Geology Reference
In-Depth Information
have to be deployed, and more measurements will have to be made, at each
field point.
5.2 Varying Currents
Varying electrical currents circulating in
transmitter
circuits can generate
subsurface currents without actual physical contact, using either inductive
or capacitive coupling. Such methods, essential in airborne work, can also
be useful on the ground, since making direct electrical contact is always
tedious and may be impossible on concrete, asphalt, ice or permafrost.
5.2.1 Induction
In the late 1800s, Ampere discovered that a circular magnetic field existed
around a current passing through a wire. A little later, Faraday realised that,
conversely, a changing magnetic field could induce current in a wire loop.
Maxwell combined and expanded these principles to provide a complete
description of the interactions between EM fields and conductors. Geophys-
ical interpretation of EM data is about visualising Ampere's and Faraday's
laws at work in the subsurface.
In a changing magnetic field, a voltage (electromotive force, or
emf
)is
induced at right angles to the direction of the change, and currents will flow
in any nearby conductors that form parts of closed circuits. The equations
governing this phenomenon are relatively simple, but geological conductors
are very complicated and for theoretical analyses the induced currents,
known as
eddy currents
, are approximated by greatly simplified models.
Eddy current magnitudes are determined by the rates of change of current
flow in the inducing circuits and by a geometrical parameter known as the
mutual inductance
. Mutual inductances are large, and conductors are said to
be well coupled, if there are long adjacent conduction paths, if the changes
in the inducing magnetic field take place at right angles to directions of easy
current flow, and if magnetic materials are present to enhance field strengths.
When current changes in a circuit, an opposing emf is induced in that
circuit. As a result, a tightly wound coil strongly resists current change and
is said to have a high
impedance
and a large
self-inductance
.
5.2.2 Permeability and permittivity
Electrical conductivity and resistivity are adequate for describing the ways
in which charges move in conductors in response to steady fields. With
varying fields, other factors become important, and Maxwell's
constitutive
equations
describe how materials respond to the fields that constitute an
electromagnetic wave. Permittivity,
ε
, describes how constrained charges
