Subsurface Methods as Applied in Geophysics 1073 



Regional Gradient 



It is an extremely rare occurrence to find a gravity anomaly not 

 influenced by nearby local structure or by density contrasts within the 

 basement itself. Frequently density variations within the basement act 

 to mask gravity anomalies due to local structure. As was mentioned in the 

 section on magnetic interpretation, the experienced interpreter is able to 

 recognize the small variations in regional gradient that betray the presence 

 of local structure. Before quantitative work may be done on the anomaly, 

 all disturbing effects must be removed. The accuracy of the final inter- 

 pretation depends upon the degree to which the interference is removed. 

 A number of methods are in use to prepare the "residual" map,^^ ^^ but 

 none gives a guarantee of removing the disturbing or regional effects and 

 nothing more. 



The work of Henderson and Zietz ^^ on the second derivatives of 

 magnetic maps would also be applicable to gravity maps. A definite 

 relationship may exist between gravity-residual maps and "second ver- 

 tical derivative" maps. The location of the maxima and minima as well 

 as the zero contours may appear in exactly the same positions on both 

 maps, the only variation being in the magnitude of the anomalies. 



Torsion-Balance Data 



It is sometimes suggested that, as the torsion balance gives data of 

 higher order (derivatives of gravity), it will render unique solutions to 

 gravity problems. Skeels ^'^ has shown that all derivatives of potential 

 are dependent upon one another and that additional data such as gradients 

 and curvatures act only to supplement the gravity picture. It is theoretic- 

 ally possible, in fact, to calculate all of the torsion-balance quantities 

 from a gravity-meter map. Skeels and Watson ^^ have calculated the 

 gravity-curvature quantities from gravity data and compared it with ob- 

 served curvatures obtained with the torsion balance for several geologic 

 features. They have also demonstrated that it is possible to calculate the 

 "deflection of the vertical" from conventional gravity maps. 



The use of the torsion balance is indicated in areas where sufficient 

 coverage with the gravity meter is not obtainable. During the war, for 

 example, only main thoroughfares across military reservations were kept 

 open to civilian traffic, and torsion-balance stations made along these 

 roads improved the gravity picture through the restricted areas. 



Gravity operators today frequently find large areas in which they 

 are refused permission to conduct surveys. In many instances, company 

 files contain torsion-balance maps made in these same areas ten or fifteen 



** Nettleton, L. h,. Geophysical Prospecting for Oil, p. 22, New York, McGraw-Hill Book Co. 

 Inc., 1940. 



^Griffin, W. R., Residual Gravity in Theory and Practice: Geophysics, vol. 14, no. 1, pp. 39-56, 

 Jan. 1949. 



^® Henderson, Roland G. and Zietz, Isidore, The Computation of Second Derivatives of Geomagnetic 

 Fields: Geophysics, vol. 14, no. 4, p. 516, Oct. 1949. 



^' Skeels, D. C, Ambiguity in Gravity Interpretation: Geophysics, vol. 12, no. 1, p. 52, Jan. 1947. 



^' Skeels, D. C, and Watson, R. J., Derivation of Magnetic and Gravitational Quantities by Surface 

 Integration: Geophysics, vol. 14, no. 2, pp. 140-147, Apr. 1949. 



