1094 



EXPLORATION GEOPHYSICS 



moved at a uniform rate of speed and each vertical trip of the Kelly produces 

 a varying pressure recording on the chart. Drilling time is proportional to the 

 length of chart between corresponding phases of the pressure curve. 



Drilling logs supply valuable information for later correlation with sample 

 logs and electrical and thermal logs. Proper correlation with sample logs is 

 dependent on an accurate evaluation of the time required for cuttings to reach 

 the surface sampling box. This time varies with the effective diameter and the 



Fig. 684. — Method for determining the approximate re- 

 sistivity of irregularly shaped materials, a, specimen; b, 

 four-electrode system. (Power applied between the two out- 

 side electrodes, and potential measured between the two 

 inner electrodes.) 



depth of hole, the rate of mud circulation, and the viscosity of the mud. In a 

 5000-foot hole, the time lag is usually about one hour. Drilling time logs are 

 finding increasing use in the industry, because they not only supply information 

 which allows the operator to choose the optimum drilling technique, but they 

 also supply a record showing time going in and out of hole, condition of bits, 

 stand-by periods, etc. 



Recent developments in mud analysis and logging electrically while drilling 

 supplement the drilling time logs. 



RADIOACTIVITY WELL-LOGGING 



Radioactivity well-logging is a conspicuous example of a successful 

 application of the physics of the nuclear atom to exploration, Tw^o early 



