WELLOG ELECTROMAGNETISM
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.
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