Analysis of Standing Waves on GPR Responses and the Improvement in Mapping Accuracy of Underground Deteriorating Coastal Structure

• Main Authors: Yun-Li Chen, 陳運理
• Other Authors: Jinder Chow
• Format: Others
• Language: en_US
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/33649945379832418548

博士 === 國立臺灣海洋大學 === 應用地球科學研究所 === 95 === The dissertation is mainly consisted of two studies: One is concerning the analysis of standing-wave curve and hyperbolic travel-time curve responses from ground penetrating radar (GPR) signals; the other is related to GPR signal processing to improve mapping accuracy of underground voids and seawater table (SWT). By employing the GPR, the main purpose of these two studies is to develop techniques for identifying the underground caves by nodal points encoded in the radargram and measuring more accurate depth of void bottom and SWT by deconvolution. Utilizing Ground Penetrating Radar, emitted electromagnetic (EM) standing waves can be generated between imperfect conductors. 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. The first study including indoor and outdoor small-scale experiments could verify 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, Keelung, Taiwan. Further study were carried out at 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 the band-pass filter with bandwidth narrower than the 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 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 HTT or arc-like curves. When emitting the radar waves toward a wall, the interval of the nodes was for estimating the wavelength of receiving GPR signals. Identifying the occurrence of nodal points in HTT or HTT-like curves in radargrams may assist the GPR interpreting work for underground tunnels, drainages, cavities, fractures or solid objects. An efficient restoration of deteriorating coastal structures requires an accurate picture of both aboveground and underground features. Although GPR can map underground features, it creates reflection artifacts. Thus, a model for deconvolution calibration was developed in an outdoor small-scale experiment in our second study. A set of GPR parameters were established, which were then applied at a deteriorating fishing port in northeast Taiwan. The deconvolution filter removed repetitive reflection patterns under the lowest part of the void creating a more accurate map. A 3D map was created from interpolated sketched void boundaries. Due to its high lossy nature at radar frequencies and large contrasting relative dielectric permittivity to the upper medium, the SWT is easily identified. The upper boundary of reflection-free area in the deconvoluted radargram, therefore, indicates the SWT. The modified methods can be employed to survey a wide range of subsurface discontinuities.