Chap. 9] 



SEISMIC METHODS 



479 



them. If the latter is the geometric mean of the other two specific acoustic 

 resistances and if the thickness of the intermediate layer is one-quarter of 

 the elastic wave length of the intermediate layer, the transmission ratio 

 of energy from water to air may be made equal to unity. By using rubber 

 for casing materials of listening devices, the intensity of sound reception 

 may be increased considerably; certain types of geophones are not placed 

 in direct contact with the ground but in holes filled with water which acts 

 as an intervening medium to step down the acoustic resistance. 



The specific acoustic resistances of the more important rocks and min- 

 erals shown in Table 56 have been computed from their velocities and 

 densities. These values are of interest in connection with reflection shoot- 

 ing, since the ratio of the reflected to the incident amplitude increases with 

 the ratio of the specific acoustic resistances of the formations involved. 



Table 56 

 SPECIFIC ACOUSTIC RESISTANCES 



2. Spreading, selective scattering, dispersion. Since the intensity of 

 sound decreases with the distance from the source, it varies, for spherical 

 waves, inversely as the surface areas of concentric spheres. For cylin- 

 drical waves it varies inversely as the surface areas of concentric cylinders. 

 Hence, formula (9-33c) becomes, for spherical waves, 



and, for cylindrical waves, 



I = 14^^' 



I = xf fR, 



(9-35a) 

 (9-356) 



where R, as before, is specific acoustic resistance, r is distance, / is fre- 

 quency, and A is amplitude. 



Selective scattering is due to reflections and refractions on prominent 

 irregularities. It is greater for high frequencies than for low frequencies, 

 since the dimensions of the disturbing objects become a controlling factor 

 compared with the wave length. The amplitude of the scattered waves 

 at any distance from the obstacle is directly proportional to the volume 

 of the obstacle and inversely proportional to the square of the wave length. 

 Hence, the intensity of scattered sound varies inversely as the fourth power 

 of the wave length. In a medium consisting of numerous small objects, 



