SECT. 1] 



THE MAGNETIC FIELD OVER THE OCEANS 



90° 75° 60° 45° 30° 



187 



0.60 



0.55 



0.50 



0,45 0.40 



Fig. 7. Section from the U.S. Hydrographic Office chart of the total field in 1955 (No. 

 1703). 



Wave-length (miles) 



100 5040 30 20 15 10 



0.02 0.04 0.06 0.08 



Wave number (1/miles) 



0.10 



Fig. 8. Power spectrum of the magnetic field on a profile in the north-western Atlantic. 

 The dashed line represents the spectrum which would be observed if the spectrum at 

 the sea bottom were "white". 



Since measurements at sea are taken at or above the surface of the sea and 

 many thousands of metres above the ocean bottom, local anomalies will be 

 smoothed and attenuated. If the anomalies are represented by a space spectrum, 

 a component of wave-length A will be reduced in the ratio exp ( —2TrhlX) if it 

 is measured at a height h above the bottom. In water of 5000-m depth the 

 attenuation is by a factor of over 2 for all wave-lengths below 14 km. There is, 

 therefore, a very substantial loss of detail as compared to a survey conducted 

 on land. A smoothed power spectrum taken by a ship along an east-west 

 profile in a depth of about 2500 fm in the eastern basin of the North Atlantic 

 is shown in Fig. 8. The profile is 379 miles in length and runs along latitude 



