414 



EXPLORATION GEOPHYSICS 



ences at the stations, in milligals, is given in column 12, The operation is 

 column 10 times k. 



The arbitrary gravity value assigned to the base station was 100 milli- 

 gals. This value is used in column 13 by adding to it the figures in column 

 12. (Column 12 plus 100 mg.). 



In column 14 the total corrections, noted as adjustments from the cor- 

 rection sheet referred to, are shown. Their application gives the final map 

 value for the field stations in milligals relative to the survey base which 

 is of record in column 15. The similarity of the above procedures to those 

 in magnetic field work is obvious. 



The correction sheet also has 15 columns, although not all of them are 

 ordinarily used. Station number and elevation are entered in columns 1 

 and 2. Space is provided in columns 3, 4, 5, and 6 to calculate the free 

 air and the Bouguer corrections separately, as is sometimes done. 



In the set of notes given here, these two corrections are combined as 

 represented in the constant 0.0683 milligals per foot, using an assumed 

 density of 2.0 for the material between the station and the datum. The 

 datum of the survey was set, for convenience, at an elevation of 1000 

 feet. 



In column 6 is shown the combined correction, obtained from multi- 

 plying the combined constant by the number of feet from the station to 

 the datum. For station 1 this was 767 feet • 0.0683, or 52.40 milligals. 



Terrain corrections for the effect of certain 

 zones at given out-distances from the station 

 can be entered in columns 7 to 10. In the survey 

 treated, the local terrain was rather flat and no 

 terrain correction was needed. 



The use of column 11, in these notes, is 

 slightly changed from its heading as given. It 

 represents the sum of the terrain zone effects. 

 As used in the notes under consideration, the 

 distances in fractions of a mile north or south 

 of the base are listed. Station 2, for example, 

 was 0.25 of a mile south of the base, as indicated 

 on the map of the locations of the stations, 

 Figure 246. 



The latitude correction comes in column 12 



and is at the rate of 1.3 milligals per mile in 



the present instance. It was determined by 



calculation for the latitude of the survey area, 



namely 38° 45' and 39° 00' north (see page 



253). At station 2 the latitude correction is 0.33 milligals (0.25 • 1.3). 



The latitude correction is applied in a proper manner in the operation 



of the form. At station 2, for instance, the latitude correction is plus 0.33 



milligals (station 2 is 0.25 miles sotith of the base, where gravity is there- 



Fig. 246. — Locations of gravity- 

 meter stations for sample calcu- 

 lations. 



