1. adj. [Formation Evaluation]
Related to a wireline log of formation resistivity based on the principle of inducing alternating current loops in the formation and measuring the resultant signal in a receiver. In the simplest device, an alternating current of medium frequency (10?s of kHz) is passed through a transmitter coil, thereby inducing an alternating magnetic field in the formation. This field creates current loops in the formation. The loops produce their own magnetic field, which induce a current when they cross the receiver coil. This signal is proportional to the conductivity of the formation, with contributions from different regions of the formation summing approximately in conductivity. As a result, the induction log is most accurate at high conductivities and with resistive invasion. However, below about 1 ohm-m skin effect becomes important.
Practical induction-logging tools use arrays of several coils, designed to achieve a specific focusing and depth of investigation. These arrays are either hardwired, such as the 6FF40, 5FF40 and others, or consist of several simple arrays that are combined in software (an array induction). For many years, the most common induction log was the 6FF40. This was often combined with the medium induction and a shallow laterolog or microresistivity log so as to correct for the effect of invasion, assuming a step profile. Induction logs also need borehole correction and shoulder-bed correction. In older tools, this was accomplished through multiple correction charts, while modern tools include software for this purpose.
H.G. Doll introduced the first practical induction-logging technique in 1949. See Doll HG: Introduction to Induction logging and Application to Logging of Wells Drilled with Oil Base Mud Journal of Petroleum Technology 1, no. 6 (June 1949): 148-162.
See: array induction, cave effect, dual induction, electrical anisotropy, polarization horn, R signal, resistivity log, skin effect, X-signal