SEISMIC METHODS 691 



spreads with reference to the same shot-point are shown in Figure 427. 

 One instrument set-up was in the north-south and the other in the east-west 

 direction. The top trace of the north-south seismogram was in the south 

 direction, while the top trace of the east-west seismogram was to the east. 

 These records were obtained on level terrain south of the city of Los 

 Angeles at an elevation of 115 feet above sea level. A charge of 12^ 

 pounds was placed at a depth of 95 feet to obtain the record in the north- 

 south direction; however only 4^^ pounds were placed at a depth of 85 

 feet to obtain the seismogram in the east- west direction. The recorded 

 amplitudes of the traces of the records are not dissimilar, in spite of the 

 large difference in the amounts of explosive. Both shots were placed well 

 below the top of the water table and below the weathered layer. The 

 difference in amplitude is small due to the action of the automatic volume 

 control in the amplifiers. 



The records were taken in an area of relatively flat-lying formations, 

 which fact is indicated by the small move-out times of the recorded reflec- 

 tions. The length of spread used was 1020 feet, which in general is much 

 too short for the mapping of flat dips. An instrument spread of 2000 or 

 3000 feet would have been better for accurate determination of dip. How- 

 ever, the problem under investigation involved the possibility of changes 

 in dip within relatively short distances. The shorter spread was chosen for 

 this reason. 



Time has been measured on these seismograms from the instant the 

 shock wave reached the up-hole seismometer recorded by the second trace. 

 By reckoning time from this instant to the instant of a recorded reflection, 

 we find the time necessary for the wave to travel from the bottom of the 

 hole to the reflecting interface and back to the equivalent elevation of the 

 bottom of the shot hole. Since the shot was placed twenty or more feet 

 below the low-velocity layer, there is no correction to be applied to the time 

 for the presence of this layer. The shot detonation was recorded by a 

 second set of seismic equipment of different frequency response character- 

 istics. The times of two seismograms, recorded independently, were com- 

 pared by reference to the up-hole time measurements on the respective 

 seismograms. 



The thickness and velocity of the low-velocity layer in this area are 

 uniform. The correction in the north-south direction was (plus) one 

 millisecond, and in the east-west direction (minus) two milliseconds. 

 These corrections were estimated by use of a least square table of the first 

 break differences similar to that illustrated by Table 22, page 727. A 

 rather high frequency disturbance on some of the traces was attributed to 

 the operation of machinery in a nearby factory. 



The reflections are marked across the records. The figures at the top 

 of the seismogram represent the reflection time, and those on the bottom 

 represent the uncorrected move-outs for the reflections. Experience of 

 interpreters of seismic data, especially during review or re-interpretation, 



