| Summary: | 碩士 === 國立交通大學 === 土木工程系 === 90 === Traditional site characterization utilizes exploratory boring, sampling, and laboratory experiment to investigate the physical and mechanical properties of the underground. These methods are time-consuming, costly, invasive, and have limited sampling volume. The materials under test are typically disturbed. Geophysical field methods, however, are non-invasive, usually fast, inexpensive, easy-to-perform, and sampling a large volume. Hence, the latter can effectively assist the traditional exploration methods or replace them at sites where borings may not be permitted.
Among several geophysical methods, multi-channel surface wave seismic survey has recently been developed to measure underground shear-wave velocity profile. A refraction seismograph records the seismic signals with geophones equally spaced in a survey line. The signals are analyzed using an advanced 2-D signal pattern recognition technique to efficiently and accurately calculate the dispersion curve of the surface wave. The shear-wave velocity profile can then be automatically inverted from the experimental dispersion curve, and the shear modulus can be further estimated. This method possesses many important applications for site investigation and earthquake engineering.
The primary objective of this research was to study the effect of field configuration on the measurement and compare different analysis methods for the dispersion curve. Parametric studies result in a general guideline for the field data acquisition. The wavefield transformation was found to be most efficient to covert surface wave on a shot gather directly into images of dispersion curves. The signal-to-noise ratio of the dispersion curve determined from the wavefield transformation was also defined. Test procedures that combine traditional seismic methods such as refraction and reflection methods are suggested. A case study was presented to demonstrate the capability of this new technique in estimating the liquefaction potential in gravelly soils. Another case study demonstrated how to measure shear wave and compression wave velocity simultaneously with a refraction array using surface wave analysis and first arrival time tomography.
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