Seismic studies of a bottom simulating reflection related to gas hydrate beneath the continental margin of the Beaufort Sea
Seismic studies of a bottom simulating reflection related to gas hydrate beneath the continental margin of the Beaufort Sea
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DOI：
10.1029/95jb00961
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发表时间：
1995-07
影响因子：
--
通讯作者：
K. Andreassen;P. Hart;A. Grantz
中科院分区：
文献类型：
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作者：
K. Andreassen;P. Hart;A. Grantz
位于阿拉斯加北部的波弗特海的上大陆边缘，被位于海底300~700米处的强似海底反射(BSR)所覆盖，对应于下面的间隙水和天然气与上面的固体天然气水合物之间的相界。类似来源的BSR在世界范围内很常见，它们通常被解释为标志着含天然气水合物的碎屑沉积物的底部，沉积物中有或没有下面的游离气。令人惊讶的是，人们对这些强烈反射的起源知之甚少。在本文中，我们通过比较不同震源-接收炮检距的合成BSR幅度和波形与波弗特海发育良好的BSR的多道地震反射数据，分析了产生BSR的对比物理性质。为了判别BSR下是否存在游离气，需要用叠前炮检距数据补充近垂直入射数据。振幅-炮检距(AVO)分析表明，BSR主要是由BSR下面碎屑沉积物中游离气的存在而产生的。根据垂直入射合成，估计游离气带薄于11-16m，如果气体浓度随深度减小，则可能厚于16m。根据AVO模拟，初步估计BSR以上沉积物中天然气水合物的饱和度小于孔隙体积的10%。
The upper continental margin of the Beaufort Sea, north of Alaska, is underlain by a strong bottom simulating reflection (BSR) that lies 300 to 700 m beneath the seafloor and corresponds to the phase boundary between interstitial water and natural gas below and solid gas hydrate above. BSRs of similar origin are common worldwide, where they are usually interpreted to mark the base of gas hydrate-bearing clastic sediment, with or without underlying free gas in the sediment. Surprisingly little is known about the origin of these strong reflections. In this paper we analyze the contrasting physical properties which produce BSRs by comparing synthetic BSR amplitudes and waveforms for varying source-receiver offsets with multichannel seismic reflection data across the well-developed BSR of the Beaufort Sea. In order to discriminate whether free gas is present under the BSR or not, it was necessary to supplement near-vertical incidence data with prestack offset data. The amplitude-versus-offset (AVO) analysis indicates that the BSR is produced mainly by the existence of free gas in the clastic sediments beneath the BSR. The zone of free gas is, based on vertical incidence synthetics, estimated to be thinner than 11-16 m. It is possibly thicker than 16 m if the gas concentration decreases with depth. Saturation of gas hydrate in the sediment above the BSR is tentatively estimated from the AVO modeling to be less than 10% of the pore volume.