“…They have been adapted to numerical techniques as different as finite differences, finite elements and spectral elements in order to reduce reflections at the outer boundaries of the computational domain and efficiently simulate unbounded media at the local or regional scale. Damping layers or 'sponge zones' (Cerjan et al 1985;Sochacki et al 1987), paraxial conditions (Clayton & Engquist 1977;Engquist & Majda 1977;Stacey 1988;Higdon 1991;Quarteroni et al 1998), optimized conditions (Peng & Töksoz 1995), the eigenvalue decomposition method (Dong et al 2005), continued fraction absorbing conditions (Guddati & Lim 2006), exact absorbing conditions on a spherical contour (Grote 2000) or asymptotic local or non-local operators (Givoli 1991;Hagstrom & Hariharan 1998) have been introduced. However, the local conditions generate spurious low frequency energy reflected back into the main domain at all angles of incidence, sponge layers require a prohibitive number of gridpoints to be relatively efficient, and paraxial conditions do not efficiently absorb waves reaching the boundaries at grazing incidence and can become unstable in some cases, for instance for a high value of Poisson's ratio.…”