Geoscience Reference
In-Depth Information
right-hand side of the figure caused by a battery. The right loop emits a magnetic field according to
Ampere's observations, with the vector direction of the magnetic lines of flux governed by the right-
hand-rule. The induced magnetic field propagates through the air and induces current to flow in
the passive loop that is shown on the left-hand side of the figure. Figure 6.2c shows how these laws
can be combined in a “source” and “receiver” arrangement, with the receiver “detecting” the EM
field, as follows: (1) current flowing in the source coil creates a “primary” magnetic field, (2) flux
lines from the primary field flow through the second coil on the right creating a flow of current in
the second coil (called an induced current), and (3) the flow of current in the second coil, in turn,
creates a “secondary” magnetic field. It logically follows that this secondary magnetic field can, in
turn, create another secondary magnetic field that will induce another current flow in the source
coil. This is called mutual coupling. It should also be noted that if the secondary coil is oriented
perpendicular to the primary coil, then no current will be induced in the secondary coil. The vector
nature of the EM field is often used to determine the maximum and minimum direction of the EM
field by rotating the receiver coil.
So far in this discussion, we have only been concerned with static, or steady, fields (i.e., con-
stant, or direct, current; magnetic moving at a constant velocity, or no acceleration, through the
loop). If we consider that the field is time varying (e.g., and alternating, or AC current), then Fara-
day's law expressed as a time-varying magnetic field can be combined with Ampere's law to form
the wave equation. We will not show the differential equations for these expressions, but they show
that any time-varying EM energy (AC current, or sinusoidal magnetic field) will move (propagate)
through time and space. The wave equation governs the propagation of all EM waves, including
radio waves. There is also a mechanical analog of the EM wave equation for acoustic and seismic
wave propagation.
Ampere's and Faraday's laws are the two fundamental laws for all of EM theory, and the gov-
erning principles for the EM induction geophysical method. The practical challenge for the geo-
physical interpreter is to visualize how this applies in the subsurface. We need to combine the
principles of Faraday's and Ampere's laws, visualizing the EM field interacting with an object in the
subsurface, as illustrated in Figure 6.3. The source, or EM transmitter, energized by a time-varying
current, emits an EM field that propagates into the subsurface. If the EM field encounters a change
in electrical conductivity, then a change in the EM field is induced in the object. The object-induced
Electromagnetic
receiver
Electromagnetic
transmitter
Magnetic field from
electromagnetic transmitter
Magnetic field from
induced eddy current
Conductive
body
Eddy current
induced on the body
fIGURe 6.3
Basic principle of geophysical induction. A
primary electromagnetic field
emitted from a
transmitter propagates through space until it encounters a conductor. Eddy currents induced in the conductor
radiate a
secondary field
that is measured at the receiver location.
