高側向解析度表面波震測法之優化

Abstract

表面波震測法在經過數十年的發展，已經成為量測地層剪力波速的重要的方法之一。而傳統表面波震測法會有施測參數與施測目標互制的情況(近站支距、展距以及接收器間距)，透過高側向解析度表面波震測法可以憑藉多個震源資料相縫合的方法加以解決。然而在一些側向變化地層下，使用原本的資料縫合手段會造成頻散曲線預期外的跳動。本研究主要目的有二，一是探討可能造成頻散曲線跳動的側向變化地層施測配置，並施以合適的方法改良，提出高側向解析度表面波震測法的優化流程；二是在此概念下對現地頻散曲線頻寬最佳化提出可行性方案。 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，來研究造成多震源資料縫合方法結果異常的地層及施測因數，以及提出方案進行改良。根據研究結果顯示，高側向解析度表面波震測法概念主要適用于單一震態主導的表面波震測訊號，當有地層施測結果存在多種模態即會造成頻散曲線跳動。本研究隨後從訊號分析的角度，對多重模態表面波進行f-k filter，再進行組合分析得頻散曲線，判斷通過f-k濾波法優化高側向解析度表面波震測法可行性，并提出高側向解析度表面波震測法優化的基本流程。 基於前述高側向解析度表面波震測法縫合的基礎，本研究提出應用多種頻寬的震源組合以達到頻散曲線頻寬最佳化的概念，透過現地試驗結果顯示，此應用概念具有可行性，通過有效組合不同能量頻寬的震測資料，不但可使得頻散曲線頻寬增大，更可使其影像圖解析度變高，大為提升現場施測成果與效率，未來可進一步朝施做標準化邁進。

The surface wave seismic method has gained popularity in engineering practice for determining shear wave (S-wave) velocity depth profiles after few decades of development. However, there are tradeoffs caused by various survey line parameters (such as near offset, receiver spacing, and offset range) in traditional surface wave test. Currently, better results can be obtained by applying high resolution surface wave method (data combination of multiple source), nonetheless in some lateral heterogeneous layers, using phase-seaming method to combine data of different sources may cause dispersion curve jumping. This study hence aimed at two objectives: (1) Exploring possible combination of survey line parameters and lateral heterogeneous underground conditions which would result in dispersion curve jumping; (2) Introducing a corresponding feasible solution for bandwidth optimization on dispersion curve. This study adopted a fourth-order velocity-stress finite difference method to simulate data based surface wave testing pseudo-section survey lines in different earth models with lateral heterogeneity. According to the simulation result, this high resolution surface wave method is mainly applicable to single-mode dominant surface wave signals. Therefore, in layer conditions which may generate more than single mode, the proposed concept would be erratic due to dispersion curve jumping after combination. Another approach proposed is to analyze by f-k (frequency-wavenumber) filter to filter out different modes and proceed with aforementioned data seaming and scanning procedure. By observing the results of filtered data, the feasibility of f-k filter is determined and high resolution signal processing of surface wave method optimization is then established in this study. This study proposed an optimization concept by applying a variety of frequency domain bandwidth surface signals, in order to achieve the best dispersion curve bandwidth based on high resolution surface wave method, comprising of f-k filter and data combination technique. Field results demonstrated the high feasibility of this application, in which the data combination of various effective energy bandwidth signals not only increased the bandwidth of dispersion curves, but also greatly enhanced the resolution of dispersion curve profile. This finding would boost the efficiency and data quality of field surveys and can be included in standard operation procedures of field surveys in near future.