GEOLOGIC AND ECONOMIC BACKGROUND 21 



Natural Field of Force Artificial Field of Force 



Geothermal Electrical (inductive) 



Magnetic Electrical (conductive) 



Gravitational Electrical (magnetic) 



Geochemical : surface soil or gas Seismic (refraction) 



analysis Seismic (reflection) 



Radioactive Slim-hole drilling, with electrical 



Electrical self-potential logging 



Slim-hole drilling with mud 



logging 

 Core-hole drilling, with paleon- 



tological studies and core 



orientation 



for sedimentary structural investigations, achieve a high resolving power 

 when applied to the location of highly magnetic subsurface bodies under 

 shallow depth of cover. 



Generally, the resolving power of the m.ethods utilizing natural fields 

 of force is less than that of methods in which the force field is artificially 

 created and subject to control in direction, intensity, and effective depth 

 range. It is characteristic of the group of methods utilizing a natural field 

 that the field of force measured at the surface is the resultant or summation 

 effect of near-surface irregularities and regional variations, as well as the 

 efifect of local geological structure concerning which information is desired. 

 The practical difficulty of separating this composite efifect into its component 

 parts is a serious disadvantage of these methods. The methods utilizing 

 artificial fields may also be subject to several component effects. In the case 

 of the latter methods, however, the extraneous efifects usually may be 

 evaluated or minimized. This is done by properly selecting the points of 

 application of the force fields. An important feature is the ability to control 

 the effective depth range of the artificially-created force fields. 



The technique of slim-hole drilling as now used commonly incorporates 

 a subdivision of the electrical methods and/or the geochemical methods. It 

 dififers from other exploratory methods in that measurements are made in 

 previously drilled deep core holes, rather than on the surface of the ground. 

 Ordinary electrical logging comprises those methods utilizing electrical 

 measurements in wells or holes drilled primarily for discovery and produc- 

 tion of oil. Slim-hole drilling, on the other hand, employs the technique of 

 making electrical measurements in small-diameter, low-cost holes drilled 

 expressly for this purpose. In the electrical logging and slim-hole drilling 

 techniques, the electrical measurements are ordinarily conducted after com- 

 pletion of drilling. The more recent techniques, however, obtain electrical 

 and mud-logging data during the process of drilling.* Another recently 

 developed method of recording subsurface data is that of constructing 

 drilling-time logs which commonly reveal any important change in lithology 

 by the change in the rate of drilling. 



*See chapter on Drill Hole Investigations. 



