It is not uncommon now for geophysical inverse problems to be parameterized by 10 to 105 unknowns associated with upwards of 106 to 107 data constraints. The matrix problem defining the linearization of such a system (e.g., ~m = b) is usually solved with a least-squares criterion (m = (~t~ -) ~ tb). The size of the matrix, however, discourages the direct solution of the system and researchers o...