Chap. 9] 



SEISMIC METHODS 



505 



mon recording point. In oil exploration there are quite a number of prob- 

 lems which can be successfully solved by refraction profiles of moderate 

 detector intervals. Whereas the detector interval in a four- to six-mile 

 fan is of the order of a mile, a refraction profile shot under similar circum- 

 stances would utilize detector spacings of the order of 1000 feet. In mining 

 applications, such as mapping of the bedrock surface on placer sites, the 

 detector spacing would be of the order of 25 to 100 feet. In engineering 

 work the detector interval depends largely on the problem and would 

 vary between 5 and 25 to 50 feet. 



In the early days of refraction shooting, mechanical seismographs with 

 individual setups, such as the Mintrop and the Schweydar seismographs 

 (page 608), were widely used. These were soon superseded by electrical 

 seismographs, such as those described in connection with the fan-shooting 

 equipment for individual setups, and later by multichannel equipment 

 with central-point recording. Reflec- 

 tion seismograph equipment is well 

 suited for refraction applications; de- 

 tails are discussed on pp. 551-56. As 

 a matter of fact, reflection equipment is 

 used for refraction work in everyday 

 reflection routine for the determination 

 of the thickness of and the time delay 

 in the weathered layer. A portable 

 refraction apparatus suited particu- 

 larly for shallow engineering and min- 

 ing applications is illustrated in Fig. 

 9-1 14a. 



3. Travel-time curve for vertical boundary 

 per sec. and Vj = 4000 m per sec. 



Fig. 9-42. Refraction travel-time 

 curves on vertical boundary. 



In Fig. 9-42 let Vi = 2000 m 

 Assume that the shot point is at and 

 that detectors are set up at the points 1 to 5 at 400-m intervals. The ex- 

 plosion wave will reach point 1, 0.2 sec. after the shot has been fired, point 

 2 in 0.4 sec, and point 3 in 0.6 sec, point 4 in 0.7 sec, point 5 in 0.8 sec, 

 point 6 in 0.9 sec, and so on. Plotting these times against distance gives 

 a curve with a break above the contact. Since a travel-time curve of the 

 same shape is obtained also for a horizontal layer, another shot may be 

 fired some distance away from the first shot point. In case of a vertical 

 fault at the surface as assumed here, the position of the break remains the 

 same. If a horizontal boundary occurs at depth, the distance of the break 

 from the shot point remains unchanged. The slopes of the two parts of 

 the travel- time curve indicate the respective velocities below: 



cotan a = 



dsi 

 dfx 



= vi; 



cotan j8 



dsz 

 dti 



= V2 



