  Existence of GZK neutrinos (ultra high energy neutrinos) have been justified
although the flux is very low. A new method is desired to use a huge mass of a
detector medium to detect them. A fundamental study of radar method was carried
out to measure microwave reflection from electromagnetic energy deposit by
X-ray irradiation in a small rock salt sample. The reflection rate of 1x10^-6
was found at the energy deposit of 1x10^19 eV which was proportional to square
of the X-ray intensity suggesting the effect to be coherent scattering. The
decay time of the reflection was several seconds. This effect implies a large
scale natural rock salt formation could be utilized like a bubble chamber
irradiated by radio wave instead of visible light to detect GZK neutrinos.
