Which type of log induces a current in the formation through the use of transmitter coils and induced magnetic fields in the formation?

• A. Spontaneous Potential
• B. Induction log
• C. Gamma Ray
• D. Resistivity log

Answer: (B)

Induction logging is all about using electromagnetic induction to probe formation conductivity. The transmitter coil is energized with an alternating current, creating a changing magnetic field that penetrates the surrounding rock. This varying field induces eddy currents in the conductive parts of the formation. The induced currents generate their own secondary magnetic field, which the receiver coils detect. By analyzing the strength and phase of this secondary field, the tool infers how easily the rock conducts electricity, giving a measure of formation resistivity.

This approach differs from other logs that rely on natural potentials or direct contact with the formation. Spontaneous Potential uses natural electrochemical differences, Gamma Ray measures natural gamma radiation, and resistivity logs inject current through electrodes in contact with the borehole wall rather than using transmitter coils to induce currents.