MAGNETIC METHODS 



215 



Fig. 103.- 



POLE 



-Magnetic triangulation for pole depth; 

 Steam's method. 



to express it more specifically, toward the pole. The resultant of the AZ 

 and the A/f forms the disturbance vector. See separate plot for station 5 

 of the figure. 



An example of the application of this procedure is given in Figure 77 , 

 a traverse at the Falconbridge pyrrhotite deposit, in the Sudbury Basin of 

 Ontario, Canada. 



Depth Finding by Mag- 

 netic Triangulation. — N. H. 



Stearn presents a somewhat 

 different solution for finding 

 the depth to a magnetic pole. 

 The procedure is termed mag- 

 netic triangulation. It is based, 

 like the preceding method, on 

 the use of magnetic vectors, 

 and is particularly well suited 

 to field measurements obtained 

 with the Hotchkiss Superdip 

 magnetometer. Figure 102 il- 

 lustrates the method. 



As is shown in Figure 102, the vector I represents by its length the total 

 magnetic intensity at a station as determined with a Superdip instrument. 

 The same vector / represents by the angle at which it is plotted the direc- 

 tion of the said total intensity. This vector is resolved into two components : 

 I^ and Ia. Component h is the normal or average total intensity for the 

 area as given by its plotted length, and it is plotted at the normal (average) 

 inclination angle for the locality. The average (or normal) would be the 

 amount and the direction of the magnetic intensity for the local area at 

 some distance from the magnetic feature involved. 



The vectorial dift'erence of / and In is the anomaly vector 7^. The 

 vectors Ia represent the components of abnormality required to produce 

 the resultant magnetic intensity and direction If. The locus of intersection 

 of the extended components of the anomaly vector I^ for a line of stations 

 locates the position of the anomalous force or pole. The depth to the pole 

 can thus be obtained graphically as shown on Figure 103 for a traverse 

 across a peridotite plug. 



The inclination of magnetic force at a series of stations can be obtained 

 using the Superdip instrument as a dip needle. With it the total magnetic 

 force can be measured at the same stations when it is operated as a magnet- 

 ometer. The depth to the pole in the figure noted is about 500 feet below 

 the top of the peridotite plug as determined by drilling. 



These vectors tend to intersect (in a theoretical case) at loci around 

 the pole, the depth of which can be found in terms of the distance scale 

 on the X axis. 



t N. H. Stearn, A.I.M.E. Geophysical Volume, 1932. 



