Signal Characteristics and Enhancement of Seismic Method for Detecting Underground Cavity

• Main Authors: Hsieh Liang Chen, 陳協良
• Other Authors: Yumin Kang
• Format: Others
• Language: zh-TW
• Published: 1999
• Online Access: http://ndltd.ncl.edu.tw/handle/26443213314808046583

碩士 === 逢甲大學 === 土木及水利工程研究所 === 87 === The purpose of this research was to discuss the signal characteristics and feasibility of various approaches of seismic method for detecting underground cavity by performing a series of numerical simulations as well as lab tests. The primary seismic method was Spectral Analysis of Surface Waves Method（SASW）. The main factors for investigation were the generation of proper seismic source, the placement of receivers, the number of times of impact, the effect of the magnitude of the time factor τ, the measurable of underground cavity, the approaches of producing various frequency and amplitudes by impact source, and the methods for processing the records of signal. The numerical simulations were performed through dispersive Green’s function. A series of lab tests were performed in a model box filling with Ottawa sand and in a poured cement plate. The series of lab tests were firstly performed in a model box filling with Ottawa sand to investigate the placement of receivers, the approaches of installing receivers on the surface of sand, and every possible seismic sources like pencils, steel balls, and steel bars, etc. Because of the cohesionless of sand, we poured a cement plate, and put a plastic pipe inside of it with 15 cm height of the center, 1 m in length, 5 cm diameter, and 80 cm away from the edge as a cavity before pouring. When performing the test on cement plate, a steel ball was used as a falling weight impact source and four receivers, two of them were installed on the surface to measure the vertical displacement, and the other two were installed on the surface to measure the horizontal part. The results of tests with Ottawa sand show that the use of coherence coefficient and cross power spectrum as threshold values for data filtering will successfully reduce large fluctuations occurred in the dispersion curve for high frequency range. And the results of tests with cement plate show the use of either fast or slow exponential functions can enhance the existence cavity. The depth of penetration of Rayleigh waves was found to be half of the wavelength of Rayleigh waves when S=D=20 cm in this research.