以多頻道表面波譜分析法建立台灣強震測站波速模型

Abstract

剪力波速為土壤動態性質之重要參數，其於工程設計上的應用範圍甚廣，但傳統 測定剪力波速的方法，例如跨孔式、下孔式或懸盪式井測法，因須鑽孔導致成本高昂， 且其代表性受取樣數量限制。相形之下，多頻道表面波譜分析法(MASW)為一個相 當具有潛力的替代方案;惟其可靠度仍待檢驗。 本研究經由數值模擬及現地試驗檢視該方法。數值模擬部分以多組假設地層之分 析結果觀察不同情境下頻散曲線的行為。而現地試驗則透過於台灣強地動觀測網 (TSMIP)中有現地調查資料的測站中實測表面波，並與以懸盪式井測法所得之結果 比對。除了用於比對之測站，本研究以多頻道表面波譜分析法為六個未調查測站建立 剪力波速資料。 分析結果顯示，由多頻道表面波譜分析法所得之頻散曲線穩定且可重現。而即便 表面波譜法所估計之vs30較懸盪式井測法之估計值低，其地盤分類的結果與懸盪式井 測法之結果大致相同。另一方面，於探測深度內若剪力波速有驟增之情形將造成反算 之剪力波速精準度下降。而波速若有隨深度降低的情況，會依其發生深度對頻散曲線 的不同頻段產生影響，造成反算時頻散曲線的有效範圍受到限制，導致反算所得之剪 力波速精度下降。

Conventionally, geological surveys for the shear wave velocity, such as down-hole, cross-hole, or suspension P-S logging, etc., are expensive, labor-intensive, and must leave a lasting mark in the field. Therefore, there is always a demand for a cost-effective and non-destructive way to conduct the survey. The multichannel analysis of surface waves (MASW) solves this scenario with the developed data-processing techniques. However, concerns regarding the reliability of this method are still under discussion. This study consists of two parts. Firstly, numerical analysis using FLAC is conducted based on hypothetical profiles and the well-investigated Lotung LSST site to observe the behavior of the dispersion curves and evaluate the applicability of MASW. Secondly, it verifies the reliability of MASW by applying the technique to sites within the Taiwan Strong Motion Instrumentation Program (TSMIP) network and comparing the results with information obtained from PS-logging. The shear wave velocity profiles are also constructed in six non-investigated sites with MASW. The findings reveal that MASW generates stable and consistent results in experimental dispersion curves, and the uncertainty in shear wave velocity profiles mainly arises from the inversion analysis or mistaking the fundamental mode. From the perspective of site classification, the MASW performs well, although it appears to underestimate the vs30 compared with PS-logging. On the other hand, the potential problems that might interfere with the continuity of fundamental dispersion curves or decrease the accuracy of the shear wave velocity profile are also discussed and displayed.