利用P波速度修正地溫梯度與天然氣水合物穩定帶底部深度

Abstract

根據中央地質調查所推動的臺灣西南海域天然氣水合物調查研究，顯示臺灣西南海域中應蘊藏有大量的天然氣水合物。由於天然氣水合物的形成受到溫度與壓力控制，故海床下的天然氣水合物在其穩定帶底部(base of gas hydrate stability zone ;以下簡稱 BGHS)以下的沉積物中便會解離成水和游離天然氣。由於含天然氣水合物的地層和含游離天然氣的地層其聲波阻抗差異很大，在震測剖面上，兩者的界面往往顯示出一個強反射面，稱為海底仿擬反射面(bottom simulating reflector,以下簡稱BSR)。理論上BSR與BGHS兩者深度應該一致，但在前人利用地熱量測資料所計算出的BGHS深度與BSR深度比較，顯示兩者間存在有一定誤差，推測主要是因為速度的掌握度不高，導致推算BSR深度不正確，加上地熱探針的量測深度有限，對於量測深度以下之地熱資料必須由外插獲得，導致計算上可能會產生誤差。 為了解決此問題，本研究利用P波速度推算地層中沉積層的孔隙率變化，再利用孔隙率的變化來求得地層中熱導係數的變化。由於地熱量測的區域並不一定有P波速度資訊，為了驗證此方法，首先使用太平洋沉積物速度與深度關係之經驗公式，對海研一號在台灣西南海域所收集到80個地熱測站的資料進行各測站之BGHS深度的估算。結果顯示除了在一些受表層沉積物影響之站位會有異常之結果外，其餘測站估算結果與BSR之深度皆相當吻合，表示熱導係數之修正相當成功。 為了進一步探討海床表層環境對於熱流值之影響，本研究利用天然氣水合物穩定曲線求出BSR深度的溫度，加上先前研究求得之熱導係數變化來計算BSR深度之熱流值。與海床表層量測到之熱流值比較後，臺灣西南海域的地熱異常可分為三個特徵區域：(1) 低熱異常區：在海底峽谷中下游、深海海盆以及增積岩體上部由於海床表層受鬆散沉積物覆蓋所影響，使地殼內部的熱能無法完全傳遞至海床表面導致表層量測到之熱流值偏低。 (2) 高熱異常區：與沉積作用相反，侵蝕作用使熱能更容易傳遞海床表面，因此峽谷上游處海床表層受到侵蝕作用而使海床表層量測到之熱流值偏高 (3) 無熱異常區：於海底峽谷間，海床表層受沉積或侵蝕的影響較小，而使量測之熱流值與BSR深度之熱流值大致相同。 排除受環境影響的地熱站位後，本研究利用2008年TAIGER航次所收集到通過地熱測站或接近測站處的長支距反射震測資料進行速度分析，以求得更精準之速度模型，希望能同時減少BSR及BGHS深度計算上之誤差，結果顯示誤差亦隨著速度精準度的增加而減小，顯示正確的速度值對計算BSR及BGHS深度之重要性。

Marine geophysical and geological/geochemical investigations in the area offshore of southwest Taiwan supported by the Central Geological Survey, MOEA, suggest the existence of a large amount of gas hydrate in that region. Because the formation of gas hydrate is controlled by temperature and pressure, gas hydrate is present only in the gas hydrate stability zone. Due to the differences of acoustic impedances of the sedimentary strata containing gas hydrate and with free gases across the base of gas hydrate stability zone (BGHS), the interface often shows a strong reflector on seismic profiles. This strong reflector is called bottom-simulating-reflector (BSR). The depth of BSR should coincide with the depth of BGHS, but earlier studies of BGHS depths derived from thermal probe data are often different from the BSR depths on seismic profile. Possible reasons include： (1) Inaccurate seismic velocities caused wrong BSR depth estimation; (2) thermal probe can measure only shallow thermal structure, if the thermal gradient calculated is not correct, the BGHS estimation could be wrong . To reduce the problem of limited heat flow measuring depths, this study uses P wave velocity to derive the change of porosity in sediments. Then, we used the estimated porosity value to calculate the change of thermal conductivities. To test this method, we first use an empirical velocity-depth relation to calculate the BGHS depth at 80 heat flow sites offshore southwest Taiwan. The results show abnormal BGHS depth at some sites. These sites are either with high sedimentation rates or covered by thick sediments, give lower in-situ heat flows than the BSR-based heat flows. At other sites, the BGHS depths are in general consistent with BSR depths. The result suggest that adding velocity information to the computation will improve the accuracy of the BGHS depth value . To study the effect of seafloor environments, this study uses gas hydrate phase boundary curve to get the temperature at the BSR, and the divided thermal conductivity value to calculate the heat flow at BSR depth. Comparing with the heat flow measured by heat probe, we can divide the study area into three zones based on their different surface environment：(1) Low heat anomaly zone：heat flow is hindered by soft sediment in lower/middle canyon、basin and accretionary wedge, so the heat flow value measured by the apparatus is lower than that at BSR depth (2) High heat anomaly zone：in area where submarine erosion is occurring, such as the upper canyon zone, higher measured heat flow value are obtained (3) No heat anomaly zone：between canyons, deposition and erosion processes on the seafloor is lower than either Low heat anomaly zone or High heat anomaly zone, so the heat flow measured by the apparatus coincide with BSR depth. Furthermore, this study uses the large-offset seismic data collected during the TAIGER survey at heat flow sites or near sites to derive the local P-wave velocities of the strata. The result shows that more accurate velocity can further decrease the discrepancy between the BSR depth and BGHS depth. We thus suggest that accurate velocity of the sediment strata is important in calculating acc BGHS depth and BSR depth.