| 摘要: | 本研究利用重力異常資料與地層層位資料來建構苗栗地區地下地質構造。在重力修正的部份，過高或過低的修正密度將導入地形所引起之修正誤差，本研究採用N-密度測定法、g-H關係圖法、布蓋密度與地形密度法等數值方法決定修正密度，並比較其優劣與適用性，以提高布蓋修正與地形修正之精度。 本研究也利用地質統計Kriging推估法來提高重力資料準確性。由於Kriging法具有最小變異數與不偏推估的特性，希望藉由該方法來降低網格插值過程所造成的誤差，並解決部份地區重力測點稀少、繁複或重疊的情形。此外，由於重力測勘所求得之地下構造並不具唯一性，本研究亦利用Kriging法來進行苗栗地區地層推估，選擇幾條垂直主要構造的剖面，利用地層資料作為控制，來模擬地下岩層之側向密度變化，並與重力測勘結果相互比對，以期獲得既能反應重力異常分佈亦有實際地層層位控制之地下構造。 研究結果顯示，將重力測點資料庫建立後，可隨時取用任意範圍的資料，並配合數值地形資料計算出該範圍的岩層平均密度，不但省去直接測量密度的麻煩，所求得之修正密度較具代表性。此外，透過剖面的模擬，顯示在研究區西北側的地層有些微向上抬升，這可能與前人認為台灣西北部中新世地層表層滑動及基底逆衝有關。至於在研究區的東側，整個沈積地層受到麓山帶邊緣造山運動的影響，構造呈現上揚並產生不少高角度的斷層。利用地層資料模擬的結果與重力觀測的結果大致相同，只有在山區無法吻合。整體上理論重力值與觀測重力值的差值皆在 ±5 mgal以內。 It is well known that the use of an incorrect density in constructing Bouguer gravity map will cause abnormal results. To reconstruct a Bouguer gravity map in the Miao-Li area, average rock density was investigated in this study. For optimization of gravity result, the density for the Bouguer correction and terrain correction of gravity was derived from N-method, g-H relations, and the method of Bouguer density and terrain density in this study. In addition, gravity data are often acquired over long periods of time using different instruments and various techniques, resulting in data sets of non-uniform accuracy. In order to obtain more precise gravity data, the Kriging technique of geological statistics was employed to determine Bouguer anomaly value exactly. With the feature of least variance and the best linear unbiased estimator, Kriging technique could take into account station accuracy and decrease interpolation error. The Kriging technique was also applied to the simulation of geological structure. The general procedure to determine geological structure is as fallows. Density structures of three cross-sections perpendicular to strike of the surface geology were obtained from modeling of well strata. We compared calculation result from modeling with observed values, which were judged by traditional correction of gravity. Moreover, the density structures of the model were fixed until the anomaly curves trend to coincide each other. After processing, however, the new method for determination of density has two advantages. First, since the density was computed with the function of the latitude, longitude, elevation, and observed gravity value of the station, the new method can save more time and has smaller error than the previously direct measured method. Second, the density of any region can be obtained form database immediately without reconstructing station data. Besides, the surface density structures obtained from modeling of three cross-sections indicated that the formations in west-northwestern side are uplift. This feature can be recognized by a step in decollement of a deep thrust in the base of Miocene formations. The fault-bend folding mountain belt in eastern side shows that deformation of the Miao-Li area was caused by the northwestward compressive force which was possible the results of the orogeny of early Pleistocene. In general, the gravity results between observed and modeling are consonance except for mountain area. |
