DIFFRACTION COEFFICIENTS FOR FLAT-BOTTOMED HOLES FROM 3-D FINITE-DIFFERENCE CALCULATIONS

Calculations have been performed with a finite difference model Swam3d to predict the time-dependent scattering in three spatial dimensions from the edge of a flat-bottomed hole. It is shown that 'diffraction coefficients' can be defined for non-normal incidence on flat-bottomed holes and that these can be calculated using 3-D finite difference techniques. It is concluded that it is appropriate to use diffraction coefficients obtained from the canonical problem of scattering from a 270-degrees vertex and to multiply these by a 'curvature term' which depends solely on the angles of incidence and scattering, the radius of curvature of the edge and on the ranges from the transmitter to the receiver, as expected from the geometrical theory of diffraction. Comparisons of results for this curvature term, determined from finite difference calculations, are made with an analytical expression. Good agreement is achieved away from specular directions. A problem exists with determining diffraction coefficients and the curvature term near specular reflection directions with the finite difference model. A solution is proposed but not tested.