Subsurface Methods as Applied in Geophysics 1051 



will be in evidence. If this condition exists, the assumption of a single 

 magnetic pole is justified. If a rather broad negative anomaly encircles 

 the sharper positive anomaly, assumption of a sphere will probably 

 more closely fit the geologic conditions. If the isoanomalic lines are 

 much longer in one dimension, a horizontal cylinder may be assumed. 

 To determine approximate edge of the disturbing mass one customarily 

 picks the inflection points on either side of the magnetic profile where 

 curvature changes from concave downward to concave upward. In the case 

 of the sphere this will give the approximate diameter and thus allow 

 estimates to be made as to the depth below the surface to the top edge 

 of the mass. 



Depth Rules 



Single pole Depth to pole = 1.305 Zj 



Sphere Depth to center = 2.0 X^ 



Horizontal cylinder Depth to center = 2.05 Zj 



Fault Depth to center = Xc 



X^ is the horizontal distance from the center of the anomaly to the 

 point where the anomaly has fallen to one half its maximum value, and Xo 

 is the distance from the center of the fault picture to the maximum or 

 minimum of the curve. 



The depth rules are subject to many errors, yet under the proper 

 conditions they do give valuable information. More accurate interpreta- 

 tive techniques are available involving the comparison of magnetic effects 

 from spheres, vertical cylinders, horizontal cylinders, and series of disks 

 with the observed magnetic anomaly. For a discussion of these the 

 reader is referred to Nettleton.^^ However, all geophysical methods in- 

 volving one of the natural potential fields of the earth are subject to 

 ambiguous interpretation. That is, a given magnetic anomaly may be due 

 to a variety of geologic conditions. Even though we are able to calculate 

 a certain distribution, depth, and polarization of magnetic material that 

 will exactly satisfy the magnetic anomaly obtained in the field, it is still 

 no guarantee that the geologic structure deduced is the correct one. By 

 using other combinations of distribution, depth, and polarization, it is 

 quite possible that equally good fits could be achieved. This ambiguity 

 means that magnetic surveys should be supplemented by other types of 

 subsurface control such as direct well control or seismograph data. The 

 magnetic method does have the advantage over the gravity method in that 

 most magnetic effects come either from within the basement or at the 

 basement surface. 



Regional Gradient — In many areas large magnetic anomalies cover- 

 ing hundreds of square miles are due primarily to polarization variations 

 within the basement rocks. Generally they are in no way related to local 

 structural conditions. These strong regional gradients act to mask mag- 



'" Nettleton, L. L., Gravity and Magnetic Calculations: Geophysics, vol. 7, no. 3, p. 293, July 1942. 



