多頻道表面波震測之施測與頻散分析標準化研究

Abstract

由於非破壞性的試驗方法及簡便操作等特性，將表面波震測運用於工址調查實務上愈來愈受歡迎。特別是多頻道表面波震測（Multi-station Analysis of Surface Wave, MASW）紀錄針對單一測線可提供較深探測深度及較多資料。但如欲得到品質良好且寬頻的頻散曲線，試驗施作時之施測參數即扮演一重要角色。選用施測參數時常因訊號分析、施測解析度及深度上的不同考量而陷入兩難。除此之外，MASW試驗者亦常對前人提出之數種頻散分析演算方法進行驗證比較。本文最主要的課題即為針對MASW中野外施測及頻散分析兩部分提出一標準步驟。針對野外施測，本文先行討論時間及空間施測參數對實驗的影響，包含時間與空間域上的映頻與資料遺漏、波場的遠近場效應、高次震態模組的影響及空間水平解析度等。之後針對各種影響加以探討分析並提出一創新施測方式及合成震測資料方法，藉以消彌各施測參數選用規則上的衝突並使MASW施測步驟標準化而有利一般工地實務應用。在頻散分析方面，本文提出一波場轉換之統一演算法。先將野外收錄之時間-空間域之二維波場轉換至頻率-空間域，並以線性迴歸之多頻道表面波頻譜分析（Multi-channel Spectral Analysis of Surface Wave, MSASW）進行初步頻散分析。透過頻率-空間域之複數頻譜及線性迴歸資料評估收錄訊號品質並消除不良訊號。再將頻率-空間域之複數頻譜以統一波場轉換方式同時得到頻率-波數（wavenumber）域、頻率-慢度（slowness）域、頻率-速度（velocity）域及頻率-波長（wavelength）域之頻散曲線。本文並提出一藉由使用離散空間傅立葉轉換（discrete-space Fourier Transform）之最佳化方法證明各域之頻散曲線皆為相同，並討論頻散曲線取樣時以等頻率及等波長進行之優劣。本文的各項結果可作為日後多頻道表面波震測實驗標準化之基礎。

The surface wave method has gained popularity in engineering practice for determining S-wave velocity depth profiles. In particular, MASW (multi-station analysis of surface wave) method permits a single survey of a broad depth range and high levels of redundancy with a single field configuration. Despite its apparent advantage over the two-channel SASW (spectral analysis of surface wave) method, the testing configuration of the MASW method remains a crucial factor that may affect the test results. Tradeoffs are involved when selecting the testing parameters. In addition, several algorithms with different preferences in the literature exit for the dispersion analysis. The objectives of this study are to establish a standard procedure for field testing and dispersion analysis of MASW. In the field testing, the influences of temporal and spatial parameters were investigated, including aliasing and leakage in both time and space domain, far and near field effects, effect of higher modes, and horizontal resolution. The investigation leads to several rules for choosing testing parameters. An innovative testing procedure and the associated signal processing was proposed to resolve the dilemma of choosing testing parameters and standardize the testing procedure. In the dispersion analysis, a unified approach was proposed. The wavefield in time-space (t-x) domain is transformed to frequency-space (f-x) domain first, in which a preliminary dispersion analysis (a new method called multi-channel spectral analysis of surface wave, MSASW) was introduced and methods for assessing data quality and data screening were proposed. The f-x domain is further transformed to f-k (wavenumber), f-p (slowness), f-v (velocity), or f-λ (wavelength). The dispersion curves obtained by different transformation are shown to be identical by a newly-proposed optimization method based on the discrete-space Fourier Transform, which allows the transformed domain remain continuous for best resolution of dispersion analysis. A wavelength-controlled sampling approach was further proposed for the dispersion curve to avoid bias in depth sampling. The results of this study may lead to further standardization of the surface wave testing.