GEOLOGICAL APPLICATION OF SEISMOGRAPHY 1325 
weathered layer. This is the refraction method applied on a miniature 
scale. If not accounted for separately in some such manner as this, it 
may easily introduce an error in depth determination of 100 feet or more. 
By taking this layer into account and also by determining elevations 
of shot and recorder points, the general, relative accuracy between two 
datums obtained by this method is found to be approximately 0.5 per 
cent. In order to accomplish this, all time measurements must be ac- 
curate to o.co1 second. The determination of distance between shot and 
recorder points need not be very accurate. 
In most places several reflecting strata are present in the sub- 
surface, as shown in Figure 8. The recorder responds to each of these re- 
flections as it arrives. 
SHOT FFE CORDEF 




FEFLECTING 
STRATA 


WLM MLL LLL 
Fic. 8.—Multiple reflections. 
