ABSTRACT1 Institute of Applied Geoscience, National Taiwan Ocean University, Keelung, Taiwan, ROC 2 Institute of Geophysics, National Central University, Chung-Li, Taiwan, ROC * Corresponding author address: Prof. Joseph Jinder Chow, Institute of Applied Geoscience, National Taiwan Ocean University, Keelung, Taiwan, ROC; E-mail: jdchow@mail.ntou.edu.tw doi: 10.3319/ TAO.2007.18.5.879(T) Utilizing Ground Penetrating Radar (GPR), emitted electromagnetic (EM) standing waves can be generated in underground voids. This phenomenon can be employed for the detection of subterranean voids and fractures when one has a proper understanding of relation between the widest inner length in an underground vacant space and half an EM wavelength. In this study, indoor and outdoor small-scale experiments verified the generation of EM standing waves. These responses were then applied in an arched-top cave covered by a single layer of backfill at Gongzihliao, Taiwan. Further studies were carried out at two other sites, including a fracture located in a granite mountain without regolith on the surface at Kinmen, and a deteriorating fishing port in Nanfangao, northeast Taiwan. Applying a band-pass filter with bandwidth narrower than a typical two-octave bandwidth produced the required standing waves with recognizable positions of minimum amplitude. A hyperbolic travel-time (HTT) curve revealing the minimum amplitude, known as standing-wave nodes, indicates the presence of an underground hollow diffractor with the widest inner length in the vacant space being larger than half an EM wavelength. However, a HTT curve without nodal points signifies a hollow object with the widest inner length smaller than half an EM wavelength or an underground solid diffractor. An underground arched-top cave was detected by nodal points in the arc-like curves. When emitting the radar waves toward a wall, the Terr. Atmos. Ocean. Sci., Vol. 18, No. 5, December 2007 880 interval of the nodes was used for estimating the wavelength of receiving GPR signals. Identifying the occurrence of nodal points in HTT or HTTlike curves in radargrams may assist the GPR interpreting work for underground tunnels, drainages, cavities, fractures, or solid objects.