MAGNETIC METHODS 155 



area and may be of such an amount as to eliminate negative magnetic 

 values in contouring. The value X for the magnetic work in a given county 

 for a survey base station may be set at 2000 y's. 



Such a base could be tied in magnetically, if desired, with a govern- 

 ment magnetic station where the true and absolute value of vertical mag- 

 netic intensity is known. The absolute vertical intensity, however, is 

 not necessary in much magnetic field work, as the values from field stations 

 are obtained relative to a base. They represent the finer variations in the 

 magnetic intensity which are of significance in geologic terms. This is the 

 main difiference between magnetic surveying for the purposes of applied 

 geophysics and that carried on by the U.S. Coast and Geodetic Survey in 

 measuring the total and absolute magnetic values at stations. 



Station Spacing 



A general rule for spacing of stations in magnetic or other geophysical 

 surveys can be stated. Stations should be set sufficiently close together so 

 that no important change in the quantity measured can occur without being 

 shown in the results of the survey. In practice, this means for placer 

 surveys stations from 5 to 25 feet apart ; for dikes and ore bodies stations 

 may be 25 to 50 or 100 feet apart, depending on the size of the body. For 

 magnetic structural studies in connection with oil accumulation, from ^ 

 to 2-mile spacing may be used.* For serpentine plugs, with which oil is 

 sometimes associated, as these features themselves are not large, ^ to ^ 

 mile stations are used. For faults and formation boundaries 100- to 300- 

 foot spacing may be necessary. 



It is customary when surveying traverses of stations to lay them out at 

 right angles to the strike of the geologic feature involved. The most rapid 

 change with distance normally occurs in such a direction for the magnetic 

 effects. Lacking information on the strike, test traverses can be run to find 

 the direction in which the most rapid change takes place. Regional strike 

 is often an aid in choosing the proper traverse direction. 



Plotting Magnetic Data. — The corrected data of a magnetic survey 

 may be presented in a number of ways. Usually, however, the results are 

 shown as : ( 1 ) magnetic profiles along traverse lines so chosen that the 

 magnetic profiles may be correlated with known geological drill-hole or 

 other data, or (2) isanomalic contour maps. In the first type of presenta- 

 tion, the data are usually plotted with the anomaly in gammas (or scale 

 divisions) as ordinate and the traverse distance in feet or miles as abscissa. 

 The second type of presentation (isanomalic contour map) is similar to 

 the conventional topographic map ; that is, the magnetic datum at each 

 station is indicated on a plan view of the area at a point corresponding 

 to the location of the station, and isanomalic magnetic contours are drawn 



*This would depend on local conditions and on the type of survey, viz., reconnais- 

 sance or detail. 



