感应测井是利用交变电磁场研究岩层导电性的一种方法。与电阻率测井的供电电极相类似,在感应测井的井下仪器中装有发射线圈 T,其中通以交变电流(通常为20千赫),这个电流将在其周围造成一个交变电磁场φ_1,处在交变电磁场中的导电介质便会感应出围绕井轴的环形电流i_1(见图14—1)。在均匀各向同性岩层中,环形电流的中心是和井轴一致的。该电流也将造成一个磁场,即二次磁场φ_2,并且在井下仪器的接收线圈 Induction logging is a method of studying the conductivity of rock strata using alternating electromagnetic fields. Similar to the resistive logging power supply electrode, a downhole instrument for inductive logging is equipped with a transmitting coil T, through which an alternating current (typically 20 kHz) is passed which creates an alternating electromagnetic field around it φ_1, the conductive medium in the alternating electromagnetic field induces a ring current i_1 around the well axis (see Figure 14-1). In a homogeneous isotropic formation, the center of the toroidal current is coincident with the well axis. This current will also cause a magnetic field, the secondary magnetic field φ_2, and the downhole instrument’s receive coil