SEISMIC METHODS 775 



and the corresponding depth distance may be obtained, as before, by using 

 the depth vs. delay time graph. 



Furthermore, if a number of seismometers are placed on a relatively 

 short line which passes through Oo and are orientated in an azimuth with 

 respect to OoSo, the offset positions associated with the common shot-point 

 will be approximately coincident. 



An extension of this arrangement which comprises a number of short 

 spreads is showai in Figure 469B. The offset positions M associated with 

 the ends of the various radial spreads lie in a small annular area. Undula- 

 tions of the relative depths below the annular area can be determined by 

 computing the delay times from the observed intercept times ( Equation 

 122), and making use of the depth vs. delay time graph. Evidently, by 

 arranging a number of such rings in a manner such that one or more 

 control points from adjacent rings overlap, it is possible to combine all 

 the data into a map showing relative depths over a large area. A triangular 

 arrangement can also be used. 



Application of Refraction Method to Salt Dome Exploration. — The 

 velocity of seismic waves in salt domes is very much greater than that 

 in the sedimentary materials surrounding the domes. Thus, a wave which 

 has traveled in part through a salt dome will have a much greater average 

 velocity than a wave traveling the same distance in a nearby area where 

 no salt dome is present. This fact was established in 1924 and led to 

 wide-spread use of refraction fan shooting. 



Fan Shooting 



In fan shooting, the seismometers are located at approximately equal 

 distances from a shot-point. The distance is usually determined by 

 shooting an auxiliary profile; it must be sufficiently great to insure that 

 the waves penetrate to the desired depth. In the Gulf Coast, it is 

 customary to use the empirical figure of one-fifth for the ratio of pene- 

 tration to shot distance.* 



The seismometers are so positioned with reference to the shot-point 

 that they cover any desired portion of the 360° surrounding the shot-point. 

 Figure 470 illustrates a common arrangement of seismometers for covering 

 a 135° sector. After the seismometers have been positioned at known and 

 preferably equal distances from the shot-point, a charge is exploded, 

 and the travel-times taken by the waves to reach the various seismometers 

 are recorded. If the waves which have traveled through any particular 

 sector have a higher velocity than the waves which have traveled through 

 the other sectors, a high velocity plug is indicated in that sector. 



The interpretation of the results of refraction fan shooting is ex- 

 tremely simple, because the time differences observed are rather great. 



* This relationship is similar tn that prevailing in electrical work where the effec- 

 tive depth of current penetration is assumed to vary from about 1/3 to 1/5 of the 

 electrode separation. 



