Chap. 2] METHODS OF GEOPHYSICAL EXPLORATION 35 



Seismic measurements. Seismic measurements in wells are made for 

 two purposes : (a) to supplement surface refraction data, and (6) to obtain 

 average velocity data for reflection shooting. In both applications a 

 vibration detector is lowered into the well to the depth desired and shots 

 are fired at the surface. In wells drilled into or near the edge of salt 

 domes, it is thus possible to obtain valuable information in regard to 

 presence of overhang and the behavior of strata along the flanks. In 

 order to make absolute depth determinations in reflection shooting, it is 

 desirable to know the velocity of seismic waves at right angles to the 

 bedding, which is likewise determined by firing several shots at or near 

 the well at the surface, with a detector lowered to various depths. 



Miscellaneous measurements. The radioactivity of strata traversed 

 by the well may be determined by lowering a small ionization chamber 

 to the desired depth. Magnetic well investigations are generally confined 

 to determinations of core orientation. Acoustic measurements are made 

 in deep wells for the location of gas and water flows. Gas detectors, 

 when used in connection with shallow holes, permit of locating leaks in 

 gas mains. 



VII. MISCELLANEOUS GEOPHYSICAL METHODS 



Radioactivity methods. Application of radioactivity methods rests on 

 the fact that zones of shattering in the earth's crust, such as faults and 

 fissures, will allow passage and accumulation of radioactive disintegration 

 products. Owing to the limited penetration power of radioactive gases 

 and radiations, the depth range of these methods is limited. It does not 

 appear to be necessary for the channels of radioactive products to remain 

 open. If they have become mineralized, it is nevertheless possible to 

 locate them by increased activity; hence, there is a possibility of locating 

 sulfide and other veins by measurement of radioactivity. Some shallow 

 oil deposits emit radioactive radiations because oil is an absorbent of 

 radon. It goes without saying that radium ore, such as pitchblende 

 and the other uranium compounds, may be located by radioactivity 

 measurements. 



Radioactivity methods fall into two groups: in the first the relative 

 content of radon in the surface soil is determined; in the second the 

 penetrating radiation of radioactive disintegration products is measured. 



Hydrocarbon (soil and gas) analysis. Oil seepages and gas emanations 

 have served for decades as indications of oil deposits. These macroscopic 

 examination methods have been supplemented recently by microscopic 

 procedures for determining small amounts of hydrocarbon in the soil air 

 and in the soil itself. The air from shallow holes is pumped into a gas 



