caliper log and the velocity log. The caliper log run in this hole showed the 

 abrupt increase in hole size so that an exaggerated velocity deflection is recorded. 

 Fortunately, pay zones are usually competent and do not wash, so that the 

 caliper effect can be neglected. In the average borehole the sum of the recorded 

 errors in over-all velocity which affects the integrated travel-time curve, and 

 which are attributed to fluid travel time in cavities, seldom exceeds 4-5 milli- 

 seconds. The magnitude of this error is comparable with that inherent in the 

 standard seismic geophone survey. 



VELOCITY -THOUSANDS OF FT. /SEC. 



Figure 21-19. Velocity log in isotropic media. 



Velocity Logs, Geographic Distribution 



Figure 21-20 shows the approximate locations of SSC continuous velocity 

 log surveys performed in the United States and Canada during the first few 

 years of service to the industry. The coverage is growing rapidly and it is 

 expected that much more will soon be learned about the velocity log. At the 

 end of November, 1956, approximately 900 logs had been run by Seismograph 

 Service Corporation throughout the United States, Canada, Mexico, Venezuela, 

 Europe, and North Africa. Logs run by Magnolia, Empire Velocity Service 

 Company, and others bring this total considerably higher. 



424 



