1092 Subsurface Geologic Methods 



through abnormally high- or low-velocity regions may be identified by 

 comparing the times to each detector in the fan with the time for a 

 corresponding distance determined in a structurally normal area. As the 

 depth of penetration of the rays increases with the distance from the 

 shot point, spreads of as great as ten miles are sometimes used, requiring 

 up to a ton of dynamite. By shooting a number of such fans and noting 

 the rays that show abnormal velocities, the approximate location of salt 

 domes may be determined. 



The fan-shooting technique has been adapted to the tracing of buried 

 river channels that have been scoured in more resistant rocks. By noting 

 rays that show an abnormally low velocity, the Imperial Geophysical Ex- 

 perimental Survey in Australia has successfully applied the method in 

 prospecting for favorable placer grounds. 



In the profile method of refraction shooting, measurements are made 

 of the travel times of waves that are refracted along high-speed subsurface 

 boundaries. A number of arrivals of refracted energy may be observed, 

 depending upon the distance between the shot point and the detectors 

 and the number of high-speed refracting horizons within the subsurface 

 that are penetrated by the rays. As the order of arrivals of refracted 

 energy is dependent upon the shooting distance, depths, dips, and velocities 

 of the refracting horizons, reliable identification of a refracting horizon 

 can be made only on the basis of well control. At the optimum shooting 

 distance for any refracting horizon, interpretations are inherently more 

 accurate, although information from the other refracting horizons is fre- 

 quently used. The refraction method does not evaluate small anomalies 

 because of the reduced accuracy of the method; however, it may be of 

 great value in locating large anomalies and investigating regional dips 

 over large areas. This technique and modifications of it are being used 

 at present in the exploration of the Edwards Plateau of west Texas and in 

 the Florida Peninsula. 



The reflection method, where adaptable, gives an actual subsurface 

 map of geologic horizons. A number of advantages make it the preferred 

 exploration tool in most regions. Among these advantages might be men- 

 tioned the greater resolving power of the method, the use of smaller ex- 

 plosive charges, the fact that the depth of penetration is not controlled 

 by the dimensions of the effective beds, and that data may be obtained 

 for a number of subsurface boundaries with a single shot. Two general 

 types of reflection shooting are practiced: the correlation and the dip- 

 shooting methods. Correlation shooting is employed where the reflecting 

 beds are persistent and readily identifiable. For reconnaissance work, shot 

 points may be located a mile or so apart with the detectors placed to 

 afford the maximum character and amplitude to the recorded reflections. 

 A spread in which the geophones and shot points are spaced regularly 

 along the entire length of a traverse line, the interval between shot points 

 being intercepted by a constant number of detectors, is used in areas 



