Faruk,
Well I don't know how to explain it better. What do you not understand?
I did notice I missed you porosity question. Seismic velocity does relate to porosity, so you could technically derive it from that, but this is complicated. You can use sonic logs (from wells) to derive predict porosity using a relationship that has been found empirically (through measurements). density, velocity and porosity are related rock properties. If you know two of them, you can predict the third one pretty accurately.
One way is using Vp n Phi (Porosity) is as follow
VP = A - B*Phi - Compaction Factor * C (units - Km/sec)
Her, A, B and C are constants and the compaction factor is a shale compaction factor that varies with pressure (eg depth). If you need to know the C. and A = 5.59, B = 6.93.
Also, please remember that check shot data is pretty useless if you have composite logs with sonic log already there. They are usually only shot if other logs are too expensive or not necessary. Synthetic seismograms are primarily useful to see a reflector's top or an impedance contrast based on the strata your well runs through. You can later compare this with the seismic that runs through the well location to get a velocity-depth relationship...
I hope that answered the rest of your questions.
Regards,
-J