77t 



EXPLORATION GEOPHYSICS 



It is necessary only to plot the deviation of the travel times from the 

 normal values. (Where it is not possible or convenient to locate all 

 seismometers at the same distance from the shot-point, a short profile 

 covering the interval from the shortest distance to the longest distance 

 is shot in order to establish the normal travel-times for the various 

 distances.) 



Fig. 470. — Diagrammatic sketch of fan shooting. 

 Seismometers are placed at ends of lines 1, 2, 3, . . . 11 

 which radiate from the shot-point O; V, mean velocity; 

 RG, probable regional velocity. 



Figure 470 shows the shot-point at and seismometers at the ends 

 of radii 1 to 11. The lower portion of the figure schematically indicates 

 the variation of velocity with orientation angle. (In an alternative pre- 

 sentation, the deviation of the travel-time from the normal value would 

 be plotted along each radius, zero deviation being plotted on an arbitrary 

 circle with the center at 0.) 



By the use of a number of strategically located shot-points and seis- 

 mometer stations, a large area can be explored fairly rapidly. If the 

 results indicate the presence of a shallow salt dome, the position and 

 outline of the dome can be located more accurately by additional shots 

 approximately at right angles to the first fan. 



Figure 471 shows the theoretical ray paths through a salt dome that 

 is located 1000 feet below the surface and has a cross sectional extent 

 of 5000 feet. 



Figure 472 shows a theoretical seismogram corresponding to a seis- 

 mometer S located a distance of 10,000 feet from the shot-point 0. It 



