How deep can georadar 'see'?

Pictures from georadars supposedly serve as proof of the existence of a secret Nazi-era train 70 meters (200 feet) inside a mountain near the Polish town of Walbrzych in Lower Silesia.

Deputy Culture Minister Piotr Zuchowski was confident of the discovery, speaking to the press on August 28, adding that pictures he saw even showed details of the train, like canons.

But three days later, the head of the local government of Wroclaw sounded much more skeptical. The only pictures authorities had received, he said, included a sketchy map and some descriptions. There were no georadar pictures.

Not only does the very existence of the train remain a mystery, therefore, but now there are questions as to whether the georadar pictures exist.

The treasure hunters, meanwhile, prefer to stay anonymous.

Multi-use

In principle, all georadars, whether used on satellites, at construction sites, or inside mines, work the same way.

"But the frequencies are different for different uses," says Uwe Meyer, who heads the department of geophysical exploration at the German Federal Institute for Geosciences and Natural Resources. "The deeper you want to look into a rock, the longer your wavelength must be. And you need more energy to penetrate the ground."

And there's a drawback to looking deeper, says Jochen Kurz, who works with georadars at the curiously named Fraunhofer Institute for Nondestructive Testing.

"The longer your waves, the sketchier the picture you get out of it."

Take construction, Kurz says. When workers look one-meter deep inside a bridge built with steel-reinforced concrete, they can identify individual rods with a diameter of one or two inches.

"But in the case of the train, we're dealing with totally different structures. If I want to look 70 meters deep, I need wavelength of well over one meter."

And this is where physics limitations come into play: It is only possible to detect objects that are bigger than half the wavelength - in this case, well over one half meter.

At such depths, it may be possible to determine that there is indeed something reflecting underground, but it will be very difficult to say exactly what it is.

Serious doubts

In fact, Kurz seriously doubts that a georadar could find anything 70 meters deep inside the rock.

It would be possible, he says, if the treasure hunters were not scanning the mountain from above or from the side, but from inside - for example, from a neighboring tunnel or shaft. Then, of course, the picture could be clearer.

"With a structure like a tunnel, I'll see the cavity. And of course, if there is indeed a train, I will have a perfect reflector - namely metal."

Provided, of course, that the tunnel itself isn't made out of steel reinforced concrete, which would provide a shield against the radar signal.

Still, while it would be theoretically possible to make a two- or even three-dimensional picture out of this radar data, it would require enormous effort, Kurz says. And part of the work would have to be done from the surface.

"You would take the antenna and drive, in a meandering way, over that surface. Every line that you record with your georadar is called a radargram. Later, you can compute all those radargrams into a three-dimensional picture."

But this requires a large, flat surface somewhere above the supposed treasure-train - something like a field.

If there's forest or a rocky landscape, then taking the radargrams will be much more difficult. And composing them into a complete picture requires more than average professional skill.

Geophysicist Uwe Mayer sees even more complications.

"If there is a layer of groundwater above the train, or if there is moist humus, the ground will be highly conductive, and will therefore suck up the radar signal like a dry sponge. The signals will dissipate right at the surface - and then, they're gone."

Other means?

Only in materials with low conductivity can a radar can penetrate well - in dry sandy soil, in salt mines or very homogenous rock formations, for example.

But even if the conditions were absolutely perfect, Mexer would still remain skeptical, simply because of the unusual depth of the object.

"I think it would be very difficult to get as deep as 70 meters with any georadar," he said.

Perhaps, then, the treasure hunters had other tools at their disposal, such as acoustic technologies similar to sonar.

Also, earth magnetism may work in finding the train.

"If there is a train, indeed, with a big locomotive, then there is a chance that the earth's magnetic field is enhanced at that place. And that can be detected," Meyer said.

And magnetism would make sense, wouldn't it?

For a train that's full of gold?

No, that would not make a difference.

Gold is not magnetic