Chap. 11] 



GEOPHYSICAL WEf-L TESTING 



863 



in areas of simple stratigraphic and structural conditions. If a detector 

 is lowered to the bottom of a well, and if shots are fired at three or four 

 equidistant pomts (with reference to the top of the well), the travel times 

 are equal, provided the bottom of the well is located exactly below the 

 top. If the travel times differ, the corresponding hole deviation may be 

 calculated from the geometric relations involved.'** The accuracy of this 

 method is not comparable to that of standard well surveying procedures. 

 In addition, its application is limited to areas in which the velocity distri- 

 bution around a well is absolutely uniform. 



fy/«5^V-''^''-i^w*ww^S'*w'»*« 



0.4 sac. 



7/me 



zow 



2500 m-set'' 



matt/ 



Fig. 11-32. Travel times (heavy curve) to various well depths from a point 500 

 meters off well top, with calculated velocity distribution (light curve) (after Dix). 



IV. MISCELLANEOUS MEASUREMENTS IN WELLS 

 A. Determination of Radioactivity 



Measurements of radioactivity of rock samples taken from wells, mine 

 workings, and tunnels have been made (since about 1905) by various 

 investigators; the literature has been compiled by Ambronn.** In most 

 instances the a radiation was examined, and radioactivity and type of 

 formation could be correlated. Continuous measurements in deep wells 

 by the wire-activation method were suggested by B. Ostermeier.*^ 



Interest in radioactive well examination was recently revived, inasmuch 

 as the penetrating 7 radiation is about the only rock property permitting 



"D. C. Barton, A.I.M.E. Geophys. Pros., 587 (1929). 

 " Elements of Geophysics, pp. 125-126. 

 "Zeit. Tech. Phys., 7, 196-198 (1926). 



