SURVEYING THE OCEANS (HYDROGRAPHIC) 39 



guidance and navigational systems ahd for studies concerning 

 the ocean bottom's geologic structure, Navy support was instru- 

 mental in the development of gravity meters capable of making 

 accurate underway measurements at sea from surface ships. At 

 first, meters suspended in gimbals proved successful only if sea 

 conditions wer^ favorable, but mounting meters on stable platforms 

 now allows continuous underway measurements of gravity to an 

 accuracy of ±5 milligals (1 gal is an acceleration of 1 cm per sec^) 

 or better, even in rough seas (up to sea state 5). This new system, 

 combined with a better positioning capability, allows the under- 

 way collection of gravity data by surface vessels in all the world's 

 oceans. The Naval Oceanographic Office now has five surface 

 ships and one submarine collecting gravity data continuously along 

 with other oceanographic data. 



As these new meters do not measure absolute gravity but only 

 the "difference in gravity" between two points, extensive surveys 

 were conducted using underwater meters set on the bottom to 

 establish precise calibration and test ranges in the approaches to 

 San Francisco, Norfolk, and Newport, R. I. Running the meters 

 over these ranges prior to and following the surveys permits 

 accuracy requirements to be met. 



Submarine iised in oceanographic surveying. 



