SEISMIC METHODS 7Z7 



lead to the same reflection times were it not for the difference in the 

 thickness of the low velocity layer at the two seismometers and the dif- 

 ference in the depth of shots. Because these differences can be deter- 

 mined, a positive correlation can be effected from the first to the second 

 record. Furthermore, when seismometers are transferred to the opposite 

 side of shot-point B for the third record, the single seismometer opposite 

 shot-point B is undisturbed. Consequently, the traces corresponding to 

 this seismometer furnish another positive correlation from the second 

 to the third records. 



Instead of making a correction on one of the records for the difference in travel- 

 times for the ray paths A-\Q and B-\, use may be made of an overall correction 

 ATi without regard to the various sources that contribute to the correction. Thus, 



^4-10 := tB-\ -\- £^T\ 



Next, one adds the same correction to fs-o and ^b-s' and averages these values. 



The next step is the tie-in between the two profiles, shot from B in both direc- 

 tions. On the two corresponding records one finds two values for ;b-9 and t^-2'. In gen- 

 eral, the corresponding values differ by a few thousandths of a second, probably due 

 to repeated shooting in the same hole with consequent changes in elastic properties 

 of the rocks at the bottom of the hole and to instrumental errors. These times are 

 corrected by a value A/2 to make the average time for the two seismometers on both 

 profiles the same. Now one adds the same correction to <b-io' and repeats the whole 

 process for the next shot-point. In this way it is possible to correlate the records 

 with little more than an occasional check on the shot-hole time (time that it takes the 

 wave to travel from the bottom of the shot-hole to the surface). The times are then 

 translated into terms of depths with the aid of data obtained in well-shooting. 



Other methods of continuous profiling are sometimes used. The term 

 itself applies only to a "continuous coverage" on a reflecting bed. Several 

 advantages are apparent for the method of continuous profiling. First, the 

 reflection points are uniformly spaced, thereby permitting most efficient 

 correlation as well as fault investigation. (Compare p. 738.) Second, 

 the common tie-in from record to record eliminates errors due to dif- 

 ferences in shot-hole depth or in the timing system, thereby allowing 

 positive correlations. Third, by use of up-hole shooting, good corrections 

 for the low velocity layer in dip determination may be obtained. Fourth, 

 the long distance from the shot-point to the extreme seismometer of the 

 accompanying spread may permit adequate velocity determination. (Com- 

 pare Figure 445.) 



Several disadvantages of the method are often cited to offset these 

 advantages. First, the close spacing of shot-points and seismometers 

 greatly increases the expense of the work. Second, the method is rather 

 inflexible, requiring continuity in seismometer line despite obstructions 

 which may be encountered. Third, the common tie-in from record to 

 record increases the tendency of the computer to force correlation, which 

 may lead to error. 



In continuous profiling it is theoretically impossible to differentiate 

 between correlation data and strictly dip, or AT, data because the series 



