Ground penetrating radar has brought to light ancient ruins, detected water leaks, and has recently helped locate a massacre site, all without anyone touching a shovel.
The traditional picture of an archeological site is a hole cut neatly into the ground. In that hole, people in tan outfits and wide-brimmed hats kneel with shovels and brushes, carefully scraping away at the dirt. Perhaps to one side there's a specimen table and to the other there's Jeff Goldblum talking about how scientists are so focused on figuring out whether or not they could do something that they didn't consider whether or not they should.
In reality, scientists often know that they shouldn't and so they don't (even if they want to). Sometimes a site is too large, to fragile, or too important to be hacked away at with pickaxes. In those cases, it's time to fall back on the old standby – waves. In this case, radio waves.
When a person shines a light on something, a certain pattern of waves comes back at them. Visible light waves travel relatively unimpeded through the air, but bounce off, or are absorbed into, most solid matter. Human eyes – with an assist from the human brain – have evolved with the ability to interpret the way those waves are bounced or absorbed and form it into a coherent picture of the world around them.
When a dolphin makes clicks or squeaks, a certain pattern of waves comes back at them. Audio waves travel through water. When they hit mass, just like light, they are either bounced back or absorbed. Dolphins have evolved the ability to interpret the way those waves are bounced back or absorbed and form it into a coherent picture of the world around them.
When a ground penetrating radar device emits a radio wave into the ground, a certain pattern of radio waves comes back at it. The radio waves, depending on their frequency, travel through certain kinds of soil, but bounce back, or are absorbed, when they hit other types of mass. The device has been programmed to use interpret the way the waves are bounced back or absorbed, and build a picture from it, the same way dolphins build a picture using sound or humans build one using light.
These pictures can combine to a three dimensional image that allows people to 'see' through soil. While they are gray and indistinct, the pictures can provide amazingly accurate information.
This is a needle hole sized leak in a pipe buried under the field of a football stadium:
The water affects the way the radio waves come back at the device, and allow geophysicists to figure out where pipes are leaking, even if there's no water on the surface of the ground.
There are certain conditions that need to be met for GPR to work. There has to be a difference in consistency between the soil it searches through and the object it's trying to find. It works best in dry, loose soil or dry hard rock. The depth is also limited to a few meters, and the lower the search needs to go, the lower the frequency needs to be, and the less detailed the picture is.
Nevertheless, GPR is a powerful, and noninvasive, searching tool. It's effective when it comes to finding the remains of ancient buildings under modern structures, or buried metal wreckage, but the most famous recent case of GPR being used is the mass grave of Duffy's Cut. Local legend had it that a number of Irish workers were buried in Duffy's Cut, Pennsylvania, after they had died of cholera. When people got curious, they started exploring the area where the graves were supposed to be. After years of searching they hadn't found any bodies. It was only once GPR was used, that they began finding human remains. Once those remains were excavated, it was determined that one of the skeletons they recovered died due to a gunshot wound to the head, and two others looked like they'd been beaten to death rather than died of illness. The newly discovered evidence makes it look like it was violence, not sickness, that killed many of the men.