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**Impedance** filters
are a special class of minimum-phase filters
that model energy-conserving devices and media.
The real part of the Fourier transform of an impedance filter is positive.
Impedances play a basic role in mathematical physics.
There are simple ways
of making complicated mechanical systems from simple ones,
and corresponding mathematical rules
allow construction of complicated impedances from simple ones.
Also, impedances can be helpful
in stabilizing numerical calculations.
Logically,
a chapter on impedance filters belongs here,
but I have little to add to what is
already found in FGDP and IEI.
FGDP describes the impedance concept in sampled time
and its relation to special matrices called ``**Toeplitz**'' matrices.
IEI describes impedances in general
as well as
their role in physical modeling
and imaging with the wave equation.

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Stanford Exploration Project

10/21/1998