Subsurface Methods as Applied in Geophysics 1069 



a thermos bottle for thermal control and weighing only a few pounds 

 (fig. 561). 



Gravity Interpretation 



The results of gravity surveys are generally shown in the form of 

 a contour map. The values plotted on the map are corrected for latitude, 

 elevation, and Bouguer effects, and for terrain and tidal effects where 

 necessary. This final map then represents the surface distribution of 

 gravity due only to density variations in the rocks beneath the surface. 



The interpretation of gravity results, like magnetic results, is usually 

 qualitative. Knowledge of the geology is even more important in the 

 interpretation of gravity data, as anomalies may be caused by density 

 variations at any point within the geologic section, including the base- 

 ment. By a careful analysis of the stratigraphic column, zones that are 

 capable of producing gravity anomalies may be noted, reducing to a 

 large extent the ambiguity of gravity methods. For example, in the 

 Permian Basin of west Texas and New Mexico the gravity anomalies 

 associated with local uplift are due in large part to the density contrasts 

 between the relatively light sands, shales, and salt of the Dockum and 

 Ochoa groups against the dense limes and dolomites of the Guadalupe 

 and lower groups. Magnetic anomalies associated with structure in the 

 same area are caused by actual uplift of the magnetic basement complex. 

 It is possible in this area, therefore, to have gravity anomalies exist with 

 or without corresponding magnetic anomalies. (See figs. 562, 563.) 



Interpretation is always aided by a knowledge of the gravity efi"ects 

 over known structural features in the same region. It is also possible 

 to calculate the magnitude and surface distribution of the gravity effects 

 to be expected for simple geometric forms and to compare these results 

 with the observed data. Another interpretation technique requires the 

 calculation of the gravity effects over known structures or fields by the 

 use of the gravity integrator. A knowledge of the density contrasts is 

 particularly desirable in such quantitative computations. 



Gravity surveys indicate large regional geologic trends as well as 

 outline areas of local structural disturbance. The areal extent and 

 magnitude of gravity anomalies depend upon a number of factors, among 

 which are the depth, size and shape, and the density contrasts involved. 

 The sharpness of gravity anomalies is related to the density contrast and 

 the depth at which such contrasts occur. Comparatively sharp anomalies 

 originate at shallow depths, whereas broad anomalies may arise from great 

 depths. Observe that broad anomalies do not necessarily originate at 

 great depths. It is quite possible to obtain a broad anomaly of large areal 

 extent from density contrasts occurring quite near the surface. In fact, 

 from any gravity profile it is possible to calculate almost an infinite 

 variety of depths, shapes, and density contrasts that would satisfy the 

 observed data. It is therefore apparent that unique solutions in gravity 

 interpretation are possible only when a great amount of additional data 



