Georadar

Radar measurement with helicopter Source: BGR

The measuring method with the abbreviation radar “radio detection and ranging” was developed at the beginning of the 20th century. It was used for the localization of objects and for the determination of their distance, first in navigation and later also in the military domain. The German engineer Christian Hülsemeyer was the first to deal with the reflexion effect and, in 1904, filed a patent for the localization of distant metallic objects. This measuring method was applied already very early in the domain of geosciences under the designation georadar measurements or electromagnetic reflexion measurements (EMR).Already in 1904, the Swedish scientist O. Trüsted proposed a method of high frequency claims in ore deposits. Today, it is internationally known as “ground penetrating radar” (GPR).

Georadar principle Source: BGR

In applied geophysics, georadar belongs to the newer measuring methods, that undergo a permanent process of technical advancement. It is being used more and more intensively for the exploration of geological structures, as it works non-destructively, is relatively cost-efficient and can be carried out logistically easy. 

In geoscience, the following radar measuring methods are applied: pulse radar, interferometry radar and the stepped frequency radar.

BGR mainly applies pulse radar and stepped frequency radar.

Pulse radar uses an electromagnetic impulse, that is emitted omnidirectionally into the surrounding medium. If this electromagnetic impulse (wave) hits heterogeneities that differ abruptly from the medium concerning conductivity (sigma) and relative permittivity (epsilon), part of the energy of the electromagnetic wave is reflected and received by the receiving aerial. These abrupt changes in most cases represent geological boundaries, changes in the mineralogical composition or changes in moisture content.

Stepped frequency radar uses an exactly defined number of frequency lines whose frequency changes gradually. A signal synthesis transforms the reflected signal from the frequency domain into the time domain.

The distance d to the reflexion objects can be calculated from the velocity of propagation v of the radar wave and the running time t of the signal to object.

High exploration depths can be achieved, where conductivity is low. As a rule, this is the case for example in salt deposits, ice or in crystalline rock.

Resolution depends on the measuring frequency used. High frequencies yield better resolution with lower penetration depth.

Georadar measurements are an important instrument for the exploration of near-surface structures, for ice-thickness measurements and for the subsurface geological exploration in mining.

Ground radar GSSI Source: BGR

BGR uses pulse radar systems made by the manufacturers Hentschel and GSSI as well as own developments.

Furthermore, in cooperation with Hamburg Technical University, a pulse radar system was developed, that can operate from aeroplanes or helicopters and is very well suited for service in polar regions (ice thickness measurements).

A pulse radar system is, due to its low resolution, only of limited use for geological purposes. Thus, BGR and the company RST Radarsysteme developed another helicopter borne SF radar system, which is based on the stepped frequency system.

Due to improvements of the helicopter and aeroplane systems, a fast and efficient exploration of near-surface structures can be carried out on extensive areas and for different assignments.