GRAVITATIONAL METHODS 415 



fore less). The 0.33 enters the sum of the corrections indicated in column 

 15 as total adjustment, and is transferred to the computation sheet. It 

 is there applied, or added, as a correction and in this way the value at 

 station 2 has heen increased by the necessary 0.33 milh'gals. The latitude 

 correction for station 5, which is a Hke distance north of the base, shows a 

 —0.33 in column 12. 



If a regional correction is used, it can be set down in column 14. It is 

 added or subtracted, as the case may be, and goes in as a part of the total 

 adjustment of the last column. 



The calculation of gravity meter notes is not difficult, but checking is 

 necessary, and errors of multiplication, addition and subtraction must be 

 guarded against, as in any operation involving numerous figures. Slide 

 rule accuracy is sufficient for certain steps, which saves considerable time. 

 Tables for certain operations can be constructed, such as for latitude cor- 

 rection or the conversion from scale points to milligals. 



Interpretation of Relative Gravity Data (Contours and Profiles.) — 



The values of the relative gravity, after the necessary corrections, are 

 plotted as isanomalic contours or as profiles. The isanomalic contours 

 plotted in direct gravity work are lines of equal value of gravity anomaly 

 referred to some point in the area, usually the main base station, where 

 the anomaly is assigned some arbitrary value. The contour interval gen- 

 erally used is one-half to two milligals.* 



Corrections for Regional Gravity Effects 



The gravity observation at a station is the measurement of a summa- 

 tion efifect — i.e., the resultant of all gravitational attractions or effects within 

 the range of the gravity instrument. The factors which contribute to this 

 summation effect are both physical and geological, and include (1) the 

 station elevation, (2) the local terrain, (3) the more distant topography, 

 (4) the normal value of the earth's gravitational field, (5) the density and 

 distribution of the near-surface materials around the station, and (6) the 

 deeper geological structure, both local and regional. 



The procedure followed in calculating quantitatively the gravity effect 

 of several of these components has been given in preceding sections. When 

 the contribution of a particular component can be calculated, it may be 

 applied as a correction to the station gravity value and thus removed as 

 one of the variables. Such correction simplifies interpretation. Terrain 

 effects are handled in this manner. As another example, the Bouguer 

 gravity anomaly values at a set of field stations are more interpretable 

 than the raw or uncorrected gravity values. The Bouguer values have 

 been reduced to datum, which considers not only the elevation and the 

 Bouguer efifect, but also the earth's normal gravity field, and if necessary 

 the local terrain and the more distant topography. This is the usual calcu- 



*Illustrations of isanomalic contours are shown in the section entitled "Petroleum 

 Surveys." 



