斷層掃描是一種經由物體外部量測的資料，以進行內部顯像的技術，斷層掃描發展至今在醫學診斷與地質探勘皆具良好成效，然而於土木工程的非破壞檢測上，由於鋼筋混凝土的高度非均質性造成波在其中傳遞行為複雜，還未能發展出實用的模式。因此本論文回顧了基本的斷層掃描理論，選擇較適用於土木結構的級數法作為理論依據，並嘗試利用應力波對一些土木基本構件做斷層掃描的檢測。 本論文之斷層掃描方法由兩部份所組成，分別為正算模式部份與反算部份，正算模式決定波的行進路徑，而反算部份再根據這些路徑算出內部的波速分布。有鑑於直線的路徑並不能良好的模擬實際應力波在混凝土中傳遞的情形，因此本研究使用三維線性走時內差法作為計算波行路徑的方法，這個方法是格點方塊模型中對最短路徑的選擇最精確者。反算部份則選擇速度分布之聯立疊代重建法作為搭配，以進行斷層掃描的分析。 為求在大型非均質混凝土構件中獲得較清晰之檢測訊號，本研究使用了彈簧槍並發展了修正觸發時間原點的裝置進行檢測之工作。根據前人之經驗，本研究提出了一種影像修正之法則，依循此一法則可有效地將土木構件中鋼筋、孔洞、裂縫的位置及尺寸顯像出來。 Tomography is an image technique which generates a cross sectional picture of an object by utilizing the object’s response to the nondestructive, probing energy of an external source. Nowadays the theories and techniques of tomography have been well developed and applied to a number of fields such as Geophysics and Medicine. However, this technique is not well applied in civil engineering. In this thesis, reviews of some basic theories and methods of seismic tomography have conducted first. Then, the series expansion method of 3D tomography has been selected to develop the nondestructive evaluation technique for civil infrastructures. In general, the tomography analysis contains two different procedures. The first one is the forward modeling for a given subsurface model. The second is the inversion for updating the subsurface model. A three-dimensional linear travel time interpolation (LTI) method, which can treat with curved paths is developed to combine with SIRT inversion technique to form a computational tomography (CT) scheme for concrete structure elements. To obtain a clearer image of the concrete element with high attenuation and inhomogenety, a BB gun is applied to generate an intensive and directional energy source into the target. Beside, a photo-diode trigger device is equipped on the nose of the BB gun to catch the time origin of input source and to measure the length of travel time to the receiver accurately. Five types of concrete elements containing voids, reinforcements and cracks are made to test and evaluate the capabilities and the accuracy of the CT scheme and code developed in this research. Analysis results indicate that this LTI/SIRT CT scheme can work well for specimens with relatively large in-homogeneity. For the specimens with smaller in-homogeneity, the images are vague. Some modification techniques are proposed to improve the vague images and make those pictures more clear.
