162 



GRAVITATIONAL METHODS 



[Chap. 7 



\/'RKluctd 



Influence of Can fad 



more 

 ea Lepfife 

 ^ Black Shales 

 E3 Overburden 



10 40 60 90 m 



{a) 



ib) 



Fig. 7-52. Gravimeter observations (a) on 

 Menstrask Lake, and (6) at Lake Lfingsele, 

 Skellefte district, Sweden (after Lindbladand 

 Malmquist). 



southern ore body may be explained by 

 body to the north. 



It is not believed that this 

 method will give so distinct re- 

 sults as electrical prospecting, 

 although it would be useful to 

 segregate electrical anomalies 

 due to noncommercial mineral 

 disseminations from commercial 

 ore indications. Fig. 7-52 shows 

 the results of gravity measure- 

 ments made on the ice at Men- 

 strask in Sweden. The ore 

 bodies below the lake, dis- 

 covered by electrical prospect- 

 ing, consist of three parallel len- 

 ticular sulfide veins of steep 

 north dip; the thickness of the 

 southern vein is about 30 feet, 

 that of the others from 10 to 

 13 feet. Leptite formation oc- 

 curs in the south, black schists 

 in the north. Owing to their 

 difference in density (0.2), a 

 correction was apphed. The in- 

 dications are shifted to the north 

 with respect to the suboutcrop 

 of the veins. This shift may 

 partially result from the dip and 

 partially from the effect of the 

 contact. The shift above the 

 the occurrence of an additional 



VI. TIME VARIATIONS OF THE GRAVITATIONAL FIELD 



Variations of gravity with time may be divided into periodic and non- 

 periodic phenomena. The former are related to the position of sun and 

 moon, the latter are caused by natural changes of geologic origin or by 

 artificial mass transports. While magnetic variations are of the order of 

 10~ to 10~ of the normal terrestrial magnetic field, gravity variations 

 represent only a 10 to 100 millionth part of normal gravity. As a rule 

 they do not interfere with gravimeter exploration but may be recorded by 

 stationary instruments. Observations of gravity variations are of scien- 

 tific value in the analysis of geologic forces bringing about slow changes in 



