Ch. 4] EXPLORATION GEOPHYSICS 81 



afford a convenient datum to which the resistivity measurements may 

 be referred. By using the conventional four-electrode method, values 

 of formation resistivity may be plotted along parallel lines of explora- 

 tion to show the subsurface attitude of the conducting horizon. Modi- 

 fications of this simple geologic setting reveal complicated pictures of 

 fine detail. The presence of a layer of high conductivity near the 

 surface will effectively mask more deeply buried features. 



A modification of the electrical method is found in electric well 

 surveys. Here direct measurements of formation resistivity and elec- 

 tric self-potential are taken from electrodes lowered into a well. A 

 log of the variations in these two quantities is of great value in the 

 development of petroleum production. It shows the presence of oil 

 or gas, fresh or salt water, and striking variations in the properties 

 of different types of formations, sandstones, limestones, and shales. 

 The electric logging method has great promise in near-surface appli- 

 cations, as yet relatively undeveloped. The instrumentation is simple ; 

 its operation involves no great cost. Problems to which the method 

 might be applied are the location of water supply, determination of 

 bedrock beneath the surface and overburden, the location of faults, 

 dikes, and other structural features related to construction engineer- 

 ing problems. 



The measurement of the radioactivity of sedimentary beds in place 

 offers striking correlation with electric measurements on the same for- 

 mations. The reasons for these correlations reside in the particle size 

 distribution of sediments and sedimentary rocks. In general, the 

 fine-grained rocks, shales, and clays show the greatest radioactivity, 

 whereas rocks comprised of coarser fragments, such as sandstones 

 and conglomerates, show lower activity. Induced radioactivity em- 

 ploying neutron radiation is also of value in identifying sedimentary 

 sequences. Both methods have undeveloped potentialities in the same 

 series of problems outlined for electric applications. 



The search for minerals and ore deposits by geophysical means in 

 the United States has not greatly advanced the science in recent years. 

 Within the mining industry there seems to exist an attitude which is 

 difficult to explain. In the past the merits of any geophysical survey 

 have been largely judged by the amount of ore the survey produced. 

 Surveys failing to produce ore have been condemned without critical 

 examination, and there has been no opportunity to profit from these 

 failures. The mining industry has not operated under the philosophy 

 which the petroleum industry has found so successful, that is, to de- 

 velop geophysics on as broad a base as possible. The result is that full 

 development of mining geophysics in the United States has not taken 



