GRAVITATIONAL METHODS 403 



rected for elevation only, still has in it this 2.6 mg., due to the 100 feet of 

 material from the surface to the datum. 



We therefore subtract the 2.6 from the 63.2, which gives 60.6 for the 

 final corrected, on datum, value of the gravity at station X. 



This pair of effects can be handled in one step by using the combined 

 constant, which for a density of 2.0 is 0.06854 mg./ft. P^or 100 feet it 

 amounts to 6.854 or 6.9. It follows that 53.8 plus 6.9 = 60.7, which checks 

 within 0.1 mg. The above may appear elementary, and so it is, but if 

 corrections are not handled properly, incorrect calculations are obviously 

 the result. 



Latitude Correction. — The correction for latitude is introduced to 

 compensate for the change in gravity with latitude. This change is approx- 

 imately a milligal per mile, increasing toward the poles of the earth. 



With the accuracy attained in modern gravimeters, it is necessary that 

 the value for the latitude correction be calculated for a survey area and 

 carefully applied. The amount of the correction can be found by calculat- 

 ing the value of gravity, using suitable formulae such as No. 21. The 

 distance in miles between the northern and southern boundaries is figured 

 and divided into the difference in gravity, as determined by calculation at 

 the north and the south extremes of the survey area. 



For example, a certain survey in Russell County, Kansas, covered terri- 

 tory lying between latitude 38° 45' and 39° 00'. The change was 1.3 

 milligals per mile. As applied, the latitude correction is handled in the 

 same manner as the north-south correction in magnetic field work, increas- 

 ing toward the north. There is no east-west correction in gravimeter work, 

 in which respect it is more simple than the torsion balance method. 



For a station a given distance north of a survey base, the gravity value 

 is the per mile rate times the number of miles. Stations to the south of a 

 base have a lower value of gravity due to this latitude change. 



Topographic Correction. — The effect of minor topographic irregular- 

 ities on the relative gravity is small and may often be neglected. In areas 

 of rugged relief, however, certain corrections pertaining to topographic 

 irregularities are usually made. Hammerf gives a comprehensive study of 

 the method of terrain corrections for gravimeter surveys. 



With the high accuracy of gravimeters and their increasing use in more 

 rugged areas, topographic corrections are becoming more important. 

 Hubbert| has recently published an article on gravitational terrain effects 

 of two-dimensional features, presenting typical cases. For example, the 

 effect of a slope of a certain degree or of a cliff of a certain height on a 

 station at a given distance from the feature is shown. Hubbert's data can 

 be readily applied in the field to determine how far a gravimeter station 



t S. Hammer, "Terrain Corrections for Gravimeter Stations," Geophysics, Vol. IV, No. 3, 

 July 1939. 



t M. K. Hubbert, "Gravitational Terrain Effects of Two-Dimensional Topographic Features," 

 Geophysics, Vol. XIII, No. 2, April 1948. 



