[CIG-SEISMO] source discretization in SW4

[Avinash Nayak]

Hello,
I am a beginner in SW4 and I am hoping for some help about source
discretization.

In SW4, is it required to put the source inside a volume of homogenous
elastic properties as in some other finite difference codes ?

How many grid points wide is the stencil for discretizing a point
source ? Given that source discretization in SW4 is mentioned to be
4th (p) order accurate (Chapter 4, Installation manual 1.1), following
requirement Q >= p-1+s (in paper Petersson and Sjogreen [2009] Stable
grid refinement ..,LLNL-JRNL-419382) , is the stencil 4/5 points wide
for single force (s=0) /moment sources (s=1) (assuming equality sign)
along a particular dimension ?

How close can we put a source to a sharp velocity contrast ? What is
the discretization procedure for a source on the boundary (i.e.
vertical force on free surface in Lamb's problem).
Is there a reference where I can read about these things ?
I will really appreciate your help.

Thanks.
Regards,
Avinash