MAGNETIC Magnetic susceptibility is a universal property, 



SUSCEPTIBILITY LOGS and rocks are classified as being diaraagnetic 



if they reflect a negative susceptibility, and 

 paramagnetic if they have a positive susceptibility. Relatively few rocks are dia- 

 magnetic (i.e., rock salt, anhydrite, coal, etc.), whereas most rock is para- 

 magnetic. Generally, certain oxides of iron control the susceptibility because they 

 are high compared to other oxides. The range of typical values for rock is 

 from —0.4 x 10 -6 to + 4000 x 10 ~ 6 cgs units. Susceptibility is a natural property 

 and is not necessarily specific to rock type. Certain rocks having high 

 susceptibility have been correlated over miles, and the log has proven to be 

 capable of identifying horizons that are peculiar to this logging method. 



A comparison of a magnetic susceptibility log with other standard logs is 

 shown in Figure 22-4. It has been found desirable in comparisons of logs to 

 plot the magnetic susceptibility in the track normally used for the self-potential 

 log and with increasing susceptibility to the left. This comparison illustrates the 

 effect of bore-hole caliper on the log. In this particular case the drilling-mud 

 susceptibility was higher than the average value of the sedimentary rocks. Thus, 



Figure 22-3. Block diagram of total-field logging system. H = total field. 



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