SEISMIC METHODS 



835 



face. Also, the relative amplitude and time of persistence apparently vary 

 with the physical properties of the bed and its adjacent environment. There 

 is good experimental evidence to indicate that it is possible to identify and 

 follow certain of these beds by frequency correlation from station to 

 station. In areas where this is possible, it will aid in the mapping of 

 stratigraphic traps, such as pinch-outs, lensing, changes in porosity, etc., 

 where much oil has no doubt accumulated. In many areas trouble is 

 encountered with reflections from successive strata or between the bound- 

 aries of a single stratum, giving rise to multiple reflections, which often 

 cause errors in interpretation. With the frequency analysis technique such 

 multiple reflections usually are easily recognized due to the periodic 

 reoccurrence of the same frequency wavelet. 



VACUUM 



TUBE 



VOLTMEITEIR 



SWEEP 

 CURRENT 



MOTOR DRIVEN 

 /VNAUVZER 



Fig. 517. — Schematic diagram for frequency analysis. (Courtesy of International 

 Geophysics Company.) 



Frequency Pattern Correlation, — Another type of instrumental 

 analysis which offers interesting possibilities is based on the comparison of 

 frequency patterns. In this technique the original record is played back 

 through a relatively narrow band-pass filter. The filter is narrow enough 

 (usually from 5 to 15 cycles pass) to give clean-cut reflection discrimina- 

 tion, but not sufficiently sharp to introduce undesirable distortion or delay 

 in passing through the filter. 



