Pseudo-section概念於表面波震測應用之數值模擬探討

Abstract

目前表面波震測法藉由對施測參數(近站支距、展距以及接收器間距)的適度配合，已可在施測參數與施測目標的互制情況下獲得較佳結果，然而對於互制的行為卻依舊沒有得到解決。本研究將針對最近所提出的Pseudo-section概念解決方案以數值模擬方式評估此概念應用於表面波震測的可行性。 本研究採用四級速度-應力有限差分法作為數值模擬工具，以不同的側向變化地層為例，對表面波在側向變化地層的行為，以及pseudo-section概念在側向變化地層中的表現進行模擬及討論。根據研究結果顯示，表面波的頻散曲線在遭遇側向變化地層時會有假頻散以及調整帶的現象產生，此現象間接的造成以水平層狀地層為假設的反算結果錯誤。此外，以pseudo-section概念應用於表面波震測中施測，數值模擬結果顯示在無反向行進的反射波訊號存在或此反向行進的反射波與前行的表面波訊號有足夠的距離而無干擾的情況下，可得到與傳統測線所得頻散曲線差不多或更為良好的結果。對於有反向行進的反射波的情形，應加以發展訊號分析方法將其分離出，以使pseudo-section概念於表面波的應用能有更加的表現，即使無法將其分離出，也可採用pseudo-section測線作為側向變化地層存在的檢驗手段。

Presently, by adjusting the survey line parameters, such as near offset, receiver spacing, and offset range, surface wave testing can provide reasonable results even though tradeoffs are involved when determining the field configuration parameters. However, we still have problems with the tradeoffs. We use numerical simulation to study the application of pseudo-section concept, which is introduced to provide a solution of the tradeoffs, in surface wave testing. Forth-order velocity-stress finite difference method is used to simulate the surface wave testing data based on conventional and pseudo-section survey line in earth models with different lateral heterogeneity. According to the results, false dispersion and interference zone exist when surface wave propagating in the earth with lateral heterogeneity. Furthermore, comparing the results based on conventional survey line and pseudo-section concept with no backward reflecting waves or well separation of the backward reflecting and forward propagating waves, the performance of pseudo-section survey line is the same or even better. When pseudo-section survey is interfered by backward reflecting wave, it is necessarily to develop a new signal processing to filter out the backward reflecting wave to achieve better performance. In its current form, pseudo-section survey is still a good way to detect the existence of lateral heterogeneity.