GRAVITATIONAL METHODS 



387 



With an underwater gravimeter the operations of leveling, of releasing 

 and clamping the system, and taking readings (all so simple on land) must 

 be accomplished by remote control, from a vessel at the surface. Leveling 

 is done by a dual motor-driven precision gimbal arrangement in which 

 a single revolution of the motor equals 3 seconds of arc. 



To ascertain when the instrument is level, the regular pair of levels of the meter 

 is used in combination with 2 photo-voltaic cells for each level vial. The level vials 

 are illuminated and a photo cell is set over each end and connected in parallel with a 

 25-0-25 micro-ammeter. When the micro-ammeter indicates a balance, the instrument 



Fig. 233. — Gulf underwater gravimeter with the top half of the pressure housing 

 removed, showing the leveling mechanism. (Courtesy of Gulf Research and Development 

 Company.) 



is level (to a sensitivity of 3 seconds of arc per micro-ampere) for the azimuth of the 

 particular level involved. 



The moving systein is clatnped by a permanent magnet motor working the arrest- 

 ing mechanism through a gear box. A warning light on the operator's control panel 

 shows red when the meter is undamped and green when it is securely locked. The 

 readings are taken photographically on 35 mm. film. Records are read with a micro- 

 meter slide comparator with an accuracy of about 0.2 mg. Corrections are made for 

 film shrinkage if necessary. 



As in the case of land operations, thermostatic heat control is provided. A special 

 alarm system gives warning if the housing is leaking water. A maximum of 17 stations 

 has been made in a day, but 6 is an average run. As shown by field checks, the accuracy 

 of observations was better than 0.1 milligal. 



Diving Bell 



A diving bell so designed as to accommodate both a gravimeter and an 

 observer has been found useful in making gravity observations on rivers, 

 lakes, and the ocean floor at depths of from 8 to 70 feet.f This bell is shown 



t E. W. Frowe, "A Diving Bell for Underwater Gravimeter Operation," Geophysics, Vol. 21, 

 No. 1, Jan. 1947, pp. 1-12. 



