We have used both electrical conductivity (EM) and electrical resistivity geophysical techniques for site investigations.

EM is a rapid method for mapping surface variations in geology based upon the material's ability to conduct an electrical current.

Clays in general conduct an electrical current whereas sand and gravel resist an electrical current.

We have successfully used EM to trace a narrow buried sand and gravel channel cutting into boulder

clay and to target exploration drilling.  Resistivity basically measures the inverse of conductivity such that sands and gravels have a high resistance and clays have a low resistance to an induced electrical current.  Interpretation of the resistivity methods produces a 'cross section' through the ground. In the image below (underneath our East Yorkshire office) the red colours represent dry chalk, the pale blue colours represent chalk below the water table and the dark blue colours represent boulder clay.