22 



EXPLORATION GEOPHYSICS 



The geochemical method, as subsequently explained, holds promise of 

 being able to detect petroleum accumulations directly, in contrast to the 

 usual use of geophysics merely as a means of determining structures favor- 

 able for oil accumulation. For this reason, the geochemical method, al- 

 though possibly of minor importance in its present stage of development, 

 is potentially a method of higher detectability for oil deposits than other 

 surface methods now in use. 



The following classification shows the fundamental properties and rela- 

 tionships which form the basis for the various methods : 



Method Field of Force 



Magnetic Earth's magnetic field 



Gravitational Earth's gravitational field 



Natural potential field 



Electrical J Artificially created electric, 



electromagnetic, or mag- 

 netic field 



Seismic Artificially created seismic 



waves 



Radioactive Radioactive radiation 



Geothermal Earth temperature gradients 



Emanation of hydrocarbon 



Geochemical ^ sapors 



Ascending or descending 

 aqueous solutions. 



Physical or Chemical Property 



Magnetic permeability 



Density 



Spontaneous polarization 



Electrical conductivity or its re- 

 ciprocal resistivity; path of cur- 

 rent flow 



The velocity of transmission 

 and/or the frequency charac- 

 teristics of seismic waves, as af- 

 fected by density, elasticity, etc. 



Radioactivity, i.e., the emission 

 of electrically charged particles 

 from the nuclei of the atoms of 

 radioactive materials 



Thermal conductivity 



Hydrocarbon content of the 

 earth 



Mineral content of the earth 



The fundamental principles and general procedure are essentially the 

 same for all of the geophysical methods. All depend for their modus 

 operandi upon the following basic facts. (1) The subsurface normally is 

 composed of rock formations possessing certain differing physical prop- 

 erties. (2) Natural or artificially-created force fields within the subsurface 

 are affected to different degrees by these differing physical properties. 



(3) The extent to which certain force fields are affected depends, among 

 other factors, on the magnitude of the particular physical properties and 

 on the sizes, masses, and arrangements of the subsurface rock materials. 



(4) The effects produced by the subsurface variations in physical properties 

 upon certain force fields can be measured at the surface of the ground.** 



(5) The data obtained can be interpreted or translated into terms of prob- 



** An exception is noted in electrical logging and slim-hole drilling wherein the 

 measurements are made below the surface. 



