5. COMPARISON BETWEEN THE DIFFERENT GEOID MAPS 



A basic piirpose of this study was to determine whether short wavelength 



gravity data, in the western North Atlantic, contribute significantly to geoidal 



undulations. Figure 9, the "1x1° difference" geoid (which is the "1x1°" geoid 



referred to the "G and L" geoid) shows that these short wavelength undulations are 



indeed significant. The geoidal height differences exceed 10 m in the western 



North Atlantic and the Caribbean. A difference of 21.5 m indicated in the 



Gulf of Mexico may be somewhat exaggerated due to poor gravity data we have 



used for Mexico (this point is discussed later) but it certainly represents a 



large and significant departure from the geoid based on Gaposchkin and Lambeck's 



combination solution. The geoidal differences are further increased when the 



"10x10' difference" geoid is also considered. The total differences (obtained by 



summing the "1x1° difference" geoid and the "10x10' difference" geoid) are nearly 



equal to 20 m. Peak to peak variations associated with such features as the 



Puerto Rico Trench and the Lesser Antilles negative belt approach 25 m. 



The differences between the various geoids are illustrated along a north-south 



profile from the South American margin across the Venezuelan Basin, Puerto Rico 



Island and Trench to the North Atlantic in Figiire 14 and along an east-west 



profile from the Venezuelan Basin across the Lesser Antilles into the North Atlantic 



in Figure 15. In both figures most of the curves are self-explanatory. A 



"2-D deflection of the vertical" was obtained by computing the horizontal component 



of the gravitational attraction corresponding to the free-air gravity c\irve assuming 



that there were no gravity differences perpendicular to the profile. The "2-D 



( 

 deflection of the vertical" was integrated to get the 2-D geoid profile. The 



"10x10' difference" geoid (Fig. 13) was added to the "1x1°" geoid to get the "10x10"' 



geoid. The slope of the "10x10'" geoid gives the "10x10'" deflection curve. 



Attention is drawn to the large difference between the "10x10"' geoid and the "G and L" 



geoid in the vicinity of the trench. Significant differences also exist in the 



Venezuelan Basin and over the South American Margin. 



23-24 



