III. GEOMAGNETISM 

 by G. Burfon and R. Obrochta 



A. Summary of Operations 



Measurements of the earth's total magnetic field intensity over approximately 

 10,000 miles of track were recorded on USS BURTON ISLAND . Most of the data 

 were collected in unsurveyed waters while the ship was performing its primary sup- 

 porting mission . (Index charts of the ship's track with corresponding profile numbers 

 are shown as Figures 29, 30, 31, and 32.) Approximately half of the collected 

 data were obtained south of New Zealand, and the results include a detailed sur- 

 vey of Commonwealth Bay, Antarctica, in the general vicinity of the south magnetic 

 pole. 



Ship positions were determined by radar, celestial navigation, and dead reck- 

 oning. When the ship was within radar range of known landmarks, position-fixing 

 errors were less than 1 mile; otherwise, positioning errors varied from 5 to 50 miles 

 depending on weather conditions. The survey of Commonwealth Bay was conducted 

 using radar fixes on surrounding ice walls. 



B. Observational Technique 



Total magnetic intensity measurements were made with nuclear resonance 

 magnetometers. Two units, a modified Varian model 4901 and a Varian model 4914, 

 were utilized. To reduce the effect of the ship's magnetic field, the sensor usually 

 was towed 500 feet astern. During extreme ice conditions (February 9 through 13), 

 however, the sensor was towed 250 feet astern. 



Magnetic data were recorded in analog form on a strip-chart recorder. The 

 sensitivity of the instruments is normally 1 1-2 gammas (1 gamma = 10"^ oersted). 

 With the exception of the Commonwealth Bay survey, however, equipment mal- 

 functions limited the sensitivity to _ 5-10 gammas. Also, because of difficulties 

 with power supply units, the trace is incomplete in many places. 



C. Compilation of Data 



The total magnetic Intensity data were scaled at 50-gamma intervals and maxima 

 and minima. These values were plotted as magnetic profiles with the associated 

 bathymetric data and ship's track. Geomagnetic data from the detailed survey of 

 Commonwealth Boy were contoured at 50-gamma intervals. No corrections for 

 temporal variations of the magnetic field were made to the magnetic data. No 

 large temporal disturbances were noted during the survey period. 



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