960 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



the amplitude of the air in the rubber hose is therefore about twice the 

 amplitude of the diaphragm. 



Thus, the gain resulting from a reduction of the cross-sectional area is 

 rather small. It may be increased by the use of electro-mechanical trans- 

 ducers coupled to the geophone mass. Ackley and Ralph**^ have reported 

 that the minimum audible distance could be doubled by using a standard 

 geophone with an unbalanced reluctance transducer and a three-stage 

 triode amplifier. Undoubtedly this sensitivity can be further increased by 

 crystal transducers and higher gain amplifiers. Carbon microphones and 

 hot-wire microphones have been proposed for this application but are 

 probably not so good as crystal microphones. 



4. Geoacoustic communication. Geoacoustic methods are used as a 

 means of communication of rescue parties with entombed miners and with 

 other parties located at the surface or in near-by mine openings. Signals 

 are transmitted by striking the wall at short intervals with a sledge 

 hammer or other available tool. Transmission is better in the direction 

 of the strike of formations than at right angles thereto, and it may be cut 

 off occasionally by faults or shear zones. Cormnunication is possible in 

 this manner through distances of 2000 to 3000 feet in rock and through 

 about 500 feet of overburden. Speech may be picked up through distances 

 of Several hundred feet, although the standard geophone, being undamped, 

 is not particularly suited to a faithful reproduction of speech. It is 

 probable that an adaptation of the crystal microphone would be better 

 adapted to direct speech transmission. 



5. Geoacoustic position-finding and sound-ranging. From the discussion 

 of these topics in connection with marine-acoustic methods, it will be re- 

 called that position-finding is defined as the determination of one's position 

 by timing the sound from one or two sources of known location, and that 

 sound-ranging involves the location of a source by acoustic triangulation. 

 Both of these procedures involve the measurement of travel times and 

 have therefore no direct parallel in geoacoustic work with present equip- 

 ment. A related seismic application is the determination of crookedness of 

 drill holes by measuring the travel time from surface shot points to a phone 

 located in the hole (see page 863). Geoacoustic triangulation is possible 

 only by an application of directionl-finding methods discussed in the next 

 paragraph. 



6. Direction-finding, noise-detection. These methods are applied in mine 

 rescue, mine safety, and mine surveying work for locating entombed miners, 

 detecting and locating underground fires, determining the approach of 

 tunnels and drifts and bringing together raises and stopes (thereby pre- 



»" W. T. Ackley and C. M. Ralph, U. S. Bur. Mines Rep. Invest., Ser. No. 2639, 

 Sept., 1924. 



