Electromagnetism is used to measure Conductivity and Resistivity. The principals of electromagnetic induction are applied in Airborne, surface and borehole logging systems.
Electromagnetic methods include a number of surface geophysical applications. Surface geophysical applications include Frequency Domain systems and Time Domain Systems.
Frequency Domain systems use continuous wave, usually sine wave transmitters. The receiver usually is designed to measure in-phase (I) and quadrature (Q) portions of the signal.
Time Domain systems usually transmit an electromagnetic pulse. The pulse enters the surrounding ground and creates eddy currents in conductors.
The Eddy currents in the conductors generate a secondary magnetic field that is measured by receiver electronics.
WELLOG has webpages that are directed toward specific applications related to electromagnetic methods.
An overview of electromagnetic theory follows:
ELECTROMAGNETIC INDUCTION: (Faraday’s Law)
When electron current flows in a conductor, a magnetic field is created.
The current in amperes multiplied by the number of turns defines the magnetomotive force (Ampere-Turns).
Magnetic field FLUX intensity is directly related to Ampere-Turns x Area:
Relationship of Flux Intensity and Flux Density:
A changing magnetic field created by a sinusoidal alternating current will induce a current in a conductor.
The induced current will flow in a direction opposite to the current that is causing the magnetic field.
This opposing “SECONDARY” magnetic field and current is directly related to “CONDUCTIVITY”.
A Receiver coil measures the secondary magnetic field.
Induction logging tools are used to measure conductivity and its reciprocal, Resistivity.
Revised 11-07-2018 © WELLOG ALL RIGHTS RESERVED