There are several excellent references regarding the location of
unmarked burials in the forensic archaeology/anthropology literature.
Here are some of the most useful ones:
 
Bevan, BW
 1991   The search for graves.  Geophysics 56(9):1310-1319.
 
Bevan, BW, King, JA, and RJ Hurry
 1993   The reliability of geophysical surveys at historic-period
cemeteries: An example from     the Plains Cemetery, Mechanicsville,
Maryland.  Historcal Archaeology 27(3):4-15.
 
Brooks, ST, and RH Brooks
 1984   Problems of burial exhumation, historical and forensic.  In TA
Rathbun and JE  Buikstra (eds.): Human Identification: Case Studies in
Forensic Anthropology, pp.      64-86.  CC Thomas, Springfield, IL.
 
Duncan, J
 1983   Search techniques.  In D Morse, J Duncan, and J Stoutamire
(eds.): Handbook of     Forensic Archaeology and Anthropology, pp. 4-19.
Bill's Book Store: Tallahassee.
 
Ellwood, BB
 1990   Electrical resitivity surveys in two historical cemeteries in
northeast Texas: A method       for delineating unidentified burial
shafts.  Historical Archaeology 24:91-98.
 
France, DL, Griffin, TJ, Swanburg, JG, Lindemann, JW, Davenport, GC,
Trammell, V, Armbrust, CT, Kondratieff, B, Nelson, A, Castellano, K, and
D Hopkins
 1992   A multidisciplinary approach to the detection of clandestine
graves.  Journal of     Forensic Sciences 37:1445-1458.
 
France, DL, Griffin, TJ, Swanburg, JG, Lindemann, JW, Davenport, GC,
Trammell, V, Travis, CT, Kondratieff, B, Nelson, A, Castellano, K,
Hopkins, D, and T Adair.
 1997   NecroSearch revisited: Further multidisciplinary approaches to
the detection of        clandestine graves.  In WD Haglund and MH Sorg
(eds.): Forensic Taphonomy: The         Postmortem Fate of Human
Remains, pp. 497-509.  CRC Press, Boca Raton.
 
Killam, EW
 1990   The Detection of Human Remains.  CC Thomas, Springfield, IL.
 
Parrington, M
 1983   Remote sensing.  Annual Review of Anthropology 12:105-124.
 
Rodriquez, WC, and WM Bass
 1985   Decomposition of buried bodies and methods that may aid in their
location.  Journal      of Forensic Sciences 30:836-852.
 
 
Bruce Bevan of Geosight in Pitman, NJ is one of the best at conducting
non-invasive invstigations of unmarked graves, especially in
archaeological contexts.  He can be reached at 609-589-9294.
NecroSearch International is located in Lakewood, CO and can be reached
at 303-232-0636.
 
Although effective when used judiciously and with caution, steel probes
and augers should really be the last methods used to locate unmarked
burials, considering the potential damage these devices can inflict on
both human remains and mortuary artifacts.
 
I hope that this information is of assistance