There are two kinds of
*dependent*
variables, those that characterize the medium and
those that characterize the waves.
The medium is characterized by its velocity *v* and its reflectivity *c*.
The waves are characterized by using *U* for
an upcoming wave, *D* for a downgoing wave, *P* for the pressure,
and *Q* for a modulated form of pressure.
Let us say *P*(*t*,*x*,*z*) is the mathematical function to find pressure,
given (*t*,*x*,*z*); and *P*' (*t*' ,*x*' ,*z*' ) is
the mathematical function given (*t*' ,*x*' ,*z*' ).
The statement that the two mathematical
functions *P* and *P*' both refer
to the same physical variable is this:

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10/31/1997