5 miles. Average ship's speed in the two survey areas was 12 

 knots. 



Control in the survey areas was maintained when possible by radar 

 and visual fixes obtained at one hour time intervals. Fix accuracy 

 during these periods is estimated as being approximately one mile. 

 When out of visual and radar range of land, position was maintained by 

 celestial navigation and dead reckoning. Fix accuracy is reduced 

 during these periods to an estimated + 5 miles. 

 B. Instrumentation . 



Total magnetic intensity measurements were made with a Varian 

 marine proton precession magnetometer, model V-4931. The sensor unit 

 for this magnetometer was contained in a cylinder 24 inches long and 



6 inches in diameter. This sensor was towed 575 feet astern in order 

 to reduce the effect of the ship's magnetic field. 



Because of equipment design, magnetic data were recorded as 

 "magnetometer counts," a unit of measurement inversely related to the 

 magnetic field intensity. These units were recorded on a Varian model 

 G-11A analog recorder and printed out in digital form on a Hewlett- 

 Packard digital recorder, model 560-A. 



Bathymetric data were obtained with a Precision Depth Recorder 

 (MK-V) and an AN/UQN 1-D sonar sounding set. 



III. DATA PROCESSING 

 The magnetic data from the analog tapes were scaled and converted 

 from magnetometer counts to absolute values in gammas (1 gamma = 

 1 x 10"5 oersted) by the use of a template. These data were used to 

 construct magnetic total intensity contour charts on Mercator Pro- 

 jections at a contour interval of 50 gammas (figures 4 and 6). 



