SPECFEM3D - Geometrical Spreading (analytical vs. simulated)

Dear all,

i am still struggling with wave damping along a profile. In case of a completely homogeneous body (attenuation off), surface waves should decay in amplitude with square root of the distance. But the simulated amplitudes from SPECFEM3D decay much stronger. Is there any explanation for this?
Any ideas?

And probably any idea, why the decay curve from specfem3d is “not that” perfectly shaped? Could this be interference of different wave types and their velocities at the surface?
Thanks so much.

Source is a vertical harmonic 1Hz vibration at x=500m,y=0,z=0 .
Receivers are at the surface.