638 



Figure 7. Relationship Between Geoid and Seafloor Topography 



With sateliites, the ocean surface 's 

 relative heights can be measured. 

 The sea surface height has been 

 shown to correlate with 

 subsurface bottom structure. 



This figure shows two very- 

 different data sets depicting 

 different views of a smalt section 

 of ocean. 



At the bottom is a high-resolution 

 shipboard measurement of 

 bathymetry, with depths in meters, 

 showing several prominent 

 seamounts. Above this is the geoid 

 of the same area, clearly showing 

 manifestations of the same 

 seamounts on Earth's gravity 

 field. This effect is illustrated 

 schematically in Figure 5. 



gravitaiional attraction, which varies as the inverse square of 

 the distance, is greater than the compensating attraction of the 

 root, which lies deeper in the mantle, and a positive anomaly 

 will be recorded. 



seafloor bathymetry. In addition, the spatial relationships 

 between topography and gravity near a volcanic ridge crest, 

 where the lithosphere is very ihin, are quite different than the 

 corresponding relationship on more mature, thickerlithosphere. 



This problem is further complicated by the strength of the 

 lithosphere that overlies a more fluid mantle. When a small 

 seamount is constructed by volcanism atop the lithosphere, the 

 seamount is supported largely by the strength of the rigid 

 lithosphere, and Archimedes' Principle never comes into play. 

 The anomaly associated with such a seamount is quite large. 

 However, as the seamount grows in size (or the lithosphere's 

 strength decreases because of heating, for example), the 

 seamount and surrounding lithosphere must obey Archimedes' 

 Principle. Thus, the relationship between seafloor topography 

 and gravity (or the geoid) becomes (spatial) wavelength 

 dependent. Thus, the inference of gravity from topography or 

 vice versa becomes ambiguous in the absence of other 

 information. For example, as the seafloor ages and becomes 

 sedimented and flat, it becomes impossible to infer gravity from 



We have identified three research topics where accessibility of 

 the Navy's currently classified gravity data would lead to 

 important scientific research. 



i) Spatial Variations in Gravity at Mid-Ocean Ridges 

 Recently gravity data have been exploited in a bid to understand 

 basic geophysical processes at mid-ocean ridges. 

 Morphologically, mid-ocean ndges can be broken into at least 

 two fundamentally different classes corresponding to fast- 

 spreading and slow-spreading environments. Slow-spreading 

 ridges are generally characterized by rugged topography and a 

 spreading center bounded by steep mountains. Fast-spreading 

 ridges, on the other hand, are recognized by muted topography 

 and the absence of significant flanking mountain ranges. Detailed 

 geophysical surveys at several locations have been used to 



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