Surface waves control the peak of the seismic records at regional and teleseismic distances. In complex structures, such as basins, their amplitude is produced by the interference between the topography and the wedges of the model. Therefore, they represent a challenging target for numerical simulations. When modelling low frequencies in thin media, such as basins and waveguides, some instabilities arise from the interaction between propagating evanescent waves and the artificial layer, required to avoid spurious reflection to be sent back into the elastic volume. Here we propose to generalize the absorbing boundary conditions, by adding a cut-off frequency and an overdamping. The efficiency is demonstrated through a comparison with analytical solutions. Finally an analysis is performed by adopting a complex basin geometry, where we show that the pollution of classical absorbing conditions becomes significant near the edges of the model.