Magnetic Susceptibility Logging

Basic Information

Description

Under low-frequency, weak magnetic field conditions, the ratio of the secondary magnetic field induced in a magnetic medium to the applied magnetic field is the initial magnetic susceptibility, or susceptibility for short. Numerically, susceptibility equals the relative magnetic permeability of the medium minus one.

Because susceptibility has a linear relationship with the iron content of the mineral, the susceptibility logging curve can be used to rapidly determine the grade of iron ore. Based on the principle of electromagnetic induction, the tool uses two inductor coils of equal inductance and opposite winding direction to excite a low-frequency alternating magnetic field into the surrounding formation.

The in-phase secondary magnetic field produced by formations containing magnetic media unbalances the transmitting coil; the unbalanced bridge signal is measured, processed by a built-in microprocessor, and uploaded digitally to the surface instrument for recording. Thanks to the special design of the sensor, eddy currents in highly conductive media cancel each other out, so the measurement output directly reflects the variation in formation magnetic susceptibility.

It employs high-sensitivity sensors for accurate and reliable measurement, and the sensor outer tube is made of high-density non-magnetic non-metallic material. It is currently mainly used in magnetite (iron ore) logging.

Specification