Miscellaneous Subsurface Methods 605 



"Curie point." The Curie point for nickel is 358° C, for iron 770° C, 

 and for cobalt 1,120° C. At temperatures above the Curie point ferromag- 

 netic minerals become paramagnetic. Conversely, as the temperature is 

 lowered residual magnetism is again acquired. For pure magnetite, Fe304, 

 the Curie point is 585° C; for pyrrhotite, FeS, 350° C. Much magnetite, 

 however, is a mixture of Fe304 and FeO. The Curie point of this contam- 

 inated magnetite depends upon the amount of FeO present and decreases 

 to approximately 0° C. for mixtures containing large amounts of FeO. 

 On the other hand, the addition of Ti02 to the ferromagnetic minerals in- 

 creases the coercive force and raises the Curie point accordingly. 



The high temperatures brought about by deep burial, by neighboring 

 igneous activity, or by other means may raise the temperature of a rock 

 above the Curie point of the ferromagnetic minerals that it contains and 

 thus destroy their residual magnetism. Upon subsequent cooling the fer- 

 romagnetic minerals again acquire permanent magnetism, which is essen- 

 tially coincident in direction with the earth's magnetic field at that local- 

 ity. This new residual magnetism may form a considerable angle with 

 the direction of the earlier-generation magnetism that was destroyed by 

 the high temperature. In this manner the change of direction of residual 

 magnetism by periodic heating tends to establish alignment with the direc- 

 tion of the earth's magnetic field. On the other hand, the negative geologic 

 factors, which destroy coincidence such as tilting and secular variation, are 

 constantly in action and normally overbalance the intermittent positive 

 effects of temperature. This lag of temperature in keeping pace is often 

 measured in geologic periods or eras, so that divergence between the di- 

 rections of residual magnetism of a rock and that of the earth's field is 

 the rule and not the exception. 



Magnetic Field of Core Barrel and Drill Pipe 



The suggestion has been advanced that a strong magnetic field, pre- 

 ferably parallel to the earth's field, be applied at the bottom of a well bore 

 prior to coring, the intent being to create or strengthen the residual mag- 

 netism in the rock. Theoretically, this should permit the cores to be more 

 reliably oriented and thus allow the vital strike and dip determinations to 

 be made with greater precision. 



As the angle of inclination of the earth's magnetic field approaches 

 the vertical and parallelism with the long axis of the drill pipe, the distor- 

 tion of the earth's field about the end of the pipe decreases to a minimum 

 value. In the United States, where the angle of inclination is large, the 

 distortion of the magnetic field about the pipe is small, so that the direc- 

 tion of the earth's field may be considered essentially unaltered by the 

 presence of the pipe. The magnitude of the field, however, is materially 

 increased, the strength being a function of the permeability of the steel 

 pipe as well as of its dimensions. The effect of this field on the core, how- 

 ever, is of rather short duration, as upon entering the core barrel the core 



