34 THE NAVY OCEAN SCIENCE PROGRAM 



on depth to the mantle, and on mantle seismic velocities (see 

 "Sediment and Criistal Structure" in "Major Accomplishments" 

 section). These crustal layers affect the propagation of sound 

 of very low frequency near and in the infrasonic frequency range. 

 Their study continues to be £in important component of this 

 program. 



Variations in crustal and mantle structures produce anomalies 

 in the earth's gravity field which, at sea, Eire important to 

 navigation, particularly in determining the shape of the geoid 

 (approximate sea level surface) and the deflection of the vertical 

 (singular difference between the normals to the geoid and the 

 earth's reference ellipsoid). Gravity at sea also provides signifi- 

 cant information on crustal and mantle rock configurations and, 

 therefore, on processes that shai)e the ocean floor, thus aiding 

 in the interpretation of geologic structure. 



Gravity measurements at sea were first made with pendulums 

 in submarines and later with spring-balance gravity meters in 

 submarines. Since about 1963 reliable measurements have been 

 made with gravity meters on surface ships, which are now 

 accurate to about ±5 milligals; it is anticipated that this accuracy 

 can be increased to ±1 milligal with the advent of navigation 

 satellite control and with improved gravity meters. Regional 

 gravity anomalies being obtained from surface-ship measure- 

 ments correlate reasonably well with those based on satellite 

 measurements. 



Crustal and mantle structures are being analyzed on the 

 basis of marine gravity measurements, particularly off Oregon, 

 Hawaii, the Solomon Islands, the Mid-Atlantic Ridge, and 

 various abyssal plains and island arc trenches. Anomalies 

 are found to be associated with geologic structures; they fre- 

 quently correlate with bottom topography, but there are many 

 marked exceptions, some of which have been satisfactorily 

 explained by density distributions suggested by seismic data. 



The gravity program is fundamental to knowledge of the 

 shape of the earth, and it is a matter of military importance. 

 It also is useful as an interpretive tool with other geophysical 

 data. 



