502 



SEISMIC METHODS 



[Chap. 9 



leads. The receivers are placed at the ends of radial lines at distances of 

 1000 to 2000 feet. The location of the lead is given by the maximum 

 travel time, and this point is then made the vertex for the next fan. The 

 position of the edge of the channel, as well as the channel depth, may be 



Tret'f/ Time 

 i 



Fig. 9-40. Tracing course of placer channel by fan shooting. 



M ■ 



m 



too 







Xff 



Alluvltl Mf ClMMl 



Fig. 9-41. Travel-time curves on two gold-bearing alluvial channels in the Gulgong 

 gold field, New South Wales (after Edge and Laby). 



detailed by refraction profiles at right angles to and parallel with the 

 strike of the leads. Fig. 9-41 shows up- and down-lead travel-time curves 

 (to determine diy of bedrock surface, see page 525) . The depth of the chan- 

 nel is roughly proportional to the ordinate of the break in the travel-time 

 curve. 



