MAGNETIC METHODS 157 



Adjusting for sensitivity* 



Checking scale value to determine gammas per scale division 



Determining location of stations to allow best correlation with 



geology 

 Establishing base stations 

 Preliminary traverses : one traverse in a N-S direction to obtain 



maximum anomalies due to polarization 



Field Work at Each Station 



Setting up tripod at station 



Leveling tripod 



Orienting tripod head 



Leveling instrument, after clamped to tripod head 



Obtaining scale readings 



Recording 



Station number and map location 



Scale readings 



Temperature of instrument 



Time of observation 



Local conditions which may affect the reading 



Corrections 



Base reading to datum plane (for survey employing more than one 

 instrument) 



Latitude 



Average for U. S. : 10-12 gammas per mile for vertical component 

 5-8 gammas per mile for horizontal component 

 When movement proceeds to the North 

 Subtracted for vertical component 

 Added for horizontal component 



Longitude 



Average for U. S. : 2 to 3 gammas per mile 

 When movement proceeds to the East 

 Subtracted for east declination areas 

 Added for west declination areas 



Base station corrections 



Diurnal and other variations in earth's field 



Instrument variations (average of readings) 

 Temperature corrections 

 Correction for deflection due to use of auxiliary magnets 



* May also be done in the laboratory. 



