A borehole radar system with a range exceeding 100 m in granitic rock has been developed and tested within the Stripa nuclear waste project. This system has been applied in measuring sites all over the world and can presently be used down to 1 km. Different measurement techniques are described: in particular the recent development of a directional antenna makes the system useful in deep boreholes.
Radar methods have been applied in geological investigations for about twenty years. The pioneering work was performed by Cook, who studied the electrical properties of rocks, soils and clays at different frequencies (Cook 1975). He demonstrated that electromagnetic waves can produce radar pictures in a limited frequency window where the range will be some tenth of meters into the ground. Several commercial system have been constructed in the frequency range 100–1000 MHz. These instruments have been successful in locating pipes, measuring the thickness of an overburden, investigating roads and building sites etc. Ground radar has sometimes suffered from a poor reputation caused by over optimistic practitioners who have neglected the difficulties of interpreting the radar data: geological structures are in fact often too complex to admit a unique interpretation, but if these problems are properly taken into account the ground radar can be an efficient instrument at small depths.
The limited range of a ground radar makes it highly desirable to measure with radar from boreholes in order to cover a larger rock volume. Core mapping and borehole geophysical logging can provide information about the quality of rock but are sensitive to the very limited sampling provided by the borehole. Most logging instruments are sensitive to properties only a few cm from the borehole walls and it is not uncommon that important structures are missed because the borehole was misplaced. The risk can be reduced by drilling many boreholes in the same area, but if the borehole is very deep this method is excluded for economical reasons. A borehole radar can alter the situation radically, since the range of this instrument can be more than 100 m in crystalline rock. The range is mainly determined by the water in the rock. Dense crystalline rock will thus be more penetrable the porous rocks though radar measurements are possible even in sedimentary rock. Apart from range a logging instrument must also provide good resolution. The total amount of information collected with a borehole radar surpasses most other logging instruments, since the radar can produce hundreds of values at each measurement point in the borehole. High resolution requires a short pulse. In practice the pulse length is nearly equal to the wave length, which means that the pulse is very broadband in frequency. The resolution is, however, even better than a wavelength, since the system has a time resolution which can resolve details inside the pulse. The wavelengths in rock are 2–5 m with the RAMAC system, but reflectors can be located with a precision better than 0.5 m.
