One of the first applications of GPR was geological exploration such as salt and coal, which was started 1970s. Many tests showed it is a good method to estimate the location of mineral resources. The GPR equipment at that time was quite simple, but in specific application, it was quite useful. We can find many salt deposits in north Germany. The salt deposits in natural geology are complete dry material and it is transparent for electromagnetic waves. Therefore maximum range by GPR reaches several hundred meter very easily. GPR plays an important role to find any anomalous material in salt deposit. The same concept can be used for monitoring of water flow in crystalline rock. Crystalline rock such as granite contains very little water flow in it, therefore it is also a good material for storage. At the same time, electromagnetic waves can penetrate relatively easily in this material. GPR in salt was quite successfully used, then advanced GPR systems such as directional borehole radar were also developed. In addition, salt deposits can be used for GPR experiments for fundamental understanding of electromagnetic wave propagation, and analysis of antenna performance in material.