There is the further question whether the gravity field at these smaller wave- 

 lengths can make any significant contribution to the geoid undulations. Statements 

 exist in the published literature to the effect that the geoid height is known to a 

 few meters. Such statements are, of course, only concerned with geoid undulations of 

 long wavelengths. As we shall demonstrate in this paper, undulations of the geoid 

 with wavelengths of a few hundred kilometers can have amplitudes of a few tens of 

 meters. Thus, for the complete description of the geoid it is essential to utilize 

 surface gravity data. 



The gravity field described by the range of wavelengths from a few tens of 

 kilometers to a few thousand kilometers is of special interest to geophysicists 

 because it reflects, in part, density inhomogeneities in the Earth's upper mantle. 

 The location, nature, and magnitude of the density anomalies in the upper mantle 

 will undoubtedly help in resolving the causes of the motion of the "plates" comprising 

 the Earth's lithosphere. While the geometry of plate motions is beginning to be 

 understood, the causes of these motions are still quite obscure. 



There are reports of plans to map the geoidal undulations in the oceanic areas 

 by radar altimeters on the Skylab and the GEOS-C satellites. A major error in the 

 altimeter measurement will arise from the uncertainty in the position of the satellites. 

 However, the position uncertainties are smallest for small wavelengths and the usefulness 

 of the altimeter will be primarily for the determination of short wavelength undulations 

 of the geoid. In order to calibrate the altimeter and to test its performance, or 

 even to determine optimum locations at which the experiment should be performed, it 

 is absolutely essential to know the geoid undulations at the same locations by 

 independent means. However, no such geoid maps exist over the oceanic areas. This 

 study, although preliminary in nature is the only one to date that provides 

 detailed information about the geoid in an oceanic area. The study will have to be 

 refined on the basis of newer information which is available but which has not been 

 incorporated . 



23-2 



