Miscellaneous Subsurface Methods 597 



orienting apparatus constructed and developed by the California Research 

 Corporation, the reader is referred to the articles by Edward D. Lyn- 

 ton.^ - ^ No attempt will be made in this section to discuss laboratory 

 equipment and technique other than to give the following brief summary. 

 The magnetic orienting apparatus used by the California Research 

 Corporation is essentially an astatic magnetic balance consisting of two 

 mutually opposed bar magnets suspended by a thin wire to which is 

 attached a small mirror. The core is introduced beneath the magnetic 

 balance. A soft-iron core is then lowered about the balance and core 

 and shields the magnets from the earth's field and ambient magnetic fields. 

 When the core is slowly rotated, the magnetic field associated with the 

 core causes a deflection of the suspended magnets. This deflection is 

 amplified and recorded on photographic paper by means of a beam of 

 light focused on the mirror and reflected to a photographic drum. The 

 resulting record is in the form of a sine curve from which the direction 

 of polarity of the core can be determined. The assumption is made that 

 the direction of polarity of the core is parallel to that of the earth's field 

 at the location of the well. Once the magnetic directions of the core are 

 established and marked thereon, it is a simple matter to determine the 

 direction of strike and dip. 



Theory 



The basic observation that prompted a study of magnetic orientation 

 Avas made some years ago and was of considerable significance. The mag- 

 netism of a piece of basic igneous rock was found to have polarity which 

 agreed essentially in direction with the magnetic field of the earth at that 

 locality. Was this not a reliable way to orient a core in which directions 

 other than vertical are wholly unknown? With this in mind, a number 

 of outcrops were visited, where strike-and-dip measurements of rocks in 

 place were made. Rock samples were carefully marked and collected 

 from these outcrops and sent to the laboratory, where they were oriented 

 magnetically. The directions of strike and dip determined magnetically 

 checked surprisingly well with the field measurements. Directional differ- 

 ences of but 5° to 10° were obtained. A maximum difference of 20" 

 was found. 



To obtain a better understanding of magnetic orientation some of 

 the theoretical aspects that have a direct bearing on this problem are 

 briefly considered. All diamagnetic and paramagnetic substances must be 

 eliminated, for the latter possess magnetism only when placed in a mag- 

 netic field. Only ferromagnetic materials (of iron, nickel, and cobalt) 



' Lynton, E. D., Laboratory Orientation of Well Cores by Their Magnetic Polarity: Am. Assoc. 

 Petroleum Geologists Bull., voj. 21, pp. 580-615, 1937. 



^ Lynton, E. D., Recent Developments in Laboratory Orientation of Cores by Their Magnetic Polarity: 

 Geophysics, vol. 3, pp. 122-129, 1938. 



^Lynton, E. D., The Mechanics of the Upside Down Core: Geophysics, vol. 5, pp. 393-401, 1940. 



