When Fourier transformation is possible, extrapolation operators are complex numbers like e^{i kz z}. With complex numbers a and b there is never any question that .Then both splitting and full separation are always valid, but the proof will be given only for a more general arrangement.
Suppose Fourier transformation has not been done, or could not be done because of some spatial variation of material properties. Then extrapolation operators are built up by combinations of the finite-differencing operators described in previous sections. Let A and B denote two such operators. For example, A could be a matrix containing the second x differencing operator. Seen as matrices, the boundary conditions of a differential operator are incorporated in the corners of the matrix. The bottom line is whether AB= BA, so the question clearly involves the boundary conditions as well as the differential operators.
Extrapolation forward a short distance can be done with the operator .In two-dimensional problems A was seen to be a four-dimensional matrix. For convenience the terms of the four-dimensional matrix can be arranged into a super-large, ordinary two-dimensional matrix. Implicit finite-differencing calculations gave extrapolation operators like .Let denote a vector where components of the vector designate the wavefield at various locations. As has been seen, the locations need not be constrained to the x-axis but could also be distributed throughout the (x,y)-plane. Numerical analysis gives us a matrix operator, say A, which enables us to project forward, say,
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To march some distance down the z-axis we apply the operator many times. Take an interval , to be divided into N subintervals. Since there are N intervals, an error proportional to 1/N in each subinterval would accumulate to an unacceptable level by the time z_{1} was reached. On the other hand, an error proportional to 1 / N^{2} could only accumulate to a total error proportional to 1/N. Such an error would disappear as the number of subintervals increased.
To prove the validity of splitting, we take .Observe that the operator differs from the operator by something in proportion to or 1/N^{2}. So in the limit of a very large number of subintervals, the error disappears.
It is much easier to establish the validity of the full-separation concept. Commutativity is whether or not .Commutativity is always true for scalars. With finite differencing the question is whether the two matrices commute. Taking A and B to be differential operators, commutativity is defined with the help of the family of all possible wavefields P. Then A and B are commutative if .
The operator representing will be taken to be A+ B. The simplest numerical integration scheme using the splitting method is
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