GRAVITATIONAL METHODS 351 



TABLE 10 



APPROXIMATE METHOD FOR DRAWING GENERAL ISANOMALIC 

 CONTOURS IN RECONNAISSANCE SURVEY 



12 3 4 5 6 7 



Gravity Cf.,tion Distances Average Difference Corrected Accumulated 



Gradients -kt in Gravity in Gravity Difference anomalies in 



in E.U. ■ cm. Gradient Eotvos Units Eotvos Units m. gals. 



30000 — ^^^ = + 10 + 300000 + 260000 



16 2 r,7 + .26 



20000 ^^ = + 6 + 120000 + 100000 



8 3 + .36 



3000U ^-^ + ^Q = + 6 + 180000 + 140000 



12 4 2 + .50 



20000 ^~^ = — 3 — 60000 — 80000 



14 5 2 +.42 



^ 40000 ~^~l^ = — 9 — 360000 — 410000 



7 1 5 — 13 + .01 



+ 180000 + 10000 



After adjusting the magnitudes of the gravitational anomalies at the traverse sta- 

 tions, the remaining stations plotted on the map are tied into the traverse. Various 

 methods are employed. An approximate method for tieing in a station not too distant 

 from the closed traverse already drawn is to connect this station with a neighboring 

 station on the closed traverse by a straight line and to compute an average gradient 

 component as before. 



Finally, the stations of equal gravitational anomaly are connected by con- 

 tinuous curves. (Dotted curves of Figure 202.) The isanomalic contour intervals are 

 usually made from 0.1 to 0.5 of a milligal ; however, smaller or larger intervals are not 

 uncommon. 



An alternative method for converting a gradient map into a contour map is 

 summarized in Figure 203. 



In detailed reconnaissance surveys, one of several elaborate schemes based on an 

 application of the method of least squares to torsion balance data may be used. An 

 illustration of the application of least squares to torsion balance data is given by 

 Roman, t 



Detailed Interpretation 



Detailed surveys require a more precise evaluation of data than recon- 

 naissance surveys. For example, detailed interpretation generally requires 

 the compiling and drawing of additional profiles and maps and the com- 

 parison of the experimental data obtained in the area under investigation 

 with theoretical results obtained by computing the effects produced by va- 

 rious simple geometrical forms and simplified geological structures. More- 

 over, the final interpretation requires the application of quantitative or 

 graphical and short-cut methods, or a combination thereof, in order to 

 combine the data obtained with the torsion balance and the geological 

 possibilities as derived from a detailed study of regional and local geology. 



1 1. Roman, "Least Squares in Practical Geophysics," A.I.M.E. Geophysical Prospecting, 1932 

 p. 460. 



