A basic understanding of electromagnetic theory is essential to appreciation of the capabilities and limitations of electromagnetism in geophysics.
BASIC ELECTROMAGNETIC THEORY:
The following illustration shows a simple circuit having a power source connected to an inductor having a number of turns (N).
In this case, the number of turns is 10. The Source of power is a battery and given a certain circuit resistance, the current (I) through the inductor is 1 ampere.
It can also be appreciated that the current flow through the inductor creates a magnetic field. The magnetic field has a force known as Magnetomotive Force (MMF).
Further, the magnetomotive force is directly proportional to the current (I) and the Number of turns (N) such that MMF = N x I.
Induction coils used in Geophysical applications provide magnetic energy intensity referred to in terms of a magnetic “Moment”.
As shown in the following illustration, the Moment is the product of Number of turns (N), Current (I), and Area (A) in meters.
The area (A) is a measurement of the area of the loop. In this example, a loop having an area of 1 meter, 1000 turns and current of 1 ampere
Imposes a magnetic Moment (N x I x A) of 1000 Newtons.
Additional considerations include Magnetic intensity in CGS units and the conversion
As shown in the following illustration.
The capability of an electromagnetic system in geophysics is directly dependent on the magnetomotive force and area of the coil system employed.
Electromagnetism is applied in many geophysical methods. 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 on this website that are directed toward specific applications related to electromagnetic methods.
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Revised 12-30-2020 © WELLOG ALL RIGHTS RESERVED