Question about material definition!

[BFF-LEI-Lei]

Hi,
Can we define a region below z=0 in sw4 with extremely small rho, vp, and vs? I want to simulate a situation with an underground cavity.
For example, I define an order like this:
block vp=4000 vs=2500 rho=2000
block vp=0.0002 vs=0.0001 rho=0.0001 z1=35000 z2=100000

I've already done this, but the solution seemed to suspend.

nColumns: 4

Column 1: Time (s)

Column 2: X velocity (m/s)

Column 3: Y velocity (m/s)

Column 4: Z velocity (m/s)

0.000000e+00 0.00000000000000000e+00 0.00000000000000000e+00 0.00000000000000000e+00
7.194245e-05 0.00000000000000000e+00 0.00000000000000000e+00 0.00000000000000000e+00
......
9.352518e-03 4.19643817116784001e+20 4.83378359524465574e+20 -2.28673577542815396e+26
9.424460e-03 4.21440352362263770e+28 4.85420987903153678e+28 -2.29650169146806713e+34
9.496403e-03 -nan -nan -nan
9.568345e-03 -nan -nan -nan
9.640288e-03 -nan -nan -nan
9.712230e-03 -nan -nan -nan
9.784173e-03 -nan -nan -nan
9.856115e-03 -nan -nan -nan
9.928058e-03 -nan -nan -nan