Geoscience Reference
In-Depth Information
6.2 bASIC pRInCIpleS
The fundamental principle of EM induction is illustrated in Figure 6.1. In environmental, agricul-
tural, and engineering applications, EM methods are used primarily for locating either metallic
conductors or conductive fluids, and determining soil moisture and mineralogy variations. The
underlying concept of induction is widely known, and it is used for a variety of common tasks that
include metal detectors at airport security entrances, the measurement of electrical current flowing
in wires, and hunting for buried treasure with metal detectors.
The EM theory that underlies EM techniques has been developed since the later part of the
nineteenth century, with the work Lenz, Biot, Ampere, Faraday, and Maxwell. There are numer-
ous excellent physics and engineering texts on the subject at all levels of mathematical complexity,
including works by Stratton (1941), Balanis (1989), Morse and Feshbach (1953), and many others.
Excellent summaries are present in the geophysical literature (e.g., Keller and Frischknecht, 1966,
and Ward and Hohmann, 1987). The following discussion is intended to cover some of the funda-
mentals that are essential to a basic understanding of the concepts underlying EM methods.
Ampere was the first investigator to recognize that a current passing through a wire has an
associated magnetic field. A simple experiment, shown in Figure 6.1a, is often used to demonstrate
the principle of Ampere's law. Iron filings laying in a plane perpendicular to a current-carrying wire
(e.g., a wire sticking up through a table containing iron filings) will form circular patterns around
the wire when current flows through the wire. The path of the line integral for the case of a straight
current-carrying wire becomes a circle with its center on the wire. The integral sum of the magnetic
field along this path is 2π
r
, which is the circumference of a circle of radius r, and the relationship
between the EM field caused by current flow can be stated as follows:
B
=
I
2π
(6.1)
r
where
B
is the magnetic induction (magnetic field strength),
I
is the current flowing in the wire, and
r
is the distance from the wire.
The magnitude of the magnetic field increases proportionally to the electric current, and there
are no magnetic fields in any direction other than the circular path around the wire. Furthermore,
the direction of the magnetic field is in a path determined by the “right-hand-rule,” which states that
Source: current
flow through wire
coil
No current
flow
Current
flow
Resulting magnetic
field in space
Iron filings
Resulting magnetic
field in space
(a) Effect of electric current on iron filings
(b) Magnetic field from current
along a straight wire
(c) Magnetic field from current
through a coil
fIGURe 6.1
Ampere's law: (a) current flowing through metal filings resting on a table will cause magnetic
filings to arrange in a circumferential pattern around the current-carrying wire; (b) current flowing through
a wire causes a magnetic field that is circumferential to the wire carrying the current; and (c) current flowing
through a coil of wire causes a perpendicular magnetic field that is in the shape of a toroid around the coil.
Note the current flow and magnetic field directions in all cases of the primary and secondary fields follow the
right-hand rule.
