magnetizing force in the frequency range above 10 cps so that reasonable 

 alternating-current (AC) amplifier or detection means can be employed and 

 below 10 KC so skin effects are minimized. Such an AC measurement is free 

 from remanent magnetization effects and allows a measurement of magnetic 

 susceptibility alone. Because an external magnetizing force is required, the 

 magnetic susceptibility instrument is classified as an induction logging tool. 



Total field measurements reflect the sum of magnetic effects that are 

 responsible for the magnetic field from the rock, that is, the remanent magnetism 

 plus the field resulting from the magnetizing force of the earth's field in con- 

 junction with the magnetic susceptibility of the rock. Surface magnetic prospect- 

 ing schemes are complicated by the polarization of the remanent magnetism. 

 Often it is not in the direction of the present earth's magnetizing force. For this 

 reason the interpretation of total field borehole measurements is complicated, 

 and such measurements have not been widely used. 



INDUCTION LOGGING If an elongated solenoid with a low reluctance 



core is inserted in a borehole as shown in 

 Figure 22-1, it will have a self impedance Z at a particular frequency. The 

 resistive term reflects losses in the coil as well as eddy current losses in the rock 

 surrounding the coil. The reactive term reflects the inductance of the coil, 

 which is influenced by the susceptibility of rock in the external magnetic path. 

 Thus, changes in rock conductivity will influence the resistive component, and 

 magnetic susceptibility will influence the reactive component of the coil im- 

 pendance Z. 



If such a solenoid is placed in one arm of a bridge and excited at some 

 frequency, as, for example, 1000 cycles per second (cps), the bridge will become 

 unbalanced with changes in conductivity or susceptibility of the rock surrounding 

 the solenoid. Because the unbalance voltages are in phase or in quadrature 

 respectively, it is possible by phase detection to separate the two terms. Thus, 

 they can be measured independently. A basic block diagram of such a system 

 is shown in Figure 22-2. The coil and bridge are in the logging probe and are 

 attached to the logging cable. The oscillator is at the surface and supplies 

 excitation to the subsurface probe, as well as reference voltage to the phase 

 sensitive detectors. The reference voltage to one phase detector is shifted 90 

 degrees with respect to the other. Thus, the signals are separated and are 

 recorded on a dual-channel recorder. 



The design of the sensing coil requires a compromise between a long coil 

 for penetration into the formation and a short coil for delineating thin beds. 

 A universal probe has been standardized, having a solenoid 21 inches long. 



429 



