606 Subsurface Geologic Methods 



is shielded to a considerable extent from all magnetic fields until it is 

 finally released at the surface. 



Any effect on the core caused by the magnetic field associated with 

 the string of drill pipe and core barrel should be favorable for orienta- 

 tion. The tendency will be to reinforce the magnetic field of the core in a 

 direction parallel to that of the earth's field. Thus, little benefit may be 

 expected by applying special magnetic fields in the bottom of the well 

 bore, when in reality a strong magnetic field is already present through the 

 medium of the drill pipe. 



Cores from Deflected Wells 



The fact that bore holes always deviate a greater or lesser amount 

 from a vertical position must be borne in mind when making magnetic 

 orientation determinations. Strike-and-dip measurements are always made 

 with the assumption that the core was cut vertically. Corrections should 

 be applied for any appreciable divergence between the vertical position 

 and the true position of the core. 



Bore-hole deviations fall into two natural classifications, accidental 

 and intentional. The first type is normally of small magnitude, and cor- 

 rections for such deviations are unnecessary. The usual drilling contracts 

 specify an allowable deviation up to several degrees from the vertical, 

 and thus most holes are drilled within this permissible cone of error. In 

 the second category, however, the deviation is generally large and cor- 

 rections are mandatory when the direction and amount of deviation are 

 known. 



Several methods may be used to correct for vertical deviation. Instru- 

 ments have been constructed which duplicate the geometric relations exist- 

 ing in deflected wells. Such instrumental solutions are desirable in that 

 they are rapid as well as visual in three dimensions. Mathematical and 

 graphic solutions have also been developed. A recent paper ^ adequately 

 sums up and evaluates the various methods of correction for hole devia- 

 tion at the cored horizon. 



Experimental Results 



Little experimental work has been conducted on the significance of 

 magnetism of rocks. Much remains to be done. Investigations of the 

 "cyclic" secular change have been made as a method of geologic dating. 

 On the basis of observation and experimental evidence largely concerned 

 with investigations of ancient pottery and bricks, Koenigsberger ^ reached 

 the following conclusions which are worthy of note. The application to 

 the residual magnetism of rocks is apparent. 



(a) It was found that the magnetism of well-baked bricks and pottery is 



^ McCellan, Hugh, Core Orientation by Graphical and Mathematical Methods : Am. Assoc. Petroleum 

 Geologists Bull., vol. 32, pp. 262-282, 1948. 



^Koenigsberger, J. C, Residual Magnetism and Measurement of Geologic Time: 16® Cong. geol. 

 internal. Compte rendu, pp. 225-230, 1933. 



