Chap. 11] 



GEOPHYSICAL WELL TESTING 



865 



Measurements of radioactivity in shallow holes are made in connection 

 with the mapping of faults, contacts, dikes, radioactive ores, and the like. 

 In this case the emanation method is appUed. This involves the with- 

 drawal of soil air from the hole into an emanation chamber, as described 

 on pages 880-881. 



B. Magnetic Measurements 



Most magnetic well investigations involve laboratory tests of cores after 

 removal from the well. Procedures for determining magnetic suscepti- 

 biUties, remanent magnetization, and hysteresis curves of cores were de- 

 scribed on pages 300-309. In the magnetic core orientation method devised 

 by Lynton^" (Sperry-Sun), the direction of dip is determined by locating 

 the direction of magnetization in a well sample. With the exception of 



fdoo' 



^000' 



em' 



Open Me 

 fkcfnca/ 

 Log 



Open Hole 



damma 



Raij Log 



Cased Hole 



Oamma 



Pat/ Log 



- Impedance 

 ' Porosilg 



Padioact/\/ihj 



Fig. 11-34. Comparison of electrical (impedance and potential) logs with gamma-ray 

 logs in open and cased hole (after Howell and Frosch). 



limestones, anhydrites, and dolomites, cores with distinct bedding planes 

 generally retain a sufficient amount of magnetism after removal and ship- 

 ment. As shown in Fig. 11-35, the core, with axis horizontal, is placed 

 close to the lower needle of an astatic magnetic system which, with the 

 core, is shielded by a steel cylinder. The core is revolved slowly through 

 360°, and the deflection of the system is recorded photographically. The 

 record will show a sine wave whose extreme amplitudes are proportional 

 to twice the remanent magnetization. The ejffect of induced magnetiza- 

 tion which shows no reversal with rotation^^ may be eliminated by taking 

 a second run in reverse direction and forming the difference of the two 

 curves. To obtain absolute dip, a correction for the crookedness of hole 

 must be made. 



Magnetic measurements have also been used in wells to determine their 



"A.A.P.G. Bull., 21(6), 580-616 (1937); Geophysics, 3(2), 122-129 (1938). 

 " See Chapter 8, section ii b, p. 300. 



