GEOLOGIC AND ECONOMIC BACKGROUND 29 



oil-bearing structures in the overlying sedimentary rocks. A noteworthy 

 example is the Hobbs oil field of New Mexico, which occurs on a structure 

 overlying a pronounced uplift in the basement rocks. The location of the 

 discovery well of this field was based on the results of a magnetic survey. 

 (See Figure 106.) 



As an independent exploration technique, magnetic methods have sev- 

 eral serious disadvantages, chief of which are (1) lack of depth control, 

 (2) difficulty in separating the unrelated magnetic components due to near- 

 surface materials from those which reflect the deeper structure, (3) diffi- 

 culty in distinguishing between anomalies due to structural variation and 

 those due to lithologic and mineralogic variations, (4) difficulty in relating 

 the areal location of a magnetic anomaly to a definite location of the struc- 

 ture causing the anomaly. The two greatest deterrents to successful inter- 

 pretation of magnetic data are : (a) the difficulty of specifying whether 

 magnetic anomalies are due to structure or to depositional variations, and 

 (b) the fact that even when the magnetic anomalies are structural in origin, 

 there may be no definite relationship between the type of anomaly and the 

 structure, e.g., subsurface structural "highs" may, in different geologic 

 settings, be associated with either magnetic "highs" or magnetic "lows." 

 These unfavorable factors arise in the use of magnetic methods in search- 

 ing for favorable structure in sedimentary rocks, but are not present in 

 many local problems of specific nature, such as the location of igneous 

 dikes. 



Compensating features in the use of magnetic methods are their rela- 

 tively high speed and low cost. These are sufficiently pronounced to indi- 

 cate a definite field of usefulness for magnetic reconnaissance, preliminary 

 to further detailed studies by other methods. This is particularly true in 

 areas where information is available on the controlling magnetic properties 

 of the subsurface. 



Gravitational Methods 



In fundamental theory and in practical limitations the gravitational or 

 gravimetric methods are quite similar to the magnetic methods. In general, 

 however, the gravitational methods have been used more extensively and 

 with greater success in mapping the relative topographic relief of 

 the comparatively dense basement rocks. They are thus oftentimes 

 valuable in locating associated structure in the overlying sedimentary for- 

 mations, as well as in finding major structural features such as faults, 

 contacts, etc., in the overlying sediments. The outstanding economic use 

 of gravitational methods in the United States has been the locating of the 

 deep, lower-density salt plugs which form the cores of the numerous salt 

 domes in the Gulf Coast region. 



Precisely as with magnetic methods, the chief deterrent to more gen- 

 eral application of gravimetric methods is the difficulty of interpretation, 

 particularly in the resolution of differential depth effects. However, the 



