Subsurface Methods as Applied in Geophysics 1053 



The location of the maxima and minima as well as the zero contours may 

 appear in exactly the same positions on both residual and derivative 

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



Shift of Axis — Magnetic anomalies associated with local structure 

 generally originate in the uplifted basement rock under the structure. The 

 center of the magnetic anomaly is often shifted laterally (to the south in 

 the northern hemisphere) from the center of the structural disturbance, 

 owing to the inclination of the earth's field. The amount of shift depends 

 upon a number of factors including the depth to the basement rocks, 

 the angle of inclination of the earth's field, shape, and size. This effect 

 is illustrated in figure 552. 



Horizontal and Total Field Interpretation — It is sometimes suggested 

 in the literature that the combination of both vertical and horizontal mag- 

 netic data will afford the magnetic method the property of depth control. 

 Depth control is also claimed for the magnetic method when the observa- 

 tions are taken at different elevations as with the air-borne magneto- 

 meter. Skeels writes, ^^ 



Since a corollary to Green's theorem holds for any potential function it 

 also holds for magnetics; the magnetic potential and all of its derivatives are 

 completely defined by the vertical intensity map. If two magnetic distribu- 

 tions can be found which satisfy the same set of vertical intensities and they 

 always can be found, they will also satisfy the same set of horizontal intensity 

 data. So combining horizontal with vertical intensity data does not give us 

 a unique solution, as has been implied in some of the literature. Nor does 

 observing magnetic effects at two or more levels resolve the ambiguity, for 

 by means of the theorem it can be shown that if two magnetic distributions 

 produce identical effects on one level they will also produce identical effects 

 on all higher levels. Magnetic observations in a mine are severely restricted 

 in areal extent; in such a case the addition of horizontal data to vertical data 

 would greatly improve the interpretation. The more geological data the inter- 

 preter can bring to bear upon the problem, the closer will he be able to limit 

 the range of possibilities, and the less ambiguous his interpretation will be. 



Henderson and Zietz ^^ have taken air-borne magnetic data at several 

 levels for several regions in the country and demonstrated that it is poss- 

 ible to compute from the first-level data all essential information ob- 

 tained at higher levels. They suggest that, if information is desired at 

 several elevations, it would be more economical to compute the data for 

 the higher elevations than to take it from a plane. Skeels and Watson ^^ 

 have calculated the horizontal components from vertical magnetic data 

 over several magnetic features and have shown that the computed values 

 agreed very well with the observed horizontal data. 



"Skeels, D. C, Ambiguity in Gravity Interpretation: Geophysics, vol. 12, no. 1, p. 55, Jan. 1947 

 " Henderson, Roland, and Zietz, Isidore, The Upward Continuation of Anomalies in Total Magnetic 



Intensity Fields: Geophysics, vol. 14, no. 4, p. 534, Oct. 1949. 



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



Integration: Geophysics, vol. 14, no. 2, pp. 133-150, Apr. 1949. 



