Computer memory cells are often used to model points in a volume that contains propagating waves. Though we often wish to model an infinite volume, the number of computer cells is, regrettably, finite. Waves in the computer reflect back from the boundaries of the finite computer memory when we would prefer that the waves had gone away to infinity. To avoid the need for infinite computer capacity this section develops the theory of absorptive side boundary conditions.

There are two kinds of side boundary difficulties.
First is where we are at the end of our observations.
Second is where we somehow decided to limit the extent of our calculations.
These boundaries might be the same.
But to avoid confusion,
let us presume that the data is of more limited
extent than the computer memory.
So alongside the data,
which comprises the initial conditions for the calculation,
is a region which will be called the *data padding* .

- Data padding
- Truncation at cable ends and at survey ends
- Engquist boundaries for the scalar wave equation
- Engquist sides for the extrapolation equation
- Size of the reflection coefficient

10/31/1997