How do mines communicate underground?

Cell towers and surface radio stay on the surface. Below ground, any coverage a mine has is coverage it installed itself, along the entries and working sections where people travel and work. NIOSH’s tutorial on mine communication notes that handheld radios at conventional frequencies can link through the air underground, but the range is very limited, so systems extend the network to the miner instead.

Rock falls, fires and explosions add a second problem. A system that works on a normal shift has to keep working, or fail over, after the event that makes it most needed.

The Mine Improvement and New Emergency Response (MINER) Act of 2006 requires underground coal mine operators to keep an emergency response plan that covers communication with the surface and tracking of people underground. MSHA’s Program Policy Letter P14-V-01, still current, sets out what that looks like in practice:

• An untethered device, such as a handheld radio, available to every group of miners and to anyone working alone.
• Two-way voice or text, with an alarm on incoming messages and the ability to send an emergency message to every device.
• Coverage throughout each working section, continuous coverage along escapeways, and a zone around strategic areas that normally extends 200 feet inby and outby.
• Redundant signal pathways to the surface, so losing one path does not cut off communication.
• Standby power for the underground infrastructure, normally at least 24 hours, and handheld devices that normally last at least 12 hours in total.
• Equipment approved by MSHA under 30 C.F.R. part 23.

The same letter states that fully wireless communication technology was not developed enough at the time to permit its use throughout the industry, which is why mines combine several systems rather than relying on one.

NIOSH groups the technologies by how their signals travel:

• Pager phones. Wired phones on a shared line; pressing a button broadcasts to every phone, which suits emergencies but allows no private calls. NIOSH notes that the wires are easily broken or shorted by rock falls.
• Leaky feeder. A coaxial cable with openings in its shield runs along the entries and “leaks” radio signal in and out along its length, with amplifiers to make up for losses. These systems are marketed at VHF around 150 MHz or UHF around 450 MHz.
• Node-based systems. Small transceivers, or nodes, are installed along the tunnels and relay traffic between them. Some use Wi-Fi; others use UHF radios. NIOSH describes these as a partial mesh, where nodes can detect a lost node and route around it.
• Medium frequency (MF). Around 500 kHz, signals couple onto existing conductors such as power lines, phone lines and pipes, and travel along them. The radios are larger and heavier than handhelds, so NIOSH expects MF to serve as a redundant or emergency path.
• Through-the-earth (TTE). At frequencies below about 10 kHz, signals can pass more than 1,000 feet through the earth between loop antennas, with no infrastructure in between. Messages are small and can take several minutes to arrive, so TTE is an emergency link.

Before electronic tracking, a dispatcher kept a list of who was working where, updated by phone. NIOSH describes the electronic replacement: each miner wears an active tag, readers fixed at known points detect tags in range, and the reader reports to the surface. A miner out of range of every reader shows at the last reader that saw them.

MSHA’s guidance expects tracking to locate miners within 200 feet on working sections, refresh at least every 60 seconds, show the last known location when a tag stops reporting, and keep location data for at least two weeks.

The regulated systems exist for voice, alarms and location. Day-to-day work produces other records: pre-shift findings, equipment faults, permits, handovers between crews. These records have to move between people who are close together underground, and later reach the surface system.

Software that lets nearby devices exchange records directly, and hold them until a path appears, covers that gap. A finding logged at the face can reach a supervisor’s device in the same section without a surface round trip, and a device that later reaches a connected node, or travels out of the mine, can carry the records out to the surface system. This sits beside the mine’s approved systems, not in place of them, and any device taken underground has to meet the mine’s approval rules for equipment in that area.