Forward solution speed-up for 3D eddy current inversion

The forward solver is the bottleneck in eddy current flaw reconstruction because the solution of a dense system matrix is required. Two methods are developed to increase the solution speed. The cell merging technique gives impressive speed-up for cracks but a fully adaptive version for volumetric flaws has not been found yet. The other algorithm based on sparse representation of rank deficient matrix blocks and the Woodbury procedure is slower than the cell merging technique but it is robust and still significantly faster than the LU factorization. Creating a hierarchical partitioning tree ensures the stability of the latter algorithm