problem about absoring boundary and dispersion

[CHAO1111111]

Sorrry to bother you. I'm new at ground motion simulation,so maybe what i am curious are basic issues.
I have a question about acoustic-elastic absorbing boundary and disspersion. When I defined the different types of materials and turned the "STACEY_ABSORBING_CONDITIONS" to be true, the code will define different types of absorbing boundary for acoustic elements or elastic elements automatically?
In addition, how to avoid the appearance of dispersion. The minimum S-wave veloctity is 1700 m/s, and the maximum frequency i want to simulate is 2.5Hz,so the minimum wave length is about 600m, which means the 600m of mesh length is enougth to avoid the dispersion?The movie of surface seems to present dispersion.
Thanks!