根據台灣地區布蓋異常圖，高屏地區存在一明顯重力低區，為探討形成低區的原因，本研究藉由觀測重力資料，結合Talwani二維理論重力值公式模擬出理論地下構造，提出高屏地區的二維密度模型，並透過重磁力異常分析，期望對此區地下構造提供更好的解釋。高屏地區的剩餘重力正異常及深部磁力異常主要位於旗山斷層西北側，與泥貫入體位置分佈一致，主要是受到此地區多高角度逆斷層及地下火成岩體有關。此外，剩餘重力異常值在旗山斷層有很好的延續性，推測可能在壽山往海域延伸。再者，比較前人利用速度估算高屏地區底下的沖積層厚度，所得結果明顯不同，故可藉由重力資料模擬地下構造做為另一項參考依據。 初始模型的假設，主要參考前人陸上地質剖面及西南海域震測剖面，配合中油台南-高雄十萬分之ㄧ地質圖做為初始模型，利用嘗試錯誤法慢慢修正模型，直到模型的理論重力值與觀測重力值相符，得到一個適當的二維地下密度構造。根據模擬結果，測勘區主要為上新世至中新世的地層，沉積層厚度最深處位於里港，約8.5公里深，與學者估算的磁性基盤面深度結果一致。因此屏東平原底下是個東西對稱，北淺南深的盆地，而中心位置就在重力值最低的里港、九如，形成低區的主因為廣大且厚層的沉積物。透過重力及磁力資料模擬地下構造其結果並非唯一解，因此若有更多的鑽井、震測及速度構造資料配合，就能對深部構造有更好的控制，這麼一來，對於推求地下構造形貌將會有更大助益。 Why the Kaoping area is a low Bouguer anomaly area ? We compiled numerous gravity data to sample as “2km” to discuss the reason and carried out a gravity survey of this study area in the summer of 2005. In the modeling of subsurface density structure, available seismic and well data are used possible to constrain the geometric and/or densities. In this study, the A-A’, B-B’ and C-C’ profiles are projected on the map and the subsurface density structures are simulated by Talwani’s two-dimensional model. The distribution of mud diapir are similar to the positive residual gravity anomalies and magnetic anomalies in the vicinity of Chishan fault. Besides, we infer that Chishan fault extend to the Taiwan strait in Shoushan by the residual gravity anomalies. According to the simulated results, the average depth of the Pingtung basin is 6-8 km and the deepest depth located around the Likang to Jiouru. In short, the Kaoping area is a basin which the depth in southern is shallower than northern. By comparing with the Taichung basin, the subsurface density structure which we simulate is quite reasonable. We wish to simulate the subsurface structure better and get deep conformation in this area by using gravity data, if there’re more velocity structure, seismic and drill data.
