If the wind is blowing keeping the tarp in place is a challenge to say the least. 
I use one of those "Gopher" pick up tools they advertise on TV. I bought mine for ten bucks at Walgreen's. I also have a long handled minnow net from Academy to put the salvaged brass in - - - saves wear and tear on my old arthritic knees.
I can pick up brass with this rig quicker and easier than bending or stooping for each piece of scattered brass. 