多頻道表面波震測之研究

Abstract

傳統之工址調查常需藉助地質鑽探、土壤取樣及室內試驗，以瞭解地下土層之物理性質及力學特性，但這類試驗通常費時、昂貴且取樣體積小，同時擾動後之土壤亦對可能試驗結果產生若干影響，故均屬於破壞性探測法；至於地球物理探測法則屬於非破壞性探測法，可提供快速、經濟且施測容易之試驗方式，以作為傳統鑽探試驗之輔助調查工具，同時在無法進行鑽孔探測之工址中，更可彌補傳統鑽探試驗受地層構造限制之缺憾，並可有效量測大範圍工址之淺部地層構造及波速度分層。 新近發展之多頻道式表面波量測法，乃透過一串受波器及震源之野外試驗配置幾何中，由折射震測儀記錄其震波資料，並利用先進之二維訊號識別技術以計算頻散關係曲線，而後假設土層參數之模型並透過自動化之反算分析流程，以求得試驗土層之平均剪力波速度剖面，進而推求其剪力模數，故現已倍受學術及工程界之重視。 本研究之目的，即探討不同施測參數對震測資料擷取之影響，並決定適當之施測參數配置準則，以減少近域效應、遠域效應、空間之映頻混擾等影響，並依此判定頻散關係曲線之有效範圍；此外由不同分析方法對頻散關係曲線之結果比較中，決定最佳之分析方法，同時定義震測資料之訊號雜訊比；最後由上述之成果中，決定野外試驗之測線配置方式，並依現場之工址條件而採用不同型式之施測方式。

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.