942 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



unaided ears have a directional sensitivity of about 3 per cent, corre- 

 sponding to a time difference of 30 microseconds, it follows that to attain 

 an accuracy of ^° or better, as required in plane detection, an artificial 

 pair of ears should have a base length of six times that of the ears, or of 

 1.2 meters. Airplane detectors have base lengths of this order, consist 

 of two pairs of large "ears," and have the shape of reflectors, horns, or 

 funnels which can be aimed independently by two operators to locate the 

 target by horizontal and vertical angle adjustments. If such detectors 

 are too bulky, compensators with mechanical or electrical time delay 

 mechanisms are used in conjunction with a cluster of (coil) microphones 

 in circular or spherical arrangement (see Fig. 12-22) and may be arranged 

 to operate anti-aircraft batteries and searchlights mechanically or elec- 

 trically. 



Noise analysis, or determination of type of source by the character of 

 sound emitted, is often associated with direction-finding and sometimes 

 with sound-ranging. An analogy familiar from everyday life is the physi- 

 cian's method of diagnosing heart and lung diseases by the use of the 

 stethoscope. Another acoustic diagnostic procedure applied in medicine, 

 the determination of the condition of certain organs by tapping and lis- 

 tening to the sound with the stethoscope, is without analogy in atmos- 

 pheric-acoustic transmission measurements. 



7. Atmospheric echo-sounding. In primitive form, echo-sounding in air 

 has been employed for a long time in the navigation of narrow channels 

 during foggy weather by skippers, who estimate the distance to shore by 

 the length of time required to receive the echo from the ship's whistle. 

 Another application of atmospheric echo-sounding is made in the Behm 

 ground-distance meter. A pistol is fired on one side of an airplane and 

 the reflection from the ground is received on the other side by a micro- 

 phone. The firing of the pistol sets in motion a disk carrying a mirror 

 which projects the image of a light source on a scale. The light passes 

 through a small lens which is deflected electromagnetically at the instant 

 when the sound is received by the microphone. The Behm airplane echo- 

 sounder is not usable for distances much in excess of 500 feet and has been 

 superseded by an electromagnetic terrain-clearance indicator using fre- 

 quency-modulated short-wave radio transmission. 



Finally, atmospheric echo-sounding is applied in geophysical research 

 concerned with the constitution of the upper atmosphere (investigation of 

 the so-called "anomalous" sound propagation, see page 936) and in the 

 measurement of depth to fluid level in oil wells. "^ The sound is generated 

 in the last method by the firing of a cartridge in a chamber attached to 



"2 Bell Sys. Tech. J., 18(1), 222-234 (1939). 



"» J. J. Jakosky, Petrol. Tech., 2(2), 1-23 (May, 1939). 



