Chap. 9] SEISMIC METHODS 571 



established by correlating dips between stations. The following proce- 

 dures may be followed: (1) shoot twice in the same hole and move the 

 receivers over; (2) leave receivers in place and shoot from both sides of 

 the spread ; (3) shoot once in one hole, and use half of the receivers on one 

 side of the shot point, and the other half on the other side; (4) shoot with 

 receivers to one side and determine dip by absolute times and time gradient. 

 These procedures may be applied' in two profiles at right angles to each 

 other to determine strike. Structural mapping by small dip angles should 

 be done in closed loops, since the depth errors of closure are considerable 

 and must be distributed by suitable adjustment. Shot distance and 

 receiver interval vary as in correlation shooting. 



(c) Shot placement, surveying, drilling. Reflection locations should be 

 selected from the point of view of good energy transmission, ease of drilling, 

 and minimum variation of elevation and surface geologx'^ along receiver 

 spread. Low places are generally preferred, since water-bearing beds 

 occur closer to the surface. In many cases the selection of reflection loca- 

 tions is a matter of compromise between the above factors. Elevation of 

 shot-point and receiver locations must be detennined with an accuracy 

 of about one foot. When dip profiles are shot in more than one direction 

 to determine strike, the direction of the profiles must be recorded. 



Holes may be drilled with hand augers in favorable locations or when 

 speed is not an important consideration. However, the more general prac- 

 tice is to use rotary drilling rigs mounted on trucks, as previously de- 

 scribed (page 491). In clay, marl, and other soil which is easy to drill, 

 two crews can make as much as 500 to 600 feet of hole a day. For hard 

 formations, coarse gravel, boulder clay, and the like, spudders are used. 

 Caving of shot holes is overcome by the use of "Aquagel," or lime, or by 

 setting casing. In soft marsh ground, holes are washed down with a high 

 pressure pump operated from a small gasoHne engine (see Fig. 9-29). 



Charges may vary from a single cap to 10 pounds of dynamite. Sixty 

 per cent special gelatin dynamite and Nitramon is most frequently used. 

 Charges are forced down the hole by rods consisting of several sections 

 provided with junctions so made that they do not come apart when 

 lowered into the hole but may be dissembled readily when pulled out. 

 Holes are generally tamped with water; in exceptional cases, with mud. 

 For this purpose a water tank is carried on the shooter's truck. 



(d) Weathered-layer technique. A near-surface "weathered" or "cor- 

 rection" zone with a considerable reduction in velocity occurs in every 

 area. As this layer is not likely to be identical at all stations it has 

 become general practice to correct for the delay of the reflected wave in 

 this layer. As it has, furthermore, very poor transmission characteristics, 

 shots are placed below it or at least near its lower siu-face. 



The extensiveness of the weathered-layer correction is usually a com- 



