GEOLOGIC AND ECONOMIC BACKGROUND 27 



interpretation usually is based on theoretical considerations tempered by 

 experience gained during prior similar studies. 



The interpretation of geophysical data is a highly involved task requiring 

 ingenuity, imagination and adequate training in geology and physics. It is 

 not uncommon for the task to be divided in such a manner that the observed 

 and computed data are first translated into basic geologic terms by a geolo- 

 gist trained in the theories and methods of analyzing geophysical data and 

 thoroughly familiar with the regional and local structural details of the 

 areas under investigation. Experience teaches that it is only by the intelli- 

 gent correlation of geophysical data zvith all available geologic data that the 

 greatest ultimate practical value of a geophysical survey can be realised. 

 At present, relatively few men are qualified by adequate training and expe- 

 rience to handle without assistance the many problems of geophysical 

 interpretations.!. 



GEOPHYSICAL METHODS IN PROSPECTING FOR 

 PETROLEUM 



General Field of Application. — The initial impetus to the development 

 of geophysical exploration methods was provided by their demonstrated 

 usefulness in the search for certain types of ore bodies. The greatest appli- 

 cation, development, and utility of the methods, however, has been in the 

 field of petroleum exploration rather than in mining. 



Until the advent of geophysical methods, the search for and the dis- 

 covery of new deposits of petroleum were accomplished by mapping of 

 surface features, core drilling, and correlation of subsurface data made 

 available from drilled wells. Exploration for possible oil-bearing structure 

 was first extended to those areas in which surface evidence of one sort or 

 another indicated the probability of subsurface oil accumulation. Such 

 favorable surface signs included: (1) direct evidence of petroleum occur- 

 rence such as oil seeps, gas seeps, bituminous rocks or dikes, "paraffin dirt," 

 asphalt pits, oil-impregnated shales, etc. ; (2) exposed anticlinal folds and 

 other structural features favorable for petroleum accumulation at depth; 

 (3) topographic and physiographic evidence of favorable structure not 

 otherwise reflected at the surface of the ground. Extensive search in the 

 known oil-bearing provinces between 1860 and 1920 rapidly depleted the 

 supply of favorable prospects apparent from the use of exploration methods 

 then available. Subsurface information from drilled wells and exploratory 

 core holes became increasingly important after 1915. Although still of 

 indispensable value, these data took a subordinate place in aggressive 

 exploration following the advent of successful geophysical methods in 1922. 

 The need for new methods was pressing at that time, since the demand for 

 large supplies of petroleum was increasing rapidly. 



In many of the known oil-bearing provinces, large areas contiguous to 



t "Round Table on Geophysical Education," A.I.M.E., Geophysical Prospecting, Tech. Pub. 

 No. 950. 



