Chap. 12) MISCELLANEOUS GEOPHYSICAL METHODS 961 



venting accidents in blasting through), and locating and measuring the 

 drift of boreholes sunk from the surface with the intention of reaching a 

 definite point of the subsurface workings. Military application includes 

 the detection and location of enemy galleries in trench warfare and the 

 surveillance of underground sapping activities for escaping enemy blasts 

 and directing counterblasts. 



In the latter application, the object of acoustic observations is to de- 

 tect, identify, and locate mining operations by the noise of mining tools 

 such as hammers, picks, drills, shovels, and other machinery. Sounds 

 transmitted by mining tools are characteristic and permit definite identi- 

 fication, despite the distortion occurring in the intervening media and an 

 undamped tuned receiver. Underground fires are identified by a typical 

 hissing sound produced by air drafts, by cracking of timber, and by the 

 fall of rock from the mine roof. 



Civil engineering application of geoacoustic methods includes the loca- 

 tion of pipes and pipe leaks. Water, gas, and oil pipes can often be found 

 by the typical noise of the gas or liquid passing through them, that is, by 

 the vibration set up in the pipe, although the use of an electromagnetic 

 detector (see page 819) is preferable if the pipe itself is to be located. Leaks 

 in water pipes can be found by the noise of the water impinging on sand, 

 gravel, or rock in the cavity surrounding the leak, and by the vibration 

 produced in the pipe by the water issuing from the leak. 



Geoacoustic direction-finding makes use of the binaural effect discussed 

 on page 938. Two geophones are shifted in position until a line connecting 

 them is at right angles to the direction of the sound. The phones may 

 also be used in fixed position, when they are provided with a compensator 

 to determine direction by adjusting the phase difference. In application of 

 the first method, the two phones are first set out with their base approxi- 

 mately at right angles to the direction of sound. One phone is left sta- 

 tionary and the other is moved toward the sound source so that the sound 

 appears to come from the right. Then this phone is moved back in the 

 opposite direction from the base line until the sound appears to come from 

 the left. In this manner an intermediate position can be established in 

 which the sound comes from neither the left nor the right.^ A source 

 is then located by making direction determinations at a number of points 

 and by finding the iniersection of the rays. Pipe leaks are usually not 

 located by direction-finding but by following the course of the pipe and by 

 observing changes in sound intensity. The location of a leak is indicated 

 by the point of maximum intensity. This point may be considerably re- 

 moved from the surface evidence of the leak, since the liquid issuing from 



"* Leighton, loc. cit 



