MAGNETIC METHODS 



209 



Taking the ratio r of the readings (which are experimentally known) a 

 relation can be obtained for the "height" h of the dike (i.e., the vertical 

 distance between poles) in terms of known quantities. 



F 



ground 



=■ r = experimentally known value 



1 



1 



pip' + i'y^ (p + h) [(p + hy + n 



r~ 



1 



1 



ip+H) {{p+HY + PY- (p + h + H) [ip + h + Hy + P]V- 



where p = the depth to the top of the dike, / = one-half the length of the 

 dike, and H = height above ground at which the record reading was taken. 

 In addition to investigations of model ore bodies, laboratory experi- 

 ments have been carried out to determine the depth and geometric con- 

 figuration of regional subsurface 

 structural features. Figure 99 

 shows a schematic section of an 



n 





&-^, 



'ik 



Fig. 99. — Schematic diagram of apparatus for 

 experimental interpretation of magnetic and gravi- 

 metric anomalies. (Jenny, Terr. Mag. 40 (1), 

 1935, p. 72.) 



apparatus devised by Jennyt which 

 permits quantitative interpretation 

 of vertical magnetic anomalies. 

 Vertical bar magnets 1 are con- 

 nected with counterweights 4 by 

 means of ribbons 2 which pass over 

 pulleys 3. The displacements of the 

 dip needles 5, 6, 7 produced by the 

 magnets when they are all at 

 the same level are compensated by 

 counterweights (not shown) which 

 are attached to the needles and by 

 screws 8. To determine depths, the bar magnets are displaced by means of 

 the counterweights 4- so as to correspond to the assumed structure. For 

 example, the positions of the bar magnets shown in Figure 99 would cor- 

 respond to an anticline. Contours of equal depth may be obtained by study- 

 ing several cross sections. 



The most flexible model arrangement for magnetic studies comprises a 

 mixture of coarse iron filings and clean white sand placed in a large 

 wooden tank, preferably about 8 feet in diameter and 2 feet high. The tank 

 should be provided with a cover in order to exclude moisture and minimize 

 rusting of the iron. The orientation of the tank with respect to the mag- 

 netic field can be varied by mounting it on a turntable. The latter should 

 be placed on a wooden track with rollers so that it can be readily moved. 



t W. p. Jenny, \oc. cit., p. 74. 



