多頻道表面波震測法施測改良與案例分析

Abstract

多頻道表面波震測法可以非破壞性的方式評估地層剪力波速剖面，並持續朝二維剪力波速剖面影像分析之應用的趨勢發展。然而多頻道表面波震測法在目前現地施做程序下，常遭遇到幾個問題：傳統插入式受波器(Plant geophone) 轉換測線通常都需要耗費相當之時間與人力；表面波之探測深度取決於震波頻率、測線展距與震源支距，且需要足夠低頻的訊號與震源能量，才能獲得足夠的探測深度，目前常使用的震源常有能量不足或移動性不佳等問題。 因應上述施測不便，本研究目的為 (1) 研發一套兼顧經濟性與效率性的新式受波器系統(Land streamer)並與傳統插入式受波器比對其資料可性度與適用性；(2) 藉由人工震源改良，組裝一操作便利的落槌系統，改善能量不足所帶來對訊號的影響；(3)配合震源觀察不同墊片所產生之低頻波的品質，以使收錄之寬頻訊號更為完整。 研究結果顯示，新式受波器於平滑路面之表現良好，在瀝青路面與砂性土壤表面使用的成效已得到印證，可以有效的提升表面波震測的施測效率；新研發之自由落鎚系統，其震波訊號波長範圍約可達30m，有效的改善空間探測深度的能力；另外，以橡皮墊作為鎚墊的確可以改善低頻訊號品質，但成效並不明顯。案例研究選擇一場趾展示不同非破壞性技術之成果與MASW的二維探測成效。

MASW can be used to investigate the shear velocity profile of stratum via non destructive tests and has been extended to perform 2-D Vs tomography. However, several inconveniences are encountered via current test procedures. Switching the survey line is quite time and man-power consuming when planting geophones are deployed; the explored depth depend on the quality of lower-frequency part of signal which is affected by the survey line length, source offsets and characteristics of source energy. Commonly-used seismic sources usually have insufficient mobility or energy. To improve those inconveniences, this study aims at there aspects. First, a new acquisition system, land streamer, is developed under the consideration of efficiency. Its reliability and performance are also verified by comparing the data from traditional planting geophones. Secondly, a handy seismic source made of weight-drop system is developed to improve seismic energy while maintaining mobility. Thirdly, to widen the frequency bandwidth and improve the quality of signal, the performance of different rubber pads with corresponding seismic source are evaluated. The results show that the new receiver system has a good performance on the smooth surface (asphalt and sandy ground). The workability and efficiency of MASW are effectively increased when applying the land streamer. The weight-drop source enlarges the wavelength bandwidth signal up to 30 meters which increases the spatial measuring capacity on depth. The rubber pads make only slight improvement on the quality of lower frequency part of signal. A case study is also presented for demonstrating the results of different NDT techniques and the effectiveness of MASW 2-D tomography.