β Scribed by Alan Leighton
No coin nor oath required. For personal study only.
THE geophone, which was invented by the French during the World War to detect underground mining operations, was developed by the United States Engineers, and the instruments now used by the Bureau of Mines were made according to phns drawn by the Engineers, except for the introduction of different diaphragms.
By the use of two geophones, one instrument to each ear, it is possible to determine the direction from which a sound is coming through the earth. A perpendicular, erected halfway between the instruments, according to principles worked out by the Bureau's investigators, will give the direction of the source of sound, or, more accurately, the projection of this line of direction on the horizontal plane will pass through the source of sound.
For use in determining the best ways of operating the geophone for rescue work in mine accidents, and for mine surveying, data must show two things : ( I ) The proper place to locate the geophone, and (2) the proper place to pound, in order that sounds may be detected and their point of origin determined.
Rails and pipe lines can not be relied on to transmit the sounds from blows of different sorts, unless these lines are properly suspended throughout their length.
Clay veins or faults interfere greatly with attempts to ascertain the direction from which the sound is coming.
Observations made over the Bureau's experimental mine through the greatest depth of cover-138 feet-show that pounding on the rib with either a timber or a sledge can be perceived on the surface anywhere within a circle whose diameter is 700 to 800 ieet, the pounding being located at the centre of this circle. The experiments showed that a blow struck in a mine, with the purpose of being detected by the geophone, should be struck upon the coal. To detect sounds coming through the strata, the instrument should be placed on a niche or shelf of coal rather than on _-* Communicated by the Director.
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