SEISMIC METHODS 



777 



will be observed that at the time 1.12 there is a small disturbance which 

 corresponds to the wave traveling through the salt. The amplitude of 

 the salt wave disturbance diminishes rapidly. At the time 1.31 a second 

 disturbance starts. This disturbance corresponds to the normal wave 

 through the material surrounding the dome. The difference 0.19 seconds 

 is the deviation of the travel-time from the normal value for that distance 

 and is called the lead. 



Sooo- 



I 





■''•j^'i^'/'v^' :'^' ^' ' ' 



Fig. 471. — Theoretical ray paths through a cross section of a salt dome. 



'SHOT IHSTAMT 



SALT HVAVe ^NORMAL IVAVt 



Fig. 472. — Schematic seismogram for a refraction shot across a salt dome. 



Fig. 473. — Travel -time curve for salt dome shown in Figure 471. 



Figure 473 shows a schematic travel-time curve corresponding to a 

 traverse over the dome. A portion of the lower branch of the travel-time 

 curve corresponds to wave energy which has traveled through the salt; 

 the remainder of the lower branch and the upper branch corresponds to 

 energy which has traveled through the surrounding material. 



