30 EXPLORATION GEOPHYSICS 



relationship between structure and variations in gravity is commonly more 

 definite than similar relationships between structure and the earth's mag- 

 netic field. For example, salt dome structures are usually associated with 

 gravity minima, while ordinary domal or anticlinal structures are more 

 often associated with gravity maxima. (The gravity maxima existing over 

 certain shallow salt domes such as the Spindletop dome, the Nash dome, 

 and a few others, constitute an exception to this rule.) 



Use of gravitational methods for reconnaissance was formerly restricted 

 by their limited applicability in areas outside the salt dome province and 

 by the relatively high cost of torsion balance work. However, recent devel- 

 opment and improvement of the gravimeter have resulted in wider use of 

 this method both for preliminary reconnaissance and for detailed work. 



Electrical Methods 



The applicability of electrical methods in oil structure mapping is gen- 

 erally conditioned by limitations as to depth and their adaptation to local 

 conditions, and by appreciable cost for moderate depth of exploration. 

 Certain of the electrical techniques permit controlled variation of the 

 current field, configuration and spacing of electrodes, etc., allowing a depth 

 control which is not enjoyed by either magnetic or gravitational methods. 

 Electrical methods may be successfully adapted to the requirements of both 

 reconnaissance and detailed work in areas where the character of sub- 

 surface rocks is favorable for their operation. For reconnaissance work, 

 the cost of electrical profiling is generally less than that of any other 

 method, with the exception of the magnetic methods and the improved 

 gravimeter methods. Reconnaissance electrical profiling consists in measur- 

 ing lateral variations in resistivity, generally at a constant electrode spacing 

 or depth, in order to obtain an indication of the location of hidden struc- 

 tural features. In detailed work a depth variable is obtained by varying 

 the electrode spacing, which permits the mapping of underground structure 

 by determining the depth and configuration of some electrically prominent 

 member of the stratigraphic section. 



Electrical methods employing depth control are generally comparable 

 to the refraction seismic method in resolving power, but are inferior to 

 the reflection seismic method. Although the cost of continuous electrical 

 profiling to depths of about 4,000 feet is usually less than that for seismo- 

 graph work, the low resolving power of electrical results restricts its 

 adaptation in oil exploration. 



The electrical methods commonly have distinct advantages in the map- 

 ping of near-surface features. In many cases, they can be used as a direct 

 means of locating faults, principally because fault zones in general have a 

 relatively high electrical conductivity. 



Electrical logging of oil wells has assumed considerable importance in 

 correlation work. This method measures the vertical variation in the elec- 

 trical characteristics of the geologic section traversed by the drill hole. The 



