GEOPHYSICAL METHODS OF INVESTIGATING 245 



right angles to the overburden of thickness h. Here 

 Vi equals 1,000 m. per second, V2 equals 5,000 m. per second, 

 and h equals 200 as will be seen at once. 



The relations for inclined beds are theoretically shown in 

 Figs. 163a to d.^ Sufficient has been said to show the possi- 

 bilities of this method.^ 



This method only requires about 2 hr. for planting, and 

 survey points and measurements need only be taken every 

 300 to 600 ft. down the hole. 



The advantages of the method are : 



1. It determines the location at each depth independent 

 of other positions. 



2. It gives the horizontal and vertical location of any 

 spot desired in the hole. 



3. It will determine the bottom of the borehole without 

 the necessity of having to survey the entire hole. 



The disadvantages are mostly those due to its recent 

 adoption, lack of experience and inexpert manipulation 

 of the apparatus and computations. 



^ Heiland, C, Ztschr.f. Instrm., p. 417, Berlin, 1925, or Eng. Min. Jour.- 

 Press, Vol. 121, No. 2, p. 54, 1926. 



2 Shaw, H., Mining Mag., p. 201, April, 1930. 



