Abstract This study investigates the evidence of shallow gas from a newly collected dataset comprising 2D multi-channel seismic (MCS), single-channel seismic (SCS), Chirp sub-bottom profiler (SBP), and multi-beam echo sounder (MBES) data from the southwestern continental shelf of the Ulleung Basin, East Sea, Korea. Various indicators of shallow gas were identified in this part the shelf, including seismic chimneys, acoustic flares in the water column, pockmarks, enhanced reflections, and acoustic blanking. Seismic chimneys, which are related to fluid leakage within the subsurface, are characterized weak to high amplitude, upturned or concave-upward internal reflection with polarity reversal in MCS section, whereas they are faint or transparent on SCS section with 700 Hz and Chirp SBP sections with 3.5 kHz frequency. Acoustic flares, which are diagnostics of active gas venting, were detected only on the Chirp SBP sections. There is also evidence of small-scale depressions, immediately below the acoustic flares; these are interpreted as formed by the sudden explosion of gas. Enhanced reflections (~3 km-long) concordant with stratification are observed at different levels. Acoustic blanking anomalies, caused by absorption of acoustic energy due to the presence of gas, are seen in the range 50–200 ms TWT below seabed. Amplitude versus offset (AVO) analysis indicates the presence of free gas ca. 350 ms TWT below sea surface using the MCS dataset recorded by a 600 m-long streamer cable. The AVO cross-section shows that the lower repetitive signals in the MCS data are due to the gas plume, not lithological contrast. Our study highlights that AVO analysis is an extremely useful tool for identifying free gas, helping to discriminate water-gas contacts and bright events among the chaotic signals on the MCS data.

中文翻译：

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通过地震资料和 AVO 处理识别浅层气：以韩国东海郁陵盆地西南大陆架为例

摘要 本研究从新收集的数据集中调查浅层气的证据，该数据集包括二维多道地震 (MCS)、单道地震 (SCS)、Chirp 海底剖面仪 (SBP) 和多波束回波测深仪 (MBES)来自韩国东海郁陵盆地西南大陆架的数据。在大陆架的这一部分确定了浅层气体的各种指标，包括地震烟囱、水柱中的声耀斑、麻点、增强反射和声学消隐。与地下流体泄漏有关的地震烟囱在 MCS 剖面具有弱到高振幅、向上或凹向上的内反射和极性反转的特征，而在具有 700 Hz 的 SCS 剖面和 Chirp SBP 剖面上它们是微弱或透​​明的频率为 3.5 kHz。声光耀斑，这是主动排气的诊断，仅在 Chirp SBP 部分检测到。也有小规模洼地的证据，紧邻声耀斑下方；这些被解释为由气体突然爆炸形成。在不同水平观察到与分层一致的增强反射（约 3 公里长）。在海床下方 50-200 ms TWT 范围内观察到由于存在气体而吸收声能引起的声消隐异常。幅度与偏移量 (AVO) 分析表明存在游离气体约。使用由 600 米长拖缆记录的 MCS 数据集在海面以下 350 毫秒 TWT。AVO 横截面显示 MCS 数据中较低的重复信号是由于气体羽流，而不是岩性对比。