940 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



measurement from one or two acoustic sources of known position which 

 transmit a controlled impulse or, as in radio-acoustic position-finding, a 

 simultaneous radio and acoustic impulse. Conversely, sound-ranging is a 

 method of locating a source by acoustic triangulation, that is, by recording 

 its sound impulses on a number of receivers of known position. When 

 a lighthouse or lightship transmits simultaneous light (or radio) and 

 acoustic signals, an approaching vessel may readily determine its distance. 

 Assuming that both impulses travel by the same path and that At is the 

 time difference of arrival, V| the light velocity, v^ the velocity of sound 

 in air, and Av the difference in the velocities, the distance from the source is 



s = —- 'Vi-Vs ^t-Vs, (12-20) 



Av 



since Av ;^ V/ = 3-10 m, whereas v^ = 3-10^ m. Therefore, the distance 

 in kilometers is readily obtained by dividing 3 into the time mterval (in 

 seconds) between reception of radio and acoustic signal. Measurement 

 of distances from two sources of known position gives the true position 

 of the receiver. In seismic refraction work, use is made of this method 

 to obtain the distances of the seismic receivers from the shot point by 

 recording the shot instant by radio and the sound of the explosion by a 

 blastophone on the same film. 



The object of sound-ranging is to locate enemy guns by recording the 

 sound of their detonation. Records of arrival time are taken at a number 

 of receiving stations spread out along a base behind one's own lines. To 

 minimize errors due to local variations in sound velocity, the length of the 

 base is made as great as practicable and of the order of 15,000 to 25,000 

 feet. It is generally about 10,000 feet behind the lines. From six to 

 twelve microphones are arranged at equal intervals along the lines and 

 are connected through amplifiers to a six- or twelve-element oscillograph 

 camera constructed like the seismic cameras used in refraction or reflection 

 recording."" To be sensitive only to the low-frequency sounds trans- 

 mitted by the firing of a gun and to reject other sounds produced by the 

 activities of one's own or enemy troops, the microphones are coupled to 

 Helmholtz resonators tuned to about 12 cycles. The microphones are 

 usually of the hot-wire type, although experiments with moving-coil 

 microphones have also been verj'^ successful. 



Since the enemy batteries may be 15,000 to 50,000 feet away from the 

 microphones and must be located with an accuracy of about 100 feet, the 

 position of the microphones must be surveyed with an accuracy of 1 to 3 

 feet. Records must be taken with an accuracy of 1/100 of a second and 



"0 See p. 556. 



I 



