Sometimes you can hear loud advertising promises: “Our georadar sees to a depth of 1000 meters.” It sounds impressive, and many are ready to believe it. But how true is this? Let’s figure out how the method works, where it is effective and where the figure of a thousand meters comes from.
How does the GPR method work?
The method is based on the emission of electromagnetic pulses and registration of signals reflected from the boundaries of layers with different electrical properties. This allows you to record:
- groundwater level (contact of dry and moisture-saturated rocks),
- the boundary between thawed and frozen rocks,
- transition of loose sediments to bedrock,
- underground communications – pipelines and cables.
If the soils have high resistance, and the operating frequency of the georadar is tens of megahertz, the electromagnetic wave behaves like an elastic wave: it is reflected, refracted and diffracted according to the laws of geometric optics. For example, with resistances of 150–300 Ohm m and a frequency of 50–100 MHz, the research depth will be only 10–15 meters. In the presence of clays, silts and loams, the depth is even less.
Problems that the method solves
Today, GPR is used in many areas:
- engineering surveys and construction projects,
- archeology and burial search,
- study of permafrost and glaciers,
- mapping of underground communications,
- geological studies of the upper layers of soil.
On most rocks, the actual depth of exploration is from several meters to tens of meters.
Where did the number “1000 meters” come from?
There are indeed results in the scientific literature where the ground penetrating radar “penetrated” up to a kilometer. But such data relate to glacier studies. Ice is practically transparent to radio waves, and therefore the signal can travel great distances without much attenuation. But this is an exception, and not a universal characteristic of the method.
Where to be careful
If you are offered a “kilometer-long ground penetrating radar” to search for ore bodies or deep structures in hard rocks, this is manipulation. In practice, the method is limited to tens of meters. For deep problems, other methods are needed: seismic prospecting, electrical prospecting, magnetotellurics.
For a comparison of methods, read our material: Complex of geophysical research
Ground penetrating radar is an important tool for the geologist and engineer. But like any method, it has limitations. In soils and rocks it is tens of meters. In glaciers – up to a kilometer. Therefore, when you hear promises of “ground penetrating radar up to 1000 m”, it is important to clarify: where exactly and under what conditions.
Have you encountered inflated promises in geophysics? Share your experience in the comments.
