54 G. H. WESTBY 
the alluvium filled valley the alluvium was penetrated and the shale 
entered for a short distance. Seven pickups were used, as shown in 
Fig. 7, and this arrangement was left in place while shots were made 
from both shot points. Shooters took special care to get the shots 
exactly on the bottom of the hole. Vertical time was measured at 
the shot hole but was not used in the calculation. This method carries 
the correlation mechanically and was used here to ascertain the 
presence of small faults or steep dips which might exist over very 
short distances. It was entirely successful in both respects. 


Fic. 7 
Data for the surface correction zone were determined from the 
arrival time of the initial energy on the reflection record. At each 
shot point the first two traces on the record were used in each direc- 
tion to obtain an average determination for the depth below this 
shot point. This scheme gave depth points which were all within a 
circle of 133 feet in radius about the shot point. These were averaged 
for the depth calculation beneath the shot point. In shot point No. 3, 
shown on map C in Fig. 3, shooting was completed in five different 
directions interlocking with records shot from shot points 2, 4, 6, 10, 
and 13. Not less than three records were shot in each of these five 
directions from shot point 3, so that the resultant average from shot 
point 3 was determined from more than fifteen records, on each of 
which two traces were used. At this shot point the maximum variation 
between determinations of the depth in the five different directions 
298 
