經由工程地震學的研究，已認知到地震動會受到地震基盤和土層狀況不同而產生差異，此種現象稱之為場址效應(site effect)。場址效應往往造成某些區域受災特別嚴重，因此在工程場址的選擇，或工程結構進行耐震設計時，都須要對場址的特性有進一步的瞭解。 利用理論解或數值方法探討場址受震反應時，最主要的工作之一就是要確認場址的土層性質，包含質量密度、土層厚度和剪力波速剖面等。由於近年來現地微地動量測經常被用來了解土層的性質，但在反演運算時都是利用一維土層模式之解析解。因此本研究利用三維有限元素數值分析方法模擬不規則土層的受微地動反應，並且利用空間自相關法 (spatial auto-correlation method，簡稱SPAC法)求出頻散曲線後，再利用雷利波在一維土層傳播的頻散曲線的理論解，及向前反演法(Forward inversion)的方式反演土層的剪力波速剖面，以瞭解一維解析解的適用性。 由分析結果顯示，利用SPAC法反演不規則土層之剪力波速剖面時，若突起物之平面尺寸大於觀測陣列時，且採用的現場鑽探資料若在這一凸起物之外時，則無法正確判斷出實際土層的剪力波速剖面，進而造成顯著週期偏移現象，其主要原因為反演的土層模型剪力波速或土壤層數不等於原始模型，另一方面則是反演的土層為一維土層模式與實際的不規則土層不同所致。 It is well known that characteristics of ground motion are affected by the shape and depth of the bedrock and near surface soil conditions during earthquakes. This phenomenon is called site effect which is one of the main reasons for different degrees of damages in an area, even tens of meters apart. Thus, in site section or in the seismic design of structures, detailed information about the underground structure of a site is desired. For either theoretical solutions or numerical solutions of seismic responses of a site, the important parameters required are soil density, soil layer thickness and shear wave velocity and the configuration of bedrock. To determine these parameters microtremor measurement has frequently be used to in recent years where the inversion process uses the theoretical dispersion curve of 1D soil stratum.. In this study, a three-dimensional finite element method is employed to simulate the mircotremors of a soil stratum underlain by an irregular (protruding) bedrock. These results are used to generate dispersion curve by using spatial auto-correlation method. Finally through forward inversion process, the validity of applying 1D theoretical dispersion curve to determining the soil profile of an irregular soil stratum is investigated. The results show that if the area of protruding bedrock is larger than that of an array and the borehole location is outside of protruding bedrock, the real soil shear wave velocity profile can not be determined exactly. The inaccuracy will cause the shift in the predominant period and variation in the values of response spectra. Such a difference may be caused by variations in inversed shear wave velocity profile and the original one, and by the difference in the bedrock configuration of real soil stratum and the one by inversion process which is a 1-D soil stratum.
