Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 935 



as the transmission of audio-frequency vibrations through the ground, is 

 termed "acoustic" transmission. With this in mind, we divide the fol- 

 lowing discussion into (a) atmospheric-acoustic, (6) marine-acoustic, and 

 (c) geoacoustic methods. 



The principal applications of acoustic methods are: (1) communication 

 (signaling) ; (2) location of sound sources (acoustic triangulation, position- 

 finding, or sound-ranging) by measurements of time and distance; (3) deter- 

 mination of the direction and characteristics of a source (direction-finding 

 and noise-detection) ; (4) location of intervening media (transmission meas- 

 urements); (5) determination of distances of sound-reflecting objects (echo- 

 sounding); (6) noise-prevention. 



A. Atmospheric-Acoustic Methods 



1. Velocity and absorption of sound in air. Of the three possible sound 

 transmitting media— air, water, and earth (inclusive of solids) — the at- 

 mospheric air is, of necessity, the one most widely used and yet probably 

 the least efficient of the three. Sound propagates in air more slowly than 

 in liquid or solid media. If the ratio of the specific heats for constant 

 temperature and pressure be designated by k (= 1.405 for dry air), if P 

 is the pressure (1.013 megadynes cm~^) and S (= 0.29-10^') the density 

 of air at 0° C, the sound velocity (at that temperature) is given by 



Vo 



= j/k^, (12-16) 



or, with the above numerical values, = 331.8 meters per second. 

 The velocity increases with the absolute temperature, tab. , or v = 

 20\/tab. m-sec.~\ which for centigrade temperatures t above zero is usually 

 written v^ = 331.4„, + 0.66 t°. 



Other factors which affect the sound velocity in air are humidity, wind 

 direction, and velocity. Near intensive sources, velocity increases have 

 been observed. For long ranges the sound does not always propagate 

 along straight paths through the atmosphere. From the source the 

 sound rays may curve upward because of a decrease in atmospheric tem- 

 perature with height, up to about 15-20 km. Temperatures are likely to 

 remain uniform and then to increase again at heights of 30-40 km in the 

 ozone layer where sound velocities may reach values of 350-360 m-sec~\ 

 The vertical increase in velocity results in an advance of the upper portion 

 of the wave front and a bending back of the sound rays to the earth's 

 surface. For a uniform vertical velocity gradient dv/dz, the ray curvature 

 is given by —(l/y)-dy/dz. The bending of the sound rays in the high 

 velocity layer gives rise to the well-known "silence zones" in sound-ranging 



