SEISMIC METHODS 



779 



Hence, it is possible to determine the paths fde and kf^; that is, it is 

 possible to determine the position of the point fa on the flank of the dome. 

 The positions of the points f^, fs, etc., may be determined in a similar way. 

 Thus, the method is capable of profiling the boundary surface of the sub- 

 surface geological structure. 



W-£3 



Fig. 474. — Schematic drawing illustrating a method for profiling 

 the boundary surface of a subsurface geological structure. (After 

 McCollum, U. S. Patent 1,923,107.) 



Another method for determining the boundary of a salt dome, based 

 upon first arrival time measurements of seismic waves from a series of 

 shot-points, is given by Gardner.f The shot-points are distributed around 

 the dome and received by a well detector within a deep hole flanking the 

 dome. 



INSTRUMENTS 



Seismometers 



Seismometers are also known as geophones, receptors, seisphones, jugs 

 and pots. They may be classified in several ways, according to (1) the 

 particular ground motion to which their outputs are more or less propor- 

 tional, (2) whether they are tuned or untuned, (3) the component of 

 motion to which they are chiefly responsive, such as the vertical or the 

 horizontal component, and (4) the means for converting the ground 

 motion (usually mechanical or electrical) into a quantity that may be 

 recorded. The first classification, according to element ground motion, is 



t L. W. Gardner, "Seismograph determination of salt dome boundary, using well detector deep 

 on dome flank," Geophysics, Vol. XIV, No. 1, January, 1949, pp. 29-38. 



