I've opened a issues at the repository,seeing
I have most of the reference materials, except the first one(Lysmer, J., and Kuhlemeyer, R. L. (1969). ), for it isn't available to my library, so if someone have it please send it to email@example.com. 1D obsorbing boundary condition can be obtained by apply a force function with the varibles of wave velocity and boundary particle velocity, while it is much more difficult for 2D and 3D situation. Also, it has other methods not mensioned there. Any further investigation and discussion is highly expected.