142 EXPLORATION GEOPHYSICS 



mile for the horizontal component ; the longitude corrections average 2 to 

 3 gammas per mile. 



Due to the irregular distribution of the isodynamic lines along the 

 earth's surface, the earth's normal field strength cannot be calculated 

 theoretically from the latitude and longitude position, but must be ob- 

 tained from maps of isodynamic lines, (See Figure 33.) 



Latitude Corrections. — Calling d the distance measured perpendicular 

 to the geographical latitude between two successive isodynamic lines dif- 

 fering in intensity by AZ or by LH, as the case may be, the corrections 

 are :* vertical component = Ayz = AZ/rf ; horizontal component = Ayn = 

 A H/d. The vertical correction is to be subtracted from readings north of 

 the base station in areas north of the magnetic equator or south of the base 

 station in areas south of the magnetic equator. (The rule regarding signs 

 derives from the fact that the magnetic intensity increases positively 

 toward the north magnetic pole and negatively toward the south magnetic 

 pole.) The horizontal correction is added to readings north of the base 

 station in areas north of the magnetic equator or south of the base station 

 in areas south of the magnetic equator. (This sign rule obtains because 

 the horizontal intensity decreases toward the two poles.) 



Usually it is convenient to employ a latitude correction curve, wherein 

 distances measured perpendicular to the geographical latitude are plotted 

 as abscissa and the calculated correction from the base station as ordinate. 



Longitude Corrections. — Vertical component = Ayz = i^Z/d; horizontal 

 component = Ayn = tiH/d, where d is the distance measured perpendicular 

 to the geographical longitude between two adjacent isodynamic lines dif- 

 fering in intensity by AZ or A//, as the case may be. 



The longitude correction curves are plotted in the same manner as the 

 latitude curves. The algebraic signs of the corrections depend on the mag- 

 netic declination in the area. For west declination areas, the corrections 

 are added for all stations east of the base station. For east declination 

 areas, the corrections are subtracted for all stations east of the base station. 

 These corrections are generally quite small and often may be neglected in 

 ordinary work. Latitude and longitude corrections can be combined into 

 a single correction by using the magnetic north and south rather than the 

 geographical north and south as the direction of measurement from the 

 base station. Also, corrections for latitude and longitude are oftentimes 

 included in the "regional gradient" in final interpretation. When the latter 

 procedure is employed, the regional gradient is drawn in by inspection of 

 the magnetic profiles, and anomalies are measured using the regional gra- 

 dient as a reference line. This is illustrated in connection with Figure 112. 



*Isodynamic lines on available charts are expressed in gauss. The rate per mile or 

 kilometer must be reduced to gammas for use in field work. 



,1 



i 



