由於彈性波波速受控於介質之彈性勁度，當彈性波傳遞於不同勁度之介質時，其壓力波及剪力波之波速將有所改變。依此特性，彈性波波速之異向性一般被解釋為地層異向性之指標。然而，剪力波於岩層內傳遞時，同時受到完整岩石之異向性、岩體之不連續面位態分布與現地應力影響，而分裂成兩質點運動方向互相垂直且速度不同之快、慢剪力波，故利用波速異向性及快剪力波(Fast shear horizontal，FSH)之質點運動方向判釋最大水平應力方向時，需同時考量其他因素對波速異向性之影響程度。本研究考慮完整岩石力學特性之異向性，並利用Oda之擬連續體模式，藉由台灣車籠埔斷層深井鑽探計畫(Taiwan Chelungpu-Fault Drilling Project, TCDP)之節理資料所建立節理張量，估算岩體之彈性勁度張量。據此，透過波傳理論即可計算出剪力波於節理岩體內傳遞之速度異向性。本研究首先以現地所得之資料，定性地觀察並歸納各因子與剪力波異向性可能之相互關係，結果發現岩性及深度為影響剪力波異向性之重要因子。再根據擬連續體模式與TCDP井測資料相比，結果發現岩性依然對於剪力波波速異向性影響程度高：砂岩段剪力波波速、異向性比皆較粉岩段高，是因為TCDP場址車籠埔斷層上盤之砂岩本身含有方向一致之微裂隙造成其先天異向性較高所致。再根據參數敏感度之結果，節理長度也很大程度地控制了波速異向性之大小。因此，若欲利用FSH推估最大水平應力方向時，須謹慎考慮岩體材料先天異向性。

The direction of the fast horizontal shear wave velocity is frequently used as an indication of the direction of the maximum horizontal principal stress. However, together with the stress induced anisotropy, the wave velocity anisotropy will also be dominated by the inherent anisotropy including the effects of sedimentary and tectonic structures. This study carefully evaluates the influence factors of wave velocity anisotropy in Taiwan Chelungpu-Fault Drilling Project (TCDP) borehole. The anisotropic stiffness tensors of sandstones and mudrocks were derived from the laboratory wave measurement. The equivalently continue model was used to evaluate the stiffness tensor of jointed rocks, which considered the anisotropic distribution of discontinuities. The lithology was identified as the most influential factor on the wave velocity anisotropy. The dip angle of the bedding plan is also a dominating factor. Surprisingly, the joint orientation is not a significant factor influencing the wave velocity anisotropy. Accordingly, determining the direction of the maximum horizontal principal stress from the direction of the fast horizontal shear wave velocity should consider the influence of inherent anisotropy of rock mass base on the TCDP borehole data.

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