How can I create a realistic impact crater?

I am currently working on a trench map, and I’ve added craters all over the around the trenches. As you could expect, they are the shitty geometry depressions with paint alpha on it. In dod, you can find various realistic looking craters in some of the main maps. This old trench map (http://www.garrysmod.org/downloads/?a=view&id=6410) has what I’m talking about.

http://filesmelt.com/downloader/craters_im_talking_about.jpg

The red outlined ones are the ones I would like to learn about, blue one seems interesting also.

In the background of the next picture you can see my shitty displacement crater.

http://filesmelt.com/downloader/dod_sunny_trenches0001.jpg

Can anyone help me out?

Also here are some artistic screenshots of the trenches themselves. I’m going to scrap the map because the trenches aren’t really that great.

http://filesmelt.com/downloader/garrys_mod_dod_ww1_trenches_going_over1.png

http://filesmelt.com/downloader/garrys_mod_dod_trench_warfare1.png

It looks like each crater is actually a smaller displacement on top of the main one(s).

Thank you.

For DOD there is a crater overlay that you can use.