Some GRAVITY METERS (fig. 17.7) used at sea are mounted 

 on stabilized platforms. The platform is stabilized to within about 

 one miinute of the vertical. With it, we are able to measure gravity 

 to about plus or minus 5 milligals. We could do better than this if 

 we had better navigation. 



The navigation is the fundamental obstacle to better accuracy. 

 Edtvos' correction is required by the change in centrifugal acceler- 

 ation due to the east-west component of ship's speed. This amounts 

 to 7-1/2 milligals per knot of ship's speed in east-west directions at 

 the Equator. If the direction and the speed are not known accurately, 

 it is hard to make the correction. The gravity meter is just one 

 of many, many uses for a stable platform on a ship. I am sure when 

 we get the narrow-beam sounders, we will need to stabilize the heads, 

 which inay be done by simply telemetering the information from 

 the stable platform of the gravity meter. 



It might be much more sensible to do this than to add addi- 

 tional stable platforms, which require considerable space and up- 

 keep. 



The DATA GENERATOR (fig. 17.8) is continuously in use on 

 our ship. It puts data on depth, magnetics, gravity, course, 

 mileage, and time on the record simultaneously. This combina- 

 tion makes all the data much more useful and gives the scientist 

 on board a chance to utilize the coordinated information immedi- 

 ately. 



One of the most fundamental geophysical measurements made 

 at sea is that for SEISMIC REFRACTION. Figure 17. 9 shows the 

 method of marine measurements in less than 600 feet of water. 

 We fire charges of half a pound to 300 pounds, in ranges of zero 

 to 60 miles apart. From the timing of the return of the sound 

 waves through the water and through the sediments, we are able 

 to determine the sound velocity in the sediments and the layer 

 thicknesses. These have rather striking variations in the ocean 

 and are upsetting the geology books, which had all problems pretty 

 well solved until we started finding out what occurs at sea. In 

 general, the frequencies examined in the ground waves are be- 

 tween about 6 and 50 c.p. s. In the water they vary between about 



184 



