History of the georadar technique at the GGA Institute

In 1971, approval was given to a research and development programme applied for by Dipl.-Ing. R. Thierbach. The aim of the programme was to develop a high frequency electromagnetic reflection method (EMR) in underground exploration in salt mines. The first device operating with a radio transmitter was completed in March 1973 and produced excellent reflections of anomalies in Zechstein salt deposits from data in the frequency range between 50 MHz and 100 MHz. Figure 1 shows a result from the first survey carried out in 1973. The reflections show the position of anhydrite and clay beds within the younger rock salt of the Na3 Series.

In the following years, the equipment was continuously further developed and the technique further refined, and numerous surveys of flat and steeply dipping beds were carried out in potash and salt mines.

A second research project (begun in 1974) looked at direction finding underground to locate the origin of the reflections. This was needed because when measurements are carried out in 360 ° space, signals can arrive from any direction. Although the travel time reveals the distance, locating the reflection surface in three dimensions, and thus being able to create three-dimensional structural models of the deposit, is only possible by locating the direction from which the reflection comes from. Antennae based on the Adcock direction finder principle proved particularly valuable for this purpose.

At around the same time, development of the first borehole logging tools began. The first versions only had either one transmitting antenna or receiver, because logging was done between two boreholes. The results of this work led to the development of integrated transmitter-receiver tools. Tools with lower or higher signal frequencies, i.e. in the range between 25 MHz and 250 MHz, were developed depending on the application - large range or proximal survey with high signal resolutions. Thanks to the generally positive results, georadar surveys subsequently established themselves as a standard method for advance exploration in salt mines, and thus helped considerably reduce the expense associated with the previous techniques involved in this essential work.

The first EMR tool for deep wells with digital data recording and transmission was developed in co-operation with Prakla-Seismos GmbH between 1977 and 1980. This tool was used in all of the exploration boreholes drilled in the Gorleben salt dome, and other salt domes down to a depth of 3000 m. Special firedamp-protected tools were developed for more detailed underground exploration work in the Gorleben salt mine.

Several expeditions to North Canada between 1979 and 1981 provided impressive results on an ore deposit in permafrost. Figure 2 shows the reflections from the top of the pyrite deposit: here at approx. 1 µs, corresponding to a depth of 55 m.

At the beginning of the 1980s, the radio transmitters were gradually replaced by avalanche transmitters which benefit from higher pulse repetition frequencies. The previous analogue logging method with photographic recording was replaced by digital recording which enabled much better signal processing and evaluation.
   
Extensive investigations were carried out on determining the thickness of ice sheets during several Antarctic expeditions. Penetration depths of up to 2000 m were achieved with lightweight carriable equipment specially prepared to cope with low ambient temperatures.

In the first half of the 1990s, a piece of commercial equipment - the SIR-10 made by GSSI/USA - was acquired for surface surveys. This has now been replaced by an SIR-20. A carriable version now also exists, the SIR3000. The spectrum of antennae ranges from pipe dipoles for frequencies of 15 MHz, up to encapsulated reflector types for 1.5 GHz.