**WELLOG **** ELECTROMAGNETISM**

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** 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.**

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**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. **

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**Additional considerations include
Magnetic intensity in CGS units and the conversion**

**As shown in the following illustration.
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**SUMMARY:**

**The capability of an electromagnetic
system in geophysics is directly dependent on the magnetomotive force and area
of the coil system employed.**

**APPLICATIONS:**

**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:**

**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:**

**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.**

**Contact WELLOG at ****info@wellog.com****
with your questions on the use of EM methods in Geophysics.**

**Revised 12-30-2020 © WELLOG ALL RIGHTS RESERVED**

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