Dear Mrs. Mcdaniel,

mining residues dumped on-site, mining subsidence either showing up as a cavity or back-filled with waste and adits are detected and investigated during an initial stage of investigation in the most effective way using methods of applied geophysics. Later they may also be drilled, but this does not seem your current issue. I suggest methods related to geoelectrics, geomagnetics and seismics which are cost-effective, do not need much manpower, easy to handle and their data output is ready for a swift interpretation. There are numerous textbooks (Roy, 1966; Slatt et al., 1992; Carlson and Zonge, 1997; Mussett and Khan, 2000, Parasnis, 2008, Ernstson, 2009; Ernstson and Kirsch, 2009; Idziak and Dubiel, 2011, Milsom and Eriksen, 2011). Another way is using the AMD (acid mine drainage) trickling out of open and covered mining adits. Run in combination with minerals of the so-called post-mining mineralization you may approximate abandoned mining sites and get a clearer picture where the entrance to the underground workings might be. I am still dealing with abandoned mining sites and you may find also some papers of mine available from the  RG server. Please keep in mind that the chemical methods are very climate-sensitive.

I wish you much success

H.G.Dill

No silver bullet in geophysical techniques generally.  It very much depends upon the surface and depth to the adit.  If the adit is under the water table and relatively shallow (sy 10m or so) then I'd suggest an EM survey (EM-31 or EM-34 for example).  Geoelectrical surveys such as resistivity and IP may be useful wrt acid mine activity; however, interpreting such data can be very, very difficult.  The cobble-boulder overburden sounds a bit odd.  The most effective means I know of to find a water filled tunnel or such is to use misse-a-la-masse or similar with a magnetometer (MMR method).  The techniques places an electric current into the tunnel and another electrode far away.  The electric current tends to follow the tunnel water out towards the far electrode.  The surface electric field or magnetic field (measure the difference between current on and then off to remove background fields) indicate the presence and extent of current flow..  I have used such techniques in mapping tree root zones and with an UG research project to find water channelling near bridge supports during flooding - usually hopeless targets with conventional geophysical tools. Modern adaptations of the idea are used in "pipe finder" tools used by engineers to find buried pipes in industrial areas with a lot of other buried clutter.