Transient electromagnetics, or time-domain methods (TEM or
TDEM), refer to a branch of geophysics, among other disciplines,
that use an electromagnetic impulse excitation to investigate
the subsurface. TEM methods are generally sensitive to the
electrical properties of the subsurface in geologic
applications, but are also sensitive to magnetic properties in
applications like UXO detection and characterization. Two
fundamental electromagnetic principles are required to derive
the physics behind TEM surveys: Faraday's law of induction and
Lenz's Law. A loop of wire is generally energized by a direct
current. At some time (t0) the current is cut off as quickly as
possible. Faraday's law dictates that a nearly identical current
is induced in the subsurface to preserve the magnetic field
produced by the original current (eddy currents). Due to ohmic
losses, the induced surface currents dissipate--this causes a
change in the magnetic field, which induces subsequent eddy
currents. The net result is a downward and ou