182 EXPLORATION GEOPHYSICS 



Susceptibility Measurements of Rocks in Place. — A means of mea- 

 suring the magnetic susceptibility of rocks in place was worked out by R. 

 G. PatersonJ. It has the unique advantage that no samples need to be taken. 

 The equipment required is small in size and light in weight so that rock 

 exposures can be tested in the field where and when desired. 



The principle underlying the method is the change caused in the induc- 

 tance of a coil, carrying a current, when a paramagnetic or a diamagnctic 

 substance comes into its magnetic field. The test coil is wound in the form 

 of a circular disc, which is placed on a flat surface of the rock, susceptibility 

 of which is to be measured. Only a part of the magnetic field from the 

 coil penetrates the rock under test. The winding of the coil forms part of 

 one arm of an alternating current inductance bridge so that the change in 

 inductance in it, caused by the rock on which it lies, can be measured. 



More accurately described, the test coil is 9.4 cm. in mean diameter, 

 wound with 1000 turns of No. 31 enameled wire which occupies a slot 

 0.5 X 1.0 cm. in cross section. Its resistance is 139.1 ohms and its inductance 

 0.17995 henries. 



The circuit used is the Owen bridgef , having the usual four arms with 

 their impedances designated as Zi, Z^, Z^ and Z^. Of these Zi consists of 

 the coil, as described above, which is connected by 8 feet of flexible 2- 

 conductor lead and a 5000-ohm variable resistance. The arm Zg has a 

 similar 5000-ohm variable resistance and a 0.13 microfarad condenser. The 

 Za arm carries an 0.06 microfarad condenser. A fixed value resistor of 

 5000 ohms makes up the Z^ arm. A 500-cycle generator provides the 

 necessary power, and the bridge balance is found by minimum sound with 

 head phones and amplifier. 



The inductance of the coil is first measured in air, and again measured 

 when it is lying on the face of a rock under investigation. The difference 

 of the two inductances is a measure of the magnetic susceptibility of the 

 rock at that frequency. A calibration curve is used which gives the change 

 in microhenries in terms of susceptibility, for a given field strength. The 

 relation of change in coil inductance and susceptibility was determined by 

 tests using solutions of different percentages of iron chloride. 



This method of susceptibility measurement removes all question as to 

 the possible change in the magnetic character of a specimen caused by 

 shaping or pulverizing it. It affords an easy means for making a number 

 of tests in the field at adjacent places on an outcrop. This greatly minimizes 

 the inherent error of assuming that a given sample is representative. 



There are two disadvantages to the method, one of which is the large 

 size sample required for the test (unless it is used on a rock in place). The 

 sample must have a relatively flat surface about twice the size of the coil, 

 and a thickness about equal to 0.6 the coil dimension, in order to give the 



t R. G. Paterson, "Determination of Magnetic Susceptibilities of Rocks in Situ," A.I.M.E. 

 Tech. Pub. 1298, Feb., 1941. 



t D. Owen, "A Bridge for the Measurement of Self-induction in Terms of Capacity and 

 Resistance," Proc. Phys. Soc. (1915), Vol. 27, pp. 35-39. 



