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Chap. 10] ELECTRICAL METHODS 627 



3. Radio methods and ^'treasure finders.'^ Radio measurements fall into 

 two natural groups. In the first, the reaction of a transmitter to changes 

 in surrounding media is determined. Methods in the second group 

 measure the effect of the media, situated between transmitter and receiver, 

 upon the latter's reception characteristics. The quarter wave method is 

 based on the fact that when a reflecting surface is at a distance of one 

 quarter of the wave length from the transmitter, a maximum of the 

 emission occurs. Hence, the depth of such reflecting surfaces as water 

 under dry surface beds, or flat-lying ore bodies in dry formations, may be 

 found by varying the frequency and observing the antenna current. In- 

 asmuch as the antenna capacity is affected by the proximity of conductors, 

 they may be located by changes in wave length and damping. This 

 method has no depth control and, therefore, interpretation of results is 

 difficult except when geologic conditions are simple and interferences from 

 noncommercial conductors are absent. 



Application of radio methods in the second group requires a transmitter 

 and receiver, and measurements are made on the receiving side. By 

 absorption measurements, ore bodies in dry country rock have been out- 

 lined from underground workings. With the interference method depths 

 to reflecting surfaces may be determined by observing the change of in- 

 tensity of reception with horizontal distance. The direction of the inci- 

 dent beam is measured in the reflection method by a search coil. 



Treasure finders may be divided into low and high frequency devices. 

 The former are modifications of induction balances. In one of these, one 

 energizing coil and two pickup coils are arranged on the same coil frame. 

 Presence of metallic bodies changes the mutual inductance between the 

 lower pickup coil and the primary coil. Instruments of this type have 

 been found useful in locating fairly small buried metallic objects such as 

 bombs. In another device two search coils a few feet apart are in a 

 balanced bridge circuit and energized with current of intermediate fre- 

 quency. The inductance change resulting from metallic bodies near one 

 coil is measured. The depth range of low-frequency treasure finders does 

 not extend far beyond five feet. 



High frequency treasure finders are (1) beat frequency oscillators, and 

 (2) combinations of transmitters and receivers. In the former the fre- 

 quency of one oscillating circuit remains fixed; that of the other varies 

 with the proximity of conductive bodies. This changes the beat note or 

 the plate current in a third circuit. The def)th penetration of these devices 

 does not exceed fifteen feet. In treasure finders consisting of a transmitter 

 and receiver combination, intensity of reception is indicated by a plate 

 current meter, and a compensating circuit is provided so that for barren 

 ground the normal intensity gives a zero reading. The reading changes 



