The attempts of geological and mining engineers to decipher the structure of the outermost formations of the earth's crust have led to many comprehensive studies, which now stand as monuments to their efforts. Their studies, in general, are confined almost exclusively to systematic collection, description, and analysis of facts which they were able to obtain by immediate observation at accessible points on and near the surface of the earth. These observations have many times been supported by other researches carried on in chemical, physical, and petrographic laboratories, but many times the complete solution of the problem has been obtained by inference, this often means that only those facts which could be seen were considered a safe basis for the inferred technical conclusions reached by a process of deduction.
A disadvantage of this direct method of investigation lies in the fact that such observations can be made only at certain points where nature has provided especially favorable conditions. The seriousness of this disadvantage has stimulated attempts in the past to develop methods which do not demand direct contact with the subsurface material for the investigating of geological structure, mineral content, and the nature of subsurface formations. Only recently have these methods been developed into usable forms. They are based upon those properties of the minerals, associated minerals, rocks, or structures which produce effects observable at a distance.
Therefore, in order that these methods of geological investigation succeed, it is necessary to attach to them all the resources of modern physics or geophysics, which may lead to the determination of the range of practical application of the known physical properties of matter connected with distant effects. This requires a constant improvement of methods and continuous new design of apparatus to keep abreast with new knowledge and ever increasing difficulties in the field as the more simple relations are learned, and the more complex relations left for study.
