Subsurface Logging Methods 439 



holes, dry, producing gas, containing salty mud, conditioned mud, fresh 

 water, oil, oil-base muds, or any mixture of these fluids. 



The location of porous strata by the neutron curve, especially when 

 it is used simultaneously with the gamma-ray curve, has been described 

 and explained. The ability to determine porous strata by radioactivity 

 means has been proved beyond all doubt by the hundreds of successful 

 completions of oil wells in which radioactivity logs were employed. 



Because of the flexibility of the radioactivity log it becomes also 

 the most economical survey available. No special arrangements or hole- 

 conditioning procedures are necessary. 



Simplicity of interpretation is one of the primary advantages of 

 logging by radioactivity means. It is not necessary for the oil operator, 

 engineer, or geologist also to become a. nuclear physicist. A very basic 

 understanding of the principle upon which a radioactivity log operates, 

 the action behavior of the recorded picture obtained, and a knowledge 

 of the stratigraphy of the region are all that are necessary for accurate 

 interpretation. 



CALIPER AND TEMPERATURE LOGGING 

 WILFRED TAPPER 



The uses of caliper and temperature records are sometimes so inter- 

 related that a discussion of one log presupposes a discussion of the other. 

 Here, however, for purposes of clarity, the records and the tools used 

 to obtain them are treated separately. 



It is the purpose of this section to give an outline of the history, 

 development, construction, and uses of caliper and temperature electrodes. 

 A knowledge of the physical construction of both types of electrodes 

 results in a clearer understanding of the data obtained and more efficient 

 utilization of the logs. 



The few anomalies cited as examples do not pretend to be compre- 

 hensive. The uses for the various logs listed surely do not exhaust present 

 or future possibilities in the oil industry or in other fields. 



The writer is indebted to Mr. H. K. McArthur, Mr. J. K. Reynolds, 

 and Mr. W. D. Owsley, of the Halliburton Oil Well Cementing Company, 

 for advice and criticism in the preparation of this section. 



Caliper Logging 



Even in the early days of cable-tool drilling, oil men were well aware 

 that drill holes did not stand true to gauge. As holes were drilled deeper 

 and exploration moved southward to younger sediments, this fact became 

 painfully obvious. 



One of the factors influencing the development and use of the rotary 

 drill was its ability to cut through young, unconsolidated sediments and 



