice style. A reply could be sent to the miner in the same way. Think of it as using a cockroach to carry small written notes back and forth.
Using voice is better than trying to have the miners use a keyboard terminal. Dust and smoke and eye irritants may make it impossible to even SEE a keyboard or LCD screen after a cave-in. Such embelishments also add to the cost and create more things that can break.
A VLF radio with such voice-encoding technology would not be very expensive to make. The unit plus battery pack could be fit into a 1-foot-square "telephone boxes" that can be placed in various places within the mine. As an added safety feature against dead batteries, each unit should have power taps to which miners could jury-rig batteries from common equipment they already carry.
Keeping the units cheap means that mine owners (who seem to be notorious tightwads) would have no excuse NOT to have them. It wouldn't even be a burden for government regulations to require such units - or unions to demand them. While not "perfect" because of the slow communication speed, slow is one HELL of a lot better than "none".
- - -
There's another alternative that might be useful in some kinds of mines - "magnetic" communication. Solid-state devices for detecting magnetic fields have become exquisitely sensitive of late. Consider a "radio" which isn't much more than a microphone feeding a single amplifying transistor - which is connected to a coil of wire around a metal beam. Speaking into the mic generates a magnetic field which is modulated at the same frequency as the incoming voice. The reciever is an almost identical rig - but uses a sensitive magnetometer to pick up the incoming signal, turning it back into a voice.
This wouldn't work well in a deep mine if there were lots of iron deposits, but it might be OK for a coal mine. Not quite as good as my proposed VLF communcations system, but even cheaper.
Voice signals can also be transformed into sound frequencies that DO carry through fractured rock. VL(a)F "very low audio frequency" communications is also possible using mechanical transducers to send vibrations through the rock.
- - -
Anyway, there are three ideas to deal with the communcations problem in mine disasters. They're all quite simple, robust and inexpensive to implement. I can't be the first one to think of this - SO WHY DON'T WE *SEE* THESE ALREADY ??? Ask your congressman. Ask your local mine-workers union. Ask your local mine owner. No excuses.