以衛星測高監測永凍土區域之地層下陷：以阿拉斯加為例

Abstract

受到全球暖化之影響，永凍土會開始消融，進而造成地層下陷，所以監測永凍土之地表高程變化就成為研究氣候變遷一項很重要的議題。由於本研究區域是位於阿拉斯加北部永凍土地區，受到氣候及環境的限制下，不易使用傳統之監測方法，所以本研究是利用Envisat(2003-2010年)及SARAL/AltiKa(2013-2016年)兩顆測高衛星上之雷達測高資料來監測本研究區域。由於地形會影響雷達測高之波形，因此須進行波形重定改正來改善高程觀測精度，而本研究所使用之波形重定方法為次波形門檻值演算法，並透過實驗發現較適合本研究區域之門檻值為0.1。進行永凍土高程變化計算時，須先進行資料處理，並沿軌跡將觀測量分成bin點，再將每個bin點裡之觀測量組成時間序列，然後透過最小二乘方式進行時間和空間的擬合，就可獲得每個bin點之變化速率。從研究結果顯示大部分是呈現下降趨勢，這也證明了因永凍土消融而造成之地層下陷，並可發現在本研究區域的南邊為地形較複雜區域，其下降速率又比本研究區域北邊地形平坦區域之下降速率大。

Permafrost is sensitive to temperature change. As the global temperature rises, permafrost is likely to thaw, and the thawing will induce land subsidence. Monitoring surface elevation changes in global permafrost regions is an important issue in dealing with the impacts of global warming. In this study, we determine surface elevation changes in the permafrost region of the northern Alaska using radar altimeter data from the satellite missions Envisat (2003-2010) and SARAL/AltiKa (2013-2015). Monitoring elevation changes in this region by conventional methods is difficult because of its inaccessibility. We use the subwaveform retracking to improve the accuracies of height measurements from these two missions. Satellite altimeter waveforms here are affected by topographic variations, and we find that a threshold value of 0.1 is the optimal choice when retracking subwaveforms using the threshold method.A data pre-processing procedure is carried out before calculating surface elevation changes caused by permafrost thawing. At each of the along-track bins, we least-squares fit the time series of elevation changes by a function containing a rate and annual changes. The resulting rates are mostly negative, indicating land subsidence caused by permafrost thawing. The subsidence rates in the southern part of the study area are larger than the rates in the northern part. The southern part has a steeper terrain than the norther part.