568 SEISMIC METHODS [Chap. 9 



dip angles and errors introduced in the records by surface geological 

 "step outs." The accuracy is probably not greater than 1° or 2°. It 

 can be increased by reducing the station interval, by surveying closed 

 loops, and by adjusting the errors of closure. The most favorable range 

 is from 5° to 30°; in exceptional cases dips of as much as 55° have been 

 recorded. 



4. Average velocity determination. Notwithstanding the frequent curva- 

 ture of reflection rays, the assumption of uniform or average velocities 

 to the reflecting bed or beds is generally satisfactory. In the absence of 

 average velocity data, reflection maps may be contoured in reduced time 

 units. In areas where changes of the character of sediments occur, it 

 may be necessary to introduce an average velocity as a function of the 

 geographic coordinates. One or more of the following procedures maj'' be 

 used for the determination of average velocities : 



(a) Calculation from known depth. Where the geological section is 

 clearly defined so that there is no doubt about the nature and depth of the 

 reflecting formation, a profile is shot near a well and the average velocity 

 is calculated from 



(b) Well shooting. A well detector is lowered to various depths and a 

 number of shots are fired at the surface. Usually the distance of the shot 

 locations from the well is made equal to one-half the shot-receptor distance 

 used in the survey, in order to obtain as nearly as possible the velocity 

 along the actual wave path. Well detectors are long seismometers of 

 small diameter, corresponding in design to that of the regular pickups; 

 they have the shape of torpedoes and are well protected against water, 

 mud, and the like. They are lowered on strong steel cables. Average 

 velocities are calculated by dividing the paths from shot point to detector 

 by the respective trave) times. Recording of shot instant and accurate 

 time marks are required as in ordinary reflection work. The method is 

 laborious but in most general use at this time. 



(c) Calculation from refraction profiles. When the thicknesses da and 

 the velocities v^ of the strata composing the geologic column are known 

 from refraction profiles, the average velocity to the depth z in the column is 



Va = 





(9-816) 



This method is not particularly accurate, nor is it generally advisable 

 to shoot refraction profiles in an area to obtain average velocity data. 



