374 MAGNETIC METHOD [Chap. 8 



7. Near a railroad bed, running E-W, Barret observed on the south 

 side a negative anomaly up to about 5 yards which then increased to a 

 maximum of 4OO7, thence decreased and disappeared at 125 yards. 



8. The effect of buildings depends entirely upon size and iron and steel 

 content. Brick buildings and brick walls are often magnetic, since the 

 stones acquire magnetism in the process of roasting. 



9. The influence of slag and gravel roadbeds, drain pipes, and the like, 

 varies greatly with construction materials; it is difficult to give even 

 approximate figures. 



10. The influence of tank farms is generally complicated by associated 

 pumping, power, and pipe line equipment; general figures are not avail- 

 able. 



11. When magnetic measurements are used for detailing or extending 

 drilled or producing fields, it is necessary to know the effect of well casing. 

 The vertical and horizontal intensity anomalies of a tubular casing of 

 uniform diameter may be written 



AZ = S/cZo 



AH = SkZo'X 



\ri Ti) 



(8-50) 



where k is the susceptibility of the casing material, S its cross-sectional 

 area, Zo the normal vertical intensity (SkZo = pole strength, or magnetic 

 moment divided by the length of the casing), d\ the depth to the upper 

 and di the depth to the lower pole, r^ the distance of the point to the 

 upper and r-i, the distance to the lower pole, x the surface distance. This 

 formula corresponds to that for a vertical magnet (formula [8-53]). For 

 a very long magnet the second term in the bracket is negligible and there 

 occius only a positive anomaly. A short magnet may produce an area 

 of negative anomalies surrounding one with positive anomalies, as illus- 

 trated in Table 43. In this example the anomaly disappears at about 

 200 feet from the well, regardless of the length of the casing. Barret ^ 

 and Clifford'* conducted surveys in oil fields before and after a change 

 took place in the number of wells and concluded that the picture of the 

 magnetic anomalies was not changed within the limits of accuracy. This 

 is contrary to Van Weelden's^^ contention that cumulative magnetic fields 

 of wells may reach figures sufficient' to explain the negative anomalies 



" Barret, loc. cil. 



78 IjQQ^ cit, 



'^ O. C. Clifford, A.A.P.G. Bull., 16(12), 1171-1176 (Dec , 1932). 



'6 A. Van Weelden, World Petrol. Congr. Proc, Sec. B. I., 86-90 (1933). 



