368 



EXPLORATION GEOPHYSICS 



borne, opening up wide new fields of application. Improved and less costly 

 underwater gravimeters would likewise find wide application to geophysical 

 exploration of the continental shelf area, which awaits more effective 

 procedures and should contain rich rewards for the adventurous. 



COMPARISON OF GRAVIMETER METHOD AND TORSION 

 BALANCE METHOD 



The modern gravimeter is superior to the torsion balance in regard to the rapidity 

 with which a survey can be carried out. Under normal conditions, a crew using one 

 gravimeter occupies 12 to 20 stations per day, whereas the usual two-instrument torsion 

 balance crew occupies 6 or 7 stations per day. As a consequence of this factor, a 

 gravimeter survey generally is much cheaper than a torsion balance survey. 



CASE I 



CASE 2 





T.B 



TV 



>;DENsiTY-2.l/7 



\ \ Tv \ 

 \2 METERS 



WW 



G.M. T.B. 



MTV 



WWWWWW 



EFFECT ON TORSION BALANCE 

 « 14 EOTVOS UNITS. 

 IF PROBABLE ERROR OF T.B. 

 READING IS TAKEN AS 2 E 

 UNITS. EFFECT IS 7 TIMES 

 PROBABLE ERROR. 



EFFECT ON TORSION BALANCE 



- 35 EOTVOS UNITS, 



17.5 TIMES PROBABLE ERROR. 



EFFECT ON GRAVITY METER 

 » 0.020 MILLIDYNE 

 IF PROBABLE ERROR OF G.M. 

 READING IS TAKEN AS 0.1 

 MILLIDYNE, EFFECT IS ONE- 

 FIFTH OF PROBABLE ERROR. 



Fig. S17. 



EFFECT ON GRAVITY METER 

 - 0.012 MILLIDYNE, 

 ONE-EIGHTH OF PROBABLE ERROR. 



Comparison of the effect of disturbing masses on the gravimeter and on the torsion 

 balance. (L. M. Mott-Smith, Geophysics.) 



The accuracy of gravity maps obtained with the torsion balance is influenced more 

 by the character of the terrain than those obtained with gravimeters. In regions where 

 there is considerable relief or irregular surface materials, the use of a gravimeter is 

 preferable. Thus, a distant mass which produces comparable effects on the two 

 instruments, say one Eotvos unit on the torsion balance and one-tenth millidyne on the 

 meter, will produce a considerably greater effect on the balance than on tiie meter 

 when close to the two instruments. f This is illustrated in Figure 217 which gives two 

 examples of the effect of local disturbances on the two instruments. Case 1 illustrates 

 the effect of a thin layer which is located at or near the surface and has a greater 

 density than the normal soil. This layer is assumed to begin as shown and extend 



No. 



t L. M. Mott-Smith, "Gravitational Surveying with the Gravity Meter," Geophysics, Vol. 

 1, pp. 30-32. 



II. 



