Chap. 12] 



MISCELLANEOUS GEOPHYSICAL METHODS 



959 



same applies in the location of water leaks, whose characteristic sounds are 

 caused by the impact of the escaping water on the surrounding formations. 

 In underground communication and mine rescue operations it has been 

 found that the most effective way of transmitting soimds is to strike a 

 hard rock surface with a sledge hammer. 



3. Geoacoustic receivers. The unaided ear would be a rather ineffective 

 means of detecting earth sounds. A marked improvement may be at- 

 tained by the simple means of using a canteen almost full of water, and by 

 placing the ear as closely as possible to its orifice. The principle involved 

 here is that the contrast in radiation impedance between ground and air 

 is stepped down by the insertion of water (see page 947). Another way of 

 detecting sounds transmitted through the earth is to resonate a mechanical 

 detector or seismograph to the predominant ground frequency and to con- 

 nect this detector with the ear by a stethoscopic ampUfication device. A 

 detector of this kind is known as a geophone and is illustrated in Fig. 12-26. 



Fig. 12-26. Geoacoustic detector (geophone) (after Leigh ton), (o) Diaphragm, 

 (6) cap plate, (c) iron ring; lead weight in solid black. 



In it a lead mass weighing about one pound is suspended between two 

 nickel diaphragms about rew inch thick. The space above the upper 

 diaphragm is about 3 inches in diameter and connects to an orifice of about 

 J inch in diameter. From the orifice the sound passes into the rubber 

 hose of a stethoscope whose end fits snugly into the ear. Geophones are 

 generally used in pairs for directional hearing. To obviate phase dif- 

 ferences, the rubber hoses must be of equal length. 



The reduction of diameter of the geophone in its orifice results in an 

 increase of amplitude. Since the transmission of sound from a large to a 

 small tube is equivalent to the transmission from a dense to a rare medium, 

 eqs. 9-34 (see page 478) apply. Substituting, for the radiation impedance, 

 the products of velocity and density, considering the velocities equal on 

 both sides, and setting the densities proportional to the cross-sectional 

 areas, the transmitted ampUtude is 2»Si/(»S2 + Si) times greater than the 

 amplitude of the diaphragm. For a standard geophone in which the ratio 

 of the cross-sectional areas of diaphragm and orifice is of the order of 120, 



