Geophysics is without a doubt the way to go to locate the extent of
graves in a (now) unmarked cemetery.  These days computer assisted data
analysis of resistivity and ground penetrating radar data can define the
features in 3D with a remarkable degree of accuracy, but this takes
experienced technicians who really know something about archaeology and
who have the right equipment for the job. The cost is generally far less
that for more traditional excavation-based approaches.
 
We have recently had excellent results working on cemetery sites in Iowa
and Michigan. Our team will be working on a cemetery in Italy later this
summer.
 
If you would like more details, contact me off list and I will put you
in touch w/ our geophysics dudes.
 
John
 
John P. McCarthy, SOPA
Vice President, Sr. Archaeologist/Historian
IMA Consulting, Inc.
Minneapolis