962 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



it may travel along the pipe or follow subsurface cavities of an unpredict- 

 able course. Noise measurements for the detection of leaks should be. made 

 in the early morning hours when traffic is at a minimum. 



Another method of leak detection developed by the Western Instrument 

 Company requires contact between the pipe and a probe carrying a crystal 

 pickup at its end. The pickup is oriented in two directions, one parallel 

 with, and the other at right angles to, the pipe. Intensity of vibration is 

 measured in both directions and also as a function of frequency if neces- 

 sary. It is claimed that the ratio of longitudinal to transverse vibration 

 intensity shows significant variations when a leak is approached. 



Geoacoustic direction-finding is not so reliable nor is it applicable at 

 such great distances as marine-acoustic methods. Transmission is limited, 

 not only in range but in direction as well, by rapid changes in the elastic 

 wave speeds, particularly near the earth's surface. When underground 

 workings are situated in a district of complex geology, it may be quite 

 difficult to establish consistent directions from geoacoustic observations. 



7. Transmission measurements. Occasionally, valuable information may 

 be obtained in underground mining operations from the location of faults, 

 fissures, shear zones, clay seams, and the like. This may be done by pro- 

 ducing sounds at a given location and observing the intensity of reception 

 at a number of points so arranged in adjacent drifts, tunnels, or shafts 

 that the presence and approximate disposition of sound-absorbing media 

 may be determined. With electrical geophones, direct measurements of 

 intensity may be made by the use of a calibrated amplifier with output 

 meter. '^^ Indirect measurements of intensity may be made by swinging a 

 hammer through a predetermined arc in a mechanism especially made for 

 this purpose. By gradually reducing the arc, the minimum transmission 

 intensity necessary to produce an audible signal is obtained. As a rule, 

 however, quantitative intensity measurements are not made, and the 

 presence or absence of sound-absorbing media is ascertained by merely 

 noting at which points the sound reception is poor or entirely absent. 



8. Geoacoustic-reflection methods. Reflection of sound waves in the 

 ground is determined (1) by noting the direction of the return ray, and 

 (2) by measuring the time interval between the initiation of a sound im- 

 pulse and the arrival of the echo. 



Distance of reflecting surfaces has been measured underground by ad- 

 justing the position of two geophones in a vertical plane for equal recep- 

 tion. This is not possible at the earth's surface, and it is necessary to use 

 a compensator to establish the direction of the return ray. Attempts have 



^" A suitable instrument (sound-level meter) is manufactured by the General 

 Radio Company, Cambridge, Massachusetts. 

 1** Leighton, loc. cit. 



