Chap. 10] ELECTRICAL METHODS 815 



by variations in the observer's position and because of formation of a con- 

 ductive surface layer on the ice during daytime. Hence all observations 

 had to be made at night. To minimize surface effects and to give the 

 method some degree of depth resolution, apparent capacities for different 

 antenna elevations may be observed. Fritsch"' has published a number 

 of curves obtained in this manner on a siderite mine and on caves. While 

 the presence of two caves could be predicted correctly, there was too much 

 interference from surface moisture, topography, and rails and pipes in 

 tunnels, to make the method reliable in all cases. 



3. Measurements of antenna damping. Both the natural frequency and 

 the damping of an antenna depend on the electrical properties of the sur- 

 rounding media. These properties may be determined from the sharpness 

 of the resonance peak; for example, from the natural frequencies cor- 

 responding to half of the maximum resonance amplitude. If Cr is the 

 capacity for resonance (maximum current) and Ci and d , respectively, 

 two capacities giving half the antenna current on either side of the reso- 

 nance point, the decrement of damping is given by A = - • (C2 — Ci)/Cr. 

 It decreases with the ratio of conductivity and dielectric constant. 



C. Field-Strength Measurements 



In this group of methods both a transmitter and a receiver are required, 

 and measurements are made exclusively on the receiving end. The 

 transmitter is left untouched except for frequency alteration where called 

 for. Broadcasting stations and amateur and other transmitters are some- 

 times used, particularly for purely scientific studies. For mine examina- 

 tions, and the like, where different positions of the transmitter are neces- 

 sary, a unit specially built for this purpose is required. This unit has the 

 form of an antenna or loop transmitter. On the receiving end a direction- 

 finding coil is used, that may be rotated about a vertical and horizontal 

 axis. Fig. 10-130 shows a heterodyne receiver used by Stern for field- 

 strength measurements. In the plate circuit of the output stage is a sensi- 

 tive galvanometer provided with a circuit for bucking normal plate cur- 

 rent. Petrowsky"* has used and described a number of transmitters and 

 receivers of similar construction. 



1. Absorption method. Related methods of intensity mapping. Measure- 

 ments of reception intensity as a function of the characteristics of inter- 

 vening media have been made to determine the depth penetration of radio 

 waves, to map faults, and to locate intervening ore bodies, cavities, or 



"'Beitr. angew. Geophys., 6(1), 100-119 (1936). 



i"A. Petrowsky, Beitr. angew. Geophys., 3(2), 149-204 (1933). 



