Subsurface Methods as Applied in Geophysics 1071 



is available in the form of seismic maps, well logs, and density informa- 

 tion from well cores. 



When used judiciously, depth rules for certain simple geometric 

 shapes may prove of value to estimate the depth to the surface of the 

 density contrast causing the anomaly. The rules should be used with 

 the mental reservation that ambiguity is inherent in interpretation of grav- 

 ity data in terms of structure. They are similar to those mentioned in the 

 section on magnetic technique, in that the distance from the center of the 

 anomaly to the point at which the anomaly has fallen to one-half of its 

 maximum value is multiplied by a factor to give the depth. Where the lines 

 of equal gravity or isogals (corrected for regional gradient) are roughly 

 circular, we may assume either a sphere or a vertical cylinder, depending 

 upon which shape is more probable geologically. For buried ridges and 

 anticlines the horizontal cylinder may be selected. As before, the inflection 

 points on either side of the gravity profile may be selected to give an idea 

 of the size of the disturbing body. 



Depth Rules 



Vertical cylinder depth to top = 0.56 X^ 



Sphere depth to center = 1.30 Xj 



Horizontal cylinder depth to center = 1.00 X^ 



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 

 Xc is the distance from the fault trace to the point at which the gravity 

 has fallen to one half the total change from the fault trace to infinite 

 distance. The depth rules above may be determined from curves presented 

 by Nettleton.^^ Interpretative techniques based on spheres, vertical cylin- 

 ders, horizontal cylinders, and series of disks are included in the above- 

 cited paper by Nettleton. 



Although gravity data alone will not permit a unique solution, they 

 do limit the depth to the density interface causing the anomaly. It is 

 obvious that an anomaly of small areal extent must be caused by density 

 contrasts close to the surface. Thus we can place the maximum depth from 

 which a given anomaly could arise. It is also possible by mathematical 

 means to determine the shallowest depth from which a structure could 

 cause a given anomaly for a given density contrast. Between these two 

 limits it is impossible to predict the depth, size, or shape of the mass 

 causing the anomaly without the benefit of additional data. Quantitative 

 interpretation becomes even more involved when consideration is given 

 the several surfaces of density contrast that may be present in the geologic 

 column. 



Reliable quantitative interpretation of gravity information requires 



^^ Nettleton, L. L., Gravity and Magnetic Calculations: Geophysics, vol. 7, no. 3, pp. 296-300, July 

 1942. 



