II. DISCUSSION OF DATA 



The test data used for calculating the anomalous field components 

 were provided from an aeromagnetic survey over Plantagenet Bank 

 (Fig. 1) carried out on 7 January 1961. Plantagenet Bank is one of 

 three flat-topped guyots of volcanic origin, on the same platform as 

 Bermuda (Woollard, 1953), and rises from 2400 fathoms to 30 fathoms 

 below sea level (Fig. 2). The survey track spacing was 0.5 mile, and 

 positioning was provided by Loran-C with an estimated accuracy of 

 + 1000 feet. 



Figures 3, 4, and 5 are contour charts depicting the total intensity 

 (F) , inclination (I), and declination (D) of the magnetic field ob- 

 served at 500 feet above sea level. The survey data were measured 

 within a 4.5-hour period thus minimizing the effects due to tem- 

 poral variations. The amplitude of the magnetic anomaly in the survey 

 area is approximately 4200 gammas which is in agreement with an overall 

 magnitude of 5000 gammas reported by Keller et al (1954). 



The following procedure was used to compute the components of the 

 magnetic anomaly over Plantagenet Bank: F, I, and D data in relatively 

 undisturbed areas near Plantagenet Bank were extrapolated into the 

 survey area. These data were then averaged to approximate the normal 

 field values, (Fn = 51,280 gammas, In - 65.0°N, Dn = 13.8°W). A grid 

 overlay with an interval of 0.5 mile was used for scaling F, I, and D 

 values from Figures 3, 4, and 5. For each of 240 grid intersections, 

 north (X), east (Y) , and vertical (Z) components were computed. These 

 directional components were then subtracted from their respective 

 components of the normal magnetic field. The residuals represent 



2 



