震源分布與傾斜地層及地形對被動式表面波量測分析之影響

Abstract

近年全球氣候急遽變遷，致使短時間內極端性降雨頻率增加，山區複合性災害比例逐年上升，山區防災及減災越來越受到重視，因此大規模深層崩塌地之可能滑動面調查工作顯得更加重要，剛開始的現地調查可以利用被動式表面波震測分析法迅速地獲得地層剖面概況，提高了後續經費成本較高工程之配置效益如鑽探等。由於山區常具有較複雜之地形及地層，與被動式表面波震測之均勻震源或水平層狀地層之假設不符，有進行適用性探討之必要。本研究針對常用之三種被動式表面波震測法，利用三維數值模型探討震源分布於水平層狀地層與一般山區較常見之有坡度之地形及非水平層狀之地層，並以波譜元素法模擬被動式表面波傳遞行為，探討在山區現地施測結果影響因子及影響程度，尋求在山區最合適之被動式表面波震測方法。 本研究成果顯示，在面對有主要來源之被動震源情況下，MAPS分析法能有效修正震源角度對於頻散曲線影像圖的影響；Roadside Passive MASW分析出頻散曲線影像圖有角度修正不足之現象，造成頻散曲線相位速度偏高；SPAC無法考慮震源角度修正，在高頻處(>5Hz)仍能獲得正確頻散曲線，惟其低頻能量明顯不足，頻寬範圍會變小；在模擬傾斜地層方面，傾斜地層造成被動震源皆屬有主要來源方向之形式，因此與有主要震源方向的情形類似，惟此主要震源的方向會與隨率有關，目前本研究所採用之震源方向修正不考慮隨頻率的差異，而使頻散曲線低頻處有提早轉折現象，對於反算測線中點深度會有低估之現象；若施測位置為非水平之地形，由於表面波傳遞在自由表面上，並非傳遞於水平面，因此進行分析時採用斜距作為受波器之間距即可避免高估地層剪力波速，獲取正確之頻散曲線，另方面，反算所獲得剪力波速剖面代表方向為垂直地表之方向，並非垂直水平之重力方向。根據本研究成果做出結論，對於三種被動式表面波震測之應用，以具震測方向修正之MAPS施測分析法較適用於山坡地調查之應用。

In recent years, increase in extreme rainfall is with the global climate changes. The natural disaster happened in mountains increase year by year. Disaster prevention and reduction become very important. One of the base for disaster prevention and reduction is the site investigation. It is a trend to involve non-destructive geophysical method especially surface wave method in large deep-seat landslide case. The shear wave velocity profile obtained from the passive surface wave method would greatly contribute to the investigation planning and the bedrock mapping. It is economic and efficient. However, the application in mountainous area, passive surface wave method violates the horizontal layer model assumption. Thus , the aim of the study is to evaluate the feasibility of passive surface wave method in large deep-seat landslide investigation. The influence of passive source distribution, inclined layer and topography in passive surface wave method are studied by 3D numerical simulation. Three popular passive surface wave methods (Roadside MASW, MAPS and SPAC) are examined. According to the numerical results, source distribution of passive sources would influence the performance of the methods even in horizontal layer earth model. When dominate sources exist, roadside MASW would have higher dispersion curve, MAPS can correct the source out-liner effect and SPAC can perform normally for frequency larger than 5 Hz without any correction. However, the dispersion curve with frequency less than 5 Hz retrieved from SPAC would be usually lower or difficult to identify. For the effect of inclined layer, it would reflect in the distribution of source. Even for the uniform distributed source, it would be forced to reveal as a dominate source case after the waves propagating through the dipping layer. Thus, the examined methods would perform like preview case except for the incident angle correction in MAPS. In fact, the direction for the dominate source is frequency dependent in dipping layer. Currently, limited to the software used, only frequency-free correction is applied. This causes the higher value of dispersion curve in low frequency, which means a shallow bedrock depth would be evaluated. Finally, for the effect of topography, the results show that in simple inclined slope, the effect can be ignored if the geophone spacing set to be the slope distance between geophones instead of horizontal distance and the representing profile should be perpendicular to the slope. Concluding from the numerical studies, MAPS with passive direction correction is the better technique applied in deep-seat landslide inverstigation.