Chap. 8] MAGNETIC METHOD 373 



travelog when such changes (shocks, demagnetizing effects, and so on) 

 are likely to have occurred. Base readings should be plotted against time 

 to separate random from systematic changes (variations, changes in tem- 

 perature coefficient, and the like). 



E. Influence of Iron and Steel Objects 



Since it is impossible to correct for the magnetic fields caused by iron 

 masses, the only remedy is to keep at a distance commensurate with the 

 accuracy of the survey. For objects extending in a vertical direction, the 

 anomaly decreases approximately in proportion to the inverse square of 

 the distance. For predominantly horizontal objects, the attraction is 

 approximately proportional to the inverse first power of distance. 



1. Interference from iron objects about the observer (watch, knife, key 

 ring, belt buckle, steel frames of glasses, suspender buckles, steel rings in 

 notebooks, and the like) can be avoided since it is a simple matter to 

 determine their influence by approaching a magnetometer from various 

 sides, and from above and below. Objects found to be effective at the 

 customary operating distance should be taken off. "^^ 



2. The vertical field of the instrument case of a magnetometer with 

 compass and auxiliary magnets is about 1257 at 1 yard distance to the 

 north and disappears at 4 or 5 yards. 



3. A bicycle produces an anomaly of almost — 6O7 at a distance of 1 

 meter. An automobile has, at 3 yards distance, a vertical field of al^out 

 — 7OO7, which disappears at about 25 to 35 yards. It varies somewhat 

 with the direction in which the car is pointed and the position of the instru- 

 ment with reference to it. 



4. The field of a wire fence running N-S (the instrument being on the 

 east side) is about — SSOy at 1 yard and disappears at about 36 yards. 



5. The influence of a bridge varies considerably, depending upon con- 

 struction and material, size and direction, and whether the observer is 

 above or below it. . " 



6. Pipe lines can generally be spotted in the field by the overhead 

 telephone lines which follow the course. For a N-S traverse across an 



.8 inch line buried 4 feet below the surface, running E-W, Barret observed 

 a vertical intensity anomaly of — 5OO7 at about 2 yards (peak in curve) 

 and a disappearance of the anomaly at about 30 yards. From 2 yards 

 te a point directly over the pipe, the anomaly decreases sharply. This 

 interference varies with the direction of the pipe, polarization at the ends, 

 and existence of anticorrosion currents. 



" Oil and Gas Journal, 28(22), 14^-150,' 245-252 (Oct. 16, 1929); A.A.P.G. Bull., 

 16(11), 1371-1389 (Nov., 1931). ^ 



