292 



GRAVITATIONAL METHODS 



[Chap. 7 



torsion balance survey of the Caldwell (Renfrew Co., Ontario) pyrite 

 deposit.^' The theoretical curve (based on the assumption of a density 

 difference of 1.7, a thickness of the deposit of 50 feet, depth 20 feet, and 

 dip 60 feet) is likewise shown. Since it is calculated under the assumption 

 of a two-dimensional body, its amplitude exceeds that of the experimental 

 curve. 



A number of attempts to use the torsion balance underground have been 

 made and published. The earliest survey of this kind was made with a 

 curvature variometer by Brillouin in the Simplon tunnel in 1905. Subse- 

 quent experiments indicate that gradient observations are more suitable 

 underground. A. Birnbaum"'^ made some torsion balance observations on 

 two levels of the Beienrode potash mine in Germany. As shown in Fig. 

 7-124, the gradients point toward the anhydrite in the 700 foot level but 



Sradtenf 



•lOOE 



Cury. 



4m 



■^m 



10 20 



iZZ3 dautlf 



Fig. 7-125. Underground torsion balance measurements on basalt dike in salt (after 



Meisser and Wolf). 



away from it in the 1000 foot level, this indicating that its major portion 

 is located below the 700 and above the 1000 foot level. A rather unusual 

 occurrence of basalt (density 3.0) in salt (density 2.2) was investigated 

 by Meisser and Wolf"^ 700 meters below the surface. Supplementary 

 measurements of magnetic vertical intensity and seismic propagation 

 speed were made at the surface and underground. The thickness of the 

 basalt varied from 0.4 to 90 meters. As Fig. 7-125 indicates, its major 

 portion occurs below the 700 meter level. As has been emphasized by 

 Kumagai, the effects underground are so large that the sensitivity of 

 the instrument must be reduced; otherwise, its starting azimuth must be 

 so selected that the effects of the adjacent tunnel walls are at a minimum. 



1" A. H. Miller, Canad. Geol. Surv. Mem., 170(3), 99-118 (1930). 

 i"A. Birnbaum, Kali, 18, 144-148 (1924). 

 176 O. Meisser and F. Wolf, Zeit. Geophys., 6(1), 13-21 (1930). 

 ''«N. Kumagai, Japan. J. Astron. Geophys., 9(3), 141-206 (1932). 



