宜蘭平原位於台灣的東北方，琉球隱沒系統中沖繩海槽的最西端，介於雪山山脈地質區和中央山脈地質區之間，上面覆蓋未固結的第四紀沉積物。宜蘭地區具有高的地溫梯度與多處溫泉，可能是鄰近山區之變質岩殘餘熱或沖繩海槽弧後擴張的火成活動所影響。本研究利用震測探勘法來探究大尺度的宜蘭平原、中尺度的蘭陽溪河兩岸和再連地區的地下構造，目的在於探測宜蘭三星地區高的地熱潛能。
本研究分為三部分。首先，在整個宜蘭平原佈設119個地震儀，實施炸藥震源廣角反射實驗。第二部分，在蘭陽溪的兩岸(三星紅柴林和員山內城地區)，以雙震盪車為震源，展開數條二至四公里的測線，進行反射震測探勘，測繪地下構造。第三部分，在員山粗坑地區，以撞擊器為震源，進行折射震測與半反射震測，探測再連斷層。最後，經結合其他地球物理方法和鑽井資料，分析解釋此區域高地溫梯度的原因。
本測區地下分別為沖積層(波速800公尺/秒)、乾溝層(3000公尺/秒)、四稜砂岩(5700公尺/秒)及更深的地層，沖積層厚度100~800多公尺，往西邊變薄；乾溝層厚度約800公尺維持等厚至大洲一帶，往西變薄。四稜砂岩層厚度約800公尺，維持等厚至大洲一帶，往西側抬升，至泰雅橋附近出露。蘭陽溪北岸有一個斷層構造─再連斷層，從粗坑地區開始沿著蘭陽溪北岸向東邊延伸，其落差在三星紅柴林超過1000公尺，藉由與其他地球物理資料比對，顯示此斷層構造可能沿著蘭陽溪，繼續向東延伸至宜蘭外海。紅柴林一號井的鑽井資料顯示本研究預估的地層深度相當準確，原本認為四稜砂岩為良好的儲集層，實際為四稜砂岩層過於完整無破裂帶，使得熱水無法在岩層內移棲，導致井底溫度不如預期的高溫。此區的地熱概念模型:高地溫梯度受控於斷層構造，熱源由龜山島底下周圍岩漿庫，隨著蘭陽溪底下的斷層構造進入宜蘭平原，天水則從雪山山脈往宜蘭平原底下流動，受到火成侵入影響提高溫度，隨蘭陽溪兩岸斷層破裂帶向上移棲至淺部，使得宜蘭特定區域產生高地溫梯度

The Ilan Plain in northeast Taiwan is located at the southwestern tip of the Okinawa Trough, which extends westward into the Taiwan orogeny. The Ilan Plain covered by thick sediments is clipped by the Hsuehshan Range in the northwest side and the Central Range in the south side. High geothermal gradients with plenteous hot springs of this area may result from magmatism associated with the back-arc spreading of the Okinawa Trough. In this study, we use geophysical methods to explore underground structures in the whole Ilan Plain, especially around the LangYang river and the SanShin area. We aim to find high geothermal potential spots in the plain area.
The research is divided into three parts. First, we use explosion sources with 119 Texan instruments to detect velocity structures by tomography mapping. Second, we use two mini-vibrators to investigate the area along the LangYang river by reflection seismics. Third, we explore a significant fault by refraction seismics and semi-reflection seismics with the impactor source. Finally, we combine with other geophysical and drilling data to analyze, compare and explain the high geothermal gradients in this area.
The shallow part of Ilan Plain can be divided into three layer: 800 m/s for alluvial deposit, 3000 m/s for Kanko formation, 5700 m/s for Szeleng sandstone. The thickness of each layer are 100~800m, 800m, and 800m, respectively. From seismic survey, there is a Zailian fault with an offset of at least 1000m, along the north bank of LangYang river. After comparing with the drilling data around Sansing area, we find that the Taiyaqiao anticline in Hsuehshan Range has entered the Sansing area and is bounded by the Zhuoshui fault (south) and the Zailian fault (north). According to the report of the HCL-1 well, the formations predicted by seismic method agree well with the drill results. However, the temperature in HCL-1 well is only 70.7℃ at the 2200m deep. The low temperature may be due to the Szeleng sandstone which has very limited fractures. The geothermal reservoir system might be controlled by faults, instead of lithology. The heat sources from the magmatism formed in the upper mantle/ lower crust may come from the Ryukyu backarc system. The magma migrates into the Ilan Plain along the Lanyang river. Meteoric water flowing underground from the Hsuehshan Range to the Ilan Plain gets its temperature heated by the intruded igneous hot rock. The fault systems provide the paths of heat migrating into the shallow parts of the Ilan Plain, which may cause the high geothermal gradients in this plain area.

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