Mines generally use reliable and effective communications systems for routine operations, but these systems require hard-wired networks, power supplies, and other infrastructure that are likely to be damaged by a catastrophic event, such as a fire, explosion, roof fall, or water inundation. Technologies are needed that will enable communications systems to function in post-disaster environments. Additionally, prior to the MINER Act, communications were only required at select locations in the mine leaving a large percentage of the mine area where communications was not required and generally not available.

Systems are also needed to track the locations of mine workers during rescue operations. However, an electronic tracking system requires a reliable communications signal to relay location information to the surface. Technical barriers to in-mine or through-the-earth signal propagation must be overcome before reliable emergency communications and miner tracking will reach acceptable functionality.

The post-disaster coal mine environment creates unique safety and survivability considerations for emergency mine communications and tracking systems. This page summarizes those issues and presents an overview of mine communications and tracking technologies. More detailed information is available from communications and tracking research papers listed on the Emergency Response and Rescue Downloadable Mining Publications page.

Mine Communications

Present communications systems for underground mines can be hard-wired or wireless. Each approach has certain strengths and weaknesses. Post-disaster procedures create unique challenges in a coal mine environment for either type of system.

Underground coal mines are subject to the build-up of methane gas unless there is continual ventilation of the mine. When a disaster occurs, power to the mine is often cut and ventilation controls are sometimes compromised. This safety issue must be addressed when designing wireless communications for underground coal mines (i.e., the systems must not create sparks that could ignite a gassy atmosphere). Such permissible designs usually limit the output power of devices, making transmission from the mine to surface more challenging.

The common practice of cutting power introduces another concern; providing power to electronics during a disaster will require backup power supplies. NIOSH is focusing on low-voltage power supply technologies and related issues through a multi-discipline Working Group.

Communications systems must also be able to survive a disaster. Mine disasters such as explosions and roof collapses release destructive forces that are hard to imagine. In some cases, the forces have been so strong that it is unlikely that any exposed components would have survived over thousands of linear feet of mine entries. As other communications systems have done for years, mine communications systems will need to achieve survivability and reliability through hardening and system redundancy:

• Hardening is defined as the measures taken to improve the ability of the system components to continue to perform during and after mine incidents including, but not limited to, fires, roof falls, power outages, and explosions. Examples are explosion-proof boxes, armor cladding, burying components, and other techniques.
• Unfortunately no current available single system can withstand every disaster scenario imaginable while maintaining mine-wide communications. Alternate communications paths over diverse routes and parallel systems can provide redundancy which enables communications to continue even if parts of the system are destroyed.

Wired Communications

Wired communications require the miner's device to be tethered to some type of cabling to carry voice and or data. Examples include twisted pair, coaxial cable, CAT5 and fiber optic cables, and trolley wire (normally used to power mine locomotives, but it can also act as conveyor for medium frequency signals). Hardware includes dedicated telephones, walkie-talkies, paging devices, and similar technologies. While sufficient for routine mine communications, cables are very vulnerable as they can be destroyed or rendered inoperable by fires, explosions, roof falls, battery failure, or mine electrical power failure. Additionally, relying on wired communications such as mine phones leaves vast areas of the mine where a miner that is injured or trapped will have no communications to the surface or rescue personnel.

Wireless Communications

Wireless communications allow communications with a miner without the need for a tethered wire. Wireless communications (walkie talkie and cell phone functionality) are a unique challenge in underground coal mines. Not all radio signals will propagate down a coal mine entry due to the electrical properties of the coal and surrounding strata.

• Very high frequency (VHF) and ultra high frequency (UHF) - can suffer both attenuation and severe corner losses. Tests have shown that a mine entry appears to act as a wave guide to propagate VHF and UHF radio signals. These signals may propagate line-of-sight or slightly beyond but typically won’t turn corners very well. However, these frequencies are used for conventional radio communications because they can use small antennas (enabling wearable devices) and have high bandwidth for accommodating many users and high amounts of voice and data traffic. This will allow for multiple voice channels and sensors, monitors, and other safety improvements.

  These radio signals require a clear path or open air for signal propagation. Stoppings or roof falls can halt or impede conventional signal propagation. It is also believed that ionized air as a result of a mine fire can be a problem. Some systems use Leaky Feeder to help propagate the radio signals around twists and turns in mine openings. Another system type called wireless mesh involves installing signal repeaters called "nodes" throughout the mine. These nodes pass communications from one another to get the communications from the mine. Either system requires potentially vulnerable infrastructure to be in the mine.

• Extremely low frequency (ELF), very low frequency (VLF), and low frequency (LF) - suffer less attenuation, but can experience electrical interference from motors and other equipment. These frequencies are used for through-the-earth communication between underground workers and the surface. However, to have effective communications to the surface, natural and man-made noise sources (such as power lines and thunderstorms) must be eliminated. NIOSH is currently evaluating these techniques. These systems have very limited bandwidth, have significant distance limitations, and require large transmit antennas.
• Medium frequency (MF) - has less severe attenuation characteristics than VHF and UHF signals and does not require a leaky feeder cable. It also does not experience the high noise levels of lower frequencies. MF technology can have a range of 1000-1500 feet in conductor-free areas. Parasitic propagation effects can help convey MF signals over distances in excess of two miles. This involves propagating the signal along metal objects such as wires, pipes, and rails. These conductors may be able to provide a means of mine-wide signal distribution. These systems have limited bandwidth and require larger antennas than VHF/UHF systems.

The attributes of the various parts of the radio spectrum are discussed more fully in the various papers in the Emergency Response and Rescue Downloadable Mining Publications page. Combining the attributes of the various wireless frequencies and systems could provide a survivable mine-wide wireless communications systems but would require that the systems are interoperable. The NIOSH Communications Road Map (PDF, 955 KB, 2007) provides an example of how such a system could work.

Miner Tracking

Another issue is being able to identify the location of miners in the event of a mine emergency. Miner tracking systems use wireless communications technology and can be integrated with communication networks. There are three types of tracking systems. All rely on a communication link to the surface that may be susceptible to damage from explosions, fires, or roof falls.

• Zone or proximity based systems (including RFID tags) - Miners wear active (battery-powered) or passive radio-frequency ID tags or similar technology to identify themselves as they pass readers placed at intersections within the mine. The readers determine the locations of the miners. This technology is widely available commercially, and systems are being submitted to the Mine Safety and Health Administration (MSHA) for approval. This technology can determine the general area in which a miner is located. The size of that area is determined by the number of readers installed. The survivability of the system in a disaster is determined by the survivability of both the reader and the communication link to the surface.
• Radio location "node" based technologies - Miners carry a radio device that communicates with radio nodes. Location is determined by identifying the nodes with which the miner can communicate. Resolution is dependent on both the number of nodes and the means of signal processing. Resolution beyond the node level adds significantly to the cost of the system. This type of tracking is somewhat inherent to wireless mesh communications systems. The technology is used commercially, and systems are being submitted to MSHA for approval. Survivability of this type of system depends on survivability of the nodes and the communication link to the surface.
• Infrastructure "autonomous" systems - Miners wear a device that determines location independent of any active elements in the mine. This type of system doesn’t depend on any active mine infrastructure (except for a communication link to the surface), and accuracy can be very high. However, the technology is not widely used commercially, and is pushing state-of-the-art in many cases. Some examples include MEMS based Inertial Navigation, "Reverse RFID", Advance Pedometer system, and through-the-earth technologies.

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