SEISMIC METHODS 695 



These equations are the same as Equation 21 with the exception that the 



derivative -7- is replaced by the expressions -^ and "tt. A very small 

 ax o 1 o 2 



error is involved in this assumption, but it may be completely neglected in 



most practical cases. 



It should be noted that the value of Fi to be used in Equations 58 and 



59 is the value which gives the best fit of the velocity-depth data to the 



equation, 



F = Fi + aZ (40) 



over the area being surveyed. It may differ somewhat from the observed 

 unweathered velocity. The calculated emergence angle is thus really an 

 effective value used as a computing aid in finding the solution for the 

 reflecting bed. If the locally observed unweathered velocity is used, the 

 true emergence angle will be given, but this will generally give an incorrect 

 solution when used in the computing charts. 



Average Reflection Time. — Column 12 contains the average of the 

 reflection times of the same reflection observed at the different instrument 

 set-ups. As slightly different times are usually observed due to residual 

 error, an average gives a more accurate result. This average is weighted 

 according to the quality and recording situations of the separate records. 



Vector Resolution. — In columns 13 and 14 the vector resultants of 

 the components sin a and sin p are tabulated, in terms of the sine of the 

 emergence angle, 61, and the bearing of the dip. 



Depth, Horizontal Distance and Dip. — Using the data in columns 12 

 and 13, the values of depth Z, horizontal distance H, and dip 6, are read 

 from charts described as Figures 424, 426, and 425 respectively. Since the 

 depth Z is read with respect to datum, column 16 is provided to record the 

 depth below sea level in this case. 



The cross dip, 6, is obtained by using the value of sin (3 and Tm and 

 represents a component of the dip at right angles to the direction of the 

 profile. In this case the dip is toward the west. In columns 20 and 21 are 

 recorded the components of the dip and the horizontal distance parallel 

 to the profile. Here the profile dip is toward the south. These values are 

 obtained from the charts and values of sin a and Tm- In using these data, 

 it should be remembered that in general this information is from a point 

 not directly beneath the profile, but usually off-set from it. 



Where the dips are small it is permissible to assume that the dips pro- 

 jected into the vertical plane of the profile represent the structure along 

 the profile. If the dips are large and the profile forms a considerable angle 

 with the bearing of the dip, then a large error is involved in the location 

 of the dip by projecting into the vertical plane of the profile. In such cases 

 the actual position of the reflecting beds is so far from the plotted profile 



