SE 



NW 



400 Km 

 FILTER 



A 



Lowr 



PASSL 



highT 



PASSL 



^\Av^ 



GRAVITY EFFECT 

 FREE AIR GRAVITY 



GRAVITY EFFECT 

 FREE-AIR GRAVITY 



150 Km 

 FILTER 



B 



Lowr 



PASSL 



HIGHT 

 PASSL 



GRAVITY EFFECT 

 FREE-AIR GRAVITY 



GRAVITY EFFECT 

 FREE-AIR GRAVITY 



ABC D 



300 T 600 



c 



D 



GRAVITY EFFECT OF 

 TOPOGRAPHY 



FREE-AIR GRAVITY 



TOPOGRAPHY 



MID-ATLANTIC RIDGE 



Figure 4. The bottom curve gives the topography over a section of the north Mid-Atlantic 

 Ridge. (Scale D is in meters) The measured free-air gravity curve is 

 immediately above the topography curve (Scales A, B, and C are in milligals) . 

 The gravity effect of topography is computed using a density contrast of 



2.67-1.03 g/cm-^ 



Both the free-air gravity and the gravity effect of 



topography is filtered, using two different Gaussian filters of widths 150 km 

 and 400 km. The filters are designed in such a way that the wavelength A of 

 unit amplitude after being filtered through the low pass Gaussian filter of 

 width w has an amplitude exp [ (-. 55 (w^/X^) ] . For example for A=2w and w/2 the 

 respective filtered amplitudes are .96 and .11. We notice that for wavelengths 

 of the order 400 km the free-air gravity can be predicted well from topography 

 in the region of the Mid-Atlantic Ridge. 



23-9 



