34 EXPLORATION GEOPHYSICS 



Frequently some certain type or types of oil-bearing structure are char- 

 acteristic of a particular province or of particular districts within a 

 province. Even more frequently, the stratigraphic sequence and accom- 

 panying lithology within a province are characteristic of that province and 

 serve to distinguish it from other provinces. 



The topography and vegetation of a province have some influence upon 

 the choice of a geophysical method. Because of the wide geographical dis- 

 tribution of the petroliferous provinces, there is usually a corresponding 

 diversity in terrain conditions. In the Permian basin, high, flat, treeless 

 plains are characteristic of practically the entire province. In the Gulf 

 Coastal province, low marshy ground is characteristic of a large part of 

 the area. In certain parts of California, and in many other regions, the 

 rural terrain is characterized by high topographic relief and semi-desert 

 conditions. 



Some areas are too thickly populated or too intensely cultivated to per- 

 mit the use of certain geophysical methods. In the selection of the geo- 

 physical methods that will be most appropriate and efficient for any one 

 area, one must consider the theoretical applicability of the methods as 

 related to various structural, stratigraphic, topographic, and cultural 

 factors. 



Extensive application and the comparative successes of the various 

 geophysical methods throughout the world during the past 10 to 20 years 

 provide useful criteria in the selection of methods best adapted to the 

 geological conditions and to the specific problems under consideration. 



The usual purpose of reconnaissance by means of magnetic or gravita- 

 tional methods is to locate any prominent anomalies in the buried surface 

 of the basement rocks and to detect by this means the possible occurrence 

 of favorable structures in the overlying sedimentary rocks. Suggestive 

 anomalies are subsequently detailed by methods more adapted to obtaining 

 precise data as to the actual structure in the sedimentary section. Recon- 

 naissance may be conducted for the purpose of determining directly the 

 major structural character of the sedimentary rocks. Success in such 

 reconnaissance by use of magnetic or gravitational methods requires the 

 presence of appreciable stratigraphic variations in the magnetic or density 

 properties of the sediments, and the absence of lateral variations which 

 might be interpreted erroneously as evidence of subsurface structure. A 

 favorable condition would be the presence in the subsurface section of a 

 formation of abnormally high magnetic permeability or density, in which 

 case structural highs are located by corresponding magnetic or gravitational 

 anomalies. Ordinarily, no depth measurements are obtainable from these 

 methods, and the proper solution of the problem may justify the additional 

 cost of other methods supplying quantitative data. 



Gravitational and magnetic methods are not adapted to all conditions, 

 even as reconnaissance techniques. For example, the magnetic method 

 clearly cannot be used to map the structure of deep beds in an area where 



