There are few situations more terrifying than being trapped underground with no visible means of escape. Miners risk being caught in this situation every day they go to work. In the United States, the Mine Safety and Health Administration establishes guidelines and safety measures that help minimize the chance of a disaster. But sometimes that's not enough.
According to the Bureau of Labor Statistics, the fatality rate for miners in 2007 was 24.8 per 100,000 people employed. Only the collective industries of agriculture, fishing, forestry and hunting had a higher fatality rate. Not every fatality was the result of a collapsed mine. An equipment failure or poor safety habits can also end in tragedy. Miners may encounter dangerous gases like methane as they move underground, too.
One of the biggest challenges in mine safety is communication. It's difficult to create a network that works well underground. Most radio waves can't penetrate rock -- you need to have a line of sight between the two transmission points. That's not feasible for most mining operations. A strong communications system underground could help reduce mining accidents and increase the effectiveness of rescue operations.
A wireless radio network would be a key component of an effective communications system. Using radio waves, miners could remain in contact with surface operations. Sensors hooked up to a wireless radio network could send back environmental information to the base of operations, alerting team leaders to potentially hazardous situations before an accident can happen. And with the right tracking system, leaders could keep tabs on where miners are within the mine. In the event of an accident, leaders could determine who was in the area at the time.
Wireless technology is pervasive -- you can find hotspots in airports and coffee shops. Some cities have rolled out municipal wireless networks. You might think that every mine in the world would have them, too. But remember: Radio waves have a hard time penetrating through solid rock. When you design a wireless radio network for a mine, you have to take that into account. We'll look at two of the most common approaches to implementing a wireless radio network in a mine.
First, we'll focus on a system with a funny name: the leaky feeder.
Until recently, the most advanced mining communications solution was a leaky feeder. Leaky feeders are cables that can emit radio frequencies. They're like coaxial cables which carry a signal from one endpoint to another. Normally, coaxial cables have a copper sheath surrounding the cable itself to prevent the signal from leaking out throughout the length of the cable. If the network uses a long cable, you might not be able to detect a signal from the opposite endpoint.
Leaky feeders are different. Instead of covering the cable in a solid copper shield, a leaky feeder has small gaps in the shielding to let the signal through. The gaps create a limited wireless radio network environment. Line amplifiers and repeaters boost the signal at regular intervals along the cable to make up for signal loss.
Wireless devices can interface with leaky feeders. They can receive radio signals from the cable and transmit data back. The data can include voice, video and computer data. With the right receivers, you can even control stationary equipment like water pumps equipped with radio receivers using signals sent via the leaky feeder. But you still need to be relatively close to the physical cable -- 300 feet (91.4 meters) or so -- to receive and transmit signals [source: Mine Safety Technology and Training Commission].
One advantage of a leaky feeder cable is that you can lay it down as you excavate a mine. You can use splitters to send lengths of cable down different pathways. And since cable is flexible, there's no problem moving the network around sharp corners and turns. You can even feed cable straight down a hole if need be.
There are a couple of downsides to leaky feeder systems. If something severs the cable, communications stop beyond the break. Another problem is that multiple leaky feeder cables can sometimes cause interference within the system. And leaky feeder radio frequencies tend to be on the high end of the spectrum -- these high frequencies don't penetrate rock very well.
The Mine Safety and Health Administration (MSHA), the governmental agency in the United States responsible for overseeing miner safety issues, approves of four leaky feeder systems:
- Flexcom from Mine Radio System
- SmartCom IS from Varis Mine Technology
- RFM 2000 from DAC
- Model VHF-1 from El-Equip Inc.
Mesh networks are an alternative to leaky feeders. A mesh network doesn't require a cable like a leaky feeder system. Let's take a closer look at how wireless mesh networks may make mining safer.
Mesh Networks in Mines
A mesh network distributes a wireless signal through a system of transmitters called nodes. A node is essentially a wireless router. Special software in the node allows it to communicate with a network of other nodes. The nodes use dynamic routing to determine the best route for data exchanges. The best route can change depending on node locations, data traffic and environmental conditions. Most importantly, some nodes can be mobile.
At least one node needs to be in a fixed position. This is the node that acts as a bridge between the mesh network and a larger network like the Internet. For instance, a radio tower linked to the Internet might serve as the main fixed node for a mine. Most mines have multiple fixed nodes to give the mesh network a strong foundation.
Wireless mesh networks tend to operate under one of two radio frequencies: 2.4 gigahertz (the IEEE 802.11 protocol) or 900 megahertz (the IEEE 802.15 protocol). Both types have problems broadcasting signals around corners, so a node at each corner may be necessary to keep the network stable. The MSHA found that these protocols didn't interfere with other communications systems in mines [source: MSHA].
Nodes don't have to be very large -- many are about the same size as an average wireless router. That makes it easy for miners to attach nodes to heavy equipment. Because nodes can work even while in motion (though at a slower bit rate), various mining vehicles can carry the nodes. You can wire nodes to sensors in a mine to monitor mine conditions as well. If danger arises, the node can broadcast signals to the rest of the network.
It's also possible to build electronic badges that interact with the mesh network. That makes it possible for the network to track the location of individual miners. As the miner moves through the mine, different nodes detect the miner's badge. If part of the mine becomes hazardous, the mining company can see how many miners could be affected and evacuate that section of the mine. Miners can also use devices like personal radios or voice over Internet protocol (VOIP) phones to remain in contact with operations on the surface.
Mesh networks can help prevent accidents, but what happens if part of a tunnel collapses? Unfortunately, mesh network wireless protocols aren't very effective at penetrating earth. For that, you need to use a system that operates on very low frequencies. The Mine Safety and Health Administration (MSHA) tested a through-the-earth (TTE) system that could penetrate 270 feet (82.3 meters) of rock. Data transfer was very slow -- 20 to 30 characters per minute with 80-percent accuracy at depths of up to 630 feet (192 meters). Potentially, a TTE system may be able to broadcast even further through solid earth. But there's a downside in addition to the slow data transfer rate -- the MSHA found that TTE systems could interfere with other communications systems in mines.
Wireless radio networks can give miners a lifeline back to the surface. Considering how dangerous their occupation is, a reliable communication network could mean the difference between life and death.
To learn more about different ways to use wireless networks, take a look at the links on the next page.
Related HowStuffWorks Articles
More Great Links
- Bureau of Labor Statistics. "National Census of Fatal Occupational Injuries in 2007." Aug. 20, 2008. (Nov. 24, 2008) http://www.bls.gov/news.release/pdf/cfoi.pdf
- Business Wire. "Axcess Micro-Wireless IDs Proven in Live Miner Tracking Safety Demo." Reuters. Sep. 22, 2008. (Nov. 24, 2008) http://www.reuters.com/article/pressRelease/idUS114133+22-Sep-2008+BW20080922
- Cawley, James C. "An Assessment of Leaky Feeder Radio Systems in Underground Mines." Bureau of Mines. (Nov. 24, 2008) http://www.cdc.gov/niosh/mining/pubs/pdfs/aaolf.pdf
- Goldman, Jeff. "Wireless Mining." Wi-Fi Planet. Sept. 10, 2007. (Nov. 24, 2008) http://www.wi-fiplanet.com/columns/article.php/3698731
- Hoskins, Robert. "ActiveMine Broadband Wireless Mesh Network Provides Communications for Mining Operations." Broadband Wireless Exchange Magazine. 2006. (Nov. 24, 2008) http://www.bbwexchange.com/pubs/2008/05/09/page1411-1771043.asp
- Kramer, Bradley. "Wireless System Connects Copper Mine." Last Mile Online. Nov. 1, 2007. (Nov. 24, 2008) http://www.lastmileonline.com/index/webapp-stories-action?id=140
- L-3 Communications. "Federal Government Approves L-3's Wireless Mesh Mine Communications System." Nov. 18, 2008. (Nov. 24, 2008) http://www.l-3com.com/news-events/pressrelease.aspx?releaseID=1227831&category=NA
- Mine Safety and Health Administration, U.S. Department of Labor. (Nov. 24, 2008) http://www.msha.gov/welcome.htm
- Minecom. "Wireless Automation, Personnel Tracking and Leaky Feeder Communication Systems." (Nov. 24, 2008) http://www.mining-technology.com/contractors/communications/minecom/
- Mohanty, Pradeep K. "Application of Wireless Sensor Network Technology for Miner Tracking and Monitoring Hazardous Conditions in Underground Mines." MSHA. March 27, 2006. (Nov. 24, 2008) http://www.msha.gov/regs/comments/06-722/AB44-COMM-95.pdf
- Roberts, James F. "Improving Mine Safety Technology and Training: Establishing U.S. Global Leadership." Mine Safety Technology and Training Commission. Dec. 5, 2006. (Nov. 24, 2008) http://www.coalminingsafety.org/documents/msttc_report.pdf
- Varis. "Leaky Feeder: How it Works." 2003. (Nov. 24, 2008) http://www.gcs.com.au/docs/leaky_feeder_manual.pdf