loss point correlates with the spike labeled "C" on the Minimag log and is 

 caused by the tracer-laden mud in a fault plane. The small anomaly labeled "D" 

 is a result of variations in the natural susceptibility found in this shale member. 

 Both magnetic logs were run after the tracer-laden mud had been displaced by 

 mud devoid of tracer material. This displacement reduces the background and 

 variations as a result of differences in borehole size or caliper. Unfortunately, 

 there is no way of distinguishing between the natural susceptibility of formations 

 and the artificial susceptibility of the mud cake from a single log. Therefore, 

 background or control logs are necessary to determine if the background is 

 negligible or consistent enough so that anomalous zones on the tracer logs can be 

 qualified. As an aid, the tracer concentration is made high enough that it over- 

 rides typical variations in formation susceptibility. Consecutive logs can be run 

 to show the build-up of cake in the anomalous areas. 



Tracer logs of this type are unique in that they do not depend on the 

 electrical properties of the formation and drilling fluids as required in electric 

 logging. Therefore, tracer logs can be made in oil-base muds or muds of either 

 very high or very low conductivity that would prevent the use of electric logs. 



The magnetic susceptibility of rock containing iron is high compared to that 

 of most sedimentary rock. Therefore, the use of this log in iron-ore evaluation 

 would appear to be attractive. However, to date only experimental logs of this 

 type have been made. It is believed that, with core control to establish calibration, 

 it will be possible to make iron-ore assays in the borehole. 



The magnetic susceptibility log is not affected by the conductivity of bore- 

 hole fluid, whereas the induction conductivity component of the log is affected. 

 The mono-coil instrument as described has no shaping or focusing of the field, 

 and therefore the zone of influence is restricted to approximately two times the 

 length of the sensing coil. Because of this effect, the conductivity log is best in 

 muds having a resistivity of two ohmmeters or more. Conductivity logs from 

 such an instrument are particularly advantageous in high-resistivity muds, such 

 as oil or oil-base muds, or in open-hole logging where conventional resistivity 

 logging methods are poor. 



TOTAL FIELD LOGS To date, measurements of total field in the 



borehole have been only experimental. The 

 complexity of the equipment has restricted its use to depths less than 3000 feet. 

 In rock of low susceptibility, there is little character to the log. However, in 

 zones of high susceptibility, the total field log indicates the polarization or field 

 effect at some distance from the body, as well as the shielding effects of the body 

 when it is penetrated. Such a log should have utility in detecting nearby deposits, 

 if iron ores are not necessarily cut by the test hole. 



434 



