表面波震測施測程序之優化與標準化

Abstract

近幾年來表面波震測法應用於探測淺層地層剪力波速技術研究日 趨成熟。此量測技術屬非破壞性檢測，因無需開孔破壞地表土層故可快 速且經濟地量測地下土層之剪力波速剖面，備受工程界之重視與應用。 波譜分析表面波震測(SASW)與多頻道表面波震測(MASW)為目前較廣 為使用之表面波震測分析技術，其牽涉到頻散曲線分析方法的基本差異， 兩者之優劣比較常可見於文獻;另方面，表面波震測測線現場施測之幾 何配置與震源使用對於頻散曲線之品質與頻寬影響甚大，相關參數之使 用莫衷一是，實務上通常需要於現場進行多次測試方能取得最佳之配置。 表面波震測技術在前述現場施作參數設定與頻散曲線分析的多樣性容 易造成工程師應用上之困擾，進行影響其推廣與應用。為改善此現象， 本研究以獲取最大頻寬頻散曲線為目標，針對表面波震測之現場施作參 數的設定與頻散曲線分析方法提出標準化施作與分析流程。，首先，本 研究探討個別施測參數(如震源種類、近站支距、受波器間距)對於頻散 分析之影響，並以 Pseudo-section 的概念，於現場收錄不同震源與不同 近站支距之訊號，而後在頻散曲線分析過程中加入各頻率下最佳空間範 圍之選取，將近場效應與遠場效應對於震測資料的訊號影響程度降至最低，再以 Pseudo-section 的概念接合資料產生擁有各近站支距之訊號， 有效利用不同震源所產生之頻段，且降低施測參數互制對訊號分析的影響，獲得最佳頻寬之頻散曲線，在此應用下，達到將表面波震測之現地施作與頻散曲線分析流程標準化的目的。如此，現場蒐集震測資料時不 因施測者不同而有對現地施測參數測試結果不同的看法，造成施測成果 上的差異，以利其未來工程應用之推廣。

The surface wave method utilized in shear wave velocity profiling of the underground becomes more mature than ever in recent years. Due to its non-intrusive feature and convenient operations, it is now widely used in site investigation and earthquake engineering. Among all the seismic methods, surface wave testing is easiest to perform since surface wave contains most of the energy in the seismograms. Being able to sample a very large volume, surface wave method is suitable for larger-scale site investigation. The data reduction method for dispersion relation in a surface wave testing is conventionally asociated with a certain method of data acquisition. At present, the two-station spectral analysis of surface wave (SASW) and multi-station analysis of surface wave (MASW) are the most popular methods used worldwide. They involved different approachs in dispersion curve analysis. The debates between two methods are still undergoing. Besides, field configuration such as near offset, geophone spacing and source type would influence the performance of testing. Practically, try and error in the field need to be done before an optimum field configuration was determined. These two situations can be confusing to geotechnical engineers as to apply the method in daily work. The purpose of the study is to propose a more definitive guideline for MASW testing and analysis to optimize the effective frequency range of dispersive curves. Firstly, the influences of field parameters were fully studied based on field data. Then, a framework based on pseudo-section concept was proposed to minimize the problems. In the framework, to maximize the bandwidth of dispersion curve, multiple sources can be used for each walk-away shot. For each frequency component, the optimal combination of source and offset can be selected to mitigate the near and far field effects as well as maximize the obtainable bandwidth for the dispersion curve. An expansive offset range in the same spatial range by synthesizing seismic records with different nearest source-to-receiver offsets can be generated based on this approach. The dilemmas in the field configuration is solved and the bandwidth of dispersion curve is maximized. Three field examples demonstrated that this new approach provides a more definitive guideline for MASW testing and maximize the obtainable bandwidth for the dispersion curve.