388 



EXPLORATION GEOPHYSICS 



in Figures 234 and 235. The fact that gravity data from adjacent land and 

 water-covered areas can be contoured smoothly indicates that the under- 

 water readings are reliable. 



The use of the diving bell is advantageous in that the conventional 

 land-type gravity meters may be employed. This is of importance for the 

 surveying of limited areas involving short-term operations. For extended 

 underwater operations, however, it is generally preferable to utilize the 

 remote-controlled underwater-type meter. 



Fig. 234. — Diving bell for underwater gravity opera- 

 tions. A, diving bell; B, hatch; C, hatch cover; D, 

 locking mechanism, operatable inside and outside; E, 

 gravity meter; F, head phones and microphone for com- 

 munication to attendant on deck. (Courtesy of Robert 

 H. Ray Company.) 



The remotely operated gravity meter has several advantages over the 

 diving bell, which may be summarized as follows : — ( 1 ) The weight of 

 the equipment handled is greatly reduced. The diving bell weighs about 

 4,000 pounds, while the remote gravity meter housing, with meter, weighs 

 about 600 pounds. (2) Danger to personnel is eliminated with the remote 

 meter. Insurance for the operator of the diving bell is an expensive item. 

 (3) Better readings are obtainable with the remote meter in mud-covered 

 areas, since the gravity meter is not accompanied by an instrument man 

 whose every motion affects the level of the meter. Reading in a mud- 

 bottom area is closely akin to reading in a very soft swamp where the 

 observer literally has to hold his breath while taking a reading. (4) When 

 the remote meter is read electronically, it is possible to use electronic aver- 

 aging circuits when disturbed conditions are present. It has been found 



