26 EXPLORATION GEOPHYSICS 



computed results into a graphic three-dimensional picture of the subsurface. 

 Graphs, cross sections, contour maps, and vector diagrams are the usual 

 means of illustrating these final results. 



In field operation, it is desirable that the computation and interpretation 

 of results proceed almost contemporaneously with the securing and record- 

 ing of field data. Preliminary interpretations are used to guide the survey 

 as it progresses and to expedite land acquisition and/or the performance of 

 lease requirements, prior to completion of the survey. 



The complete results of the geophysical survey should be contained in a 

 final report, giving the location of the area, its geological setting, the rela- 

 tionship of the geology to the geophysical measurements, and the economic 

 purpose of the survey.* The field w^ork conducted should be covered in an 

 appropriate degree of detail, depending on the function of the report. If it 

 is to be presented to laymen who are not familiar with the fundamental 

 principles of exploration, it is often best to discuss briefly the limitations of 

 the work. On the other hand, if the report is being made to a group of 

 engineers who have had experience with the type of measurements em- 

 ployed, much elementary detail may be omitted, and the report confined 

 almost entirely to a discussion of the measurements made and the results 

 obtained. Such a report is often the work of several men who may be 

 conversant with special phases of the survey. Final and complete maps are 

 included, together with an interpretation of the geophysical findings. 



If the survey has been successful in gathering the desired information, 

 recommendations usually will take one of three forms: (1) test drilling or 

 mining, (2) abandonment of the venture or (3) holding the property for 

 later development. The decision to abandon an enterprise is fully as impor- 

 tant and oftentimes more important with regard to conservation of capital, 

 than the decision to mine or drill. The history of mining and oil exploration 

 clearly indicates that the greatest single factor causing loss of capital has 

 been the common human trait of "not knowing when to stop." 



The interpretation of the geophysical data is the most important step in 

 the general exploration program. The technique of interpretation covers a 

 wide range of procedures, from that relying predominantly on basic theory 

 and rigid mathematical analysis, to that relying predominantly on empirical 

 relationships. The latter are built up through extensive experience in actual 

 field problems and are often supplemented by empirical data from small 

 scale laboratory experiments. In most cases, successful interpretation in- 

 cludes both a mathematical and an empirical treatment of the field data, the 

 relative predominance of one over the other usually being a function of the 

 amount of diagnostic geological information available and the relative com- 

 plexity of the subsurface structure. Pure mathematical analysis alone is 

 seldom adequate, principally because of the many complex variations in the 

 structure, configuration and composition of underground rocks. Successful 



* The importance of final reports cannot be overemphasized. Professional men have 

 found the ability to write clear and accurate reports a most important success factor. 



