Reservoirs of ¹⁴C-depleted methane (CH₄), a potent greenhouse gas, residing beneath permafrost are vulnerable to escape where permafrost thaw creates open-talik conduits. However, little is known about the magnitude and variability of this methane source or its response to climate change. Remote-sensing detection of large gas seeps would be useful for establishing a baseline understanding of sub-permafrost methane seepage, as well as for monitoring these seeps over time. Here we explored synthetic aperture radar’s (SAR) response to large sub-permafrost gas seeps in an interior Alaskan lake. In SAR scenes from 1992 to 2011, we observed high perennial SAR L-band backscatter (σ ⁰) from a ∼90 m-wide feature in the winter ice of interior Alaska’s North Blair Lake (NBL). Spring and fall optical imagery showed holes in the ice at the same location as the SAR anomaly. Through field work we (1) confirmed gas bubbling at this location from a large pockmark in the lakebed, (2) measured flux at the location of densest bubbles (1713 ± 290 mg CH₄ m⁻² d⁻¹), and (3) determined the bubbles’ methane mixing ratio (6.6%), radiocarbon age (18 470 ± 50 years BP), and δ¹³C_CH4 values (−44.5 ± 0.1‰), which together may represent a mixture of sources and processes. We performed a first order comparison of SAR σ ⁰ from the NBL seep and other known sub-permafrost methane seeps with diverse ice/water interface shapes in order to evaluate the variability of SAR signals from a variety of seep types. Results from single-polarized intensity and polarimetric L-band SAR decompositions as well as dual-polarized C-band SAR are presented with the aim to find the optimal SAR imaging parameters to detect large methane seeps in frozen lakes. Our study indicates the potential for SAR remote sensing to be used to detect and monitor large, sub-permafrost gas seeps in Arctic and sub-Arctic lakes.

中文翻译：

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合成孔径雷达 (SAR) 检测到阿拉斯加湖泊中的大量气体渗漏

永久冻土层下方的¹⁴ C 贫化甲烷 (CH ₄ ) 是一种强效温室气体，在永久冻土层融化形成开放塔利克管道的地方很容易逃逸。然而，人们对这种甲烷来源的规模和变化或其对气候变化的响应知之甚少。对大型气体渗漏的遥感探测将有助于建立对永久冻土层下甲烷渗漏的基线了解，以及随着时间的推移监测这些渗漏。在这里，我们探索了合成孔径雷达 (SAR) 对阿拉斯加内陆湖中大型地下永久冻土气体渗漏的响应。在 1992 年至 2011 年的 SAR 场景中，我们观察到常年高的 SAR L 波段后向散射（σ ⁰）来自阿拉斯加内陆北布莱尔湖（NBL）冬季冰中约 90 米宽的特征。春季和秋季的光学图​​像显示，与 SAR 异常位置相同的冰层上有洞。通过现场工作，我们 (1) 确认了湖底大麻点处的气体冒泡，(2) 测量了最密集气泡位置的通量 (1713 ± 290 mg CH ₄ m ⁻² d ⁻¹ )，以及 (3 ）确定了气泡的甲烷混合比（6.6%）、放射性碳年龄（18 470 ± 50年BP）和δ ¹³ C _CH4值（−44.5 ± 0.1‰），它们一起可能代表来源和过程的混合物。我们对 SAR 进行了一阶比较σ ⁰来自 NBL 渗漏和其他已知的具有不同冰/水界面形状的亚永久冻土甲烷渗漏，以评估来自各种渗漏类型的 SAR 信号的变化性。提出了单偏振强度和偏振 L 波段 SAR 分解以及双偏振 C 波段 SAR 的结果，旨在寻找最佳 SAR 成像参数来检测冰冻湖泊中的大型甲烷渗漏。我们的研究表明合成孔径雷达遥感有潜力用于探测和监测北极和亚北极湖泊的大型地下永久冻土气体渗漏。