Chap. 9] SEISMIC METHODS 465 



ing device, and (3) errors in timing the impulses in the record. The shot 

 instant may be transferred to the recorder by wire or radio. For short 

 distances this method is generally not accurate enough; an insensitive 

 seismograph near the shot point is preferable, as it will record the actual 

 time when the energy is impressed on the ground. 



Errors in the timing device can generally be kept down sufficiently to 

 make possible time determinations with an accuracy of 1 in 10,000 if 

 necessary. The "timing" of the impulses in the record is generally the 

 most difficult part of the problem (as seen in some of the records published 

 by Weatherby, A.A.P.G. Bull., Jan. 1934). Unless the sharpness of the 

 first impulses can be improved, there is no object in increasing the accuracy 

 of the timer and shot instant transmission to more than tuws of ^ second. 

 When a number of seismographs are used, it is necessary to balance their 

 phase shifts and to determine the corresponding parallax corrections. 



More serious than these instrumental difficulties are geologic factors 

 which often make it difficult to determine reliable velocities. The surface 

 of the rock whose wave speed is to be determined must be free from cover 

 and reasonably unweathered so that true velocities may be obtained. It 

 is, of course, possible to obtain velocities of inaccessible layers from the 

 travel time curve. However, these velocities are true velocities only if the 

 layers are horizontal. Dip may be eliminated by shooting the profile up 

 and down dip. 



Vertical velocity determinations through formation (that is, generally at 

 right angles to the bedding planes) are made in connection with weathered- 

 layer-correction shooting in shallow (25 to 150 foot) holes and in connection 

 with average-velocity determinations in deep wells. For the latter a de- 

 tector is lowered into the well to various depths and shots are fired either 

 near the well or at a distance corresponding to one-half the general spread 

 distance, to simulate the direction of the ray in its travel to or from the 

 reflecting bed. By means of the travel time to various depths, the average 

 velocity from the surface to the formation in question or the differential 

 velocities between strata are calculated. This method is discussed further 

 in Chapter 11. 



Tables 4^5 through 55 list the elastic moduli and wave velocities for the 

 more important minerals, rocks, and formations. In Table 45 are listed 

 the elastic moduli of minerals, and in Table 46 are given elastic moduli of 

 igneous, metamorphic, and sedimentary rocks." Table 47 shows velocities 

 of longitudinal waves for the formations nearest the surface, particularly 

 the weathered layer; Table 48, the longitudinal wave speeds for alluvium 

 and glacial drift; Table 49, the longitudinal wave velocities for sands, 



" Largely after Adams, Adams and Gibson, Adams and Williamson, Zisman, 

 Richards, Don Leet, Born and Owen. 



