i am given to understand that there is also a radiation measurement that is able to check decomposition for several yards of depth. And there is an electric wave test that measures the varied resistivity of the ground, that can locate variations in ground density. It is used in looking for oil, water, large gold veins, terrorist tunnels, etc. and is extremely expensive. (Phoenix software, markham) Actually, an invasive method of digging a four-inch test hole to withdraw a soil core is probably the cheapest