MAGNETIC DISTURBANCE 1387 
positive magnetic pole because of its remote depth. It is seen that the 
lower parts of the curves are in good agreement, negative anomalies being 
absent in each case, and that the calculated value is decidedly too small 
for the position directly above the casing. The failure to observe negative 
values in the field supports the supposition that the effect of the lower 
pole may be disregarded, though it is well to mention that this same 
effect would be noticed had the casing extended a sufficient distance above 
the ground to establish the upper pole at a position above the magnetic 
system of the variometer. For points near the casing head the discrep- 
ancy between the curves is exaggerated because of the tremendous ratio 
of L to x. 
It has been repeatedly argued that the pronounced magnetic “‘highs”’ 
associated with certain producing fields may be largely attributed to 
magnetized casing. As we have seen that theory and experiment are in 
complete accord regarding the highly localized nature of the disturbance 
traceable to this cause, we are naturally led to a consideration of the 
possible areal distortion of the magnetic elements in conjunction with 
producing fields. This, in fact, brings to the attention the third assump- 
tion upon which the theoretical treatment has been based, that is, the 
symmetry of the normal earth’s field suffers no distortion by virtue of 
the presence of the magnetized pipe. While this interpretation is support- 
ed by its general acceptance for mathematical analysis,' the most 
conclusive evidence is that results derived in this manner may be verified 
by precise experiments. Only observational data regarding the possible 
areal effect will be considered here. 
In December, 1928, at the writer’s direction, a vertical intensity 
survey was made of the Sligo field, located in T. 17 N., R. 12 W., Bossier 
Parish, Louisiana. At that time there were 17 producing wells in the 
field, this number being increased to 45 by January, 1931, when a second 
series of measurements were made at the same group of stations. It is 
felt that this field offered exceptional opportunities for determining the 
possibility of an areal disturbance for the reason that only minor mag- 
netic reflections of the subsurface relief were obtained; hence any extran- 
eous influence would be easily recognized. The locations of the wells, 
together with a selected series of magnetic stations, are shown in Figure 
7, and for each survey the anomalies, with respect to station No. 1, are 
tA. Gray, op. cit., 
W. S. Franklin waa B. “MacNutt, Advanced Theory of Electricity and Magnetism 
(Macmillan, New York), p. 85. 
105 
