Application of vertical radar profiling technique to Sendai Castle

The vertical radar profiling (VRP) technique is able to explore much deeper than conventional surface ground‐penetrating radar (GPR) because it uses boreholes. It has been successfully applied at the Sendai Castle site in Japan to investigate the extent of an old stone wall and strata buried by a more recent stone wall. The transmitter of a polarimetric radar system was moved within a borehole, and the receiver was fixed on the ground surface several meters away from the borehole head. Cross‐ and copolarization data were measured at a receiver position with a different orientation to the receiver. Ten data sets were acquired by placing the receiver in five directions. The depolarization is strong, indicating the subsurface contains a great amount of gravel. To get clear and intuitive images of the subsurface, we applied data processing techniques, including the separation of direct and reflected waves of raw VRP data using f-k filtering approach and Kirchhoff migration of separated reflected waves. By com...