You beat me to it. This is exactly what I was thinking. I wonder if there is such a thing as a commercially available, reasonably priced machine that does this, or if it is only a hugely expensive military piece of hardware.Ground penetrating radar???
Edit: Found it, Bing search. http://penetradar.com/iris-gpr-systems.html. Look particularly at the man-portable units on this page.
Look at these guys, too. They were actually the first result, so maybe better, more popular? https://www.geophysical.com/products
Plus, the whole Bing results page. https://www.bing.com/search?q=ground+penetrating+radar+for+sale&qs=AS&pq=ground+pen&sk=ONR2AS2&sc=8-10&cvid=CDAB15DA791F4485BE790ACD552FD214&FORM=QBLH&sp=5&ghc=1