An important category of non-invasive site investigation techniques involves geophysical survey methods. In addition to finding buried objects (as discussed under asset location), these methods can identify boundaries among soil and rock zones and infer some aspects of the geotechnical properties of those zones. The methods can quickly investigation large volumes of the ground and work well together with soil sampling boreholes that provide more detailed data but at widely-spaced discrete locations. These geophysical survey methods mostly introduce seismic and electromagnetic waves or signals into the ground and track geological boundaries and properties by wave reflections, wave refractions, electrical resistance, speed of wave travel, etc.