SEISMIC METHODS 



693 



12./' 13 14,.. 15 , IS 17 ,18 19 , 20 ,1 21 22 ^-" 23 24 



16 1.7 IS 19 20 21 Z2 



Los Angeles County, California. (Courtesy of Geophysical Engineering Corporation.) 



columns are not used. The capital (T) designates the reflection time in 

 milliseconds and small (t) designates the move-out time difference in 

 milliseconds. The subscripts refer to the spread positions shown in a 

 free-hand sketch in the upper left of the computing form. At the head of 

 each pair of columns the bearing of the spread is written, the direction 

 always being from the last towards the first seismometer on the seismogram 

 (in this case from No. 14 to No. 1 seismometer). 



Designation of the direction of the component dip is by the algebraic 

 sign preceding the move-out figure, the plus sign being used to show that 

 the emergence angle dips in the direction of trace number one. In other 

 words, subtracting the time of the bottom trace from that of the top one 

 will give the algebraic sign of the move-out time when using this convention. 



Columns 10 and 11 contain the sines of the component emergence 

 angles, alpha representing in-line and beta the cross component. These are 

 calculated by multiplying the move-out times by the ratio of the datum 

 velocity (Fi) to the effective spread length. 



sm a 



sin^ = 



Si 



S2 



(58) 

 (59) 



