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- 6 - chemical light sources, medical supplies, methane detectors, carbon monoxide detectors, oxygen level detectors, and a limited number of 45-minute breathing devices to permit those in the refuge chamber to egress to aid others who may be injured reach the refuge chamber. This equipment would be mounted on a wagon that could be easily pulled by a shuttle car to the new location. A detailed discussion of survival systems is contained in Appendix A. Although there have been few occurrences of multiple explosions in mines, it would be desirable to provide in any refuge chamber some protection from subsequent explosions. For the portable chamber, two types of bulkheads are being considered, rigid metal and inflatable. For metal bulkheads, trade-off studies are being conducted of aluminum versus steel, type of stiffening, weight versus size, weight versus pres- sure, and convex versus flat configuration. Actual design of the bulk- head, construction, and testing of a prototype are beyond the scope of this study. Inflatable bulkheads would be more portable than metal and, if inflation is rapid enough, can be left deflated until actually needed. Their more complex construction will make them higher priced. COMMUNICATIONS SUBSYSTEM "Communications, " as used in this report, means locating and communicating with workers trapped underground either behind barri- cades or in refuge chambers. Electromagnetic and seismic communica- tion techniques are being investigated. Signal location accuracy requirements depend upon whether the mine is equipped with refuge chambers at the working sections. If chambers are used, only sufficient accuracy to determine the section signaling will be required. The exact location of the chamber can be found from the mine map. If refuge chambers are not used, the signal source must be located to within _+50 ft. Electromagnetic Communications Electromagnetic communications techniques have the advantage that, in the long term, they might evolve as a means of operational communication within the coal mine. If properly designed, enough of
- 7 - the operational system could survive an explosion to provide emergency communications. Moreover, electromagnetic or radio communications is the only technique that would permit emergency voice contact between men on the surface and those underground. For the short-term system, electromagnetic beacons are proposed. Beacons would be located in refuge chambers, near the working faces, or in locations where miners would be likely to construct barricades if they were unable to egress from the mine. The beacon would operate at a low frequency, 500 to 1000 Hz, to ensure penetration to the surface even under the most adverse conditions. It would consist of a battery, a buzzer, and a key to interrupt the signal. Power would be applied to either a grounded wire or to a coil of wire. With the beacon underground would be a low frequency receiver. Surface searchers could locate the source of the field radiating from the beacon then communicate with the trapped individuals by laying out a large coil on the surface and transmitting downward at a higher frequency with very high power levels. The underground receiver would have a band width of only a few hundred Hertz which would make the construc- tion simple. Two-way communication could be effected by having voice transmission downward and keyed code transmission upward. (The trapped individuals would not have to know code because the proper response could be indicated by the downward voice transmission. ) A detailed discussion of both the possible mine radio communi- cations system and the emergency beacon system is contained in Appendix B, Part 1. Seismic Communications The requirements for a seismic system, as for an electromagnetic system, are that it be capable of rapid deployment, that it have sufficient sensitivity to detect the miner-generated signals at distances comparable to the dimensions of the area of possible entrapment, and that it be capable of locating the miners with sufficient accuracy that they can be reached by holes drilled from the surface or that fairly exact direction can be given to rescue teams. Although generally desirable, the ability to establish two- way communication between the miners and the surface is of secondary importance. An interim system consisting entirely of currently available instruments has been described by the Committee and its performance predicted. This system is not expected to be optimum but will be adequate to provide a location capability that does not now exist during the time required for development of a more advanced system.
- 8 - The interim system is based on miner location and communication by means of seismic pulses generated by striking the mine roof or walls with a hammer. The source of the signal is located by an analysis of the relative arrival times of seismic signals at the various elements of a large seismometer array. Two-way communication is by simple coded messages based on sequences of hammer blows. The surface system for initial location consists of four subarrays of 19 seismometers each deployed in a square approximately 1/2 mile across a diagonal. Each subarray is recorded on a strip-chart recorder. Analysis of the arrival time of the P-waves at each of the subarrays will give approximate location, after which the array size will be reduced to a 500-ft square to minimize the effects of an irregular topography, geologic structure, and other inhomogeneities. With the smaller array, knowledge of the depth of the mine at this location, and knowledge of rock velocity in this area, it should be possible to locate the signal source within +25 ft. The smaller array will probably have one or more subarrays with a signal-to-noise ratio sufficient to provide good communication capability. If not, the four subarrays may be moved to a point above the best location estimate and their outputs combined to give an improved signal-to-noise ratio. The miner below ground will have a single seis- mometer, an operational amplifier, thresholding circuitry, an audio oscillator, and an earplug speaker. These devices have been successfully used in personnel intrusion detection. Their total weight is about 4 lb and the seismometer is equipped with a spike that can be driven into the roof of the mine for best reception. Any tool can be used for hammering--a 10 lb hammer would be preferable if available. The signaling code and instructions for using the system could be pasted inside each miners helmet. When ready to communicate, a series of hammer blows on the ground surface will tell the miners that they have been located and that they should stop sending their location signal and start send- ing the prearraged coded messages regarding their condition. With this system it would be possible to determine the number of miners trapped, their physical condition, the condition of the air in their vicinity, and perhaps an estimate of their survival time to provide the rescue team guidance on priorities with which they should explore various areas of the mine.
